Q8P - Organic Chemistry (Mcmurry)

Question

How many peaks would you expect in the ¹H NMR spectrum of 1,4-dimethylbenzene(para-xylene, or p-xylene)? What ratio of peak areas would you expect on integration of the spectrum? Refer to Table 13-3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2-4 that aromatic rings have two resonance forms.)

Step-by-Step Solution

Answer

1, 4-dimethylbenzene produces two peaks as it contains two different types of environment of protons.

The ratio of peak integration is 4:6 which is equivalent to 2:3.

Step 1: Number of peaks in a 1 H NMR spectrum

The number of peaks in each signal of the ¹H NMR spectrum is termed as the multiplicity.  The multiplicity of each signal signifies the neighboring protons. It is evaluated using the n+1 rule when the neighboring protons are chemically equivalent to each other.

Step 2: Number of peaks given by 1,4-dimethylbenzene

1, 4-dimethylbenzene produces two peaks as it contains two different types of environment of protons. These protons can be given as:

 

The chemical shift for these protons is as follows:

 

For -CH₃ the chemical shift value is 2.35 ppm.

The aromatic hydrogens have a chemical shift value of 6.94 ppm.

                      Different protons in the molecule

Chemical shift values of the different protons

 

The spectrum can be given as:

NMR spectrum of the compound

 

The area under each peak is proportional to the number of protons causing that particular peak. By measuring the area electronically or integrating, the number of protons causing the peak can be determined.

The ratio of peak integration is 4:6 which is equivalent to 2:3.