An inexperienced graduate student was making some 4-hydroxybutanoic acid. He obtained an excellent yield of a different compound, whose ¹³CNMR spectrum is shown here.

1. Propose a structure for this product.
2. Assign the peaks in the ¹³CNMR spectrum to the carbon atoms in the structure.

There are two elements of unsaturation in C₄H₆O₂ one of which is a carbonyl and there is no evidence of presence of C=C in the spectrum, which proves that the ring must be present. The product is obtained from 4-hydroxybutanoic acid with a loss of molecule of water means formation of lactone took place. This gives the structure which is butyrolactone.

The values of chemical shifts have been assigned to the respective carbons in the obtained structure. The carbonyl carbon has highest value of chemical shift of 180 and this is due to presence of electronegative oxygen near to it and also carbonyl carbon is electron deficient in nature, thus this makes less electron density available on it and hence increasing the value of chemical shift.

Carbon-b has chemical shift value 70 as due to presence of nearby oxygen which abstracts electron density from carbon and increasing the chemical shift value. Chemical shifts of two methylene carbons-c and d are 28 and 22 respectively. Carbon-c has more chemical shift value than carbon-d because of presence of nearby carbonyl group which is electron withdrawing in nature and makes electron density less available on carbon-c, thus increasing value of chemical shift.

Values of chemical shifts of carbons in butyrolactone