Chem 233 Final Exam (Labs 5-10)
Aromatic rings on IR spectra show up as?
(C-H) sharp band at around 2950.
Carbonyl group on IR spectra show up as?
(C=O) sharp band at around 1750 cm-1.
The boiling point of the by-product is 67-69 ºC. If a few drops of the final distillation product are collected at 69 ºC, based on your previous experiments in this class, what simple chemical test can you employ to identify that fraction as a diene or the 2,3-dimethyl-2,3-butanediol? Briefly explain. (Lab 9)
- We can do the bromine test to test for dienes. The bromine test shows us if there are any carbon-carbon double bonds present. A positive test is indicated by the solution going from an orange color to a clear solution. For the presence of an alcohol group, we can use the Lucas test. The Lucas test helps us see the difference between primary, secondary, and tertiary alcohols.
Why does phenacetin remain in the organic layer during work-up, but acetaminophen is extracted to the aqueous layer?
Acetaminophen contains a phenol in its structure. The oxygen will end up having a negative charge on it because of deprotonation by NaOH. The negative charge will cause the phenol group to be more soluble in the aqueous layer than the organic layer because it is polar. The phenacetin does not have a phenol group that we can deprotonate so it stays in the organic layer.
Why is the boiling point of the parent alcohol higher than that of the product alkene?
Alcohols participating in hydrogen bonding, which are the strongest of the dipole-dipole interactions. Alkenes, on the other hand, are non-polar and make intermolecular attractions primarily through induced dipole-induced dipole interactions (London dispersion forces), which are much weaker than hydrogen bonds (alcohols have London dispersion forces too). The more/stronger intermolecular attractions a molecule makes, the higher its boiling will be.
What is the reason why semicarbazide was added as a hydrochloride salt in the pinacolone semicarbazone derivative synthesis procedure?
Because it is a salt which is more water soluble than neutral semicarbazide. Also, to increase the electrophilicity when attacked by a semicarbazide nucleophile.
Why is an aqueous solution of potassium hydroxide an unsuitable medium for effecting the base promoted elimination of the E2 reaction procedure?
Because it is stable in an aqueous medium and isn't a base-promoted elimination of this procedure.
Why is sodium bicarbonate used in Lab 6: Preparation of Alkyl Halides by Substitution Reactions, if it's difficult to handle?
Because it's a strong enough base to promote the formation of alkene (so that only one organic product was produced).
What indicates a positive Lucas test?
Cloudy white precipitate forms.
What does a positive bromine test look like?
Color goes from orange to colorless. (Alkenes present)
What does a positive baeyer test look like?
Color goes from purple to brown. (Alkenes present)
What does doubling the nucleophile do to the rate of reaction?
Doubles the rate of reaction.
Why does the excess of base used in these eliminations favor the E2 over the E1 mechanism for elimination? (Lab 7)
E2 mechanism because they are bimolecular, meaning they include the substrate with the base. -E1 reactions only have the substrate -E2 is favored because the base adds to the strength of the rate
Define the Le Chatelier principle.
If a chemical system at equilibrium experiences a change ("stress") then the equilibrium shifts to counteract the imposed change & a new equilibrium is established.
In Lab 7, why is the head temperature kept below 90ºC in the initial step of the dehydration reaction?
If the temperature went over 90 ºC, the alcohol could possibly co-distill. This would take away from the formation of alkenes from the alcohol.
Why is cold 2M NaOH recommended for Lab 6: Preparation of Alkyl Halides by Substitution Reactions?
It doubles the concentration which leads to increased rate of reaction.
In Lab 7: Preparation of Alkenes by Elimination, what is the solid material that gets precipitated from the organic solvent, as the eliminations proceed?
KCl.
Why is it proposed that the alcohol functional group is protonated by acid before dehydration can occur via either an E1 or an E2 mechanism?
Leaving group ability is inversely proportional to basicity, i.e. weaker bases are better leaving groups. The alcohol functional group must be protonated during an E1 or E2 reaction to turn -OH, a very strong base and therefore poor leaving group into H2O, a much weaker base and therefore much better leaving group.
Esters are normally unreactive toward sodium borohydride, but they react readily with lithium aluminum hydride to produce alcohol. Why?
LiAlH4 is more reactive than NaBH4, therefore esters cannot be reduced by NaBH4. The carbonyl group in esters are not that polar due to the resonance effect provided by the presence of -O- adjacent to the carbonyl group. Hence, the strong hydride donor (LiAlH4) can only reduce esters whereas the weak hydride donor NaBH4 cannot reduce it.
How do retention factor and polarity relate?
Low Rf value = stronger polarity
Why is the pH of the solution used to form the semicarbazone derivative important?
Low pH slows the rate because the electrophile is protonated.
If a melting point of a sample is broad and depressed, it is a: -Pure sample -Mixture
Mixture.
How would doubling the concentration of the nucleophile affect the rate of an SN1 reaction?
NO effect.
Does an impurity always lower the melting point of an organic compound?
No, not always. (Usually this statement is true, except if the impurity has a significantly higher melting point and is present in large quantities.)
If we tried the same reaction with acetaminophen and potassium carbonate but instead used t-butyl iodide as the alkyl halide, would the reaction still proceed as expected? If not, explain why. (Lab 10)
No, the reaction will not proceed as expected because tert-butyl iodide in the presence of strong bases such as alkoxides, will undergo elimination instead of substitution to reform acetaminophen and an alkene as the product instead of the desired ether.
If we tried the same reaction with iodoethane and potassium carbonate but instead used cyclohexanol as the alcohol, would the reaction still proceed as expected? Yes or no, explain why. (Lab 10)
No, the reaction would not proceed as expected because cyclohexanol is not as acidic as the alcohol. - Deprotonation would not take place between the reaction of cyclohexanol and K2CO3. - We cannot get to the next step of the Williamson ether synthesis until deprotonation occurs, so this reaction would not proceed.
Is bisulfate a good leavening group for substitution reaction?
No.
What is the objective of Lab 6: Preparation of Alkyl Halides by Substitution Reactions?
Prepare tertiary alkyl halides by SN1 reaction and a primary alkyl halide by SN2 reaction.
If a melting point of a sample is sharp and the same temperature, it is a: -Pure sample -Mixture
Pure sample.
What is the purpose of washing the crude 2-chloro-2-methylbutane with cold sodium bicarbonate solution?
Sodium bicarbonate neutralizes excess HCl from the reaction.
How many molar equivalents of hydride does sodium borohydride contain?
Sodium borohydride contains 4 equivalents of hydride.
In principle, the equilibrium in the dehydration of an alcohol could be shifted to the right by removal of water. Why is this tactic not a good option for the dehydration of 4-methyl-2-pentanol and cyclohexanol?
The alkene products of these reactions can form polymers by reacting with concentrated sulfuric acid and heat. By removing all of the water (which couldn't be accomplished by distillation anyway, since the alkene products will be removed first since their boiling points are lower than water's anyway) and leaving only alkene and concentrated sulfuric acid you lose your product to this reaction.
What is the purpose of adding 5% sodium hydroxide to the reaction during the workup? How can one tell that adding the NaOH served its purpose? (Lab 10)
The basic compound will combine with the acidic solution which results in the formation of a salt. The salt will not dissolve in the organic layer and during the work up, the amide group will be the base.
What is the driving force for the rearrangement of intermediate carbocations during the dehydration of alcohols via the E1 mechanisms?
The driving force for carbocation rearrangement is stability. The more stable a carbocation, the lower in energy it is.
What are sodium iodide tests (NaI) and silver nitrate tests (AgNO3) used for?
To distinguish between primary, secondary, and tertiary alkyl halides.
What is the objective of Lab 7: Preparation of Alkenes by Elimination?
To find out if alkenes were formed from the reaction using the baeyer test and bromine test to check for formation.
What is the purpose of the sodium chloride wash in Lab 6: Preparation of Alkyl Halides by Substitution Reactions?
To remove residual water from the organic layer.
What is the objective of Lab 10: Williamson Ether Synthesis?
To successfully prepare phenacetin via Williamson ether synthesis with acetaminophen using potassium carbonate and iodoethane
A sharp melting point for a crystalline organic substance does NOT always indicate a pure single compound. (True or False)
True.
If the addition of a sample of compound A lowers the melting point of compound X, X and A cannot be identical.
True.
9-fluorenone is colored (bright yellow color), but 9-fluorenol is not (white). What accounts for this difference? (Lab 8)
We see molecules as colored when they absorb light of visible frequency. When comparing the structures of 9-fluorenol and 9-fluorenone, we see that 9-fluorenone is fully conjugated, whereas 9-fluorenol is not (reducing the ketone to the alcohol breaks the conjugation). The full conjugation of 9-fluorenone is what allows it to be colored when 9-fluorenol is not.
When does a substitution reaction compete with elimination?
When a primary alkyl halide is treated with alcoholic potassium hydroxide.
Indication of a positive silver nitrate test?
White or yellow precipitate forms (Negative test= clear)
Indication of a positive sodium iodide test?
White precipitates forms, indicating alkyl bromides and chlorides are present. -Precipitate should dissolve when mixed.
What is another term for the major product? And the minor product?
Zaitsev = major (most stable) Hoffman = minor
Looking at each reagent used during the synthesis and work-up, decide if the following substitutions could be made, and note why or why not they are appropriate: a) Sodium hydroxide for potassium carbonate b) Propanol for 2-butanone c) Ether for ethyl acetate d) Water for 5% NaOH (aq)
a) Sodium hydroxide for potassium carbonate -substitution works b) Propanol for 2-butanone - substitution does not work c) Ether for ethyl acetate -substitution works d) Water for 5% NaOH - substitution does not work
Describe errors in procedure that may cause an observed capillary melting point of a pure compound: a) to be lower than the correct melting point b) to be higher than the correct melting point c) to be broad in range (over several degrees)
a) heating the sample too quickly, using an uncalibrated thermometer, measuring a sample with an impurity, measuring a wet sample. b) using too large a sample size, packing a sample too loosely in the capillary tube, using an uncalibrated thermometer, measuring a sample with significant quantities of an impurity with much higher melting point than the sample itself. c) measuring an impure sample, measuring a wet sample, heating the sample too quickly, using too large a sample size, measuring a sample made up of large crystals.
What is the objective of Lab 8: Preparation of Alcohols - Reduction of Fluorenone & Lucas Test for Alcohols?
the goal is to form 9-fluorenol by reducing 9-fluorenone with sodium borohydride.
What is the objective of Lab 9: The Pinacol Rearrangement?
to examine how the carbocation is rearranged in the pinacol structure.
