Question

What are possible structures for an organic molecule whose mass spectrum gives `m"/"z=128, 85, 71, 57, 43`?

Answer 1

This could be anything (well anything with a mass of 128 amu). You need to do more experiments.

Explanation:

What is the melting point of the hydrocarbon? What are the melting points of its derivatives? You will not identify an unknown organic compound with only a mass spectrum.

Answer 2

I'm guessing that one possibility is 2-methyloctane.

Explanation:

You know that the compound is a hydrocarbon.

Let's assume that it's an alkane. Then its molecular formula is `C_nH_(2n+2)`.

Its molecular mass is

`12n + 2n+2 = 128`

`14n = 126`

`n=9`

So the molecular formula is `"C"_9"H"_20`.

The base peak at `"m/z" = 43` corresponds to a 3-carbon fragment, almost certainly an isopropyl group, because that is the more stable cation (2°).

The other fragments must be less stable (1°?).

The peaks at `"m/z" = 57, 71`, and `85` correspond to radical cations with formulas `"C"_4"H"_9`, `"C"_5"H"_11`, and `"C"_6"H"_13`.

A possible structure is 2-methyloctane

Parent

If the molecule cleaves between `"C-2"` and `"C-3"`,

Scheme 1

we get an isopropyl fragment (`43`) as the base peak and a less stable 1° 6-carbon fragment (`85`).

If the molecule cleaves between `"C-3"` and `"C-4"`,

Scheme 2

we get a 1° isobutyl fragment (`57`) and a less stable 1° 5-carbon fragment (`71`).

The isobutyl cation should be more stable because of the inductive effect of the isopropyl group.