June 29, 2011
If a chemist can(not) recognize the pattern, then they can(not) understand the structure. Remember pattern recognition comes with practice.
From the 1H NMR spectrum, the coupling pattern at 3.10 ppm exhibits four signals in a 1:2:2:1 ratio. The coupling pattern is most consistent with a doublet of doublets (dd) and an AB spin system poised as two doublets (d). Pure first-order dd patterns are expected to be in a 1:1:1:1 ratio but the contribution from second-order effects can produce patterns that deviate from this ratio. The non-uniform coupling constants (14.1 and 15.8 Hz) rules out a quartet (q).
Note#1: If the integral was given as 1 for 1 hydrogen atom, then the pattern is not consistent with an AB spin system, thus leaving a dd pattern.
Note#2: If the wrong coupling pattern was assigned, then a chemist would be chasing the wrong group. For example, if we treated the pattern as a quartet (q), then the unknown contained either a CH-CH3 or CH2-CH3 group. If we considered the pattern to be a doublet of doublets (dd), then the unknown contained one of the following groups: CH-CH2 or CH2-CH or CH-CH-CH. If we treated the pattern as two doublets (d), then a C-CH2-C would be expected (assuming only carbon atoms are adjacent to the underlined CHn group).
Thanks Serge for your comment.