Q13-43E - Organic Chemistry (Mcmurry)

Question

How could you use ${}^{\mathbf{1}}\mathbf{H}$NMR to distinguish between the following pairs of isomers?

Step-by-Step Solution

Answer

a.Ethyl cyclopropane shows no signal and pent-2-ene has two vinylic protons which show a chemical shift at 5.4-5.5$\mathrm{\delta }$.

b. ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{OCH}}_2{\mathrm{CH}}_3$Produce 2 peaks one is triplet and quartet whereas ${\mathrm{CH}}_3{\mathrm{OCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3$in which methyl group bonded to oxygen produce singlet unsplit absorption.

c. Each compound shows three peaks but ester produce the downfield peak due to${\mathrm{CH}}_2$ next to oxygen.

d. Each compound has four protons two are methyl protons and two are vinyl protons

Isomer 1 produce singlet and 2^nd produce 1 singlet and doublet.      

Step-by-Step Solution Step 1 :

Nuclear magnetic resonance is the technique that is used to identify the hydrogen in the compound and produce the corresponding peaks for its detection.

Step 2: 1 H   NMR for compound a

Ethyl cyclopropane shows no signal and pent-2-ene has two vinylic proton which shows chemical shift at 5.4-5.5$\mathrm{\delta }$ so it is easy to distinguish isomers from each other.

Compound A

Step 3: 1 H   NMR for compound b

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{OCH}}_2{\mathrm{CH}}_3$produce 2 peaks because of two kind of protons one is methyl and other is methylene one is triplet and quartet where as ${\mathrm{CH}}_3{\mathrm{OCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3$  in which methyl group bonded to oxygen produce singlet unsplit absorption and its spectrum is very complex.

Compound A

Step 4: 1 H   NMR for compound c

Each compound shows three peaks but ester produce the downfield peak due to ${\mathrm{CH}}_2$ next to oxygen as shown :

COMPOUND C

Step 5: 1 H   NMR for compound d

Each compound has four protons two are methyl protons and two are vinyl protons Isomer 1 produce singlet and 2^nd produce 1 singlet and doublet as per the structure of the isomers.

Compound D