IR/MS Key
Practice
Problem
1
O-H
stretch
C=C
triple bond
stretch
3
sp C-H
stretch
3
C H4O
UN = (6+2-4)/2 = 2
2
C=O
stretch
m/z = 58
m/z = 43
m/z = 113
m/z = 128 (M)
3
UN = 0
Odd mass – contains N
No N-H stretches
Must be tertiary amine.
m/z = 58
m/z = 73 (M)
4
From MS: contains Br
From IR: carbonyl
Two possibilities:
m/z = 43
m/z = 150 (M)
m/z = 107
• Propose structures for compounds
that
meet
the
following
descriptions:
a. An optically active compound C5H10O
with an IR absorption at 1730 cm-1
b. A non-optically active compound C5H9N
with an IR absorption at 2215 cm-1
a. UN = (12-10)/2 = 1
IR – must be carbonyl
Only one optically active structure can be drawn:
b. UN = (12-9+1)/2 = 2
IR – must be triple bond
IR – no mention of N-H stretch
5
You are carrying out the dehydration of 1methylcyclohexanol to yield 1-methylcyclohexene. How
could you use IR to determine when the reaction is
complete?
Look for:
• Loss of O-H stretch ~3300 (broad)
• Gain of C=C stretch ~1650
• Gain of sp2C-H stretch >3000
6
Propose a structure consistent with the following IR
spectrum and molecular formula
UN = (16-6)/2 = 5
sp2C-H
C=O
stretch
aldehyde
C-H
C7H6O
aromatic
ring
7
• Nitriles undergo a hydrolysis reaction
when heated with aqueous acid. What is
the structure of the product of hydrolysis
of propanenitrile (CH3CH2CN) if it has IR
absorptions at 2500 to 3100 cm-1 (br) and
1710 cm-1 (s) and has a parent peak in MS
at 74?
8
Propose a structure that is consistent with the following data:
Mass spec shows a molecular ion at m/z = 69
m/z = 69  contains N
Formula = C4H7N
UN = 2
N
From IR:
triple bond (2246)
No N-H
9
Propose a structure that is consistent with the following data:
10
Mass spec shows a molecular ion at m/z = 98
O
From IR: C=O (1716)
No aldehyde CHO
MW 98  C6H10O
UN = 2
Back to IR – no alkene (1650)
must be ring