Ill: Review: Passages III, VII, VIII, XII, XIV, & Questions 95-100 Focus on reviewing the concepts. Do not worry about timing.
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Specializing in MCAT Preparation
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Alcohols, Acidity, and nucleophilicity
Alcohol Reactions Acetals and Ketals
Carbonyl Reactivity Study
Thermodynamic versus Kinetic Control
Alcohol Oxidation
Unknown OxygenContaining Compounds Wolff-Kishner versus Clemmensen Reduction Grignard Reaction
Aldol Condensation and Alpha Hydrogen
Claisen Condensation Reaction Transesterification Reaction
Malonic Ester Synthesis Carboxylic Acids
Questions not Based on a Descriptive Passage
Carbonyls and Alcohols Scoring Scale
Raw Score MCAT Score
83 - 100 13- 15
65-82 1 0 - 1 2
46-64 7 - 9
33-45 4 - 6
1 -32 1 - 3
(1 -6) (7 - 12) (13- 18) (19-25) (26 - 32) (33 - 39) (40 - 46) (47 - 53) (54 - 60) (61 -67) (68 - 73) (74 - 80) (81 -87) (88 - 94) (95- 100)
Passage I (Questions 1-6)
Alcohols are a common protic solvent used in many organic reactions. Alcohols have organic properties (due to the alkyl chain), and they exhibit hydrogen bonding (not traditionally thought of as an organic property). Low-weight alcohols are the only organic solvents that can be used to simulate aqueous conditions while dissolving organic compounds. They are not an ideal solvent, because they can react as a nucleophile, as an acid, or as a base. Alcohols are chosen in many reactions involving esters as a reactant, where the ester leaving group is the conjugate base of the alcohol. This is done to eliminate the chance for a transesterification reaction.
In some reactions, alcohols serve as both the reactant and the solvent. For instance, when an alkoxide is used as a base (as it is in elimination reactions), it is convenient to use the conjugate acid of the alkoxide as the solvent. This is done, because the conjugate acid (an alcohol) is formed when the alkoxide is protonated. One drawback to alcohol solvents is their relatively high boiling point, due to hydrogen bonding.
When the solvent has a high boiling point, it is hard to distill the solvent away to isolate the product mixture. Of organic compounds with equal carbons (and therefore roughly equal molecular mass), alcohols and carboxylic acids generally have the highest boiling and melting points.
1. The pKa of methanol (H3COH) is 15.5, while the pKa of phenol (H5C6OH) is 10.0. Which of the following
statements are true?
I. Phenol is less acidic than methanol due to the electron donation from the benzene ring through
resonance.
n.
m.
Phenol is more acidic than methanol due to electron withdraw by the benzene ring through
resonance.
Methanol is more acidic than phenol, because of electron withdraw through the inductive effect.
IV. Methanol is less acidic than phenol, because of electron donation through the inductive effect A. I and IV only
B. II and m only C. I and m only D. II and IV only
2. Which of the following bases will deprotonate methanol completely?
A. H3CC02Na B. NH3
C. NaOH
D. H3CCH2Li
Copyright © by The Berkeley Review® 53
Which of the alkoxides is the BEST nucleophile when undergoing a substitution reaction with methyl
bromide?
A. H3CCH2ONa B. H3CCCl2ONa C. H5C6ONa D. para-ClHUCgONa
Which of the following phenols is the strongest acid?
A. para-ClC6H40H B. para-02NC6H40H C. para-H3COC6H40H D. C6H5OH
The pKa of H3COH is 15.5, and the pKa of CI3COH is 11.2. Which of the following values is the BEST approximation for the pKa of F3COH?
A. 19.4 B. 13.8 C. 11.8 D. 10.4
In which of the following reactions would ethanol be
the solvent of choice?
A. H3C
H3N + Ji Br
¥ H
B. O
(H3C)2CHMgBi
C.
H3C
- •.. .AH3C CH2CH3
+ Cr03/H2S04 •
O
H3CH2CH2CX H
D. O O
. MOCH2CH3 2. H3CCH2I1. KOEt
GO ON TO THE NEXT PAGE
Passage II (Questions 7-12)
Alcohols are useful reagents in a vast number of organic reactions, most often used as a nucleophile. Their reactivity as a nucleophile correlates directly to steric hindrance. For instance, because of steric hindrance, a primary alcohol is a better nucleophile than a secondary alcohol, which is in turn a betternucleophile than a tertiaryalcohol.
Alcohols do not readily react as electrophiles, because the hydroxyl group is a poor leaving group. To increase the electrophilicity of an alcohol, the hydroxyl group is treated with a Br0nsted-Lowry acid or a Lewis acid to form a better leaving group. Once a hydroxyl group is protonated to form a water molecule, it becomes a stronger leaving group that can be displaced by weak nucleophiles. A hydroxyl group can also bemade into a better leaving group by converting it into a tosylate group according to the reaction inFigure 1.
ROH + Cl ROS02C6H4CH3
Figure 1 Tosylation of an alcohol
After a hydroxyl group is converted into a better leaving group, the compound readily undergoes dehydration reactions at elevated temperatures, which yield an alkene. A dehydration reaction is commonly known as an elimination reaction. Elimination reactions compete with substitution reactions in most cases where an alcohol nucleophile is
present. The elimination reaction is most favorable with
tertiary alcohols under both basic and acidic conditions, because the alkene product is highly substituted in that case.
The nucleophilicity of alcohols is enhanced by treatment with a strong base, which converts it into an alkoxide anion after it is deprotonated. A nucleophilic substitution reaction with an alkoxide acting as the nucleophile also has complications with the competing elimination reaction, which proceeds by an E2 mechanism.
7. Which ofthe following alcohols would MOST readily
undergo an E\ elimination reaction?
8.
A. B.
C.
OH
D.
o r o r
OH R,CH3C OHOH
Which of the following alcohols will MOST readily undergo transesterification with ethyl acetate and H+?