The peak at 4.2 ppm in the 1H NMR spectrum of Molecule A is a quartet. How many protons are on the adjacent carbon?

The splitting pattern in a 1H NMR spectrum can provide information about the number of protons on the adjacent carbon atom. The number of peaks in a multiplet (in this case, a quartet) is determined by the "n + 1" rule, where n is the number of protons on the adjacent carbon atom.

Here are the steps to reach this conclusion:

1. Identify the splitting pattern: In this case, the peak at 4.2 ppm is a quartet.

2. Apply the "n + 1" rule: According to this rule, the number of peaks in a multiplet is equal to the number of protons on the adjacent carbon atom plus one. If the peak is a quartet (four peaks), then the number of protons on the adjacent carbon atom is 4 - 1 = 3.

So, there are three protons on the adjacent carbon atom to the one responsible for the peak at 4.2 ppm in the 1H NMR spectrum of Molecule A.