OAT Organic Chemistry 5 — Questions and Answers
Question 1: Which of the following correctly describes the relationship between enthalpy of combustion and molecular structure for straight-chain alkanes?
- Enthalpy of combustion decreases as chain length increases.
- Enthalpy of combustion increases as chain length increases. (Correct answer)
- Enthalpy of combustion is independent of chain length.
- Branched alkanes have higher enthalpy of combustion than straight-chain isomers.
Correct answer: Enthalpy of combustion increases as chain length increases.
Each additional CH2 unit contributes approximately 659 kJ/mol to the enthalpy of combustion, so longer chains release more energy.
Question 2: Which type of reaction converts an alkyl halide into an alkene by treatment with a strong, bulky base such as KOtBu?
- SN1
- SN2
- E1
- E2 (Correct answer)
Correct answer: E2
Bulky bases favor E2 elimination because steric hindrance prevents the nucleophilic SN2 pathway.
Question 3: What is the term for the energy difference between reactants and the transition state in a reaction?
- Enthalpy of reaction (ΔH)
- Gibbs free energy (ΔG)
- Activation energy (Ea) (Correct answer)
- Entropy change (ΔS)
Correct answer: Activation energy (Ea)
Activation energy is the minimum energy required to reach the transition state and is the energy barrier of the reaction.
Question 4: Carboxylic acid derivatives in decreasing order of reactivity toward nucleophilic acyl substitution are:
- Acid chloride > anhydride > ester > amide (Correct answer)
- Amide > ester > anhydride > acid chloride
- Ester > acid chloride > amide > anhydride
- Acid chloride > amide > ester > anhydride
Correct answer: Acid chloride > anhydride > ester > amide
Reactivity decreases as the leaving group ability decreases: Cl⁻ > RCOO⁻ > RO⁻ > NH2⁻.
Question 5: What product is formed when a primary amine reacts with excess methyl iodide followed by Ag2O/H2O and heat?
- Secondary amine
- Tertiary amine
- Quaternary ammonium salt
- The least substituted alkene (Hofmann elimination product) (Correct answer)
Correct answer: The least substituted alkene (Hofmann elimination product)
Hofmann exhaustive methylation converts a quaternary ammonium hydroxide to the least hindered alkene via elimination.
Question 6: Which orbital interaction is responsible for the aromatic stability of benzene?
- Overlap of sp3 orbitals around the ring
- Continuous cyclic overlap of six p orbitals forming delocalized π electrons (Correct answer)
- σ-bond resonance between alternating single and double bonds
- Hybridization of d orbitals with ring carbons
Correct answer: Continuous cyclic overlap of six p orbitals forming delocalized π electrons
Benzene's six p orbitals overlap continuously around the ring, delocalizing six π electrons and conferring exceptional stability.
Question 7: An unknown compound shows a broad O–H stretch near 3300 cm⁻¹ and a C=O stretch at 1710 cm⁻¹ in its IR spectrum. What functional group is most likely present?
- Alcohol
- Ketone
- Carboxylic acid (Correct answer)
- Ester
Correct answer: Carboxylic acid
A broad O–H stretch combined with a carbonyl peak near 1710 cm⁻¹ is the hallmark IR signature of a carboxylic acid.
OAT Organic Chemistry 5
Which of the following correctly describes the relationship between enthalpy of combustion and molecular structure for straight-chain alkanes?