Organic Chemistry Laboratory I - Chem 341

Quiz 8c

1. Define or give an example to explain the following NMR term: homotopic hydrogen atom. (2 pts)

Protons that are on the same atom and are free to rotate are said to be homotopic. Thus, they will not split each other because they are identical on the time-average. Recall it takes about 1 second to acquire an NMR signal, so over that time a methyl group would rotate many times, so each proton on the methyl group is essentially identical.

2. Circle the structure most consistent with the following ¹H NMR spectrum. (2 pts)

3. How many different types of protons are present in the following compound? (2 pts)

Since the molecule is symmetric the top half is the same as the bottom half. Thus, this compound has three unique types of protons.

4. Consider the compound below and answer the following questions:

(a) What would the splitting pattern for the protons on carbon a look like? (e.g. singlet, doublet, triplet, etc.) (2 pts)

Triplet because it has two adjacent protons, so n = 2 and n+1 = 3.

(b) Specify where the protons on carbon d would show up on the ¹H NMR spectrum, i.e. what ppm? (2 pts)

Proton d is adjacent to a carbonyl, which is an electron withdrawing group, so it will show up from about 2 ppm. 2 pts for 2.5 ± 0.5 ppm. Since d is not adjacent to any other protons it will not be split, and will show up as a singlet.