Question

What are the four possible constitutional isomers for the molecular formula C3H5Br?

Answer 1

Here are the steps I would take to figure them out.

Explanation:

1. Decide how many ring or double bonds are in the molecule.
2. Draw all the possible acyclic isomers.
3. Draw all the possible cyclic isomers.

1. Rings or double bonds

The molecular formula is `"C"_3"H"_5"Br"`.

Replacing the `"Br"` with an equivalent `"H"` gives the formula `"C"_3"H"_6`.

The formula of an alkane with 3 carbons is `"C"_3"H"_8`, so the compound is missing two `"H"` atoms.

∴ `"C"_3"H"_5"Br"` must contain a double bond or a ring.

2. Acyclic isomers

(a) Draw all possible alkenes with 3 carbon atoms.

(b) Add a bromine atom in every possible location.

These isomers are 3-bromopropene, 2-bromopropene, and 1-bromopropene.

3. Cyclic isomers

(a) Draw all possible cycloalkanes with 3 carbon atoms.

(b) Add a Br atom in every possible location.

This isomer is bromocyclopropane.

And we have our four isomers.