Q13-13P - Organic Chemistry (Mcmurry)

Question

How many kinds of electronically non-equivalent protons are present in each of the following compounds, and thus how many NMR absorptions might you expect in each?

$$\begin{gathered} \left(\mathrm{a}\right){\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{Br}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{b}\mathbf{\right)}\mathbf{ }{\mathbf{CH}}_3{\mathbf{OCH}}_2\mathbf{CH}{\mathbf{\left(}{\mathbf{CH}}_3\mathbf{\right)}}_2 \\ \mathbf{\left(}\mathbf{c}\mathbf{\right)}\mathbf{ }{\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2{\mathbf{NO}}_2\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{d}\mathbf{\right)}\mathbf{ }\mathbf{Methylbenzene} \\ \mathbf{\left(}\mathbf{e}\mathbf{\right)}\mathbf{ }\mathbf{2}\mathbf{-}\mathbf{Methyl}\mathbf{-}\mathbf{1}\mathbf{-}\mathbf{butene}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{f}\mathbf{\right)}\mathbf{ }\mathbf{cis}\mathbf{-}\mathbf{3}\mathbf{-}\mathbf{Hexene} \end{gathered}$$

Step-by-Step Solution

Answer

1. 2 signals
2. 4 signals
3. 3 signals
4. 4 signals
5. 4 signals
6. 3 signals

Step-by-Step Solution Step 1: Types of protons

Homotopic means identical. If replacing two protons with a different group (X) gives the same compound then protons are homotopic.

If by replacing two protons with two different groups and if the pair shows an enantiomeric relationship then protons are enantiotopic.

If replacing two protons with different groups give  a pair of diastereomers then the protons are diastereotopic.

Step 2: Signals given by different types of protons

Homotopic and enantiotopic protons gives one signal. If the asymmetric center is present then the protons of ${\text{-CH}}_{\text{2}}$ are diastereotopic. Diastereotopic protons give different signals.

Step 3: Number of signals given by the following compounds

(a) ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{Br}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence, give only one signal. As there is no asymmetric center is present therefore protons of ${\text{-CH}}_{\text{2}}$ are enantiotopic hence giving one signal.

Bromoethane

Therefore, the above compound gives only one signal.

(b)  ${\text{CH}}_{\text{3}}{\text{OCH}}_{\text{2}}\text{CH}{\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence give only one signal. As there is no asymmetric center present therefore protons of ${\text{-CH}}_{\text{2}}$ are enantiotopic hence, giving one signal.  

 

Therefore, the above compound gives four signals.

2-methoxy sec-butyl

(c) ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{NO}}_{\text{2}}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence give only one signal.  As there is no asymmetric center is present therefore protons of ${\text{-CH}}_{\text{2}}$ are enantiotopic hence, giving one signal.

Nitropropane

Therefore, the above compound gives three signals.

(d)  $\text{Methylbenzene}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence give only one signal. As methylbenzene contains a plane of symmetry hence, protons that are opposite to each other become identical and give the same signal as shown below

Methylbenzene

Therefore, the above compound gives four signals.

(e)  $\text{2-Methyl-1-butene}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence give only one signal. As there is no asymmetric center  present therefore, protons of ${\text{-CH}}_{\text{2}}$ are enantiotopic. Hence, they  giving one signal.

2-Methyl-1-butene

Therefore, the above compound gives four signals.

(f) $\text{cis-3-Hexene}$

As the three protons of ${\text{-CH}}_{\text{3}}$ are homotopic hence, give only one signal. As $\text{cis-3-Hexene}$ contains a plane of symmetry hence protons that are opposite to each other become identical and give the same signal as shown below:

 As there is no asymmetric center is present therefore protons of ${\text{-CH}}_{\text{2}}$ are enantiotopic hence, giving one signal.

Cis-2-Hexene

Therefore, the above compound gives three signals.