Engineering Chemistry S Datta

INSTRUMENTAL METHODS OF ANALYSES 533
Number of signals
A set of protons with the same environment are said to be equivalent. The number of
signals in the nmr spectrum tells us, therefore, how many sets of equivalent protons, how
many kinds of protons.
Judging the equivalent protons
Each proton in a molecule is in turn to be replaced by some other atom Z. If replacement
of either of two protons by Z would yield the same product or enantiomeric products—then the
two protons are chemically and magnetically equivalent.
Example. CH 3
CH 2
—Cl (CH 3
protons are non-equivalent to the CH 2
protons)
The environment of these two protons are neither identical nor mirror images of each
other, magnetically these protons are non-equivalent and we expect the nmr signal from
each one.
Cl
Cl
Cl
H
1
H
H
Z
Z
H
H
Cl
H
Cl
H
Cl
CH Cl 3
CH 3
CH 3
1, 2-dichloropropane ( a )
( b)
C–1 are diastereomeric, magnetically non-equivalent and give separate nmr signals
Examples: The number of signals indicate the presence of the number of sets of
magnetically nonequivalent protons in the molecule of the compound.
CH ⎯CHCl⎯CH
→ 2 nmr signals.
3 3
a b a
aCH 3
aCH 3
C=C
Hb
Hb
→
2 nmr signals.