Question

Which of the highlighted carbon atoms in each molecule absorbs farther downfield?

a.

b.

c.

d.

Short answer

a.

b.

c.

d.

Step-by-step solution

Upfield and Downfield in 13C NMR

The carbon atoms that absorb downfield are the ones that shows signal to the left of the NMR spectrum (higher chemical shifts).

The carbon atoms that absorb upfield are the ones that shows signal to the farther right of the NMR spectrum (lower chemical shifts).

Downfield shift in 13C NMR

• The presence of electronegative elements shifts the absorptions downfield.

• The sp² hybridized carbon atoms show downfield absorptions.

• All the electron deficient carbon atoms (for example the carbonyl carbon) shows downfield absorptions.

Carbon atoms showing downfield absorptions in the given compounds.

a.

Carbon atom absorbing downfield in a.

The carbon atom adjacent to the oxygen atom is electron-deficient and hence it absorbs downfield.

b.

Carbon atom absorbing downfield in b.

The carbon atom bonded to two bromine atoms is highly electron-deficient compared to the other and hence it absorbs farther downfield.

c.

Carbon atom absorbing downfield in c.

The carbonyl carbon absorbs downfield.

d.

Carbon atom absorbing downfield in d.

The double-bonded carbon ( hybridized) absorbs farther downfield.