Question 1

Which of the following best describes the difference between a sawhorse representation and a Newman projection of ethane?

Accepted Answer

A sawhorse representation views the C–C bond from an oblique angle showing all C–H bonds, while a Newman projection views the C–C bond directly end-on, representing the two carbons by a circle.

Question 2

What is the energy barrier to rotation around the C–C bond in ethane, and what is this energy called?

Accepted Answer

$$12\ \mathrm{kJ/mol}$$, called torsional strain

Question 3

In the Newman projection of butane, which conformation is the lowest in energy and why?

Accepted Answer

The anti conformation, because the two methyl groups are $$180^\circ$$ apart, minimizing steric interactions

Question 4

Which of the following interactions contributes the most to the energy of an eclipsed conformation in butane?

Accepted Answer

CH$$_3$$ $$\leftrightarrow CH$_3$$ eclipsed

Question 5

What is the total energy cost (in kJ/mol) for the fully eclipsed conformation of butane, considering two H $$\leftrightarrow CH$_3$$ and one H $$\leftrightarrow H $$interactions?

Accepted Answer

$$16\ \mathrm{kJ/mol}$$

Question 6

Which type of strain is primarily responsible for the higher energy of the gauche conformation of butane compared to the anti conformation?

Accepted Answer

Steric strain

Question 7

A molecule of propane is rotated about its central C–C bond. What is the energy difference between its staggered and eclipsed conformations?

Accepted Answer

$$14.0\ \mathrm{kJ/mol}$$

Question 8

Which of the following best describes the general formula for cycloalkanes?

Accepted Answer

$$\mathrm{C}_n\mathrm{H}_{2n}$$

Question 9

Which of the following is the correct IUPAC name for a cycloalkane with a methyl group on carbon 1 and an ethyl group on carbon 2 of a cyclopentane ring?

Accepted Answer

1-Ethyl-2-methylcyclopentane

Question 10

Which of the following statements about the stability of cycloalkanes is correct?

Accepted Answer

Cyclohexane is more stable than cyclopropane because it can adopt conformations that minimize angle and torsional strain.

Question 11

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

Accepted Answer

Torsional strain

Question 12

Which conformation of butane has an energy cost of $$3.8\ \mathrm{kJ/mol}$$ due to steric strain?

Accepted Answer

Gauche conformation

Question 13

A student draws a Newman projection of butane with the two methyl groups $$60^\circ$$ apart. What is the name and energy of this conformation?

Accepted Answer

Gauche conformation, $$3.8\ \mathrm{kJ/mol}$$

Question 14

Which of the following is the correct IUPAC name for a cyclohexane ring with a bromo group on carbon 1, an ethyl group on carbon 3, and a methyl group on carbon 5?

Accepted Answer

1-Bromo-3-ethyl-5-methylcyclohexane

Question 15

Which of the following best explains why the anti conformation of butane is more stable than the gauche conformation?

Accepted Answer

The anti conformation has less steric strain between the methyl groups.

Question 16

What is the correct IUPAC name for a cyclobutane ring with a bromo group on carbon 1 and a methyl group on carbon 2?

Accepted Answer

1-Bromo-2-methylcyclobutane

Question 17

Which of the following is NOT a source of strain in cycloalkanes?

Accepted Answer

Hydrogen bonding strain

Question 18

Which of the following best describes the relationship between conformational isomers (conformers)?

Accepted Answer

They are different arrangements of atoms resulting from rotation around single bonds.

Question 19

A chemist analyzes the energy profile of ethane as it rotates about the C–C bond. At which dihedral angles does ethane reach its maximum energy, and what is the value?

Accepted Answer

$$0^\circ,\ 120^\circ,\ 240^\circ,\ 360^\circ$$; $$12\ \mathrm{kJ/mol}$$

Alkanes and Cycloalkanes: Conformations, Stereochemistry, and Nomenclature

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