Question 1

Which general formula correctly represents the molecular formula of alkanes?

Accepted Answer

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

Question 2

What is the primary reason staggered conformations of ethane are more stable than eclipsed conformations?

Accepted Answer

Staggered conformations minimize torsional strain due to reduced electron repulsion.

Question 3

In a Newman projection of ethane, what is the dihedral angle between two adjacent C-H bonds in the eclipsed conformation?

Accepted Answer

$0^\circ$

Question 4

Which type of strain is minimized in the staggered conformation of ethane?

Accepted Answer

Torsional strain

Question 5

What is the energy barrier (in kJ/mol) for the rotation from staggered to eclipsed conformation in ethane?

Accepted Answer

$12\ \mathrm{kJ/mol}$

Question 6

Which statement best describes conformational isomers (conformers) in alkanes?

Accepted Answer

They are different spatial arrangements of a molecule generated by rotation about single bonds.

Question 7

In butane, which conformation is the most stable and why?

Accepted Answer

Anti conformation, because it minimizes both torsional and steric strain between methyl groups.

Question 8

Which representation is best for visualizing the spatial arrangement of bonds in a molecule when looking directly down a bond axis?

Accepted Answer

Newman projection

Question 9

What is the main difference between torsional strain and steric strain in alkanes?

Accepted Answer

Torsional strain arises from repulsion between electron clouds in eclipsed bonds, while steric strain arises from atoms being closer than their van der Waals radii.

Question 10

If the rotation about a C-C single bond in ethane occurs every $10^{-11}$ seconds, what does this imply about the energy barrier for rotation?

Accepted Answer

The energy barrier is low, allowing rapid rotation at room temperature.

Question 11

Which conformation of butane has the highest potential energy?

Accepted Answer

Eclipsed conformation with methyl groups aligned

Question 12

A molecule of butane is found in a conformation where the two methyl groups are $60^\circ$ apart. What is this conformation called?

Accepted Answer

Gauche conformation

Question 13

Which of the following best explains why alkanes can form long chains (catenation)?

Accepted Answer

Carbon atoms can form stable covalent bonds with other carbon atoms.

Question 14

In a real-world scenario, why might the stability of different butane conformers be important in the design of pharmaceuticals?

Accepted Answer

The preferred conformer can influence the shape and binding of the drug to its target.

Question 15

Which of the following statements about the rotation around single bonds in alkanes is correct?

Accepted Answer

Rotation around single bonds is generally free, allowing different conformations.

Question 16

Which conformation of ethane has all dihedral angles equal to $0^\circ$?

Accepted Answer

Eclipsed conformation

Question 17

If a chemist wants to minimize both torsional and steric strain in a butane molecule, which conformation should they favor?

Accepted Answer

Anti staggered conformation

Question 18

Which of the following is NOT a property of saturated hydrocarbons (alkanes)?

Accepted Answer

They contain at least one double bond.

Question 19

A student draws a Newman projection of ethane and notices that the hydrogens on the front and back carbons are offset. What conformation is this?

Accepted Answer

Staggered conformation

Question 20

Which of the following best describes the energy profile for ethane as a function of dihedral angle?

Accepted Answer

Energy is highest at $0^\circ$ (eclipsed) and lowest at $60^\circ$ and $180^\circ$ (staggered).

Alkanes: Structure, Conformations, and Strain

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