Animated Solution for Chemistry - Hydrocarbons: Which branched chain isomer of the hydrocarbon with molecular mass 72 u gives only one isomer of mono substituted alkyl halide?

The first step in unraveling this chemical mystery is to identify the hydrocarbon based on its molecular mass. We are given a mass of $72\text{ u}$. In organic chemistry problems like this, unless specified otherwise, it is a safe bet to start by assuming the hydrocarbon is an alkane.

The general formula for an alkane is $C_n{H}_{2n+2}$. We can set up a simple algebraic equation using the atomic masses of carbon ($12\text{ u}$) and hydrogen ($1\text{ u}$):

Simplifying this, we get:

This tells us that our mystery molecule is a pentane, with the molecular formula $C_5{H}_{12}$.

Now we arrive at the core constraint of the problem: the hydrocarbon must yield only one isomer of a mono-substituted alkyl halide.

What does "mono-substituted" mean? It means we are replacing exactly one hydrogen atom in the molecule with a halogen atom (like chlorine). If a molecule gives only one possible product regardless of which hydrogen is replaced, it implies a profound structural property: all the hydrogen atoms in the molecule must be chemically equivalent.

If even one hydrogen was in a different chemical environment (e.g., attached to a secondary carbon instead of a primary carbon), replacing it would yield a different structural isomer.

Let's evaluate the structural isomers of pentane ($C_5{H}_{12}$) to find the one with perfect symmetry:

1. n-pentane: This straight-chain isomer has three different types of hydrogens (those on the end carbons, those on the adjacent carbons, and those on the middle carbon). It would give three different mono-substituted products. 2. Isopentane: This branched isomer has four different types of hydrogens. It would give four different mono-substituted products. 3. Neopentane (2,2-dimethylpropane): This highly branched isomer features a central quaternary carbon bonded to four identical methyl ($-C{H}_3$) groups.

In neopentane, all 12 hydrogen atoms are located on the four methyl groups. Because these methyl groups are perfectly symmetrical around the central carbon, every single hydrogen atom is in the exact same chemical environment.

Because all 12 hydrogens are equivalent, chlorination of neopentane will always result in the exact same product: neopentyl chloride.

Therefore, the branched-chain isomer that satisfies all the conditions is Neopentane.