Question 1

Which of the following best describes the electron-pushing mechanism for the transformation of a terminal alkyne to a conjugated diene using a strong base and subsequent alkylation?

Accepted Answer

Deprotonation of the terminal alkyne forms an acetylide anion, which then attacks an alkyl halide via $$\mathrm{S_N2}$$ mechanism.

Question 2

In the reaction of 1-bromo-2-methylpropane, which conformer (A or B) is expected to have the lowest energy and why?

Accepted Answer

Conformer A, because it minimizes steric interactions between the bromine and methyl groups.

Question 3

Which of the following statements about the resonance structures of the trans-1-methoxy-1-butene cation is correct?

Accepted Answer

There are 6 $$\pi$$ electrons in a conjugated system, shared between C2, C3, C4, and the O atom.

Question 4

Given the potential energy surface for the reaction of HBr with allyl benzene, which intermediate corresponds to the lowest energy on the reaction coordinate diagram?

Accepted Answer

The benzylic carbocation intermediate.

Question 5

Which of the following best explains the regioselectivity observed in the second protonation of an alkyne by HBr?

Accepted Answer

Protonation occurs at the carbon that leads to the most stable carbocation intermediate, typically the more substituted carbon.

Question 6

Estimate the $$pK_a$$ values for the most acidic hydrogen atoms in 2,4-pentanedione and explain which atom is deprotonated first by a strong base.

Accepted Answer

The hydrogen atom between the two carbonyl groups (at C3) with $$pK_a \approx 9 is $$deprotonated first.

Question 7

Which of the following is the correct electron-pushing mechanism for the formation of a carbocation intermediate in the reaction of benzyl chloride with a strong acid?

Accepted Answer

The chloride ion leaves, generating a benzylic carbocation stabilized by resonance with the aromatic ring.

Question 8

In the conformational analysis of 1-bromo-2-methylpropane, what is the spatial relationship between the bromine atom and the methyl group in conformer A?

Accepted Answer

Anti (180° apart)

Question 9

Which of the following best rationalizes the lack of energy difference between conformational isomers A and B of 1-bromo-2-methylpropane?

Accepted Answer

Both conformers have similar steric interactions and torsional strain, resulting in comparable energies.

Question 10

Which of the following is true regarding the hybridization of the oxygen atom in trans-1-methoxy-1-butene?

Accepted Answer

The oxygen atom is $$sp^3$$ hybridized due to two lone pairs and two sigma bonds.

Question 11

In the reaction of HBr with an alkene, which carbocation intermediate is expected to be the most stable?

Accepted Answer

A tertiary carbocation adjacent to an aromatic ring.

Question 12

Which of the following best describes the effect of conjugation on the acidity of 2,4-pentanedione?

Accepted Answer

Conjugation stabilizes the enolate anion formed after deprotonation, lowering the $$pK_a of $$the central hydrogen.

Question 13

Which statement correctly describes the delocalization of positive charge in the resonance structures of the trans-1-methoxy-1-butene cation?

Accepted Answer

The positive charge is delocalized over C2, C4, C5, and the O atom.

Question 14

Which of the following is the correct ranking of carbocation intermediate energies formed during the reaction of HBr with allyl benzene?

Accepted Answer

Benzylic \u003c Allylic \u003c Secondary

Question 15

Which of the following is the most acidic proton in 4-aminophenol, and what is its approximate $$pK_a$$ value?

Accepted Answer

The phenolic OH proton, $$pK_a \approx 10$$

Question 16

Which of the following best explains why the benzylic carbocation is more stable than the allylic carbocation?

Accepted Answer

The benzylic carbocation is stabilized by resonance with the aromatic ring, allowing delocalization of the positive charge.

Question 17

In the reaction mechanism for the formation of regioisomers A, B, and C from the addition of HBr to styrene, which intermediate leads to the major product?

Accepted Answer

The benzylic carbocation intermediate.

Question 18

Which of the following is the correct electron-pushing arrow notation for the formation of an enolate from 2,4-pentanedione?

Accepted Answer

Arrow from the base to the central hydrogen, then from the C-H bond to form a C=C bond and push electrons onto the oxygen.

Organic Chemistry Exam Study Guide: Mechanisms, Conformations, Resonance, and Acidity

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