Orgo lab Final
The solubility of solid W in water is: 1.68 g/100 mL at 0°C, 20.9g/100 mL at 100°C. a) How many mL of boiling water are required to dissolve 128.0 g of W?If solution were cooled to 0°C, how many grams of W would crystallize out? What is the percent recovery?
(128 g/20.9 g/mL) x 100 = 621.4 mL 621 mL of boiling water is required to dissolve 128 grams of W. (621.4/100) x 1.68 = 10.44 grams When the solution is cooled to 0C, 10.4 grams of solid would crystallize out. 128-10.4= 117.6 (117.6/128) x 100 = 91.9% The percent recovery is 91.9%.
sources of error for percent recovery
- if the minimal amount of hot solvent was not used - if solute if not properly dried it adheres to the filter paper. -Unavoidable errors include some solute always soluble in cold solvent so it dissolved during the washing process.
sources of error distillation
- loose connectors causing vapors to escape, loss of liquid. -Temperature variations due to airflow in hoods -large surface area of microscale apparatus making it easy to lose heat to the environment - thermometer calibration, positioning of thermometer and sidearm of apparatus not adhered to -hold up from fractional distillation - significant amount of distillate left adhering to glass surfaces of apparatus.
Sources of error of melting point
- moisture in air absorbed by sample - thermometer in Mel-Temp not calibrated -determination made too fast, sample not dry -sample not packed tightly, sample not finely powdered - sample more than 2-3mm high in capillary - contamination of sample.
esterification sources of error
-Incorrect insulation of apparatus - Bumping of solution -CaCl2 pellets removed too quickly - Contamination of reagents - Loose connectors -Reaction was not taken to completion
sources of error TLC
-The spots are too close to the edge -The spots are too close to together -Contamination of samples -Solvent is above the spotting line -The cap is missing from the developing chamber -TLC plates leans against the filter
Sources of error purity
-if impurities cannot be removed due to solvent not having proper characteristics -impurities forming due to cooling solution being disturbed -impurities crashing out of solution due to fast cooling. Impurities can also occur - If solvent is not completely removed - if the crystals are wet, filter paper could adhere to crystals as they are scraped off with a spatula.
You have a mixture of 2-chlorobenzoic acid (pKa 2.92) and 4-Bromophenol (pKa 10.2) dissolved in ether. You will need the following information: The pKa of HCl is -7; pKa of carbonic acid (H2CO3) is 6.52 and the pKa of water is 15. Which species do you use to separate the mixture? Which species will remain in the ether layer?
-sodium bicarbonate -4-bromophenol
SN2: Rank fastest to slowest theoretically Explain theory with respect to: - structure of alkyl halide - leaving group
1) 1- bromobutane 2) 1-chlorobutane 3) 2-bromobutane 4) 2-chlorobutane In SN2, the reactivity of alkyl halides is CH3>1>2>3 as SN2 prefers sterically unhindered alkyl halides to have a successful backside attack. Better leaving groups are ranked from I>Br>Cl>F
Pure R,R - hydrobenzoin has a mp of 148 degrees C: 1) What do you expect the mp of pure S,S - hydrobenzoin to be? 2) Do you expect the racemic mixtures of these enantiomers to have the same or dif mp than the pure enantiomers?
1) 148 as well 2) different
epoxidation: 1) Why was column chromatography used in this experiment? 2) why was this a good technique to achieve the purpose for ^ ?
1) Column chromatography was used to separate 3-chlorobenzoic acid from the cholesterol epoxide. 2) this was a good technique to use because 3-chlorobenzoic acid is very polar and will get absorbed by the silica gel whereas the cholesterol epoxide is non-polar and will rapidly migrate on the silica gel which will allow the two to separate.
epoxidation: 1) Why was TLC used? 2) Why did you need to use 2 visualization techniques for the TLC? 3) Did the reaction go to completion based on the given TLC?
1) TLC was used to monitor the progress of the reaction and to determine how effective the purification was. This was done by spotting the TLC in the beginning and in the end of the experiment. 2) Two visualization techniques were required for the TLC because UV can visualize MCPBA and 3-chlorobenzoic acid since they are both conjugated. I2 is required to visualize the non-conjugated compounds, cholesterol and cholesterol epoxide. 3) based on the given TLC, the reaction did go to completion because all of the spots are visible using both visualization techniques.
Match the problems that arise when a particular situation is present when running a spectrum of a neat liquid: 1) using a low viscosity liquid 2) using a viscous liquid
1) weak peaks arise 2) peaks are too intense
What is the order of reagents that were added to the test tube used to carry out the synthesis of dibenzalacetone?
1.NaOH 2. benzalaldehyde 3. acetone in ethanol
An amount of the dibenzalacetone crystals formed are recrystallized using 70:30 ethanol/water solution. What are the next 2 immediately sequential actions AFTER a boiling stick is added to the test tube once a few drops of solvent is added to the solid? (Don't skip steps)
1.heating solution with boiling stick while adding hot solvent till solid is dissolved 2. removing boiling stick
You have 132.0 g of solute Q dissolved in 110-mL of water. The distribution coefficient between ether and water for the solute is 6. What volume of ether would you need to extract 50.0 g of solute Q in one extraction from the original 132.0 g dissolved in 110 mL water? Provide the answer to the closest mL (show calculations).
132-50= 82/110 = 0.745 6=(50/x) / 0.745 x= 11 mL
From figure 5.4 on pg. 91, you distill a liquid mixture of 50% ethanol: 50% water. What would the temperature when the distillate begins to condense in the sidearm (the distillate's boiling point) be? (Select the best representative answer).
79 C
Look at Fig. 5.3 on pg 90: If you boiled the liquid mixture of 30%cyclohexane and 70% toluene, condensed the vapor mixture it generated and then redistilled that condensed mixture, what would the vapor composition of the mixture resulting from the 2nd distillation be? (Select the best representative answer).
87% cyclohexane: 13% toluene
Since boiling stones cannot be used to control the bumping of liquids in vacuum distillation, other methods are used. What is specifically done with glass wool (one of the implements used to control bumping) when small quantities of material are being distilled via vacuum distillation?
Add enough amount of glass wool into the flask to fill the space above the liquid when small quantities of material are being distilled via vacuum distillation.
Explain the adiabatic process as it specifically pertains to a fractional distillation.
An adiabatic process involves there being no gain or loss of heat to and from the surrounding system. In terms of fractional distillation, the heat is transferred between the ascending and descending vapor with no heat being exchanged with an outside system.
You come across a white solid and take its melting point and find it to be 104-107°C. You decide to identify it. If the thermometer is calibrated, compounds of what general melting point range can you automatically eliminate and why? (For the general melting point range, indicate above or below a certain melting point or in between melting ranges. Don't provide specific compounds as examples)
As the compound is an unknown, any temperatures below the melting range of 104-107 should be eliminated. Your sample may already contain an impurity which lowered the range to 104-107, so its suggested to use compounds with higher melting ranges to identify the unknown.
What specifically causes the vaporization of the lower boiling liquid on the packing in a fractional distillation?
As the vapor of the higher boiling point component condenses, it releases heat and causes the vaporization of the lower boiling point component to move up while the higher boiling vapor component moves down. The large surface area of packing material allows for the exchange between ascending and descending vapor.
Why would the presence of the side product 4-hydroxy-4-methyl-2-pentanone be minimized?
Benzaldehyde is more reactive than acetone which is a ketone and they are less open to nucleophilic attack at carbonyl carbons compared to aldehydes making them the minimized side product.
Why would boiling stones not be able to control the bumping of liquids in vacuum distillation as they are able to for simple and fractional distillations?
Boiling stones cannot control the bumping of liquids in vacuum distillation because the reduced pressure vacuum will suck the air that gets trapped in the boiling stone capillaries making them ineffective for creating bubbling which prevents bumping.
What character of the actual elements (aluminum, silicon, oxygen) present in alumina and silica cause the stationary phase to be very polar?
Both alumina and silica are polar because oxygen is more electronegative in both molecules making both aluminum and silicon partially positive, having partially negative and positive charges creates a polar molecule.
In the reduction demo video, you would have noticed some bubbling occurring when NaBH4 was added to the benzil in ethanol. What caused the bubbling?
Bubbling would occur when NaBH4 was added to the benzil in ethanol because NaBH4 would react with some of the solvent (ethanol) which would cause the bubbling in the test tube.
In what case would column chromatography be used to purify a desired compound because techniques like recrystallization, extraction and distillation are not adequate to do so?
Column chromatography would be used when the impurities are closely related to in structure.
What is the definition of efficient stacking when it pertains to how molecules stack depending on the shape of the molecule? How does efficient stacking ensure a higher melting point?
Efficient stacking refers to the ability of molecules to pack tightly into a crystal lattice. Efficient stacking maximizes Van der Waal interactions, which requires more energy to break the lattice leading to a higher melting point.
From observations in video and lab session: Which reactions undergo SN2 Which reactions undergo SN1 Which don't undergo either Compare these to theory in prelab
From observations 1-bromobutane goes through both SN1 and SN2 reactions, when in theory it should only undergo SN2. Both 2-bromobutane and 2-chlorobutane underwent SN1 reactions when in theory they were supposed to do both SN1 and SN2. In theory, 1-chlorobutane was only supposed to do SN2 but instead it did the opposite and went through SN1. Following the theory of only doing SN1, 2-chloro-2-methylpropane only did SN1. Bromobenzene also followed theory by not undergoing any reaction, both SN1 and SN2. In conclusion, 1-bromobutane was the only SN2 reaction. SN1 reactions included 1-bromobutane, 1-chlorobutane, 2-chlorobutane, 2-bromobutane, 2-chloro-2-methylpropane.
Why are glass, quartz and plastics not used as IR sample holders?
Glass, quartz, and plastics are not used because these are opaque to IR radiation and will absorb the radiation instead of refracting. Sample holders must be made out of transparent material.
Since benzoin and meso-hydrobenzoin have similar melting points, how would you confirm that the product you obtained from this reaction is indeed meso-hydrobenzoin using only melting point techniques? Describe briefly how you would carry out this experiment.
I would carry out a mixed melting point experiment to confirm the identity of meso-hydrobenzoin as the product. Assuming that pure meso-hydrobenzoin is available to us in the lab, I would mix that with the product and take the melting point of this mixture. If the melting point stays consistent with the literature melting point of meso-hydrobenzoin (134 - 136 °C), then we know that the product does not have benzoin in it as an impurity. However, if the melting point decreases, this would be an indication that benzoin is present in the product.
Why would IR and TLC not be useful for differentiating between meso-hydrobenzoin and its diastereomers?
IR and TLC would not be good to use in order to differentiate between meso-hydrobenzoin and its diastereomers because both of them have the same functional groups and the same polarities so differentiating them on IR and TLC would be very difficult.
In vacuum distillation, how you do ensure that the liquid does not boil too suddenly upon reduction of pressure?
In vacuum distillation, you must introduce the vacuum to the system first before applying heat to ensure that the liquid does not boil too suddenly.
What is the character of the vapor after each condensation-vaporization cycle in a fractional distillation of a mixture?
Vapor is richer in the more volatile fraction
What aspect of water makes it favorable to use in steam distillation?
Its low molecular weight
Recrystallization Purpose
Learn the techniques of recrystallization and to compare the two methods of crystallizing acetanilide or salicylic acid.
distillation lab purpose
Learning a technique of distillation and comparing simple and fractional distillation of cyclohexane/toluene mixture.
Purpose of melting point lab
Learning a technique of melting point. Figuring out eutectic point of cinnamic acid and urea mixture, also identifying an unknown.
TLC purpose
Learning the technique of thin-layer chromatography and identifying the unknown.
aldol condensation sources of error
Melting range inaccurate due to impurities ● Loss of product during washing/recrystallization ● Washing done inaccurately ● Loss of product during tube shaking ● Crystals not dried properly ● Side product formation and the inability to detect them Include melting point and recrystallization sources of error ( on their own cards)
Why do the NaCl plates used for IR need to be handled with care? What precautions need to be taken when handling them?
NaCl plates used for IR need to be handled with care because they can break easily and are very fragile. The plates are also attracted to moisture present in the air. Precautions for the plates include handling it with care, only holding the edges, and keeping it away from moisture and other aqueous solutions,
Why was partial recrystallization necessary for this experiment?
Partial recrystallization is necessary for this experiment to remove the starting materials and side products.
SN1: Rank fastest to slowest theoretically Explain theory with respect to: - structure of alkyl halide - leaving group
Ranking Fastest to Slowest: 1) 2-chloro-2-methylpropane 2) 2-bromobutane 3) 2-chlorobutane Better leaving groups are ranked from I>Br>Cl>F As primary carbocations are very unstable, the reactivity of alkyl halides for SN1 is 3>ally~benzyl~2>1>CH3 ~vinyl.
epoxidation: Why was recrystallization used in the experiment?
Recrystallization was used in this experiment to purify any remaining starting material or some side product such as diastereomers.
Alkyl haildes undergo SN1, SN2, neither pre-lab
SN1 : -2-chlorobutane -2-bromobutane -2-chloro-2-methylpropane SN2: -1-chlorobutane -1-bromobutane -2-chlorobutane -2-bromobutane Neither : -Bromobenzene
Define the term separation it relates to general chromatography.
Separation, in terms of general chromatography, measures the migration rate of compounds (Rf) also known as elution.
extraction sources of error
Sources of error during extraction: ● Unreactive weak base left behind ● Material left behind from transfers ● Incomplete reactions ● Contaminations from the pipettes ● Wrong chemicals used ● Temperature was not regulated Sources of error during recrystallization: ● Leftover solute in solvent ● Losing crystals when it is transferred from the filter paper ● Lack of drying, wet crystals
Let's consider that ethyl acetate is the ideal solvent for this TLC experiment. What might have happened to the spots if you had used acetone as the eluent instead of ethyl acetate. WHY?
Spots move towards top of TLC plate - acetone more polar, overcomes attractions of sample to adsorbent better
Let's consider that ethyl acetate is the ideal solvent for this TLC experiment. What might have happened to the spots if you had used t-butyl methyl ether as the eluent instead of ethyl acetate. Spots remain towards bottom of TLC plate
Spots remain towards bottom of TLC plate
What is the purpose of having very small bores on the spotters?
Spotter bores should be small because once a liquid is drawn into them they should not be able to flow out to form a droplet
Define steric hindrance in a nucleophilic substitution reaction.
Steric hindrance, in regards to nucleophilic substitution, especially refers to SN2 reactions as they work the best with sterically unhindered nucleophiles. Steric hindrance usually refers to molecules that are more substituted which make the backside attack in the SN2 reaction more difficult. More substituted molecules have more electron clouds making them more hindered when trying to react with the incoming nucleophile.
In the solvent pair experiment that is being carried out with the solid that is being recrystallized, why do you NOT have to worry about sudden vigorous boiling of crystal solution when you add the second solvent to the solution?
Sudden vigorous boiling only occurs when the second solvent has a lower boiling point than the first. Water is the second solvent and ethanol is the first solvent, as water has the higher bp we don't have to worry about sudden vigorous boiling.
esterfication lab purpose
Synthesize butyl acetate via esterfication
aldol condensation purpose
Synthesize of dibenzylacetone via aldol condensation
What are the intensity of the IR absorption bands proportional to?
The IR intensities are proportional to the change in dipole moment that a bond undergoes when stretching.
In the synthesis of isobutyl propionate (shown on pg. 521), excess propanoic acid is used in the reaction. a) What purpose does the excess acid serve?
The addition of excess acid will favor the forward reaction as the reaction is in equilibrium, increasing the forward reaction rate. The excess acid also causes the OH (alcohol) to act as the limiting reagent leading to the maximum yield of products.
How was the identity of your unknown confirmed? (mixed melting point)
The identity of my unknown was confirmed because the temperature remained the same after being added to the standard. If the unknown was different from the standard, the melting point would go down as they mixture is now impure.
When comparing 2 hydrocarbons, why would the larger, higher molecular weight molecule have a higher melting point?
The larger, higher molecular weight molecule has a higher melting point because the larger the molecule, the more the Van der Waal interactions which causes a higher melting point as it requires more energy to break apart the molecule.
What does the melting point data of the product indicate about the product?
The literature melting point range for the product is 140 - 145 °C. However, the observed melting point 145 - 148 °C. The higher melting point range indicates that there are impurities within the product.
reduction: What does the melting point data of the product indicate about the product?
The melting point data that is given indicates the presence of impurities in the product since the range is lower and wider than the literature melting point range (134 - 136 °C). Several sources of error can lead to this such as contamination of the sample.
The mole fraction of A (XA) in the vapor of a mixture is 0.57 and the sum of the partial pressures is 770 torr. Calculate the mole fraction of A (NA) in the liquid if the vapor pressure of pure A (PA∘∘) is 673 torr. (Hint: Find PA first) Show calculations and give answer to 2 decimal places (if not reporting as a percentage):
The mole fraction of A in the liquid if the vapor pressure of pure A is 0.65. PA = (0.57 x 770) = 438.9 NA = PA / PA∘
Calculate the mole fraction of B (XB) in the vapor if a mixture of liquids is 58%A and 42%B. The vapor pressure of pure A (PA∘∘) is 810 torr and the vapor pressure of pure B (PB∘∘) is 759 torr. Show calculations and give answer to 2 decimal places.
The mole fraction of B in the vapor mixture is 0.40. (PA x XA) + (PB x XB) = (810 x 0.58) + (759 x 0.42) = 469.8 + 318.78 = 788.58 318.78/788.58 = 0.40
You dissolve an amount of solid Q in 197.0 mL solvent, forming an unsaturated solution. The solubility of Q at 0°C is 1.73 g/100 mL. The amount of solid remaining in solution at 0°C is 23.0% of the original amount of solid dissolved . What is the original amount of solid that was dissolved
The original amount of solid dissolved was 14.8 grams. 197mL x (1.73g/ 100 mL) = 3.4 g 100 x (3.4g/ 23%) = 14.8 grams
extraction lab purpose
The purpose is to separate the premade mixture of benzoic acid, 4-tertbutylphenol, and biphenyl through the process of extraction.
Other than solubility of sample in the solvent, what is the criteria for the solvent (other than cheap and nontoxic) used for preparing the sample that impacts the experiment and why would this criteria be important in the spotting phase if you have to make repeated applications of the sample?
The solvent should have a low boiling point because it must be able to evaporate easily from sample, leaving sample on TLC plate. should also dry easily between repeated additions
What is the vapor pressure of a sugar solution dependent on?
The vapor pressure of a sugar solution is dependent on the number of water molecules present in a given volume of sugar solution.
Explain theoretically why SN1 of 2-chloro-2-methylpropane with silver nitrate in ethanol/water would be faster than reaction of 2-chloro-2-methylpropane with silver nitrate in ethanol alone
Theoretically, SN1 of 2-chloro-2-methylpropane with silver nitrate in ethanol/water would be faster because the ethanol and water mixture is more polar protic which is favored by SN1 reactions. Polar protic solvents stabilizes the transition state which causes the reaction to occur faster.
In the citric acid cycle when an aldol addition occurs in a biological system, how do the amino acids contained in the Citrate Synthase enzyme help facilitate the reaction?
They protonate and deprotonate
In the synthesis of isobutyl propionate (shown on pg. 521), excess propanoic acid is used in the reaction. b) What does this indicate with regards to the relative costs of the two starting materials in this synthesis?
This indicates that the propanoic acid is cheaper out of the two starting materials in this synthesis.
nucleophilic substitution purpose
To compare SN1 and SN2 reactions of various alkyl halides.
What can one do to obtain a second crop of crystals after initial crystals have been collected? What is the drawback of this method?
To obtain a second crop of crystals, you can combine and evaporate the filtrate and washings to the point of saturation. The second crop will help increase the overall yield, but the drawback to this method is that the crystals would not be as pure compared to the first crop.
Besides a large spot and the TLC sample being too concentrated, what two other factors can cause the developed spots to appear as a streak?
Too many components in the sample and not enough drying time between the first and second spotting can cause streaks to appear. also some solvents cause streaking
Why would the presence of the side product benzalacetone be minimized?
Using a reactant with no alpha-H, benzaldehyde allows for only one carbonyl to form an enolate which is acetone. We use an excess amount of benzaldehyde because benzalacetone now has an alpha-H and it reacts with another benzaldehyde to form dibenzalacetone thus minimizing benzalacetone.
Why would water not be used to elute a column (besides it polarity) in column chromatography?
Water would not be used to elute a column because of its high boiling point and medium viscosity, water would also dissolve some of the silica gel being used.
When preparing to collect the crystals, you would have used vacuum filtration (not demonstrated in the video). What must be put on the frit of the Hirsch funnel and why?
When preparing to collect the crystals, using vacuum filtration would require the use of a filter paper on top of the frit of the Hirsch funnel to prevent the smaller crystals from going through the funnel into the filtrate. Additionally, the filter paper prevents the frit from being clogged with impurities.
What do you have to do to ensure the crystals are completely dry after remaining on the Hirsch funnel for a few minutes before taking the melting point?
You must complete drying by squeezing crystals gently in between the filter paper and letting crystals air dry on a watch glass.
In column chromatography (microscale), after loading it with solvent and adsorbent and prior to loading the sample, what level should you allow the solvent to drop to? Why should you not allow the sample to run dry and what is the consequence if you had allowed it to do so?
You should allow the solvent to drop at the 5mm mark. By allowing a sample to run dry it would allow air bubbles and channel formation to form leading to poor quality separations.
You have a 1:1 mixture of 2,2,4-trimethylpentane (boiling point: 99∘∘C) and methoxybenzene (boiling point: 154∘∘C) that you wish to separate. c) Why would the distillation type chosen in question b) be ideal for this mixture?
boiling point difference and composition
disposable test tube disposal
broken glass container
disposable test tube disposal?
broken glass container
UV visualizes -
conjugated compounds
You have a 1:1 mixture of 2,2,4-trimethylpentane (boiling point: 99∘∘C) and methoxybenzene (boiling point: 154∘∘C) that you wish to separate. What kind of distillation (between simple and fractional) would be efficient for this mixture?
fractional
You have a 1:1 mixture of 2,2,4-trimethylpentane (boiling point: 99∘∘C) and methoxybenzene (boiling point: 154∘∘C) that you wish to separate. a) What common criteria would this mixture have to satisfy in order to be separated by either simple or fractional distillation?
homogenous
what are the major and minor products of aldol condensation
major - dibenzalacetone minor - benzalacetone and 4-hydroxy-4-methyl-2-pentatone
the washing step of recrystallization removes which category of impurities?
on the surface of the solid
used filter paper disposal
regular trash
used paper towels
regular trash
used weighing paper disposal
regular trash
Melting Point Capillary after use Disposal ?
sharps container
glass pipette after use disposal
sharps container
pipette disposal
sharps container
leftover solid organic compound disposal
solid chemical waste
ethanol filtrate disposal
solvent/liquid waste
excess solvent disposal
solvent/liquid waste
Why? t-butymethyl ether less polar, cannot overcome attractions of sample to adsorbent
t-butymethyl ether less polar, cannot overcome attractions of sample to adsorbent
The crystallization step of recrystallization removes which category of impurities?
within the crystal lattice
Nucleophilic Substitution sources of error
● If reaction conditions are not properly met resulting in low yield ● Pipettes causing contamination in test tube ● Not the same amount of reagents used for each reaction ● Contamination between reactions