What is the total number of structural isomers possible for the compound with molecular formula \( C_4H_9Br \)?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
Structural isomers have the same molecular formula but a different arrangement of atoms. To count them for \( \ce{C4H9Br} \), first write the possible carbon skeletons of four carbon atoms, then place the bromine on every non-equivalent carbon of each skeleton.
The straight chain (butane) has two kinds of carbon, end and middle:
\( \displaystyle \ce{CH3CH2CH2CH2Br}\ (\text{1-bromobutane}) \qquad \ce{CH3CH2CH(Br)CH3}\ (\text{2-bromobutane}) \)
The branched chain (2-methylpropane) also has two kinds of carbon, the three equivalent \( \ce{CH3} \) carbons and the central carbon:
\( \displaystyle \ce{(CH3)2CHCH2Br}\ (\text{1-bromo-2-methylpropane}) \qquad \ce{(CH3)3CBr}\ (\text{2-bromo-2-methylpropane}) \)
So \( \ce{C4H9Br} \) has 4 structural isomers.