Pentanedioic acid, diethyl ester

• Formula: C₉H₁₆O₄
• Molecular weight: 188.2209
• CAS Registry Number: 818-38-2

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.