PCAT Organic Chemistry 4

Question 1

Which of the following is the strongest acid?

Accepted Answer

Acetic acid (pKa ~4.75)

Answer

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?

Accepted Answer

Rotation is zero (racemic mixture)

Answer

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?

Accepted Answer

Silver ammonia complex [Ag(NH3)2]+

Answer

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?

Accepted Answer

The alkyl C–O bond (O from alcohol)

Answer

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?

Accepted Answer

Cleaves a C=C double bond to give carbonyl compounds

Answer

Ozonolysis cleaves C=C double bonds; reductive workup (Zn/H2O) gives aldehydes or ketones depending on substitution.

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