An alkene X ($\mathrm{C}{}_7\mathrm{H}{}_{14}$) gives propanone and butanal on ozonolysis with reductive work-up. X is treated with HBr in the presence of an organic peroxide. The major product is

Joining the two carbonyl carbons gives $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_3$, i.e. 2-methylhex-2-ene. With peroxide, Br${}^{\bullet }$ adds to the less substituted alkene carbon (C-3) to give the more stable tertiary radical at C-2. H-abstraction from HBr then gives $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CHBr}-\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_3$, i.e. 3-bromo-2-methylhexane. The Markovnikov product 2-bromo-2-methylhexane is the ionic-addition product.