MASTERCLASS ORGANIC CHEMISTRY
REAGENTS EXAM LONG FORM WITH BOLD
ANSWERS AND DEEP RATIONALES
Question 1
A research chemist attempts to synthesize an alpha,beta-
unsaturated aldehyde from a primary allylic alcohol without
causing over-oxidation to the corresponding carboxylic acid or
inducing the unwanted migration or cleavage of the delicate
carbon-carbon double bond present in the molecular framework.
Which of the following reaction conditions and chemical reagents
would be the most synthetically viable choice to achieve this
highly selective transformation while maintaining mild operating
conditions?
A) Potassium permanganate (KMnO4) buffered in an aqueous
sodium hydroxide solution at elevated reflux temperatures.
B) Chromium trioxide dissolved in aqueous sulfuric acid and
acetone, commonly known as the Jones Reagent, maintained at
zero degrees Celsius.
C) Pyridinium chlorochromate (PCC) suspended in anhydrous
dichloromethane (CH2Cl2) at room temperature.
D) Sodium dichromate (Na2Cr2O7) combined with concentrated
sulfuric acid and water under vigorous stirring.
CorreCt Answer: C) Pyridinium chlorochromate
(PCC) suspended in anhydrous dichloromethane
(CH2Cl2) at room temperature.
,rAtionAle: Pyridinium chlorochromate (PCC) is a mild,
chromium-based oxidizing agent that is soluble in organic
solvents like dichloromethane. Because the reaction is performed
under strictly anhydrous (water-free) conditions, the aldehyde
intermediate cannot form a hydrate (gem-diol), which is a
mandatory prerequisite for further oxidation to a carboxylic
acid. Consequently, PCC selectively oxidizes primary alcohols to
aldehydes and secondary alcohols to ketones without over-
oxidizing sensitive functional groups like alkenes. In contrast,
reagents like Jones Reagent (CrO3/H2SO4) and potassium
permanganate (KMnO4) are highly acidic or strongly basic
aqueous oxidants that would immediately over-oxidize the
aldehyde to a carboxylic acid and potentially cleave or alter the
double bond.
Question 2
During the multi-step total synthesis of a complex pharmaceutical
agent, an intermediate molecule contains both a highly reactive
aliphatic ester linkage and a relatively hindered secondary amide
functional group. The synthetic pathway requires the completely
chemoselective reduction of the ester group into a primary
alcohol, while leaving the secondary amide perfectly intact and
unreacted. Which of the following reducing agents must be
selected to successfully execute this transformation?
A) Lithium aluminum hydride (LiAlH4) dissolved in anhydrous
diethyl ether followed by a careful aqueous acid workup.
B) Sodium borohydride (NaBH4) dissolved in a polar protic
solvent mix of ethanol and water at room temperature.
C) Diisobutylaluminum hydride (DIBAL-H) utilized in a strict 1:1
,stoichiometric ratio in toluene maintained at minus seventy-eight
degrees Celsius.
D) Lithium borohydride (LiBH4) utilized in an anhydrous
tetrahydrofuran (THF) solvent system at moderate temperatures.
CorreCt Answer: D) Lithium borohydride (LiBH4)
utilized in an anhydrous tetrahydrofuran (THF) solvent
system at moderate temperatures.
rAtionAle: Lithium borohydride (LiBH4) is a reducing
agent of intermediate reactivity, making it more powerful than
sodium borohydride (NaBH4) but significantly milder than
lithium aluminum hydride (LiAlH4). It is uniquely suited for
chemoselective applications because it smoothly reduces esters to
primary alcohols but is completely unreactive toward amides.
Sodium borohydride (Option B) is too weak and generally
reduces only aldehydes and ketones, leaving esters and amides
untouched under standard conditions. Lithium aluminum
hydride (Option A) is extremely powerful and would
unselectively reduce both the ester to an alcohol and the amide to
an amine. DIBAL-H at low temperatures (Option C) would
selectively reduce the ester to an aldehyde rather than a primary
alcohol.
Question 3
An organic synthesis project requires the transformation of an
unsymmetrical internal alkyne, specifically 2-methylhex-3-yne,
into a highly stereopure (Z)-alkene (cis-alkene) configuration
without creating any traceable amounts of the corresponding
alkane over-reduction product or the thermodynamically favored
, (E)-alkene (trans-alkene) isomer. Which of the following specific
catalytic systems and reaction conditions will yield this exact
geometric outcome?
A) Sodium metal safely dissolved in liquid ammonia (Na / NH3)
maintained at a constant temperature of minus thirty-three
degrees Celsius.
B) Hydrogen gas (H2) provided at high pressure over a standard,
unpoisoned palladium-on-carbon (Pd/C) heterogeneous catalyst
matrix.
C) Hydrogen gas (H2) delivered at one atmosphere of pressure
over a Lindlar catalyst system consisting of palladium on calcium
carbonate poisoned with lead acetate and quinoline.
D) Borane-tetrahydrofuran complex (BH3·THF) followed by a
basic oxidative workup utilizing hydrogen peroxide and sodium
hydroxide.
CorreCt Answer: C) Hydrogen gas (H2) delivered
at one atmosphere of pressure over a Lindlar catalyst
system consisting of palladium on calcium carbonate
poisoned with lead acetate and quinoline.
rAtionAle: The Lindlar catalyst is a heterogeneous
palladium catalyst that has been intentionally deactivated
("poisoned") with lead acetate and quinoline. This deactivation
slows its catalytic activity down to the point where it can easily
reduce highly reactive alkynes to alkenes, but completely lacks
the chemical power to reduce the resulting alkenes into alkanes.
Because the hydrogenation mechanism occurs via syn-addition
on the solid surface of the catalyst, both hydrogen atoms are
delivered to the exact same face of the alkyne triple bond,
exclusively generating the (Z)-alkene or cis-isomer. Option A
REAGENTS EXAM LONG FORM WITH BOLD
ANSWERS AND DEEP RATIONALES
Question 1
A research chemist attempts to synthesize an alpha,beta-
unsaturated aldehyde from a primary allylic alcohol without
causing over-oxidation to the corresponding carboxylic acid or
inducing the unwanted migration or cleavage of the delicate
carbon-carbon double bond present in the molecular framework.
Which of the following reaction conditions and chemical reagents
would be the most synthetically viable choice to achieve this
highly selective transformation while maintaining mild operating
conditions?
A) Potassium permanganate (KMnO4) buffered in an aqueous
sodium hydroxide solution at elevated reflux temperatures.
B) Chromium trioxide dissolved in aqueous sulfuric acid and
acetone, commonly known as the Jones Reagent, maintained at
zero degrees Celsius.
C) Pyridinium chlorochromate (PCC) suspended in anhydrous
dichloromethane (CH2Cl2) at room temperature.
D) Sodium dichromate (Na2Cr2O7) combined with concentrated
sulfuric acid and water under vigorous stirring.
CorreCt Answer: C) Pyridinium chlorochromate
(PCC) suspended in anhydrous dichloromethane
(CH2Cl2) at room temperature.
,rAtionAle: Pyridinium chlorochromate (PCC) is a mild,
chromium-based oxidizing agent that is soluble in organic
solvents like dichloromethane. Because the reaction is performed
under strictly anhydrous (water-free) conditions, the aldehyde
intermediate cannot form a hydrate (gem-diol), which is a
mandatory prerequisite for further oxidation to a carboxylic
acid. Consequently, PCC selectively oxidizes primary alcohols to
aldehydes and secondary alcohols to ketones without over-
oxidizing sensitive functional groups like alkenes. In contrast,
reagents like Jones Reagent (CrO3/H2SO4) and potassium
permanganate (KMnO4) are highly acidic or strongly basic
aqueous oxidants that would immediately over-oxidize the
aldehyde to a carboxylic acid and potentially cleave or alter the
double bond.
Question 2
During the multi-step total synthesis of a complex pharmaceutical
agent, an intermediate molecule contains both a highly reactive
aliphatic ester linkage and a relatively hindered secondary amide
functional group. The synthetic pathway requires the completely
chemoselective reduction of the ester group into a primary
alcohol, while leaving the secondary amide perfectly intact and
unreacted. Which of the following reducing agents must be
selected to successfully execute this transformation?
A) Lithium aluminum hydride (LiAlH4) dissolved in anhydrous
diethyl ether followed by a careful aqueous acid workup.
B) Sodium borohydride (NaBH4) dissolved in a polar protic
solvent mix of ethanol and water at room temperature.
C) Diisobutylaluminum hydride (DIBAL-H) utilized in a strict 1:1
,stoichiometric ratio in toluene maintained at minus seventy-eight
degrees Celsius.
D) Lithium borohydride (LiBH4) utilized in an anhydrous
tetrahydrofuran (THF) solvent system at moderate temperatures.
CorreCt Answer: D) Lithium borohydride (LiBH4)
utilized in an anhydrous tetrahydrofuran (THF) solvent
system at moderate temperatures.
rAtionAle: Lithium borohydride (LiBH4) is a reducing
agent of intermediate reactivity, making it more powerful than
sodium borohydride (NaBH4) but significantly milder than
lithium aluminum hydride (LiAlH4). It is uniquely suited for
chemoselective applications because it smoothly reduces esters to
primary alcohols but is completely unreactive toward amides.
Sodium borohydride (Option B) is too weak and generally
reduces only aldehydes and ketones, leaving esters and amides
untouched under standard conditions. Lithium aluminum
hydride (Option A) is extremely powerful and would
unselectively reduce both the ester to an alcohol and the amide to
an amine. DIBAL-H at low temperatures (Option C) would
selectively reduce the ester to an aldehyde rather than a primary
alcohol.
Question 3
An organic synthesis project requires the transformation of an
unsymmetrical internal alkyne, specifically 2-methylhex-3-yne,
into a highly stereopure (Z)-alkene (cis-alkene) configuration
without creating any traceable amounts of the corresponding
alkane over-reduction product or the thermodynamically favored
, (E)-alkene (trans-alkene) isomer. Which of the following specific
catalytic systems and reaction conditions will yield this exact
geometric outcome?
A) Sodium metal safely dissolved in liquid ammonia (Na / NH3)
maintained at a constant temperature of minus thirty-three
degrees Celsius.
B) Hydrogen gas (H2) provided at high pressure over a standard,
unpoisoned palladium-on-carbon (Pd/C) heterogeneous catalyst
matrix.
C) Hydrogen gas (H2) delivered at one atmosphere of pressure
over a Lindlar catalyst system consisting of palladium on calcium
carbonate poisoned with lead acetate and quinoline.
D) Borane-tetrahydrofuran complex (BH3·THF) followed by a
basic oxidative workup utilizing hydrogen peroxide and sodium
hydroxide.
CorreCt Answer: C) Hydrogen gas (H2) delivered
at one atmosphere of pressure over a Lindlar catalyst
system consisting of palladium on calcium carbonate
poisoned with lead acetate and quinoline.
rAtionAle: The Lindlar catalyst is a heterogeneous
palladium catalyst that has been intentionally deactivated
("poisoned") with lead acetate and quinoline. This deactivation
slows its catalytic activity down to the point where it can easily
reduce highly reactive alkynes to alkenes, but completely lacks
the chemical power to reduce the resulting alkenes into alkanes.
Because the hydrogenation mechanism occurs via syn-addition
on the solid surface of the catalyst, both hydrogen atoms are
delivered to the exact same face of the alkyne triple bond,
exclusively generating the (Z)-alkene or cis-isomer. Option A
