Question 1

Which of the following best explains why the staggered conformation of ethane is more stable than the eclipsed conformation?

Accepted Answer

The staggered conformation has less torsional strain due to minimized electron repulsion between adjacent C-H bonds.

Question 2

What is the energy difference between the staggered and eclipsed conformations of ethane?

Accepted Answer

$$3.0\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 3

Which of the following statements about Newman projections is correct?

Accepted Answer

Newman projections are the best way to judge the stability of different conformations of a molecule.

Question 4

In the butane molecule, which staggered conformation is the most stable?

Accepted Answer

The conformation where the two methyl groups are anti to each other.

Question 5

What is the estimated energy difference between the anti and eclipsed conformations of butane?

Accepted Answer

$$3.8\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 6

Which interaction contributes the most to the torsional barrier in butane's eclipsed conformation?

Accepted Answer

Me–Me eclipsing interaction

Question 7

A monosubstituted cyclohexane (e.g., methylcyclohexane) is more stable when the substituent is in which position, and why?

Accepted Answer

Equatorial, because it minimizes 1,3-diaxial interactions.

Question 8

What is the expected destabilization energy for a methyl group in the axial position of cyclohexane compared to the equatorial position?

Accepted Answer

$$1.8\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 9

Which of the following best describes the energy profile for the rotation about the C–C bond in ethane?

Accepted Answer

It alternates between maxima at eclipsed and minima at staggered conformations.

Question 10

If a cyclohexane ring has two methyl groups in a trans-1,2-disubstituted arrangement, what is the most stable conformation?

Accepted Answer

Both methyl groups equatorial

Question 11

What is the energy penalty for a 1,3-diaxial Me–Me interaction in cyclohexane?

Accepted Answer

$$3.7\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 12

Which of the following is a correct estimate for the energy difference between the two chair conformations of methylcyclohexane?

Accepted Answer

$$1.74\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 13

Which of the following best describes the effect of gauche interactions in butane?

Accepted Answer

They destabilize the molecule due to steric hindrance between methyl groups.

Question 14

A 1,2-disubstituted cyclohexane with both substituents equatorial will have what kind of interaction compared to the diequatorial conformation?

Accepted Answer

It will have one gauche butane interaction and be less stable.

Question 15

Which of the following is the correct formula for estimating the 1,3-dimethyl eclipsing interaction energy in cyclohexane?

Accepted Answer

$$\Delta G = X + 2Y$$

Question 16

If the A-value for a methyl group is $$1.74\ \mathrm{kcal\ $$mol^{-1}$$}$$, what is the expected energy difference for a trans-1,4-dimethylcyclohexane with both methyl groups equatorial?

Accepted Answer

$$0\ \mathrm{kcal\ $$mol^{-1}$$}$$

Question 17

Which of the following is a real-world application of conformational analysis?

Accepted Answer

Designing pharmaceuticals that fit specific protein conformations.

Conformational Analysis of Alkanes: Ethane, Propane, Butane, and Cyclohexane

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