Draw all the structural isomers for the molecular formula C3H7Br.
The Correct Answer and Explanation is:
The molecular formula C₃H₇Br represents bromopropane, a compound that contains three carbon atoms, seven hydrogen atoms, and one bromine atom. There are two structural isomers for C₃H₇Br:
Structural Isomers of C₃H₇Br:
1. 1-Bromopropane (n-propyl bromide)
Structure:
Br–CH₂–CH₂–CH₃
- In this structure, the bromine atom is attached to the first carbon of a straight-chain propane molecule.
- This is the normal (n-) isomer.
2. 2-Bromopropane (isopropyl bromide)
Structure:
CH₃–CH(Br)–CH₃
- Here, the bromine atom is attached to the second (middle) carbon in the chain.
- This results in a branched structure, also referred to as the iso form.
Explanation
The molecular formula C₃H₇Br consists of three carbon atoms, seven hydrogen atoms, and one bromine atom. This formula corresponds to haloalkanes, also known as alkyl halides, specifically the brominated derivatives of propane (C₃H₈). When one hydrogen atom in propane is substituted by a bromine atom, the result is a monobromopropane isomer.
To determine the number of structural isomers, we analyze all the unique ways a bromine atom can be bonded to the carbon skeleton of propane.
Propane (C₃H₈) has a straight chain of three carbon atoms. The bromine can be attached either to the end carbon (C-1) or the middle carbon (C-2). Attaching Br to either of the terminal carbons gives 1-bromopropane. However, because of molecular symmetry, attaching Br to the first or third carbon yields the same compound. Attaching the bromine to the middle carbon gives 2-bromopropane.
There are no other arrangements of the carbon skeleton with only three carbon atoms that would result in a different structure. Unlike longer carbon chains, three-carbon chains do not allow for more complex branching.
Therefore, only two structural isomers exist:
- 1-bromopropane, a straight-chain compound with Br on the end.
- 2-bromopropane, a compound where Br is attached to the central carbon, creating a branched appearance.
These isomers differ in their physical and chemical properties, such as boiling point, reactivity, and steric effects, due to the position of the bromine atom within the molecule.
