Question

γ−Butyrolactone (C4H6O2, GBL) is a biologically inactive compound that is converted to the biologically active recreational drug GHB by a lactonase enzyme in the body. Since γ−butyrolactone is more fat soluble than GHB, it is more readily absorbed by tissues and thus produces a faster onset of physiological symptoms. γ−Butyrolactone shows an absorption in its IR spectrum at 1770 cm−1 and the following 1H NMR spectral data: 2.28 (multiplet, 2 H), 2.48 (triplet, 2 H), and 4.35 (triplet, 2 H) ppm. What is the structure of γ−butyrolactone?

Answer 1

Here's what I get.

Step-by-step explanation:

The name tells me the compound is a lactone (a cyclic ester).

1. IR spectrum

1770 cm⁻¹

Esters and unstrained lactones normally absorb at 1740 cm⁻¹.

This peak is shifted to a higher frequency by ring strain.

A five-membered lactone absorbs at 1765 cm⁻¹, and a four-membered lactone at 1840 cm⁻¹.

The compound is probably a five-membered lactone.

2. NMR spectrum

2.28 m (2H)

2.48 t (2H)

4.35 t (2H)

This indicates three CH₂ groups arranged as X-CH₂-CH₂-CH₂-Y.

The X-CH₂- and -CH₂-Y signals would each be triplets, being split by the central -CH₂- group.

The central -CH₂- signal would be a multiplet, split by the non-equivalent hydrogens on either side.

The peak at 4.35 ppm indicates that the group is adjacent to an oxygen atom ( -CH₂- = 1.3; -CH₂-O- = 3.3 - 4.5).

The peak at 2.42 ppm indicates that the group is adjacent to a carbonyl group (-CH₂-C=O = 1.8 - 2.5.

The only way to fit these pieces together is if γ-butyrolactone has the structure shown below.

Confirmation:

(a) The IR spectrum shows a carbonyl peak at 1770 cm⁻¹.

(b) The NMR spectrum matches that given in the problem.