Animated Solution for Chemistry - Organic Chemistry: On mixing a certain alkane with chlorine and irradiating it with ultraviolet light, it forms only one monochloroalkane. This alkane could be

Imagine you are a chemist in a lab, tasked with a very specific mission: you need to chlorinate an alkane, but you are only allowed to produce exactly one type of monochloroalkane. No mixtures, no separation required. Just one pure product.

What does this strict condition imply? It means that every single hydrogen atom in your starting alkane must be chemically identical. If you were to blindfold yourself, pick any hydrogen atom at random, and replace it with a chlorine atom, the resulting molecule must be exactly the same as if you had picked any other hydrogen. In chemistry, we call this having chemically equivalent hydrogens.

Let's put our options under the microscope and count the distinct types of hydrogens they possess.

First, we have propane ($C{H}_3-C{H}_2-C{H}_3$). If you look at its structure, it has two ends and a middle. The hydrogens on the terminal methyl groups are identical (primary hydrogens), but the two hydrogens on the central carbon are different (secondary hydrogens). Replacing a terminal hydrogen gives 1-chloropropane, while replacing a central one gives 2-chloropropane. That's two products. Not what we want.

Next is pentane ($C{H}_3-C{H}_2-C{H}_2-C{H}_2-C{H}_3$). This chain is even longer. It has terminal hydrogens, hydrogens on the carbons next to the ends, and hydrogens on the exact middle carbon. That's three distinct environments, leading to three different monochloro products.

What about isopentane? This branched molecule is quite asymmetrical. If you draw it out, you will find four completely different types of hydrogens. Chlorinating isopentane would give you a messy mixture of four different structural isomers!

Neopentane

Finally, we arrive at neopentane ($C(C{H}_3{)}_4$). Visualize this molecule: it features a central quaternary carbon atom. This central carbon is bonded to four other carbons, meaning it has absolutely no hydrogens of its own to substitute.

Surrounding this central carbon are four methyl ($-C{H}_3$) groups. Because of the perfect tetrahedral symmetry of the molecule, these four methyl groups are completely indistinguishable from one another. All 12 hydrogen atoms reside in the exact same chemical environment.

If you replace any one of these 12 hydrogens with a chlorine atom, you will always get the exact same molecule: neopentyl chloride.

This problem beautifully illustrates a core principle of organic chemistry: symmetry dictates the number of structural isomers. The higher the symmetry of the starting material, the fewer the possible substitution products. Always draw the structure and look for those hidden planes of symmetry!