Question

Does the length of the carbon chain affect the boiling point of alkanes? If so, how? provide evidence to support your answer? (b) If the chain contains a functional group, would you make the same conclusion? Again provide evidence.

Answer 1

(a)

Yes. B.p. tend to increase with the number of carbon atoms in the backbone.

Explanation

Straight-chain alkanes have the general molecular formula C$_n$H$_{2n+2}$. Adding carbon atoms to the chain increases the number of electrons in each molecules.

Alkane molecules are not polar without any functional groups. The only possible force between them is London Dispersion Force (a.k.a. induced dipole). Electrons shift within molecules to create instantaneous dipole in this type of force.

Molecules with a large number of electrons experience the most significant shift. London Dispersion Force is strongest in those molecules. They shall have the highest boiling points.

Examples: (SynQuest)

• C₂H₆ (ethane)- b.p. -88 °C
• C₄H₁₀ (butane)- b.p. -1 °C ~ 1 °C
• C₆H₁₄ (hexane)- b.p. 68 °C ~ 70 °C

(b)

The conclusion in (a) likely holds molecules in the same homologous series.

Explanation

Molecules in the same homologous series have the same types and numbers of functional groups. However, they differ only in the number of repeating units (-CH₂ in this case) that they contains. The number of such units in each molecule is directly related to the length of its carbon backbone.

Functional groups introduce extra types of forces between the molecules. For instance:

• Halogens, e.g., Cl, forms polar bonds with carbon. In most cases they make the molecule polar enough to form dipole-dipole interactions.
• Hydroxyl groups -OH can lead to hydrogen bonds between the molecules.

Functional groups tend to have similar effects on the b.p. in the same homologous series. The extra interaction due to the functional groups stays generally the same. Trends in the strength of dispersion forces likely follow the reasoning in (a). There shall be a similar conclusion. Molecules with the longest backbone in the same homologous series would have the highest b.p.

Example: (FooDB)

• CH₃COOH (ethanoic acid)- b.p. 118 °C
• C₂H₅COOH (butanoic acid)- b.p. 163.5 °C
• C₃H₇COOH (hexanoic acid)- b.p. 205 °C

Note, that it's only the number of repeating units in the carbon backbone that differs. Functional groups shall be on the similar positions on members of the series. For instance, 1,1-dichloroethane Cl₂-CH-CH₃ and 1,1-dichloropropane Cl₂-CH-CH₂-CH₃ are on the same series; whereas 1,3-dichloropropane Cl-CH₂-CH₂-CH₂-Cl is not.