Q13.47E - Organic Chemistry (Mcmurry)

Question

How many absorptions would you expect to observe in the NMR

spectra of the following compounds?

 

(a) 1,1-Dimethylcyclohexane

 (b)  CH₃CH₂OCH₃

(c) tert-Butylcyclohexane

 (d) 3-Methyl-1-pentyne

(e) cis-1,2-Dimethylcyclohexane 

(f) Cyclohexanone

Step-by-Step Solution

Answer

(a) Five different absorptions are observed in the ¹³C NMR spectra for 1,1-Dimethylcyclohexane.

(b) Three different absorptions are observed in the ¹³C NMR spectra for .

(c) Six different absorptions are observed in the NMR spectra for tert-Butylcyclohexane.

(d) Six different absorptions are observed in the ¹³C NMR spectra for 3-Methyl-1-pentyne.

(e) Four different absorptions are observed in the ¹³C NMR spectra for cis-1,2-Dimethylcyclohexane.

(f) Four different absorptions are observed in the ¹³CNMR spectra for cyclohexanone.

Step 1: Number of absorptions in the 13 C NMR spectra of the compounds

The number of absorptions in the¹³C  NMR spectra of the compounds is based on the number of non equivalent carbons in the compound. Each different kind of carbon produces a different signal in the ¹³C NMR spectra. 

Step 2: The number of absorptions in the 13 C NMR spectra of the compounds

(a)The structure of the compound 1,1-dimethylcyclohexane can be given as: 

The compound 1,1-dimethylcyclohexane has five different kind of carbon atoms. Hence, five different absorptions are observed in the ¹³C NMRspectra.

(b)The structure of the compound ethyl methyl ether can be given as:

                                            Structure of compound (b)

The compound has three different kind of carbon atoms. Hence it produces three different signal. Hence, three absorptions are seen in the ¹³C NMR spectra.

(c)The structure of the compound tert-butylcyclohexane can be given as:

                             Structure of compound (c)

The compound tert-butylcyclohexane has six different kinds of carbon atoms. Hence, six absorptions are observed in the ¹³C NMR spectra.

(d)The structure of the compound, 3-methyl-1-pentyne can be given as:

                                Structure of compound (d)

The compound, 3-methyl-1-pentyne contains six different carbon atoms. Hence six different absorption is observed in the ¹³C NMR spectra.

 

(e)The structure of cis-1,2-dimethylcyclohexane can be given as:

                                                Structure of compound (e)

In cis-1,2-dimethylcyclohexane, the two methyl groups have same chemical environment, so these two carbon are equivalent. It has four different kind of carbon atoms. Hence, four absorptions are observed in the ¹³C NMR spectra.

(f)The structure of cyclohexanone can be given as:

                                         Structure of compound (f)

The compound cyclohexanone has four different carbon atoms. Hence, four absorptions are observed in the ¹³C NMR spectra.