CHE 3238 Test 1 Study Guide

Which of the following statements about melting point determination is NOT true?

The presence of impurities introduces entropy, which raises the energy required to melt the substance.

A student uses a TLC plate laced with fluorescent dye to spot their compounds in order to visualize these compounds under a UV. light, the compound must?

-absorb UV light -Quench the fluorescence of the absorbent

The stationary phase in GC is:

Liquid

Based on the total vapor pressure, is the mixture a liquid below boiling, boiling, or is it a gas?

Liquid below boiling. Since the total vapor pressure was found to be below atmospheric pressure of 760 mmHg, the liquid must be below boiling.

Calculate percent recovery

(total mass recovered/total mass of sample)*100=%

After a distillation a student runs an IR. He thinks it's methanol. Which stretches might lead him to this conclusion?

-C-H stretch @ 2990-2850 -O-H stretch @ 3200-3500

After a distillation a student runs an IR of their compound. The student thinks it is acetone. Which stretches in the spectrum might lead the student to make this conclusion?

-C-H stretch at 2990-2850 cm-1 -C=O stretch at 1710 cm-1

What stretches in IR would lead to toluene. (benzene ring with methyl off it)

-CH stretch at >3000 -C=C stretch at 1600

Esters contain

-COOR =O and -OR group to same carbon

What are the pros of using hydrogen as a carrier gas in a GC separation?

-Cheaper than helium -greater difference in mass between the carrier gas and sample than Nitrogen (lighter than N) -Can be generated at the GC instead of from gas tanks

Which lab glassware are appropriate for collecting distilate?

-Erlenmeyer flask -Round Bottom Flask -Conical Vial -Graduated cylinder Because of the small openings they have, they are considered appropriate for use as collecting vessels for distillate.

One type of detector used in GC is a thermal conductivity detector (TCD). What two elements commonly make up the wire (filament) used in TCDs?

-Tungsten -Rhenium Tungsten and rhenium are common elements used to make wires (or filaments) for thermal conductivity detectors because of the ability to allow both heat and electricity to be transferred.

Which types of molecular interactions determine a molecule's melting range?

-Hydrogen bonding -dipole-dipole -Van der Waals Hydrogen bonds are present when a compound contains an electronegative atom like oxygen or nitrogen with which a hydrogen can interact. Dipole-dipole interactions are present when there is a permanent dipole in a molecule which can interact with the dipole of another molecule. Van der Waals forces are the sum of both attractive and repulsive forces that are otherwise unaccounted for. Because all of these forces contribute to the overall strength of intermolecular forces, all play a role in determining the melting point of a solid.

How can the injection time be indicated on a gas chromatograph?

-If the GC has analysis software it will automatically be shown on the printout. -If the detector is a TCD, inject several microliters of air with the sample. -Use a pen to mark on the recorder base line immediately after the injection

A young researcher set out to develop a method for quantifying the polycyclic aromatic hydrocarbon (PAH) content in a contaminated soil sample by gas chromatography. During the initial stages of method development, the researcher was unable to separate anthracene and naphthalene with baseline resolution. From the options below, select all possible modifications the researcher could make to improve the resolution.

-Increase the column length -Choose a different stationary phase -Use a column with a smaller inner diameter

Drying agents

-Magnesium Sulfate, Mg2SO4 -Sodium Sulfate, Na2SO4 -Calcium Sulfate, CaSO4 -Calcium Chloride, CaCl2 -Potassium Carbonate, K2CO3 -Brine, NaCl (aq) Drying agents remove water from organic solutions. The effectiveness of these drying agents is usually measured by their capacity to absorb water, what volume of water per gram of drying agent the drying agent holds, and how fast the water is removed from the solution. Most drying agents work well with several types of compounds, but should not be used for specific types of compounds as they may react with functional groups on the compounds of interest. One drying agent, brine or saturated sodium chloride in water, is used to wash organic solutions. The salt in the aqueous solution moves water from the organic solvent to the aqueous layer because salt has a stronger attraction to water than to organic solvents and salt moves towards the more stable state of being diluted with water. The brine solution is then removed by extraction and the organic solution is kept and dried further. Potassium carbonate, K2CO3, is a drying agent with an average capacity for water. It is also average in its rate of drying a solution. Potassium carbonate should not be used in acidic conditions or with acid containing compounds. Calcium chloride, CaCl2, is a drying agent with a high rate of drying, but it has a low capacity for water. Calcium chloride is mainly used to dry solutions containing hydrocarbons. Calcium chloride will also absorb methanol and ethanol from a solution. Calcium sulfate or Drierite, CaSO4, is a drying agent with a low capacity, a high rate of drying, and is useful for most all compounds. Sodium sulfate, Na2SO4, is a drying agent that has a high capacity for water and a high rate for drying an organic solution. Sodium sulfate is useful for most all compounds. Magnesium sulfate, Mg2SO4, is a drying agent with a high capacity and a high rate of drying. Since magnesium is a strong Lewis acid, acid sensitive compounds, such as epoxides, should not be dried with magnesium sulfate

What are the products of the methyl salicylate hydrolysis reaction shown below?

-Methanol -Salicylic Acid

What mistakes during the extraction procedure could lead to a percent recovery rate above 100%

-Not of the drying reagent was filtered out -the samples were not dried enough

What physical properties prevent a sample mixture from being analyzed by gas chromatography?

-One or more of the components is thermally unstable -One or more of the components is not volatile

Not Drying Agents

-Ozone, O3 -Graphite, C -Acetylene, C2H2 -Aginine C4H14N4O2 -Glucose -Hydrogen Peroxide -Chlorine -Nicotine

Why is a flask never distilled to dryness?

-The flask could explode because of residue. -The flask could crack or break -The flask could become charred (When heated to dryness, it creates unsafe conditions)

True statements about melting point determination:

-The lower limit of the melting point temperature range occurs when the first crystal starts to melt. -The melting point is a physical measure of the forces holding the material into its crystal lattice. Pure substances typically have a melting range of less than 2 C. -The upper limit of the melting point temperature range occurs when the last remaining crystal melts.

When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample?

-The melting temperature range of an impure sample is higher than that of a eutectic. -The melting temperature range of an impure sample is broader than that of a eutectic. A eutectic occurs when a single composition of a two component mixture melts at a temperature that is lower than either of the melting points of the two pure components.

During sublimation, what two reasons explain why the cold finger should be kept 0.5-1.0 cm away from the bottom of the flask?

-The solid may decompose before it can condense. -impurities could splatter onto the cold finger

Which statements are true about the principles of thin layer chromatography?

-The solvent moves by capillary action. -if the concentration of the component is too high, the spot will become a smear or begin to tail. -separation will be achieved if one component adheres to the stationary phase more than the other component does. In a chromatographic separation, the components in a mixture are separated by virtue of the fact that they have differing solubilities. Each component adheres to the stationary phase, and is continually washed from the stationary phase by the mobile phase. The separation occurs when one component adheres to the stationary phase more than the other component. In some cases the mobile phase moves by capillary action, while in others it may be pushed along by pressure. The concentration of a component if too high, will cause a streak, smear, or tail in chromatographic techniques.

How is it possible to separate two liquids with the same exact boiling point by gas chromatography when it cannot be done by fractional distillation?

-The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility

List the seven procedural steps from start to finish, that are required to perform a thin layer chromatographic separation.

-add just enough developing solvent to cover the bottom 3 mm of the TLC Chamber. Cover with a watch glass. -Make a pencil mark on the TLC plate 10mm from the bottom -Use a capillary to deliver a 2mm diameter spot onto the pencil mark. -Stand the TLC plate upright in the chamber, and replace the lid. -Remove the TLC plate when the solvent is near the top of the plate and mark the solvent front with a pencil before the TLC plate dries -Mark the outline of the spots on the plate with a pencil. Unless the spots are colored, some visualization method will be needed. -Measure the distance that the spot and the solvent have moved. The TLC chamber is set up first to allow the solvent vapor to fill the chamber before the separation begins. The TLC plate is marked gently with a pencil near the bottom. A small spot of sample solution is delivered over the pencil mark. The TLC plate is stood upright in the developing solvent, and the lid is replaced to allow the solvent vapor to fill the chamber. The TLC plate must be removed before the solvent front reaches the top of the plate. The position of the solvent front must be marked before the TLC plate dries, because it may not be visible when dry. Most organic substances don't have any appreciable color. It may be necessary to spray the TLC plate with a dye, or to view the plate under UV light in order to reveal the spot. The Rf value can be determined once the distance traveled by the spot and the distance traveled by the solvent front have been measured.

Which lab glassware are inappropriate for collecting distillate?

-beaker -Petri dish -condenser -evaporating dish They are not appropriate to use because of their wide openings which allow the distillate to evaporate faster.

Which procedures can be used to identify a compound in Gas Chromatography?

-enhancing peaks -measuring and analyzing retention times -analyzing with other spectroscopic methods Without additional methods, a determination of a compound cannot be made. Retention times can be compared to a known standard to confirm identify. By "spiking" a sample, or adding a concentration of a compound with a known retention time, a single peak can be enhanced and identified. Additional spectroscopic methods, such as mass spectrometry, can help narrow down and eliminate unknown substances. A compound must be a gas to be used in gas chromatography.

In what ways can simple distillation be useful in a lab setting?

-to purify more than 60% -to remove solids from a liquid - to removed dissolved, non-volatile components from a liquid -when 2 components of a system have a BP difference of 60°C or more -as the last step in the purification of a liquid Simple distillation only has 2 or 3 vaporization/condensation cycles, only allowing for separation of two components with large boiling point differences. However, it is very useful if there is a non-volatile liquid or a solid present that a volatile liquid can be distilled from. This process of removing non-volatile and solids from a liquid can allow purification of higher than 60% and is typically the last step in trying to purify the volatile liquid.

How wide is the typical melting point range for a pure compound?

.5°C to 1.5°C

ortho

1,2

para

1,4

If 0.9804 g of maleic anhydride is used, how much 1,3-cyclohexadiene should be added for the reaction to have a 1:1 mole ratio?

Maleic anhydride: moles=mass/molar mass=.9804g/98.06g/mol=.0099mol mass of 1,3-cyclohexadiene= moles*mm=..0099*80.13g/mL=.9431mL

How can the efficiency of fractionating columns be improved?

1. Pack the column with wire spiral 2. Pack the column with thin metal strips 3. Pack the column with glass helices 4. Pack the column with metal sponge. NO ICE

Which of the following melting point ranges would be appropriate for an impure aspirin (acetylsalicylic acid) sample that is contaminated with salicylic acid?

121 °C - 130 °C If impurities are present in our aspirin sample, the melting point range for our product will be lower than the range of pure aspirin.

A TLC plate was run in 1:1 ethyl acetate:hexane, but when developed, the plate only showed baseline spots. What change could be made to get the spots to move further up the plate?

2:1 Ethyl acetate:hexane (make eleuent more polar) The interactive animation shows that all spots move farther as the polarity of the mobile phase increases. Switching to a solvent system that includes more ethyl acetate than hexane will allow the spots to separate from each other and move farther from the baseline.

Stereoisomer equation

2^n (n=# of chiral centers)

What is the maximum boiling point range gap between two solvents that simple distillation can successfully separate?

60-70°C Because there are only 2 or 3 condensation and vaporization cycles, the boiling point range gap between two solvents is large compared to a system with more cycles.

EWG ex

=O

Identify first and last compounds distilled from a graph.

A fractional distillation curve is created from data collected by counting the number of drops of solvent being distilled correlated to the temperature during the distillation. Where the curve becomes horizontally flat indicates the two boiling boints of the solvents.

The binary mixture of water and dichloromethane is centrifuged.

A layer of emulsion may form between the aqueous layer and the organic layer.

Determine which solvent has the boiling point shown from a graph

A simple distillation curve is created from data collected by counting the number of drops of solvent being distilled correlated to the temperature during the distillation. Where the curve becomes horizontally flat indicates the boiling boint of the solvent.

How does the rate of heating affect the melting point measurement?

A slow rate of heating produces a more accurate measurement.

If we use anthracene and maleic anhydride, what is the molar mass of the product?

Add molar masses

An extraction was performed using mixture of the compounds below. First the organic mixture was dissolved in dichloromethane, treated with acid, and separated. The organic layer was then treated with base and separated. Sort the compounds based which layer it was in at the end of the extraction.

Amine functional groups are easily protonated by acids. Carboxylic acids and phenols are easily deprotonated by bases. Esters, ketones, and carbon rings with no other groups will not be affected by either acids or bases and thus will remain only in the organic layer.

Which of the following will facilitate a normal Diels-Alder reaction?

An electron withdrawing group on the dienophile.

Calculate Rf

Rf=distance traveled by spot/distance traveled by solvent.

A 80:20 (mole %) ratio of acetone:water is heated to 32.84 °C at 760 mmHg. What is the total vapor pressure of the system? The vapor pressures for acetone and water at 32.84 °C are 318 mmHg and 33.3 mmHg respectively.

Begin by converting the mole percent of each component to a mole fraction. Xa=80/100=0.800 Xb=20/100=0.200 Then vapor pressure can be defined by the equation Ptotal= (Xa*P°a)+(Xb*P°b) Ptotal= (0.800*318mmHg)+(.200*33.3mmHg) Ptotal=261mmHg

which compounds were left in the organic layer?

Benzil and aniline Since NaOH will not deprotonate the aniline or the benzil, they will both stay as neutral organic compounds and remain in the organic layer.

Tea leaves are stirred in boiling water.

Caffeine is forty times more soluble in boiling water than room temperature water.

Purpose of DMAP in esterification?

Catalyst. Since dimethylaminopyridine (DMAP) speeds up the reaction, is not used up, and is reformed during the reaction, DMAP must be a catalyst.

In an eluent solution of 70% hexanes and 30% ethyl acetate, compound A moves farther on the TLC plate than compound B. Compound A is ______ polar than compound B.

Compound A is less polar than Compound B.

Why must the cyclopentadiene be freshly distilled and kept cold prior to a Diels-Alder reaction?

Cyclopentadiene dimerizes at room temperature

Determine which peak is which compound in a GC.

Determined by boiling point order

Pair of solvents that make for a bad extraction system

Diethyl Ether & Dichloromethane Water & Ethanol Acetone & Diethyl Ether The combinations of ether and dichloromethane, water and ethanol, and acetone and diethyl ether are unsuitable for use as solvent extraction systems as there are insufficient differences in polarity to create the needed immiscibility.

In infrared spectroscopy, what must chemically occur within a compound to have a signal appear in the IR spectrum?

Dipole moment

endo products favored because ________? EWG is closer to the e-rich transition state.

EWG is closer to the e-rich transition state. Compound A is the major product because the transition state leading to it allows more interaction between the ED diene and the EWG NO2

A layer will form between the aqueous and organic phases if the extraction is shaken too vigorously. What is this the term used to describe this layer?

Emulsion

What are the cons of using hydrogen as a carrier gas in a GC separation?

Explosion hazard

High compound concentration facilitates visualization T or F?

False

The stationary phase will always be more polar than the mobile phase T or F?

False

T or F: Simple distillation is effective at separating mixtures of liquids with very similar volatilities?

False. Simple distillation is only capable of separating mixtures that have a significant difference in volatilities. . In order to separate mixtures with similar volatilities, fractional distillation is required.

T or F: Over time, the amount of the most volatile component in the distillation flask increases?

False. The most volatile component leaves the flask during the process of distillation.

Which distillation technique would provide the best method to separate acetophenone from cyclohexanol?

Fractional Distillation

Select the analytical technique where peak integration gives useful chemical information.

H NMR spectroscopy The intensity of the peaks in 1H NMR spectroscopy are proportional to the number of equivalent hydrogen atoms that correspond to those peaks. One can determine the relative number of equivalent hydrogen atoms by integrating each peak. Because of this, 1H NMR spectroscopy is widely used to determine the structures of small organic molecules. While the absorbance at maximum peak height is often used for quantification in UV-vis spectroscopy, peak integration is not typically performed. Similarly, peak integration in IR spectroscopy and mass spectrometry does not yield useful chemical information.

While heating a vessel at approximately 65°C, which contains a reaction using a hexanes and water solution, a student notices the volume in the vessel has significantly decreased. What should the student add to the reaction to continue the reaction properly?

Hexanes Since the boiling point of hexanes is close to the temperature of the vessel, it is most likely that the hexanes have boiled off leaving just the aqueous layer, appearing as a loss of volume in the vessel.

Determine a limiting reactant in a TLC.

If there is none of the product shown in the product spot

What effect do impurities have on the Melting Point of a substance?

Impurities lower the MP. The melting point value can be used to identify a substance, as well as to establish its purity. Impurities have two distinct effects on the melting point of a substance. The more narrow the melting point range, the more pure a substance. Similarly, the higher the temperature at which the melting point range occurs, the more pure the substance is. Therefore, impurities both depress and widen the melting point range

How does the degree of chemical purity of a substance affect the melting point measurement?

Impurities make the melting point range lower and broader.

In a TLC, if you have two compounds what can be said about the less polar substance?

It travels the longer distance.

A water bath should be used in which of the following situations?

Keeping a flask nor test tube containing a chemical substance at a specific, constant temperature. A water bath can be programmed to maintain a particular temperature for a specific time duration. This is useful to chemists when needing to control specific reaction conditions such as temperature. It should not be used to wash glassware, sterilize equipment, or dispose of chemicals, trash, or glassware.

Arrange the following solvents from least dense to most dense.

Least Dense -Hexanes -Diethyl ether -Water -Dichloromethane -Chloroform Most Dense Hexanes is the least dense solvent with a density of 0.66 g/mL. Then diethyl ether (d=0.71 g/mL), water (d=0.998 g/mL), dichloromethane (d=1.33 g/mL), and the most dense is chloroform with a density of 1.50 g/mL.

A fractionating column is more efficient than simple distillation bc it provides ________, where several liquid-vapor cycles can occur.

More surface area Having more surface area increases the efficiency of fractional distillation because it causes the theoretical plates to be more efficient. The greater number of theoretical plates present in a column, better separation will occur.

Rank the carboxylic acid derivatives from the most reactive to least reactive.

Most reactive -Acid Chlorides -Acid Anhydrides -Esters -Amides The rate of reaction of carboxylic acid derivatives depends on the basicity of the leaving group. The weaker the basicity of the leaving group, the faster the reaction proceeds.

Calculate the number of theoretical plates used in a distillation when the column is 3.0 m long, with a HETP (height equivalent to a theoretical plate) of 50 cm.

NT=L/HETP=(3.0m*(100cm/1m))/(50cm)=6 plates

A reaction was performed, and the dichloromethane solvent was dried by adding magnesium sulfate drying agent. when the reaction flask was shaken, it was observed that the magnesium sulfate swirled around freely at the bottom of the flask. What does this observation indicate?

Not enough drying agent has been added Magnesium sulfate powder will clump together at the bottom of the reaction flask if there is still excess water in the organic solvent, and will swirl freely when they have dried the solvent effectively.

EDG ex

O-CH3

If there is a reaction mixture in organic solvent that contains an organic acid, an organic base, neutral organic compounds, and base containing impurities, what is the aqueous layer after complete treatment with NaOH(aq)?

Organic acid Organic bases, neutral organic compounds, and base containing impurities will not change polarity in the presence of a base and will therefore stay in the organic layer. An organic acid will be deprotonated by the hydroxide group and become negatively charged, as shown below. (draw compound) This will greatly increase the compounds polarity, becoming ionic, and it will then move to the aqueous layer

Extraction

Separation of substances

A reaction mixture in an organic solvent, contains an organic acid, an organic base, neutral organic compounds, and base containing impurities. The organic solvent is treated completely with NaOH(aq), followed by removal of the water layer. The organic layer then undergoes complete treatment with HCl (aq). What is left in the organic layer after this treatment?

Organic bases, neutral organic compounds, and base containing impurities will not change their polarity in the presence of a base and will therefore stay in the organic layer. An organic acid will be deprotonated by the hydroxide group and become negatively charged, as shown below. (draw) This will greatly increase the acid compounds polarity and it will then move to the aqueous later. After the basic washing is complete, washing with an aqueous solution of hydrochloric acid (HCl) will cause protonation of the base containing compounds and impurities, which will cause a charge, and salt, to form and these compounds will move to the aqueous layer.

A student is comparing two samples of crystalline organic solids.l A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. What would provide the most conclusive evidence that the two samples are the same compound?

Perform a mixture melting point. Performing a mixture melting point analysis will identify if the two samples have the same identity or not. The introduction of an impurity results in depression of melting point. Running the TLC on a longer plate or using more developing solvent will not be effective since Rf values are independent of length or volume of solvent. Mixing the substances to run the TLC as a single spot will only give the same results of identical Rf values.

Filtration

Physically blocking certain objects or substances while letting others through

Decantation

Pouring off a liquid but keeping the solid

What happens to the RT in a GC if the carrier gas flow is increased?

Retention time decreases. Increasing the carrier gas flow decreases the retention times because the sample will spend less time in the vapor phase.

What happens to the RT if the column temp is decreased?

Retention time increases. Decreasing column temperature increases retention times because the sample will spend more time in the vapor phase.

What happens to the RT in a GC if the amount of sample injected is doubled?

Retention time is unaffected. Increasing the amount of sample injected doesn't affect the retention time, but will cause tailing because the column is overloaded.

A student develops their TLC plate and places it under a UV light, but nothing appears. What mistake might the student have made?

Spotted the compound below the level of eleuent. Spotting the compound below the level of eluent may allow the eluent to wash away the sample. Spotting above the level of eluent would give a normal TLC. Using too much sample or too concentrated of a sample would result in a streaking or tailing effect on the TLC plate.

Select the function of TMS in NMR spectroscopy

TMS is an internal standard defining a chemical shift of 0.00 ppm TMS, or tetramethylsilane, is an internal standard that resonates at a value defined as 0.00 ppm in 1H and 13C NMR spectroscopy. The δ values of other peaks in these spectra are measured relative to that of TMS. Due to its relatively electropositive silicon atom, TMS resonates upfield from most organic compounds and does not complicate their spectra. In 1H NMR spectra, deuterated solvents are used to prevent 1H solvent protons from resonating with strong signals. Solvents such as D2O,CDCl3, and C6D6 are used in place of their 1H analogs.

Given a representation of a minimum-boiling azeotropic system of two compounds, A and B. Assuming a sufficiently long and efficient fractionating column is used, determine the percent composition of the azeotrope being distilled?

The azeotrope composition that will be distilled is indicated by the minimum point (which is when the composition boils) on the figure shown

What best describes a eutectic composition?

The composition of a mix of two components that generates the lowest possible MP. A eutectic composition occurs when a single composition of a two component mixture melts at a temperature that is lower than either of the melting points of the two pure components.

What happens if you put a plate with spots 1 cm above the bottom in a chamber with 1.2cm of solvent

The compounds to be analyzed will end up in the solvent

Select the reason why chemical shifts are expressed in units of ppm in NMR spectroscopy

The frequency of a chemical shift is dependent on the operating frequency of the instrument Chemical shifts are expressed in units of ppm, or parts per million, because the chemical shift frequencies are dependent on the operating frequency of the instrument. A chemical shift of 200 Hz on an instrument operating at 100 MHz will correspond to a chemical shift of 600 Hz on an instrument operating at 300 MHz. Therefore, chemical shift, δ, is expressed as a ratio of the frequency shift to the operating frequency of the instrument. This allows the chemical shifts of NMR spectra obtained on different instruments to be directly comparable. The ratio is shown in the equation below: δ=Chemical shift frequency (Hz)/instrument operating frequency (MHz) The ppm unit comes from the fact that the numerator in the equation above is in units of Hz, while the denominator is in units of MHz, or 106 Hz. The chemical shifts of some nuclei, such as acidic protons, can vary with sample concentration. The chemical shifts of some nuclei will also vary in differing NMR solvents, though this variance is typically only ±0.1 ppm. The use of the ppm scale is not used to address either of these phenomena.

Calculate the number of theoretical plates used in a distillation when the column is 4.5 m long, with a HETP (height equivalent to a theoretical plate) of 50 cm.

The number of theoretical plates, NT, is equal to the length of the column (L) divided by the HETP NT=L/HETP=(4.5m*(100cm/1m))/(50cm)=9.0 plates

Assume an organic compound has a partition coefficient between water and ether equal to 8.52. If there are initially 7.91 grams of the compound dissolved in 75.0 mL of water, how many grams will remain in the aqueous layer after extraction with one 20.0 mL portion of ether?

The mass of a compound that will remain in the aqueous layer can be calculated with the equation below. x=(volume of original solvent*initial mass of compound)/((partition coefficient*volume of extraction solvent)+volume of original solvent)=(75mL*7.91g)/((8.52*20mL)+75mL)=2.42g

For an impure substance..

The melting point is lower than that of a pure sample

How would the melting range of a pure organic material be changed there was a tiny shard of broken glass inside the MP capillary?

The melting range would not be affected. The addition of impurities to a pure substance will normally result in a depression and broadening of the observed melting range. Because the tiny shard of glass does not have any appreciable solubility in the liquid phase of the organic material, it will not change the observed melting range.

Label a fraction distillation apparatus

The mixture of liquids in the round bottomed flask are heated by the heating mantle until the most volatile component boils. The vapor passes up the fractionating column, through the T-joint (distillation head), past the bulb of a thermometer, and condenses upon cooling. The cooled liquid slides down the angled condenser, through the vacuum adapter and into the receiving flask. The distillate is the name given to the pure liquid of the most volatile component that drains into the receiving flask. Proper cooling of a condenser involves a slow trickle of cold water into the lower inlet of the condenser, with slightly warmed water emerging from the upper outlet. K clips are used to ensure joints are sealed to allow no vapor to escape, and clamps are used to secure the setup to the bench top

look at diagram for a simple distillation apparatus and know each piece

The mixture of liquids in the round bottomed flask are heated by the hot plate until the most volatile component boils. The vapor passes through the T-joint (distillation head), past the bulb of a thermometer, and is condensed by cooling. The cooled liquid slides down the angled condenser, through the vacuum adapter (receiving head) into a collecting flask. The distillate is the name given to the pure liquid of the most volatile component that drains into the collecting flask. Proper cooling of a condenser involves a slow trickle of cold water into the lower inlet of the condenser, with slightly warmed water emerging from the upper outlet.

Peaks in the Proton NMR spectroscopy can be split into multiple lines. Select the way in which the number of lines can be predicted.

The number of lines is equal to the number of hydrogens on the adjacent atom plus one. Peak splitting, also known as multiplicity, arises when hydrogen atoms in adjacent positions interact via coupling. In 1H NMR, the number of lines that a peak splits into is the number of hydrogens on the adjacent atom plus one. This is known as the n 1 rule. In the molecule shown below, the peak representing the highlighted hydrogen is split by the three neighboring hydrogens of the adjacent methyl group. Following the n 1 rule, where n = 3 in this case, the peak representing the highlighted hydrogen is split into (3 1) = 4 lines. Conversely, the peak representing the three equivalent hydrogens of the methyl group is split by the single highlighted hydrogen into (1 1) = 2 lines. (Draw molecule)

What is extraction?

The process of transferring a solute from one solvent into another solvent

What error is introduces if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?

The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor. If the bulb of the thermometer or tip of the thermocouple is placed too high in the condenser, then the hot vapors are likely to condense before they reach the thermometer. This will result in the recorded temperature being lower than the actual boiling temperature of the vapor. In order to find the boiling point that is more accurate, the bulb of the thermometer has to be positioned in a way that all the bulb will be completely exposed to the hot vapor before passing into the condenser.

When determining melting point, you need to be aware of several factors that can affect the accuracy of the measurement. How does the size of a sample affect the melting point measurement?

The sample should not be large, because a large sample would produce a higher and broader MP range.

Define "lower end" and "upper end" of a melting point range.

The upper end is the temp at which the last trace of crystals disappears. The lower end is the temp at which liquid first appears.

During a fractional distillation of acetone and toluene, a student notices that their data shows drop in temperature from 56°C to 51°C. What does the student conclude?

There is no more acetone vapor and the toluene has not started to distill. Acetone distills off at 56.2 °C, while toluene doesn't distill off until 110.6 °C. After all of the acetone has distilled off, there is no longer enough vapor to continue to warm the thermometer. At this point, the student should increase the temperature so the toluene will begin to distill.

Which of the following extraction regiments is the most effective for extracting a mixture of compounds X and Y with either?

Three extractions of 25mL of ether each One extraction will not extract enough product from the reaction mixture, and significant product will be left behind. Two extractions may leave a significant amount of product behind, but is better than one extraction. Three extractions is the most efficient for removal of the product, without wasting solvent trying to get insignificant product amounts. Although extraction efficiency depends on the choice of solvent and the partition coefficient, it can be generally said that one extraction will isolate 65-75% of the compound. Two extractions will isolate 85-95% of the compound. Three extraction will isolate 95-99% of the compound. Four extractions will yield another 0.5% of compound and, generally speaking, is not worth the time, effort, and solvent to perform.

An organic compound was extracted into dichloromethane (DCM) and then the aqueous layer is shaken with saturated sodium chloride solution. What is the purpose of the sodium chloride?

To decrease the solubility of the organic product in water. Adding a strong ionic electrolyte, like sodium chloride, to the aqueous solution decreases the solubility of organic molecules in the aqueous layer and promotes the transfer of the organic molecule to the organic layer.

In an aqueous workup of a reaction, determine which solvent is located in the separatory funnel. Drag the labels to the appropriate layers in the beaker. Refer to the densities of solvents.

Top layer: Organic phase diethyl ether Bottom layer: aqueous phase water Because the density of diethyl ether is less than that of water, diethyl ether is located in the top layer while the water is located in the bottom layer. Diethyl ether is an organic solvent, thus the organic layer is on top of the aqueous (water) layer.

A separatory funnel contains the two immiscible liquids water and tetrahydrofuran. Use the given densities to determine which layer is on top and which is on the bottom in the binary mixture. Tetrahydrofuran=.89g/mL water=.998g/mL

Top layer=Tetrahydrofuran Bottom layer=water When immiscible liquids occupy the same container, they separate into layers. The more dense liquid will be the bottom layer. Because water is more dense than tetrahydrofuran it will be the bottom layer.

If the concentration of the component is too high, the spot will begin to smear or tail T or F?

True

Separation will be achieved if one component adheres to the stationary phase more than the other component does T or F?

True

The solvent moves by capillary action T or F?

True

T or F: Over time, the amount of the most volatile component in the distillate increases/

True. The most volatile component will be the first to distill. The temperature increases during a distillation as the lowest-boiling component is driven off.

T or F: Distillation is a physical separation, not a chemical reaction?

True. Distillation is a physical process; the identities of the components do not change over time.

A mixture of two compounds on a TLC plate will move a short distance (Rf = 0.1) and appear as a single spot only if a very polar solvent is used. Which two ways should the system be adjusted for better results (Rf = 0.4 and separation)?

Using a less polar eluent and a less active adsorbant (stationary phase)

Based on the fractions of acetone and benzene in the liquid phase, calculate the fractions in the vapor phase.

Using one of the values previously found, the fraction of benzene in the vapor phase can be found using: Xb(vapor)= (P°b*Xb(liquid))/(Ptotal) =(521*0.61)/(760)=0.42 And finally, Xa (acetone) in the vapor phase can be found using: Xa(vapor)=(1-Xb(vapor))=0.58

A mixture of acetone and benzene is heated at 68.3 °C and at 760 mmHg. The vapor pressures for acetone and benzene at 68.3 °C are 1130 mmHg and 521 mmHg respectively. Calculate the fractions of acetone and benzene in the liquid phase.

Vapor pressure can be defined by the equation: Ptotal= (Xa*P°a)+(Xb*P°b) After inserting the partial pressures given, and Xa and 1-Xa, the equation can be shown as: 760mmHg=(1130*Xa)+(521*(1-Xa)) Xa=0.39 Xb=(1-Xa)=0.61

Pair of solvents that make a good extraction system.

Water & Dichloromethane Water & Diethyl ether When dielectric constants have a large difference, the solvents polarities are different enough to cause them to be immiscible. Water and dichloromethane and water and diethyl ether form immiscible two-component solvent systems that are ideal for solvent extraction.

Sublimation

When a solid turns into a gas without turning into a liquid

During a solvent extraction, a student lost track of which layer was the aqueous layer and which was the organic layer. What simple, non destructive test could be used to determine which layer is which.

add a little distilled water to see which layer the water adds to. Water is polar. Therefore, when adding a little distilled water, if the drops of water fall through the upper phase and mix with the one on the bottom, the organic phase is on the top. If the drops falling through the layer, the aqueous phase is probably on the top. If the drops mixing with the top layer, then the top layer is aqueous. Another option is to mark the levels of the layers on the separatory funnel with a sharpie or wax pencil, and then add a volume of water there are two possible observations: If the level of only the top layer rises, then the top layer is the aqueous layer. If the level of both layers rise, then the bottom layer is the aqueous layer.

The three compounds below were disolved in a solution of diethyl ether. aniline, benzoic acid, benzil __________can be extracted into the aqueous layer using 3M HCl. __________can be extracted into the aqueous layer using 3M NaOH.

aniline and benzoic acid HCl is a strong acid and will protonate the most basic compound, aniline, to form anilinium chloride. NaOH is a strong base and will deprotonate the most acidic compound, benzoic acid, to form sodium benzoate. Both p-nitroanilinium chloride and sodium benzoate are water-soluble salts and are thus extracted into the aqueous layer. Benzil is neither acidic nor basic and therefore will remain neutral and water-insoluble through both extractions.

At what point during the process of extraction is a "drying agent" used?

before the solvent is evaporated to yield the final product

The three compounds below were disolved in a solution of diethyl ether. aniline, benzoic acid, benzil After an extraction with 3M NaOH________ is now a salt.

benzoic acid The carboxylic acid functional group on the benzoic acid is the most acidic group present and will therefore be deprotonated first.

This Diels-Alder reaction does not proceed at high yield because it ________ an________ on the _______

contains; EDG;dienophile

a ______ proton displays a (upfield/downfield) signal.

deshielded; downfield

Which of the following will facilitate. normal Diels-Alder reaction? EWG on?

dienophile

When the vapor pressure equals the _________, a liquid will boil.

external pressure. Vapor Pressure is the pressure exerted by a vapor on its surroundings. The boiling point is defined by both internal and external pressures and melting point has no correlation to the vapor pressure. The vapor pressure is equal to the internal pressure. Therefore, when the internal (vapor) pressure is equal to the external pressure, the liquid will boil

The mobile phase in a GC is:

gas The mobile phase in gas chromatography is an unreactive carrier gas, usually helium or nitrogen.

recrystallization want solvent in which solute

is moderately soluble

As time progresses during an hydrolysis reaction, the m-tolyl acetate spot on a TLC plate will become..

less intense

As time progresses during an esterification reaction, the m-tolyl acetate spot on a TLC plate will become..

more intense

Select the mathematical formula that predicts the splitting of a proton NMR signal by n adjacent protons.

n+1 The mathematical formula that predicts the splitting of a 1H NMR signal by adjacent protons is n 1. The signal is split into n 1 lines by n adjacent protons. For example, the signal corresponding to the terminal methyl (-CH3) protons of CH3CH2OH is split into a triplet, or three lines, by the two adjacent methylene protons. In this case, n = 2 and n+1 = 3.

Will the following isotope be visible in an NMR? 14N7

no, because it has a nuclear spin of 1

major product...

one with a TS that leads to more interaction between the electron donor diene and the EWG

How to calculate peak area in a GC

peak are=height*width @half height

configuration of diene is ______

retained

Dienophile _____cis and trans?

retains

If EDG on dienophile then?

rxn will be slow

Which distillation technique would provide the best method to separate anthracene from xylene. Values for MP and BP are given in °C MP=-48°C BP=139°C

simple distillation. because there is a significant difference in the boiling points of the two components, and because those boiling points are easily achieved using simple distillation apparatus.

For a pure substance...

the melting temperature range is narrower than that of an impure sample

During lab, a closed system should not be heated because...

the setup could explode A closed or sealed system should never be heated because as temperature rises, so does pressure, which may cause the setup to explode.

a strongly electronegative group will_______ electron density from a proton, causing the proton to be ______

withdraw; deshielded

Is the following isotope visible and easily analyzed by an NMR experiment? 19F9

yes, bc it gives a nuclear spin of 1/2 Nuclei that have nuclear spin I = 1/2 give sharp, detectable signals in the NMR experiment. Nuclear spin I = 1/2 arises when there is a difference of one between the number of protons and the number of neutrons in the nucleus. Nuclei that have nuclear spin I = 0, are not detectable by the NMR experiment. Nuclei that have nuclear spin I > 1/2, possess a quadrupole and give broad signals that are difficult to detect by the NMR experiment.