Question

Describe two ways in which the octane number of a gasoline consisting of alkanes can be increased.

Short answer

The octane number of gasoline containing alkanes can be increased by: (1) increasing the carbon chain length of the alkanes, resulting in improved knock resistance; and (2) increasing the branching of the alkanes, leading to better auto-ignition resistance and a higher octane rating.

Step-by-step solution

1. Increasing the carbon chain length of alkanes

One way to increase the octane number of gasoline containing alkanes is to increase the carbon chain length of the alkanes. Longer-chain alkanes have higher boiling points, lower vapor pressures, and are generally better at resisting auto-ignition. Increasing the carbon chain length of the alkane will generally result in improved knock resistance, which directly contributes to an increase in the octane number.

2. Increasing the branchiness

Another way to increase the octane number of gasoline consisting of alkanes is to increase the branching of the alkanes. This means modifying the alkane molecules so that there are more carbon atoms branching off of the main carbon backbone. Branched alkanes, also known as isomers, have a higher octane number due to their molecular structure. The more branchiness the alkane molecules have, the better they resist auto-ignition, leading to a higher octane rating. In summary, the octane number of gasoline containing alkanes can be increased by increasing the carbon chain length and increasing the number of branches of the alkanes. Both methods result in better knock resistance and contribute to a higher octane rating for gasoline.