The peak at 2.1 ppm in the 1H NMR spectrum of Molecule A is most likely to be a:

The chemical shift at 2.1 ppm in a 1H NMR spectrum is typically indicative of a methyl group (CH3) adjacent to a carbonyl group (C=O). This is because the electronegative oxygen in the carbonyl group deshields the hydrogen atoms in the methyl group, causing them to resonate at a lower field/higher ppm value.

Here are the steps to reach this conclusion:

1. Look at the chemical shift value: The chemical shift of 2.1 ppm falls into the range typically associated with protons in a methyl group adjacent to a carbonyl group.

2. Consider the electronegativity of neighboring atoms: The carbonyl oxygen is highly electronegative, which means it pulls electron density away from the neighboring methyl group. This deshielding effect causes the protons in the methyl group to be more exposed to the magnetic field, resulting in a downfield shift (higher ppm value).

3. Consider the splitting pattern: If the peak at 2.1 ppm is a singlet, this would further support the conclusion that it represents a methyl group adjacent to a carbonyl group, as there would be no neighboring protons to cause splitting.

So, the peak at 2.1 ppm in the 1H NMR spectrum of Molecule A is most likely to be a methyl group adjacent to a carbonyl group.