Chapter 4: Alkanes and Cycloalkanes

Question 1

What are saturated hydrocarbons (alkanes)?

Answer 1

Hydrocarbons that lack pi bonds

Question 2

What are the four steps for assigning an IUPAC systematic name to a compound?

Answer 2

1. Identify the parent compound
2. Name the substituents, which can be either simple alkyl groups or branched alkyl groups
3. Number the carbon atoms of the parent and assign a locant to each substituent.
4. Assemble the substituents alphabetically, placing locants in front of each substituent. For identical substituents, use di, tri, tetra, penta, or hexa.

Question 3

What is a bicycloalkane?

Answer 3

An alkane that has two rings that are connected to one another.

Question 4

How does the number of possible constitutional isomers for an alkane change with molecular size?

Answer 4

The number of possible isomers increases dramatically with increasing molecular size.

Question 5

What is the heat of combustion for an alkane?

Answer 5

The heat of combustion is the negative of the change in enthalpy (-∆H˚) associated with the complete combustion of 1 mol of alkane in the presence of oxygen.

Question 6

In a Newman projection, what does the dihedral (torsional) angle describe?

Answer 6

The relative positions of one group on the back carbon and one group on the front carbon.

Question 7

How does the energy of staggered and eclipsed conformations compare?

Answer 7

Staggered conformations are lower in energy while eclipsed conformations are higher in energy.

Question 8

Describe the degeneracy of ethane’s conformations.

Answer 8

For ethane, all staggered conformations are degenerate (equivalent in energy) and all eclipsed conformations are degenerate.

Question 9

What is torsional strain?

Answer 9

The difference in energy between staggered and eclipsed conformations of ethane.

Question 10

Compare the torsional strain of propane and ethane.

Answer 10

The torsional strain for propane is larger than that of ethane.

Question 11

Describe the degeneracy of butane’s conformations.

Answer 11

One staggered conformation (the anti conformation) is lower in energy than the other two staggered conformations because they possess gauche interactions.

Question 12

When does angle strain occur in cycloalkanes?

Answer 12

Angle strain occurs when bond angles are less than the preferred 109.5˚

Question 13

How are angle strain and torsional strain related?

Answer 13

Angle strain and torsional strain are components of the total energy of a cycloalkane, which can be assessed by measuring heats of combustion per CH2 group.

Question 14

Describe the strain of the chair conformation of cyclohexane.

Answer 14

The chair conformation has no torsional strain and very little angle strain.

Question 15

Describe the strain of the boat conformation of cyclohexane.

Answer 15

The boat conformation has significant torsional strain (from eclipsing H’s as well as flagpole interactions). The boat can alleviate some of its torsional strain by twisting, giving a conformation called a twist boat.

Question 16

What are the two positions a substituent can occupy on a carbon on a cyclohexane ring?

Answer 16

1. Axial position
2. Equatorial position

Question 17

What is a ring flip?

Answer 17

The term ring flip is used to describe the conversion of one chair conformation into the other.

Question 18

The axial-equatorial equilibrium favors which chair conformation?

Answer 18

It favors the chair conformation with the substituent in the equatorial position.

Question 19

Why is the equatorial position favored in the chair conformation?

Answer 19

An axial substituent generates 1, 3-diaxial interactions.

Question 20

How can the two chair conformations of a disubstituted cyclohexane be drawn?

Answer 20

Each substituent must be identified as being either up or down. The 3D orientation of the substituents (up or down) does not change during a ring flip.