Question

Without consulting tables, arrange these compounds in order of decreasing (less negative) heat of combustion: hexane, 2 -methylpentane, and 2,2-dimethylbutane.

Short answer

Question: Arrange the following compounds in the order of decreasing heat of combustion: hexane, 2-methylpentane, and 2,2-dimethylbutane. Answer: Hexane > 2-methylpentane > 2,2-dimethylbutane

Step-by-step solution

Write down the structural formulas of the compounds

The first thing to do is to write down the structural formulas for each compound. 1. Hexane: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}\) 2. 2-methylpentane: \(\text{CH}_{3}\text{CH}\text{(CH}_{3}\text{)CH}_{2}\text{CH}_{2}\text{CH}_{3}\) 3. 2,2-dimethylbutane: \(\text{CH}_{3}\text{C}\text{(CH}_{3}\text{)}_{2}\text{CH}_{2}\text{CH}_{3}\)

Compare the compounds based on branching

Now that we have the structural formulas, we will compare the compounds based on the branching. The more branching a compound has, the more stable it is, and the lower its heat of combustion will be. 1. Hexane has no branches. 2. 2-methylpentane has one branch. 3. 2,2-dimethylbutane has two branches.

Arrange the compounds based on heat of combustion

Based on the stability and branching, we can now arrange the compounds in order of decreasing heat of combustion: 1. Hexane (least branched, least stable, highest heat of combustion) 2. 2-methylpentane (moderate branching, moderate stability, moderate heat of combustion) 3. 2,2-dimethylbutane (most branched, most stable, lowest heat of combustion) So, the final order of compounds in decreasing heat of combustion is: Hexane > 2-methylpentane > 2,2-dimethylbutane