Question

Which of the following is formed when propene is heated with bromine at high temperature?

• A. 1,2-Dibromopropane
• B. 1-Bromopropane
• C. 2-Bromopropene
• D. 3-Bromopropene

Hint:

Chemistry Tip: Halogenation of alkenes: Cold/Room Temp $\rightarrow$ Addition across the double bond. High Temp / UV Light $\rightarrow$ Allylic substitution.

Answer 1

The Correct Option is D

Concept: Chemistry (Organic Chemistry) - Allylic Substitution.

Step 1: Identify the reactants and reaction conditions. The reactants are propene ($CH_2=CH-CH_3$) and bromine ($Br_2$). The reaction condition is specified as "high temperature" (typically around $500^{\circ}C$).

Step 2: Recall the reactivity of alkenes with halogens at different temperatures. At room temperature or lower, halogens like $Br_2$ typically undergo electrophilic addition across the carbon-carbon double bond, which would form 1,2-dibromopropane. However, at high temperatures, the reaction pathway changes completely.

Step 3: Determine the high-temperature reaction mechanism. At high temperatures, the $Br-Br$ bond undergoes homolytic cleavage to form bromine free radicals. This initiates a free radical substitution reaction rather than an addition reaction.

Step 4: Identify the target site for radical substitution. Free radical substitution in alkenes preferentially occurs at the allylic position (the carbon atom adjacent to the double bond) because the resulting allyl radical is highly resonance-stabilized. In propene ($CH_2=CH-CH_3$), the terminal methyl group ($-CH_3$) is the allylic carbon.

Step 5: Determine the final product and its IUPAC name. A hydrogen atom from the allylic methyl group is substituted by a bromine atom.
The reaction is: $CH_2=CH-CH_3 + Br_2 \xrightarrow{\Delta} CH_2=CH-CH_2Br + HBr$.
The resulting molecule, $CH_2=CH-CH_2Br$, is commonly known as allyl bromide. According to IUPAC nomenclature, numbering starts from the double bond, making the bromo group at position 3. Thus, its systematic name is 3-bromopropene. $$ \therefore \text{The product formed is 3-Bromopropene.} $$