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Organic Chemistry II CHEM 2325 Packet #2

1. Predict the 1H NMR spectra for isopropyl butyrate.

O

O

There are a number of things that should be considered when trying to predict a 1H NMR spectrum for a given compound.

1) Determine the number of symmetrically different positions. This will represent the number of different signals expected. In isopropyl butyrate there are 5 different types of .

2 4

H H O H CH3

H3C O CH3 H H

1 3 5 two methyl groups are symmetrically equivalent

2) Need to predict downfield shift for each signal. This prediction is based entirely on how much electron density is around the hydrogen in question. Need to be able to predict the amount of shift for typically functional groups.

adjacent to beta to carbonyl isopropyl bonding ~1.6 ppm ~5 ppm

H H O H CH3

H3C O CH3 H H

terminal methyl terminal methyl gamma to carbonyl beta to ester ~0.9 ppm ~1.2 ppm

alpha to carbonyl ~2.2 ppm

3) The amount each signal is split is due to the number of magnetically active hydrogens are present on the adjacent . This follows the N+1 rule, where the splitting observed is N+1 the number of hydrogens causing the splitting.

5 adjacent H's, 6 adjacent H's, therefore hextet therefore heptet

H H O H CH3

H3C O CH3 H H

2 adjacent H's, 1 adjacent H, therefore triplet therefore doublet

2 adjacent H's, therefore triplet

4) The intensity of each signal (and thus the integral for each signal) is due to the number of hydrogens causing the signal

These predictions allows one to either predict a 1H NMR or assign positions in a given 1H NMR.

Actual spectrum:

2. Predict the position of the downfield shift in a 1H NMR for the following functional groups:

halides typically low 3 ppm allylic hydrogens low 2 ppm α-carbonyl hydrogens low 2 ppm aromatic hydrogens 7-8 ppm 9-10 ppm mid 3 ppm low to mid 4 ppm alkyl hydrogens 1-2 ppm terminal methyl groups below 1 ppm vinylic hydrogens 5-6 ppm

Effects are cumulative. If a given hydrogen is adjacent to a carbonyl and geminal to a halogen, then the downfield shift will be between 4-5 ppm.

b. Predict changes in downfield shift with the following on a monosubstituted ring. Explain the relative downfield shift for each, and explain the physical basis for the relative shift.

O

NO2 CH3 OCH3

While an unsubstituted benzene has only one signal and a downfield shift of low 7 ppm, adding substituents will affect the amount of downfield shift. Electron withdrawing substituents will cause the signal to be further downfield, while an electron donating will cause the signal to be more upfield. In these compounds, nitro group is the most electron withdrawing and thus will have the furthest downfield shift. The is also electron withdrawing. The methoxy substituent is the most electron donating and thus will have the furthest upfield aromatic signals for these compounds (one signal is at 6.88 ppm). The is also electron donating, but not as much as the . 1 3. Below are shown H NMRs for five (C4H10O1). Indicate the proper structure for each spectrum. The relative integration is shown in each spectrum.

9

OH

1

6

2

! "!

HO 1 1

3

1

3

OH 1 2

2 signals, 4 total 3

2

1 HO

3

O

2 2 3