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

Question

Christian Anderson · Accepted Answer

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

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.

Skylar Alexander · Answer

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

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

If the molecule cleaves between #"C-2"# and #"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 #"C-3"# and #"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.

Brooklyn Barber · Answer

undefined Possible structures for an organic molecule with a mass spectrum showing peaks at $m/z = 128$, $85$, $71$, $57$, and $43$ could include:

1. $m/z = 128$: This could correspond to the molecular ion peak (M+) or a fragment containing the entire molecule.

2. $m/z = 85$: This could represent a fragment containing a benzene ring, such as a phenyl group (C6H5).

3. $m/z = 71$: This could correspond to a fragment containing a methyl group (CH3).

4. $m/z = 57$: This could represent a fragment containing a propyl group (C3H7) or a butyl group (C4H9).

5. $m/z = 43$: This could correspond to a fragment containing a methyl group (CH3).

Based on these mass spectral peaks, possible structures for the organic molecule could include benzene-containing compounds with substituents such as methyl and propyl groups.

Christian Anderson · Answer

undefined The possible structures for an organic molecule with a mass spectrum showing m/z ratios of 128, 85, 71, 57, and 43 are:

1. C8H8O (molecular formula) - This could be benzaldehyde.
2. C6H5COCH3 (molecular formula) - This could be acetophenone.