Select the correct structure for the following compounds A and B :
A). 1-Bromobut-2-ene B). 4-Bromo-3-methylpent-2-ene
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A: CH3-CH=CH-CH2Br, B: CH3-CH=C(CH3)-CH(Br)-CH3
To draw a structure from an IUPAC name, read the name from the end:
The word root and suffix give the length of the main chain and the double bond (but-2-ene: four carbons with C=C between C-2 and C-3).
The prefixes give the substituents and the carbon atoms they are attached to.
Finally, fill in hydrogen atoms so that every carbon has four bonds.
A. 1-Bromobut-2-ene: a four carbon chain, C=C between C-2 and C-3, Br on C-1:
\( \displaystyle \ce{Br-CH2-CH=CH-CH3} \)
B. 4-Bromo-3-methylpent-2-ene: a five carbon chain, C=C between C-2 and C-3, a methyl group on C-3 and Br on C-4:
\( \displaystyle \ce{CH3-CH=C(CH3)-CH(Br)-CH3} \)
Only Option (3) shows both these structures correctly.
Why the other options are incorrect
Option (1) — Incorrect. Structure A is correct, but structure B is \( \ce{CH2=C(CH3)-CH2Br} \) (3-bromo-2-methylprop-1-ene), which has only four carbon atoms.
Option (2) — Incorrect. Structure B is correct, but structure A is \( \ce{CH2=C(CH3)-CH2Br} \), a branched chain and not but-2-ene.
Option (4) — Incorrect. Structure A is the cis form of 1-bromobut-2-ene, which also fits the name, but structure B is again \( \ce{CH2=C(CH3)-CH2Br} \).