Organic 2 Lab : Techniques
Rotary evaporators
A rotary evaporator consists of a hot water bath, a vacuum pump, a spinning flask and a control unit.
In lab you will carry out a similar dehydration reaction, however you will use Montmorillonite KSF clay. What role does Montmorillonite KSF clay play in this reaction and how is this modification an example of green chemistry?
Montmorillonite KSF clay will serve as a heterogenous catalyst in the dehydration reaction. This heterogenous catalyst will work to speed up the rate of the reaction by decreasing the temperature, pressure and energy consumption. This displays an example of green chemistry. Additionally, the use of this solid-catalyst reduces any negative impact on the environment by avoiding any additional chemical reagents or toxic chemicals.
Retention time
Retention time can be calculated by taking the distance traveled by peak x the chart speed. Area of peak = Height of peak × Width of peak (at 1⁄2 of the height) Percent Composition = area of peak 1 /area of peak 1 and peak 2 * 100
Shutting down:
Shutting down: After observing the evaporation of the sample, pull the handle up, and click the stop button to turn off the vacuum & stop the rotation. Close the valve on the vacuum pump. Next, remove the flask from the adaptor. Lastly, turn off the rotary evaporator.
Why do you think you need to add the 0.4 g of sodium borohydride in 4 portions and add each portion slowly over 5-10 minutes?
Sodium borohydride can react with protic solvents (and especially water) to form hydrogen gas. Absolute (100%) ethanol is used but molecules of water still could be found due to humidity and other factors. Adding in small portions prevents a vigorous reaction.
Spectroscopy
Spectroscopy is the interaction of matter and light (electromagnetic radiation). Spectroscopy can use visible light (colorimetry), ultraviolet light (UV), infrared light (IR), or radio waves (NMR).
What does the spinning of the flask prevent and how does it prevent it?
Spinning prevents bumping because it breaks the surface tension between liquid molecules, further preventing bumping and superheating.
Starting up:
Starting up: First turn on the vacuum and power unit. Then, prepare the water bath, and set the desired temperature. Clamp the round bottom flask with the desired sample onto the adaptor. Turn the valve on the vacuum pump and press start. Lower the handle, and immerse the flask into the water bath. Lastly, observe the sample evaporate.
The Wittig Reaction
The Wittig reaction has been used as a good general method for preparing alkenes from aldehydes or ketones. The Wittig reaction is one of the most widely used methods for forming carbon-carbon double bonds, because it is easy to carry out and often gives high yields of pure product. It is named after its discoverer, German chemist Georg Wittig. The reaction involves the addition of a phosphorus ylide to an aldehyde or ketone to form double bond with the elimination of phosphine oxide
Stereochemistry
While NMR of the diol cannot be used to determine if the reduction yields the syn- or anti-diol, the conversion to the acetal creates a product whose stereochemistry can be identified by NMR. Once you know the stereochemistry of the acetal (from the diastereotopic methyl groups in the NMR), you can infer the stereochemistry of the diol, which CANNOT be identified using NMR.
A reaction that is diastereoselective is
a reaction in which a mixture of diastereomers is possible but only one diastereomer (or mostly one) is formed.
What is the purpose of adding sulfuric acid in the acetal formation reaction? Why is sodium carbonate added? Why is it added slowly?
acid is needed to catalyze the acetal formation reaction neutralize the sulfuric acid; to allow CO2 to escape slowly
percent yield formula
actual yield/theoretical yield x 100
Aldol condensation
an aromatic aldehyde will react with a symmetric aliphatic ketone in the presence of a base to form one crossed aldol condensation product. In the presence of excess aldehyde, each mole of ketone can condense with two moles of the aldehyde.
Rotary evaporators
are devices that use a combination of heat, reduced pressure and spinning to remove solvent quickly and efficiently from your compound
If heating occurs very slowly, the melting point you determine may be..
close to the expected
In gas chromatography, a liquid mixture is injected and converted into a mixture of gases that are separated based on their boiling points, the separation occurs in the
column oven
Another way you could improve yield is to shorten the total number of steps that are required by finding an alternative synthetic route. Generally, which do you think is most effective at increasing overall yield?
decreasing the total number of steps requried
If the sample is packed too losely, the melting point you determine may be... If the tube is packed with too much material, the melting point you determine may be.. If heating occurs too fast, the melting point you determine may be..
higher than expected
On the electromagnetic spectrum shown
in Figure REV.1, high frequency (high energy) light are higher energy. Gamma rays are very dangerous to humans; one exposure can cause damage and be fatal. X-rays are also danger- ous; this is the reason that you wear a lead vest when getting x-rays and the technicians leave the room. Ultraviolet radiation is a little weaker than x-rays. However, exposure to UV rays over a long period of time can cause chem- ical changes that lead to skin cancer.
If a compound is wet with water or residual solvent, the melting point is expected to be... If a compound is impure, containing traces of starting material or byproduct, the melting point is expected to be...
lower than expected
m-nitro acetophenone NAOCL NaOH Acetone HCl
methyl ketone: starting material source of halogen for haloform reaction acts as both base and nucleophile methyl ketone, neutralize excess bleach protonation to form COOH
The reaction between sodium borohydride and benzoin is diastereoselective. This means...
one diastereomer will be produced.
Benzoin
oxidizing agent
NaBH4
reducing agent It can be used in some protic solvents such as methanol It can reduce ketones and aldehydes but not esters or acids It is a milder reducing agent than lithium aluminum hydride
Ethanol
solvent
What is the function of the heated water bath?
speed up the removal of solvent
The TLC plate is placed into the developing chamber at an angle...
the Rf values will be inaccurate because the solvent front will move unevenly
Too much blue food dye was spotted...
the spot will be too large and may smear
Not enough yellow food dye was spotted..
the spot will be too small, light and hard to see
Instead of spotting in one tight and consistent spot, the red food dye was spotted as a loose cluster of multiple spots..
the spot will smear and be difficult to measure the distance
The baseline is too low and there is too much developing solvent in the chamber, so the solvent level is above the baseline and spots when the TLC plate is first placed into the chamber...
the spots may dissolve in the developing solvent instead of moving up the plate
The TLC is not taken out of the chamber quickly enough and the solvent front runs past the end of the TLC plate instead of stopping about 1 cm from the top of the plate...
the spots may either converge at the top or run off the plate entirely
There chamber is not adaquately sealed and the TLC plate dries out before it is done developing..
the spots will stop moving once it is dry
The TLC plate is removed fom the developing chamber too soon, when the solvent front is less than half-way up the plate...
there may be inadequate separation between the spots
Which statement correctly describes the effect multiple steps has on the overall yield of a reaction?
with each added step, the overall yield will be lowered
Can an IR identify a compound? Why or why not?
you can only identify the fictional group in a compound, not the complete identity of the compound with IR
Gas chromatography (GC)
Gas chromatography (GC) is a chromatographic method that involves a stationary (non-moving) phase, which is a high boiling organic compound in liquid state adsorbed on solid inert packing in a column and a mobile (moving) phase, which is the inert carrier gas such as He or N2 gas.
Gas chromatography (GC)
Gas chromatography (GC) is an analytical technique used to separate the chemical components of a sample mixture and then detect them to determine their presence or absence and/or relative quantities. In gas chromatography, the organic molecules are vaporized and then carried through a column with an inert carrier gas such as helium or nitrogen (mobile phase). The gas chromatography column (stationary phase) is a glass or metal tube containing a microscopic layer of a polymer. The molecules will move through this column at different speeds based on polarity and boiling point.
Green Chemistry
Green chemistry is the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. Green chemistry applies across the life cycle of a chemical product, including its design, manufacture, use, and ultimate disposal. Green chemistry is also known as sustainable chemistry. To make this experiment 'greener', we will use a benign Lewis acid catalyst (Montmorillonite clay) instead of a toxic Bronsted acid catalyst. The Montmorillonite KSF clay acts a Lewis acid catalyst. It will protonate the alcohol and promote dehydration. In addition, we are also able to capture and recycle our catalyst from semester to semester.
Review of Homotopic, Enantiotopic, and Diastereotopic Hydrogen Atoms
Homotopic and enantiotopic hydrogen atoms are chemically equivalent and will give only one signal in a 1H NMR spectra. Diastereotopic hydrogen atoms are NOT chemically equivalent and will give two UNIQUE signals in a 1H NMR spectra.
Difference between refluxing and distilling
Refluxing is the return of processed fluid after it has been cooled, condensed, heated, or boiled. Whereas, distilling is the separation of components based on boiling point
Relate retention time to boiling point
The lower the boiling point is, the higher the vapor pressure of the compound and the shorter retention time usually is because the compound will spent more time in the gas phase.
Benzoin + NaBH4-->
1,2-diphenyl-1,2-ethanediol hydrobenzoin (a diol) : 214.26g/mol
Review of Homotopic, Enantiotopic, and Diastereotopic Hydrogen Atoms
A - CH3 will always be homotopic and will always appear as one signal in the 1H NMR. A - CH2 - can either be enantiotopic (chemically equivalent) and will appear as one signal in the 1H NMR if there is NOT another chirality center anywhere in the molecule OR if they are not part of a stereogenic center. Otherwise, it can be diastereotopic (not chemically equivalent) and appear as two individual signals in the 1H NMR. Diastereotopic hydrogen atoms that result because of another chirality center will sometimes appear as the same signal depending on the complexity of the molecule. A CH is always one single, unique proton and we do not classify it as homotopic, enantiotopic or diastereotopic.
Aldol condensation
A crossed aldol condensation is a result of two different carbonyl compounds containing 𝛼-hydrogen(s) under-going aldol condensation. Ordinarily, this leads to four possible products as either carbonyl compound can act as the nucleophile and self-condensation is possible, which makes a synthetically useless mixture. However, this problem can be avoided if one of the compounds does not contain an 𝛼-hydrogen, rendering it non-enolizable. In addition, the use of symmetric ketones means that there is only one unique nucleophilic site.
Explain how to take a mixed melting point and how you can use a mixed melting point to determine the identity of an unknown.
A mixed melting point can be completed by taking the melting point of the unknown substance 'A' and known substance 'B'. The scientist must then mix both the unknown and known samples to create a mixture. After taking the melting point of the mixture, the scientist must record the melting point range of the mixture. If the melting point range is similar to substance A, then the substance is pure to substance A. This is also true for substance B. This is a way of identifying unknown samples.
Improving a Reversible Reaction Yield
Alcohol dehydration is a reversible process, as the presence of water could recreate the alcohol from the alkene. To optimize the yield of the alkene, the reversibility can be addressed by distilling off the product as it is made. The removal of the alkene from the reaction mixture has two benefits. First, decreasing the alkene concentration pushes the equilibrium toward the product side. Second, the alkene is removed from the water in the reaction flask to reduce the reverse reaction.
Explain fractioning distillation
An important refinement of the basic technique is fractional distillation, which adds a fractionating column to give better separation of liquids with similar boiling points. The fractionating column is included right above the heated flask, as shown in Figure LF.1. The column contains extra surfaces on which the high boiling point component can condense back to liquid into the flask, releasing heat that helps vaporize the low boiling point component and move it through the system. Fractional distillation allows for a better separation of liquids with a boiling point difference of 40 degrees or less.
After performing an experiment, you take the melting point of your purified compound. The melting point for your purified compound is 180-195oC. If the literature value for your compound is 185-186oC, then your purified compound is pure and the right compound .
An impurity in your compound will make your melting point depress and broaden .
Analysis or Characterization
Analysis or Characterization is the process by which organic compound's structures and properties are mea- sured or determined. Characterization allows you to determine the structure of the product of your experiment. Characterization techniques include polarimetry, melting point, TLC, IR, NMR, GC, GC/MS, and crystallography.
Gas chromatography (GC)
As the compounds emerge from the end of this column, a detector will "detect" the presence and amount of an organic molecule. Retention time is the amount of time it takes for a molecule to move through the column. The chromatographic data is presented as a graph of detector response (y-axis) against retention time (x-axis). This gas chromatogram is a visual representation of each organic sample eluting from the column. The retention times can be used to identify the compound when compared to the retention time of the known compound run under the same conditions.
Identify 3 of the principles that are present in this experiment. Give a short explanation of how they are being applied in this procedure compared to the traditional Wittig reaction.
Atom economy and reduce derivatives are two of the principles present in the experiment. The traditional Witting has a poor atom economy, generating an 18-C side product. To make this a greener reaction, we recycle this side product through reduction. Another principle present in this experiment is safer solvents. The traditional Wittig uses butyl lithium in ether or DMF, however; in this greener reaction, sodium hydroxide solvent was used.
Boiling point : distillation
Boiling point depends on atmospheric pressure, which is not likely to be 760 torr, and thus the observed boiling point is not the normal boiling point of the liquid. If the final boiling point does not exactly correspond to the literature boiling point, it is most likely due to the pressure and/or the thermometer, and not to "operator/exper- imental error" or some impurity in the remaining mixture.
An acetal can be formed by the reaction of a carbonyl compound with alcohol in acid. Why are you adding 2,2-dimethoxypropane to your reaction to form the acetal?
DMP reacts with alcohol in acid to form a binary azeotrope which is broken down by hydrocarbon solvent to form the desired acetal. DMP is used to form the acetal since it acts as a water scavenger; thus preventing the water from causing any unwanted reactions.
Diastereotopic hydrogen
Diastereotopic hydrogen atoms (or protons) are not identical to each other AND are not chemically equivalent. Diastereotopic hydrogens will show up as two different signals in a 1H NMR spectrum.
Examples of isolation techniques
Distillation is used to isolate a liquid from a liquid solution based on a difference in boiling points. Filtration is used to isolate a solid from a liquid solution using filter paper or membranes. This is a key step in a recrystallization. Extraction is used to isolate a liquid from a liquid solution based on different solubilities. Another important isolation technique is the use of a rotary evaporator to remove solvent. It is important to remove solvent thoroughly as the solvent can remain in the "dried" crystals and interfere with characterization.
Spinning
During this process, the flask is spinning. Spinning the solvent in a flask, increases the rate at which molecules of solvent will convert from liquid to gas and thus increases the vapor pressure, which decreases the time needed to evaporate the solvent.
Enantiotopic hydrogen
Enantiotopic hydrogen atoms (or protons) are not identical to each other. However, they still show up as one signal in a 1H NMR spectrum. If you have a - CH2 - in a compound and both hydrogen atoms in that one - CH2 - show up as just one, unique signal, then they are enantiotopic (also chemically equivalent).
The faster a flask spins on the rotary evaporator the quicker the solvent will be removed.
FALSE
When getting pH paper from a vial or bag, ALWAYS use gloves.
FALSE : Always remove gloves before getting pH paper to avoid contaminating the entire package of pH paper.
Summary of MP procedure
First, obtain the equipment needed to complete the experiment. Then, retrieve a capillary tube, and tap the open end of the capillary tube onto the benzoic acid sample. Also, retrieve a packing tube, and drop the capillary tube until the sample has reached the closed end. Next, place the capillary tube into the melting point apparatus. Make sure to adjust the melting point apparatus temperature. Observe the sample, and record the melting point range. Lastly, discard the capillary tube in the glass waste after completion.
Homotopic hydrogen
Homotopic hydrogen atoms (or protons) are identical to each other. In an NMR spectrum, they show up as one signal. When you have a - CH3 in a compound, all the hydrogen atoms bonded to that carbon show up as just one, unique signal. These three hydrogen atoms are chemically equivalent or homotopic.
How can an IR indicate if a dehydration reaction was successfully completed?
IR can indicate a dehydration reaction was successfully completed by observing the spectra for a double bond formation. A scientist should perform an IR before the dehydration reaction, and an IR after the completion of a dehydration reaction. After obtaining both spectra, the scientist should check for a loss of hydroxyl group before the dehydration reaction, and formation of a double bond in the completed reaction. If both criteria were shown, then a dehydration reaction was successfully completed.
IR spectra
IR spectra are the recordings of the infrared light absorbed by a molecule. The X-axis provides the energy of absorption for each bond vibraton type which is reported in wavenumbers, (cm−1). This is a frequency term. The x-axis in an IR spectrum ranges from 4000 cm−1 to around 500 cm−1. The y-axis is labeled as "Percent Transmittance" and ranges in number; this number is a relative value of the intensity of the absorbance. IR spectra can provide a good sense of the bonds present and thus the functional groups in a molecule. IR spectroscopy can be a great diagnostic tool when combined with other techniques to determine a molecule's structure.
What does IR spectroscopy measure?
IR spectroscopy measures the interaction of matter (your sample) with infrared light, which causes vibrations, stretches, and bending in the covalent bonds of molecules, which can be interpreted to determine the functional groups in your compound. To take an IR in the lab, you need to place your sample directly on the crystal.
Distillation
Many organic compounds are volatile, which means they have relatively high vapor pressures and low boiling points. Distillation uses this volatility by boiling the liquid mixture in a single flask and condensing the vapors that travel into the apparatus, allowing a liquid to be collected at the other end. As long as there is a difference in boiling points between the liquids in the mixture, one component will distill over before the other.
Melting point
Melting point is the temperature at which a solid transforms into a liquid. Typically, this value is reported as a melting point range: the temperature at which the solid begins to turn to liquid to the temperature at which the entire sample becomes liquid.
Gas chromatography (GC)
In GC, a solution is injected and vaporized in the injection port. This vaporized mixture moves through through the column carried the carrier gas. Gases with a higher boiling point will move slower through the column. Interactions with the column can also slow down an analyte moving through the column. After the analytes leave the column, a detector measures the amount and retention time of the gas, which is displayed on a chart.
IR spectroscopy
In IR spectroscopy, infrared radiation causes covalent bond to vibrate. Each of these vibrations has a specific range that correlates to the vibration of a specific type of covalent bond and a specific type of vibration (bending, stretching, scissoring, rocking, and twisting). Analyzing the peaks present in an IR spectrum helps us identify the types of bonds present in a compound and, ultimately, the functional group.
Spinning
In addition, spinning prevents 'bumping'. When boiling point is reached too quickly that the liquid will pass the boiling point without ever boiling due to the surface tension of the liquid. If the sur- face tension is suddenly broken, the boiling solvent can erupt. Spinning disrupts the surface tension preventing superheating and bumping.
IR light
In infrared (IR) spectroscopy the light is infrared light, which is light from around 700-800 nm to around 1 mm (1,000,000 nm). IR light is a weak form of light. It is much weaker than ultraviolet light and even weaker than visible light (the light from a lamp or light bulb produces more dangerous light than infrared light). Infrared light is associated with heat. IR radiation from the sun is what heats the earth and causes climate change (due to absorption of IR by greenhouse gases). IR light is also what is seen on the radars from weather apps or local news (they are called infrared weather radar).
Improving a Reversible Reaction Yield
In the dehydration of cyclohexanol performed in this lab, distillation during the reaction works well as there is a large difference in the boiling points of the reactant (cyclohexanol bp=165-166 ∘ C) and the product (methyl- cyclohexene bp = 106 ∘ C). Thus, the cyclohexanol mostly remains in the reaction flask during the distillation to continue reacting with the acid. If any of the acid happens to distill over with the methylcyclohexene, it can easily be neutralized with aqueous sodium bicarbonate. Traces of water can also be removed by using a drying agent, such as anhydrous magnesium sulfate or calcium chloride.
Green Variation
In the original Wittig reaction, a triphenylphosphonium halide was added to a solution of n-butyllithium in diethyl ether. Once the ylide had formed, the carbonyl compound was added and heated to reflux overnight. In this particular Wittig, the alpha protons of the phosphonium salt are sufficiently acidic to be deprotonated by aqueous NaOH, allowing the reaction to be carried out in aqueous solution. This makes this method much greener than the normal Wittig conditions of using n-butyllithium in ether or DMF. It should be noted, however, that the traditional Wittig reaction has very poor atom economy, generating an 18- carbon side product (Ph3P=O). Thus, to make this a greener reaction, we would also need to recycle this side product through a reduction process.
Isolation
Isolation is a set of techniques for separating the desired product of an organic reaction from solvent and most byproducts and impurities. Isolation does not usually lead to a purified product
Distillation
Many organic compounds are liquid at room temperature. If a reaction results in a mixture of miscible liquids, the separation of the liquids requires a new technique known as distillation. Distillation uses the different boiling points of liquids in a mixture to separate the components.
Is IR spectroscopy alone sufficient for differentiating between all of the types of carbonyl groups explored above?
No, only carboxylic acids can be reliably differentiated
Chemically Equivalent
Protons (or hydrogen atoms) that are chemically equivalent will EXPERIENCE the same chemical environment and have the same or identical linkage. Because of this they resonate the same and show up exactly the same in an NMR spectra. Protons (or hydrogen atoms) that are not chemically equivalent will EXPERIENCE different chemical environments and will NOT have the same or identical linkage. Because of this they resonate differently and show up as different signals in an NMR spectra.
Azeotrope
Pure organic compounds will distill over a very narrow boiling point range. If the boiling point range or distilling range is too wide, the collected compound is likely impure and a complete separation was not obtained. However, not all compounds with narrow boiling point ranges are pure. An azeotrope is a mixture of two liquids which has a constant boiling point and composition throughout distillation. An example azeotrope is a mixture of ethanol and water; ethanol has a boiling point of 78.4 °C and water has a boiling point of 100 °C but a mixture of 95% ethanol and 5% water has a boiling point of 78.1 °C. When distilling ethanol and water, the collected ethanol will be 95% with 5% water due to the azeotrope.
Boiling point : distillation
Pure organic compounds will distill over a very narrow boiling point range. If the boiling point range or distilling range is too wide, the collected compound is likely impure and a complete separation was not obtained. However, not all compounds with narrow boiling point ranges are pure. An azeotrope is a mixture of two liquids which has a constant boiling point and composition throughout distillation. An example of an azeotrope is a mixture of ethanol and water; ethanol has a boiling point of 78.4 °C and water has a boiling point of 100 °C but a mixture of 95% ethanol and 5% water has a boiling point of 78.1 °C. When distilling ethanol and water, the collected ethanol will be 95% with 5% water due to the azeotrope.
Pure samples
Pure samples melt over a narrow range. Samples containing impurities have a melting point range that is broader and lower than the melting point range of a pure sample. Samples containing insoluble impurities have a melting range very similar to that of a pure sample but you may observe small particles that do not melt.
Purification
Purification is a set of techniques for taking an isolated but impure product and removing the remaining im- purities to yield a single (usually) pure product.
Examples of purification techniques
Recrystallization is used when the impure organic product is a solid and has a different solubility from the impurity. Column chromatography is used to separate solids or oils with different polarities. This technique will often work with structures that can't be purified with recrystallization. Fractional distillation is used when the impure organic product is a liquid miscible with liquid impurities.
Role of the Acid Catalyst
The dehydration of an alcohol requires a catalyst, usually an acid, for the reaction to occur. The acid plays two roles in the mechanism. First, the acid protonates the hydroxy group, making a good leaving group of water. Then, the conjugate base of the acid removes a hydrogen cation (proton) from a carbon adjacent to the cation in the intermediate. Therefore, the acid is regenerated throughout the reaction as required to be a catalyst. The most common acid catalysts for the alcohol dehydration are sulfuric or phosphoric acid dehydration are sulfuric (H2SO4) or phosphoric acid (H3PO4). Other acids, such as hydrochloric or hydrobromic acid, are not used because the small conjugate base ions (halide ions) could participate in a competing nucleophilic attack on the carbocation intermediate.
Explain what is meant by "efficiency" with respect to a distillation. Comment on the efficiency of your fractional distillation as seen by your graph.
The efficiency of simple distillation refers to the amount of heat and time required to distill the solution. It also refers to the accuracy of the fractionation. The fractional distillation seen by my graph is good since one can clearly see the two peaks for ethyl acetate and butyl acetate at their expected boiling points. To make my graph more accurate, I would make more fractions, especially one near the literature boiling point of butyl acetate.
What is the function of the vacuum pump?
The function of the vacuum pump is to decrease the pressure within the system and allow for faster and more efficient evaporation of the solvent.
Dehydration Reactions
The goal of this experiment is to synthesize an alkene from an alcohol using acid catalyzed dehydration. Purity of the alkene product will be analyzed using gas chromatography. The dehydration of an alcohol is the loss of water from the alcohol, resulting in an alkene. The reaction is one method to prepare alkenes.
Rotary evaporators
The hot water bath is used to heat your mixture and help evaporate the solvent. The vacuum pump creates a reduced pressure environment that allows for quicker and more efficient evaporation of the solvent.
Explain how the length of fractionating column can impact the quality of separation and/or efficiency.
The longer length of the fractioning column can efficiently impact the separation quality. There is a direct relationship found between column length and retention time. As column length increases, retention time increases. In other words, the vapors take time to reach the top of the column allowing for a more pure fraction to be produced due to the efficient separation.
Regiochemistry
The major product is the usually the more substituted alkene (Zaitsev), while the minor product is the less sub- stituted alkene (Hofmann). In this experiment, you will perform the dehdyration of 2-methylcyclohexene as shown in Figure DH.2 which can result in three different alkenes. GC will be used to determine the actual alkene product ratios.
peak of an IR represents
The peak of an IR represents the absorption of specific frequencies caused by the molecular vibrations of functional groups present in the molecule.
Gas chromatography (GC)
The peaks displayed in a gas chromatogram represent the detector response to that compound. The area under a peak is proportional to the amount of compound present in the solution.
A Wittig Reaction of trans-Cinnamaldehyde
The purpose of this experiment is to synthesize 1,4-diphenyl-1,3-butadiene from cinnamaldehyde.
Based on the five pieces of information (melting point, TLC, proton NMR, carbon NMR, IR), indicate which product was formed in your experiment.
The recrystallized melting point produced was sharp. This is indicative of a single geometric isomer in the trans-form. That is the E, E-1,4-diphenyl-1,3-butadiene. TLC for the crude product shows a mixture of isomers ( E, Z having a higher Rf, E, E having a lower Rf. However, when the product is recrystallized, we see the product only for E, E-1,4-diphenyl-1,3-butadiene. Next, the IR corresponds to the (E, E)-1,4-diphenyl-1,3-butadiene structure being formed. This is because the C=C stretch frequency corresponds to that of trans and not cis. Lastly, the H-NMR and C-NMR correspond to (E, E)-1,4-diphenyl-1,3-butadiene since the trans isomer is typically more downfield.
Simple distillation
There are many types of distillation that differ in the components of the distillation apparatus. All distillation setups do have similarities, though. A simple distillation has the most basic set-up of the distillation options. The flask with the mixture is attached to some form of condenser, which empties into a new container. The setups also include a thermometer to monitor the vapor temperature compared to the expected boiling points. All set-ups must also be open to a gas environment in some way, whether that is open to air or another gas. A closed distillation system could allow the pressure to build up and cause an explosion
How does retention time relate to boiling point?
There is a direct relationship between retention time and boiling point. When the boiling point is higher than the column temperature; the compound will spend a higher retention time condensed as a liquid at the beginning of the column.
Comparison of simple and fractional distillation temperature behavior over a distil- lation
There is no sharp transition in the black line representing the temperature behavior during the simple distillation, indicating that there is a wide range of collected distillate containing a mixture of both liquids. The red line has a much sharper transition, which indicates a cleaner separation between the two liquids.
Examples of characterization techniques
Thin-Layer Chromatography (TLC) is a quick method to assess the success of purification techniques. A single spot usually indicates success. This technique is often used to determine when a reaction is complete. Infrared Spectroscopy (IR) is used to determine the presence or absence of functional groups. Gas Chromatography (GC) is another method to assess the success of purification techniques. A single peak usually indicates purity. This technique is often used to determine when a reaction is complete. Nuclear Magnetic Resonance (NMR) is used to identify the exact structure of your product. NMR is the most common technique used in characterization in research and industry. Melting point (mp) and mixed melting points are used to confirm the identity and purity of a product in comparison to previously reported literature values.
Green Variation
This reaction proceeds through two steps. First, the phosphorus ylides are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide In the second step, phosphorous ylide (also called a phosphorane) reacts with a carbonyl compound with to form a 4-membered ring intermediate, oxaphosphetane. This intermediate then undergoes a bond rearrangement which leads directly to the alkene and the triphenylphosphine oxide
Why is water and 3 M HCl added to the reaction mixture from the reduction of benzoin, once the reaction is done?
Water and HCl are both protic and can help neutralize any excess sodium borohydride. Water and HCl are added to protonate the alkoxide (O-) to an alcohol.
