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Saturday, 22 November 2014

Pentanedioic acid, diethyl ester



Pentanedioic acid, diethyl ester


In this spectrum the two methyls labeled (a) are homotopic, as are the two methylenes labeled (b).
  • The three Hs within each methyl are homotopic, and the two Hs within each methylene are enantiotopic.
  • Hence we get single absorptions for these groups, displaying the typical ethyl pattern of methyl triplet and CH2 quartet.
  • Note the numerical values for the integration shown under the chemical shift scale, which fit the 3 : 2 ratio of CH3 to CH2.
The two CH2 groups labeled (c) are homotopic (enantiotopic within the group) and therefore have the same chemical shift.
  • They are split to a triplet by the CH2 at (d).
The two Hs of (d) are enantiotopic and have the same chemical shift.
  • With a total of four equivalent neighbors (both (c) groups), (d) should be a quintet. The enlarged view shows this clearly.
  • The apparent small additional couplings seen are probably artifacts. The integration values fit the 2 : 1 ratio of (c) to (d).
Part of the use we make of nmr spectra is simple pattern recognition. An ethyl is always triplet + quartet, although the separation between the two groups will vary according to the rest of the structure. A chain of three CH2 with no other neighbors is always triplet + quintet, again with variation in the separation on the chemical shift axis.


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