Identify the reagent R used in following reaction?
Show Hint
Whenever you see a hydrogen atom on a saturated alkane chain being cleanly replaced by a halogen atom, look for molecular halogen paired with light energy ($\text{h}\nu$ or UV light) to trigger that free-radical path!
Step 1: Understanding the Question:
We are given a chemical conversion step where an alkane or an alkyl side chain undergoes selective bromination at a specific position. We need to identify the exact reagent and reaction conditions designated as "R".
Step 2: Detailed Explanation:
The conversion shown involves free-radical substitution of a hydrogen atom on a saturated carbon center by a bromine atom.
• Molecular bromine ($\text{Br}_2$) combined with ultraviolet light (UV light) or heat induces homolytic cleavage of the Br-Br bond to form highly selective bromine free radicals.
• Bromine radicals selectively abstract a hydrogen atom from the most substituted carbon position (preferring tertiary or secondary positions, or allylic/benzylic sites) due to the higher thermodynamic stability of the resulting intermediate free radical.
• Reagents like HBr undergo electrophilic addition across double bonds rather than substitution on saturated carbons, while $\text{Br}_2$ in the dark requires a Lewis acid catalyst to substitute onto aromatic rings. Thus, $\text{Br}_2\text{/UV light}$ is the correct choice for this free-radical alkane substitution pathway.
Step 3: Final Answer:
The reagent and conditions represented by R are $\text{Br}_2\text{/UV light}$, corresponding to option (D).
