MCAT Organic Chemistry 2 — Questions and Answers
Question 1: Which reagent converts a primary alcohol to an aldehyde without over-oxidizing to a carboxylic acid?
- KMnO4
- PCC (pyridinium chlorochromate) (Correct answer)
- H2CrO4
- K2Cr2O7
Correct answer: PCC (pyridinium chlorochromate)
PCC selectively oxidizes primary alcohols to aldehydes and stops there because it lacks aqueous conditions needed for further oxidation.
Question 2: In an E2 elimination reaction, what is the required geometric relationship between the leaving group and the beta hydrogen?
- Syn-periplanar (0°)
- Gauche (60°)
- Anti-periplanar (180°) (Correct answer)
- Eclipsed (120°)
Correct answer: Anti-periplanar (180°)
E2 requires an anti-periplanar arrangement (180°) so that the breaking bonds can align with the forming pi bond in a concerted step.
Question 3: Which type of isomerism is exhibited by cis- and trans-2-butene?
- Enantiomerism
- Conformational isomerism
- Constitutional isomerism
- Geometric (cis/trans) isomerism (Correct answer)
Correct answer: Geometric (cis/trans) isomerism
Cis- and trans-2-butene are geometric isomers because they have the same connectivity but differ in the spatial arrangement of groups around the double bond.
Question 4: What product forms when an alkene reacts with ozone (O3) followed by a reductive workup (Zn, H2O)?
- Epoxide
- Diol
- Two carbonyl compounds (aldehydes/ketones) (Correct answer)
- Carboxylic acid
Correct answer: Two carbonyl compounds (aldehydes/ketones)
Ozonolysis with reductive workup cleaves the double bond to give two carbonyl compounds; aldehydes if the carbon was =CH2, ketones if disubstituted.
Question 5: Which statement best describes the inductive effect of a fluorine substituent on benzoic acid?
- It donates electrons via resonance, raising pKa
- It withdraws electrons inductively, lowering pKa (Correct answer)
- It has no effect on acidity
- It donates electrons inductively, raising pKa
Correct answer: It withdraws electrons inductively, lowering pKa
Fluorine is highly electronegative and withdraws electron density inductively, stabilizing the carboxylate anion and lowering the pKa (increasing acidity).
Question 6: A compound with the molecular formula C4H8O shows a strong IR absorption near 1715 cm⁻¹ and no broad O–H stretch. What functional group is present?
- Alcohol
- Ether
- Ketone or aldehyde (Correct answer)
- Carboxylic acid
Correct answer: Ketone or aldehyde
A carbonyl stretch near 1715 cm⁻¹ without an O–H peak indicates a ketone or aldehyde, not an alcohol or carboxylic acid.
Question 7: Which of the following best explains why sp² hybridized carbons are shorter than sp³ C–C bonds?
- More p-orbital character increases bond length
- Greater s-orbital character increases electronegativity and shortens bonds (Correct answer)
- Pi bonds repel sigma bonds, compressing the bond
- Hybridization has no effect on bond length
Correct answer: Greater s-orbital character increases electronegativity and shortens bonds
Increased s-character in sp² orbitals means electrons are held closer to the nucleus, resulting in shorter, stronger bonds compared to sp³.
MCAT Organic Chemistry 2
Which reagent converts a primary alcohol to an aldehyde without over-oxidizing to a carboxylic acid?