PCAT Organic Chemistry 4 — Questions and Answers
Question 1: Which of the following is the strongest acid?
- Ethanol (pKa ~16)
- Acetic acid (pKa ~4.75) (Correct answer)
- Phenol (pKa ~10)
- Cyclohexanol (pKa ~17)
Correct answer: Acetic acid (pKa ~4.75)
Acetic acid has the lowest pKa (~4.75), making it the strongest acid among the options because its conjugate base (acetate) is resonance-stabilized.
Question 2: What happens to optical rotation when equal amounts of two enantiomers are mixed?
- Rotation doubles
- Rotation is zero (racemic mixture) (Correct answer)
- Rotation is unpredictable
- Only one enantiomer contributes
Correct answer: Rotation is zero (racemic mixture)
A 50:50 mixture of enantiomers is called a racemic mixture and shows zero net optical rotation because the rotations cancel.
Question 3: Which reagent is used to distinguish an aldehyde from a ketone in Tollens' test?
- Fehling's solution only
- Silver ammonia complex [Ag(NH3)2]+ (Correct answer)
- Benedict's reagent only
- 2,4-dinitrophenylhydrazine
Correct answer: Silver ammonia complex [Ag(NH3)2]+
Tollens' reagent ([Ag(NH3)2]+) oxidizes aldehydes to carboxylic acids, depositing a silver mirror; ketones do not react.
Question 4: In the Fischer esterification reaction, which bond in the ester product comes from the alcohol?
- The C=O bond
- The O–H bond of the product
- The alkyl C–O bond (O from alcohol) (Correct answer)
- The acyl C–O bond
Correct answer: The alkyl C–O bond (O from alcohol)
In Fischer esterification, the oxygen in the ester's alkyl–O bond is derived from the alcohol (confirmed by isotope labeling studies).
Question 5: Which type of organic reaction does ozonolysis (O3 followed by Zn/H2O) perform?
- Adds OH across a double bond
- Cleaves a C=C double bond to give carbonyl compounds (Correct answer)
- Reduces a triple bond to a double bond
- Forms an epoxide from an alkene
Correct answer: Cleaves a C=C double bond to give carbonyl compounds
Ozonolysis cleaves C=C double bonds; reductive workup (Zn/H2O) gives aldehydes or ketones depending on substitution.
Question 6: Which factor most stabilizes a carbocation?
- Increased s-character of the orbital
- Adjacent electronegative substituents
- Hyperconjugation and inductive donation from alkyl groups (Correct answer)
- Negative inductive effect
Correct answer: Hyperconjugation and inductive donation from alkyl groups
Carbocations are stabilized by alkyl groups through hyperconjugation and positive inductive effects, explaining the order: tertiary > secondary > primary.
Question 7: What is the hybridization of the carbon atoms in a C≡C triple bond?
- sp3
- sp2
- sp (Correct answer)
- dsp2
Correct answer: sp
Each carbon in a triple bond forms two σ bonds (one to the other carbon, one to a substituent) using sp orbitals, leaving two unhybridized p orbitals for the two π bonds.
PCAT Organic Chemistry 4
Which of the following is the strongest acid?