Question

A chemist has at his disposal the following hydrocarbons: (a) 3,3 -dimethylpentane, (b) n- heptane, (c) 2- methylheptane, (d) n-pentane, and (e) 2-methylhexane. Arrange these compounds in order of decreasing boiling points (without referring to tables).

Short answer

The hydrocarbons can be arranged in the order of decreasing boiling points as follows: n-heptane (b) > 2-methylheptane (c) > 2-methylhexane (e) > n-pentane (d) > 3,3-dimethylpentane (a).

Step-by-step solution

Identify the molecular size and branching for each hydrocarbon

Let's identify the number of carbon atoms and the amount of branching in each of the given hydrocarbons: (a) 3,3-dimethylpentane: Molecular size: 5 carbon atoms in the main chain + 2 methyls (branching) (b) n-heptane: Molecular size: 7 carbon atoms in the main chain (no branching) (c) 2-methylheptane: Molecular size: 7 carbon atoms in the main chain + 1 methyl (branching) (d) n-pentane: Molecular size: 5 carbon atoms in the main chain (no branching) (e) 2-methylhexane: Molecular size: 6 carbon atoms in the main chain + 1 methyl (branching)

Compare molecular sizes and branching to determine boiling points

We can now compare the molecular sizes and branching of these hydrocarbons to determine their boiling point order: 1. n-heptane has the largest molecular size among the hydrocarbons, which will result in the highest boiling point. 2. 2-methylheptane has the same molecular size as n-heptane but has one branch, which will reduce the boiling point compared to n-heptane. 3. 2-methylhexane has one less carbon atom than 2-methylheptane but has similar branching, resulting in a lower boiling point than both n-heptane and 2-methylheptane. 4. n-pentane has two less carbon atoms than n-heptane and no branching. This will have a lower boiling point than the larger hydrocarbons. 5. 3,3-dimethylpentane has the same number of carbon atoms as n-pentane but has more branching, which will result in the lowest boiling point among the hydrocarbons.

Arrange the hydrocarbons in the order of decreasing boiling points

Now that we have compared the molecular sizes and branching of the hydrocarbons, we can arrange them in the order of decreasing boiling points: 1. n-heptane (b) 2. 2-methylheptane (c) 3. 2-methylhexane (e) 4. n-pentane (d) 5. 3,3-dimethylpentane (a) So, the order of the hydrocarbons in terms of decreasing boiling points is: (b) > (c) > (e) > (d) > (a).

Key concepts

Molecular Size and Boiling Points

Understanding the relationship between molecular size and boiling points is fundamental in organic chemistry. Larger molecules typically have higher boiling points due to the increased van der Waals forces, a type of intermolecular attraction. In the case of hydrocarbons, these forces are directly related to the length of the carbon chain. As the chain lengthens, the surface area for these forces increases, leading to stronger attractions between molecules and consequently higher boiling points.

For example, n-heptane, with its long chain of seven carbon atoms, has a higher boiling point than n-pentane, which only contains five carbons. This is because n-heptane's larger size allows more significant contact between molecules, intensifying the van der Waals attractions and requiring more energy—in the form of heat—to separate them during the transition from liquid to gas.

Branching of Hydrocarbons and its Effects

Branching in hydrocarbons introduces a twist in the tale of boiling points. Although one might think that a larger molecule always equates to a higher boiling point, branching changes the game. The presence of branches reduces the boiling point of a hydrocarbon, even if its molecular size is comparable to a straight-chain counterpart. The reason lies in the compact nature of branched molecules which decreases the surface area in contact with neighboring molecules, thus diminishing intermolecular van der Waals forces.

For instance, 3,3-dimethylpentane, despite possessing a total of seven carbon atoms, boils at a lower temperature than n-heptane. This phenomenon is a result of its two branches causing a compact shape that cannot pack as efficiently as the straight chain of n-heptane. The reduced surface area means weaker intermolecular forces and a lower boiling point.

Comparing Boiling Points

Comparing boiling points of different hydrocarbons requires looking at both molecular size and branching. It’s like balancing scales: on one side, we consider the larger molecule with more significant van der Waals forces and on the other, the effect of branching that weakens these forces. For chemists, predicting the order of boiling points without looking at tables is a task of understanding these principles.

Given a set of hydrocarbons, a chemist will first look for the longest carbon chain, expecting a higher boiling point. Then, they assess the degree of branching; more branching suggests a lower boiling point. By considering both factors, chemists can accurately predict the boiling point order: n-heptane, with its larger size and no branching, will have the highest boiling point, followed by less branched isomers and ultimately, the most branched and hence, the least boiling point. In our example, the correct order from highest to lowest boiling points is n-heptane (b), 2-methylheptane (c), 2-methylhexane (e), n-pentane (d), and then 3,3-dimethylpentane (a).