Question 1

Which of the following statements best explains why the base peak in a mass spectrum often corresponds to the most stable carbocation fragment?

Accepted Answer

The base peak corresponds to the fragment that forms the most stable carbocation after bond cleavage, leading to its higher abundance.

Question 2

In infrared (IR) spectroscopy, which bond stretch is most diagnostic for identifying the presence of a carbonyl group in an organic molecule?

Accepted Answer

A strong absorption near $$1700\ \mathrm{cm}^{-1}$$ due to the $$\mathrm{C=O}$$ stretch.

Question 3

Which conformation of ethane is most stable and why?

Accepted Answer

The staggered conformation, due to minimized torsional strain and favorable stereoelectronic effects.

Question 4

In butane, which conformation is highest in energy and what is the main reason?

Accepted Answer

The syn-periplanar (eclipsed) conformation, due to steric strain between two $$\mathrm{CH_3$}$$ groups only $$60^{\circ}$$ apart.

Question 5

During a cyclohexane ring flip, what happens to the positions of axial and equatorial substituents?

Accepted Answer

Axial substituents become equatorial, and equatorial become axial.

Question 6

Which isomer of 1,2-dimethylcyclohexane is more stable and why?

Accepted Answer

The trans isomer, because it allows both methyl groups to occupy equatorial positions, minimizing 1,3-diaxial interactions.

Question 7

A molecule has two stereocenters. How many possible stereoisomers can it have?

Accepted Answer

$$4$$

Question 8

Which of the following best describes the difference between enantiomers and diastereomers?

Accepted Answer

Enantiomers are non-superimposable mirror images, while diastereomers are stereoisomers that are not mirror images.

Question 9

Which carbocation is most stabilized by sigma conjugation?

Accepted Answer

A tertiary carbocation ($$\mathrm{R_3C^+})$$

Question 10

Which of the following statements about radical stability is correct?

Accepted Answer

Tertiary radicals are most stable due to hyperconjugation and sigma conjugation.

Question 11

Which IR absorption would you expect for an alcohol ($$\mathrm{O-H}) $$group?

Accepted Answer

A broad, intense peak above $$3000\ \mathrm{cm}^{-1}$$

Question 12

Given the following priorities for groups around a stereocenter: $$1 \u003e 2 \u003e 3 \u003e 4$$, and group 4 is pointing away, what configuration is assigned if the sequence $$1 \rightarrow 2 \rightarrow 3 is $$clockwise?

Accepted Answer

$$(R)$$

Question 13

Which of the following best describes the stabilization energy in ethane due to stereoelectronic effects?

Accepted Answer

Stabilization is $$2 	imes \Delta E$$ per interaction between filled $$\mathrm{C-H}$$ orbitals and empty $$\mathrm{C-H}\ \sigma^*$$ orbitals.

Question 14

Which of the following is the correct formula for the change in free energy ($$\Delta G) in a $$chemical reaction?

Accepted Answer

$$\Delta G = \Delta H - T\Delta S$$

Question 15

In mass spectrometry, what is the role of electron bombardment?

Accepted Answer

It ionizes the molecule to produce a radical cation.

Question 16

Which of the following best explains why equatorial substituents are favored in cyclohexane chair conformations?

Accepted Answer

Equatorial substituents experience less 1,3-diaxial repulsion than axial substituents.

Question 17

A molecule with the formula $$\mathrm{C_4H_{10}} is $$analyzed by mass spectrometry. Which fragment ion is most likely to be observed as the base peak?

Accepted Answer

A fragment corresponding to a secondary carbocation ($$\mathrm{R_2CH^+})$$

Question 18

Which of the following is true about the hybridization and orbital occupancy of carbon radicals?

Accepted Answer

Carbon radicals are $$sp^2$$ hybridized with a singly occupied molecular orbital (SOMO).

Organic Chemistry I – Exam 2 Key Concepts Study Guide

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