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

Sep 28, 2015

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.

Sep 28, 2015

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 ${C}_{n} {H}_{2 n + 2}$.

Its molecular mass is

$12 n + 2 n + 2 = 128$

$14 n = 126$

$n = 9$

So the molecular formula is ${\text{C"_9"H}}_{20}$.

The base peak at $\text{m/z} = 43$ 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 $\text{m/z} = 57 , 71$, and $85$ correspond to radical cations with formulas ${\text{C"_4"H}}_{9}$, ${\text{C"_5"H}}_{11}$, and ${\text{C"_6"H}}_{13}$.

A possible structure is 2-methyloctane

If the molecule cleaves between $\text{C-2}$ and $\text{C-3}$,

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

If the molecule cleaves between $\text{C-3}$ and $\text{C-4}$,

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.