ORGANIC CHEMESTY AND BIOCHEM PCAT |VERIFIED QUESTIONS AND
CORRECT DETAILED ANSWERS|RATED AND GRADED A+ NEW
UPDATE| 2026/2027
Ester Formation (Carboxylic acid) - ANSWER✔ 1. Because Carboxylic acids are less reactive than acyl
chlorides, they must be activated towards nucleophilic substitution by protonation from acid. This is why
this reaction is called Fischer acid-catalyzed esterification. The electrophile is the carbonyl carbon and
the nucleophile is the alcohol. The lone electron pairs on the alcohol attack the carbonyl carbon to form
the ester with water acting as the leaving group.
Hydrolysis (Ester and Amides to Carboxylic Acid) - ANSWER✔ 1. Both amides and esters can react
with water to cleave them back into either an alcohol for an ester or an acid or amine for an amide. In
this reaction, water serves as the nucleophile, kicking of the amine or alcohol leaving group
Acyl Halide formation - ANSWER✔ Formation of the acyl halide follows the same reaction mechanism
as the esterification. The nucleophile in this reaction is the thionyl chloride. Creating an acyl halide is
synthetically useful as it is the most reactive derivative.
Carboxylic Acids - ANSWER✔ Carboxylic acids contain both a carbonyl group and an alcohol group.
The carbonyl carbon is vulnerable to nucleophilic addition, and the alcohol group can become a leaving
group in acid. Oxygen is more electronegative than carbon, so there will be an uneven distribution of
charge in the carbonyl. The result is that the carbon atom has a partial positive charge and the oxygen
atom has a partial negative charge. Nomenclature: "-oic acid" Electrophile: carbonyl carbon Leaving
Group: hydroxide Derivatives The different derivatives of carboxylic acids are shown below in order of
increasing reactivity. Acyl halides are the most reactive, while amides are the least reactive and thus
most stable.
Aldol Addition - ANSWER✔ 1. can be catalyzed with either base or acid and will form the same
product
2. In the first step, the electrons on the oxygen will resonate to reform the carbonyl of the keto from,
creating a carbanion on the alpha carbon. This carbanion will attack the carbonyl of the aldehyde,
forming a new carbon-carbon bond. The color coded circles show the carbons on the new molecule that
correspond to the reactants.
, Keto-enol tautomerism - ANSWER✔ 1. Acidity and the Alpha Hydrogen: if deprotonated, the carbonyl
is much more stable than a regular alkane because of its ability to resonate. Because of this stability, a
carbonyl can lose an alpha hydrogen much more easily, and is thus acidic.
2. Resonance upon the loss of a proton and the protonation of the oxygen is called Keto-enol
tautomerism. Keep in mind that the keto form is more thermodynamically stable. An enol can be formed
by adding either base or acid to a ketone. The enol is less stable than the ketone; thus the equilibrium
will favor the formation of the ketone The interconversion between the ketone form and the enol is
called tautomerism. It is important to know that alpha hydrogens are acidic and are located on the alpha
carbon, which is the carbon one carbon away from the carbonyl.
Grignard Reagents - ANSWER✔ 1. Each of the reactions below follows the general mechanism
described for carbonyls! A Grignard reagent is an alkyl group attached to MgX (usually MgCl) and is an
important tool for forming new carbon‐carbon bonds. The R group of the Grignard reagent acts as a
nucleophile, which attacks the electrophilic carbonyl carbon. Pi electrons are pushed onto the carbonyl
oxygen, which is then protonated.
2. The propyl Grignard will attack the carbonyl of the ketone to generate an alkoxide anion in the first
step. In the second step, H2O serves as a proton source to convert the alkoxide anion to the
corresponding alcohol. The net result of reacting with a grignard reagent, regardless of the type of
carbonyl (aldehyde or ketone) is a new carbon carbon bond and an alcohol.
Acetal Formation - ANSWER✔ Ketones, hemiketals, and ketals all have a central carbon that makes
two carbon‐carbon bonds and two carbon‐oxygen bonds. In the ketone the two carbon‐oxygen bonds
happen to be to the same oxygen atom, while hemiketals and ketals have these two carbon‐oxygen
bonds to different oxygen atoms. The difference between a ketone and an aldehyde boils down to the
carbonyl carbon making two carbon‐carbon bonds vs. one carbon‐carbon bond and one carbon‐
hydrogen bond. The same is true of the difference between a hemi‐ketal and a hemi‐acetal and
between a ketal and an acetal.
2. A geminal diol is an organic compound with two hydroxyl groups bound to the same carbon.
("Geminal" comes from the Latin word "Gemini", which means "twins", and is also the name of an
astrological constellation depicting twins.) The first mechanism below shows a pathway that such
geminal diols can be formed.
3. This pathway follows the same general mechanism for reactions of carbonyls: Here, H2O is the
nucleophile, which attacks the carbonyl carbon, and so on. In the second pathway below, a similar
CORRECT DETAILED ANSWERS|RATED AND GRADED A+ NEW
UPDATE| 2026/2027
Ester Formation (Carboxylic acid) - ANSWER✔ 1. Because Carboxylic acids are less reactive than acyl
chlorides, they must be activated towards nucleophilic substitution by protonation from acid. This is why
this reaction is called Fischer acid-catalyzed esterification. The electrophile is the carbonyl carbon and
the nucleophile is the alcohol. The lone electron pairs on the alcohol attack the carbonyl carbon to form
the ester with water acting as the leaving group.
Hydrolysis (Ester and Amides to Carboxylic Acid) - ANSWER✔ 1. Both amides and esters can react
with water to cleave them back into either an alcohol for an ester or an acid or amine for an amide. In
this reaction, water serves as the nucleophile, kicking of the amine or alcohol leaving group
Acyl Halide formation - ANSWER✔ Formation of the acyl halide follows the same reaction mechanism
as the esterification. The nucleophile in this reaction is the thionyl chloride. Creating an acyl halide is
synthetically useful as it is the most reactive derivative.
Carboxylic Acids - ANSWER✔ Carboxylic acids contain both a carbonyl group and an alcohol group.
The carbonyl carbon is vulnerable to nucleophilic addition, and the alcohol group can become a leaving
group in acid. Oxygen is more electronegative than carbon, so there will be an uneven distribution of
charge in the carbonyl. The result is that the carbon atom has a partial positive charge and the oxygen
atom has a partial negative charge. Nomenclature: "-oic acid" Electrophile: carbonyl carbon Leaving
Group: hydroxide Derivatives The different derivatives of carboxylic acids are shown below in order of
increasing reactivity. Acyl halides are the most reactive, while amides are the least reactive and thus
most stable.
Aldol Addition - ANSWER✔ 1. can be catalyzed with either base or acid and will form the same
product
2. In the first step, the electrons on the oxygen will resonate to reform the carbonyl of the keto from,
creating a carbanion on the alpha carbon. This carbanion will attack the carbonyl of the aldehyde,
forming a new carbon-carbon bond. The color coded circles show the carbons on the new molecule that
correspond to the reactants.
, Keto-enol tautomerism - ANSWER✔ 1. Acidity and the Alpha Hydrogen: if deprotonated, the carbonyl
is much more stable than a regular alkane because of its ability to resonate. Because of this stability, a
carbonyl can lose an alpha hydrogen much more easily, and is thus acidic.
2. Resonance upon the loss of a proton and the protonation of the oxygen is called Keto-enol
tautomerism. Keep in mind that the keto form is more thermodynamically stable. An enol can be formed
by adding either base or acid to a ketone. The enol is less stable than the ketone; thus the equilibrium
will favor the formation of the ketone The interconversion between the ketone form and the enol is
called tautomerism. It is important to know that alpha hydrogens are acidic and are located on the alpha
carbon, which is the carbon one carbon away from the carbonyl.
Grignard Reagents - ANSWER✔ 1. Each of the reactions below follows the general mechanism
described for carbonyls! A Grignard reagent is an alkyl group attached to MgX (usually MgCl) and is an
important tool for forming new carbon‐carbon bonds. The R group of the Grignard reagent acts as a
nucleophile, which attacks the electrophilic carbonyl carbon. Pi electrons are pushed onto the carbonyl
oxygen, which is then protonated.
2. The propyl Grignard will attack the carbonyl of the ketone to generate an alkoxide anion in the first
step. In the second step, H2O serves as a proton source to convert the alkoxide anion to the
corresponding alcohol. The net result of reacting with a grignard reagent, regardless of the type of
carbonyl (aldehyde or ketone) is a new carbon carbon bond and an alcohol.
Acetal Formation - ANSWER✔ Ketones, hemiketals, and ketals all have a central carbon that makes
two carbon‐carbon bonds and two carbon‐oxygen bonds. In the ketone the two carbon‐oxygen bonds
happen to be to the same oxygen atom, while hemiketals and ketals have these two carbon‐oxygen
bonds to different oxygen atoms. The difference between a ketone and an aldehyde boils down to the
carbonyl carbon making two carbon‐carbon bonds vs. one carbon‐carbon bond and one carbon‐
hydrogen bond. The same is true of the difference between a hemi‐ketal and a hemi‐acetal and
between a ketal and an acetal.
2. A geminal diol is an organic compound with two hydroxyl groups bound to the same carbon.
("Geminal" comes from the Latin word "Gemini", which means "twins", and is also the name of an
astrological constellation depicting twins.) The first mechanism below shows a pathway that such
geminal diols can be formed.
3. This pathway follows the same general mechanism for reactions of carbonyls: Here, H2O is the
nucleophile, which attacks the carbonyl carbon, and so on. In the second pathway below, a similar
