Q.In the following reaction,
1-methylcyclopentane (a cyclopentane ring bearing one -CH3 substituent on a ring carbon) $\xrightarrow[h\nu]{Br_2}$ ?
The major product obtained is

Step 1. Br2/hv is a free-radical halogenation: a light-generated bromine radical abstracts a C-H hydrogen, and the site abstracted is whichever gives the MOST STABLE carbon radical (tertiary > secondary > primary), exactly as for methane chlorination but now with a choice of several different C-H environments.

Step 2. Methylcyclopentane has three distinct kinds of C-H: the ring carbon bearing the methyl group (C1, a TERTIARY C-H -- attached to the methyl branch, two ring carbons, and one H), the other four ring CH2 carbons (SECONDARY C-H's), and the methyl group's own hydrogens (PRIMARY C-H's, on the exocyclic -CH3). …

Classify every distinct C-H in the substrate as primary/secondary/tertiary, then predict the major free-radical halogenation product at whic …

• Assuming free-radical halogenation is unselective/random -- it is strongly selective for the most stable radical, tertiary over secondary over primary, by a well-established relative-rate ordering. …