Question

Use the data below from an electron impact mass spectrum of a pure compound to deduce its structure.

m/z	Relative intensity
110	59
108	60
81	8
79	9
29	100
28	29
27	95
26	44                                     given CH4 as starting material, How to begin with the correct structure? I know how to use Rule of 13 to find the molecular formula.

Answer 1

the molecular ion peak is 110 (M),

from that we we can calculate the molecular formula by using rule of thirteen, M/13,

=110/13=n+r/13=8.46, FROM THAT numenator=8 and remainder =6,

puting the values in CnHn+r=C8H14, from that we can calculate DBE,

Using CnHn+2=4 hydrogen less than parent alkane =2 sites of unsaturation.

THE PEAK AT 100 ABUNDANCE IS AT 29, which corresponds to CH3-CH2- fragment,

the given is CH4 as a starting material means , this reaction might be free radical substitution reaction of haloalkanes.

there is also peaks from isotopic halogens, at 1:1 intensity(59:60) which must be corresponds to Bromine atom, also there are peaks at 79 and 81 which are about 1:1 intensity(9:8) , this is also the indication of presence of bromine in the structure. there fore the correct strcture from above data must be 2-bromoethane.

1.CH3-CH2-Br Mol wt =108 ( for Br atom 79),

110 (for Bromine atom 81),

2.fragments are CH3-CH2- (29) with 100 % intensity,

3.Br- (79,81),

therrefore, THE CORRECT STRUCTURE IS CH3-CH2-Br