Consider the organic molecule allylbenzene (). If this molecule is exposed to highly controlled free — Haloalkanes and Haloarenes Chemistry Question
Question
Consider the organic molecule allylbenzene ($C_6H_5-CH_2-CH=CH_2$). If this molecule is exposed to highly controlled free-radical bromination conditions using $Br_2/h u$, determine the absolute total number of hydrogen atoms in the entire molecule that will completely resist abstraction due to their attachment to $sp^2$ hybridized carbons.
💡 Solution & Explanation
Free radical halogenation exclusively targets $sp^3$ hybridized aliphatic carbons, particularly those forming stable radicals (like benzylic or allylic positions). Hydrogens attached to $sp^2$ carbons (aryl and vinylic) are completely inert. In allylbenzene ($Ph-CH_2-CH=CH_2$), we count the inert hydrogens: 1. The phenyl ring ($C_6H_5-$) has 5 aryl hydrogens. 2. The vinylic portion ($-CH=CH_2$) has 1 internal vinylic hydrogen and 2 terminal vinylic hydrogens, totaling 3 vinylic hydrogens. Total inert hydrogens = $5 + 3 = 8$. Only the 2 benzylic/allylic hydrogens on the $-CH_2-$ group will successfully react.