Chapter 14, Problem 31P

Each of the following compounds is characterized by a 1H NMR spectrum that consists of

only a single peak having the chemical shift indicated. Identify each compound.

${\text{C}}_{\text{8}}{\text{H}}_{\text{18}}\text{;δ 0}\text{.9}$

${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{;δ 1}\text{.5}$

${\text{C}}_{\text{8}}{\text{H}}_{\text{8}}\text{;δ 5}\text{.8}$

${\text{C}}_{\text{4}}{\text{H}}_{\text{9}}\text{Br;δ 1}\text{.8}$

${\text{C}}_{\text{2}}{\text{H}}_{\text{4}}{\text{Cl}}_{\text{2}}\text{;δ 3}\text{.7}$

${\text{C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{Cl}}_{\text{3}}\text{;δ 2}\text{.7}$

${\text{C}}_{\text{5}}{\text{H}}_{\text{8}}{\text{Cl}}_{\text{4}}\text{;δ 3}\text{.7}$

${\text{C}}_{\text{12}}{\text{H}}_{\text{18}}\text{;δ 2}\text{.2}$

${\text{C}}_{\text{3}}{\text{H}}_{\text{6}}{\text{Br}}_{\text{2}}\text{;δ }2.6$

Interpretation Introduction

Interpretation:

The compounds gives only single peak in 1H NMR spectrum as indicated by the chemical shift is to be identified.

Concept introduction:

The information related to the proton in the compound can be deduced by the number of signals in a spectrum.

The proton chemical shift in the compound is due to the environment of the proton.

The signal of a particular proton can be split by the presence of protons in vicinal position.

The NMR spectrum of a compound will consist of a single peak if all protons in the compound are structurally equivalent.

Index of hydrogen deficiency is calculated as

$\text{Index}\text{of}\text{hydrogen}\text{deficiency=}\frac{\text{1}}{\text{2}}\left({\text{C}}_{\text{n}}{\text{H}}_{\text{2n+2}}{\text{-C}}_{\text{n}}{\text{H}}_{\text{x}}\right)$

Here $C_n{H}_x$ is the molecular formula of the compound.

Oxygen atoms do not affect the index of hydrogen deficiency.

Answer to Problem 31P

Solution:

$$ ${\text{C}}_{\text{8}}{\text{H}}_{\text{18}}\text{; δ}\text{0}\text{.9}$

${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{; δ}\text{1}\text{.5}$

${\text{C}}_{\text{8}}{\text{H}}_{\text{8}}\text{; δ}\text{5}\text{.8}$

${\text{C}}_{\text{4}}{\text{H}}_{\text{9}}\text{Br; δ}\text{1}\text{.8}$

${\text{C}}_{\text{2}}{\text{H}}_{\text{4}}{\text{Cl}}_{\text{2}}\text{; δ}\text{3}\text{.7}$

${\text{C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{Cl}}_{\text{3}}\text{; δ}\text{2}\text{.7}$

${\text{C}}_{\text{5}}{\text{H}}_{\text{8}}{\text{Cl}}_{\text{4}}\text{; δ}\text{3}\text{.7}$

${\text{C}}_{\text{12}}{\text{H}}_{\text{18}}\text{; δ}\text{2}\text{.2}$

${\text{C}}_{\text{3}}{\text{H}}_{\text{6}}{\text{Br}}_{\text{2}}\text{; δ}\text{2}\text{.6}$

Explanation of Solution

Chemical formula: ${\text{C}}_{\text{8}}{\text{H}}_{\text{18}}$; Chemical shift: $\text{δ}\text{=0}\text{.9}$

A single peak at $0.9$ indicates all protons in the compound are equivalent protons. Further, they must be symmetrical alkyl (methyl) protons.

Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}$; Chemical shift: $\text{δ}\text{=1}\text{.5}$

A single peak at $1.5$ indicates all protons are equivalent. Further, they must be alkyl protons. The chemical formula shows the index of hydrogen deficiency to be $1$.

Therefore, it must be a symmetric cycloalkane:

Chemical formula: ${\text{C}}_{\text{8}}{\text{H}}_{\text{8}}$; Chemical shift: $\text{δ}\text{=5}\text{.8}$

A single peak at $5.8$ indicates all protons are equivalent. The chemical shift of $5.8$ indicates that the protons are all vinylic protons. The chemical formula shows the index of hydrogen deficiency to be $5$. Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{4}}{\text{H}}_{\text{9}}\text{Br}$; Chemical shift: $\text{δ}\text{=1}\text{.8}$

The chemical formula shows it is a saturated alkyl halide. A single peak means that all protons are equivalent. The presence of the bromine atoms accounts for the higher chemical shift of the protons. Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{2}}{\text{H}}_{\text{4}}{\text{Cl}}_{\text{2}}$; Chemical shift: $\text{δ}\text{=3}\text{.7}$

A single peak shows all protons to be equivalent. The formula is of a saturated alkyl halide. The presence of two chlorine atoms on the same carbon as the protons will increase the shift to $3.7$. Considering this, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{Cl}}_{\text{3}}$; Chemical shift: $\text{δ}\text{=2}\text{.7}$

A single peak means all protons must be equivalent. The chemical formula shows it to be a saturated alkyl halide. The higher shift of $2.7$ indicates that the alkyl protons are all on one carbon (methyl group) with the three chlorines on the carbon next to it. Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{5}}{\text{H}}_{\text{8}}{\text{Cl}}_{\text{4}}$; Chemical shift: $\text{δ}\text{=3}\text{.7}$

A single peak shows all protons to be equivalent. The chemical formula shows it to be a saturated alkyl halide. The high chemical shift of $3.7$ indicates that the chlorine atoms are on the same carbon atom as the protons. Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{12}}{\text{H}}_{\text{18}}$; Chemical shift: $\text{δ}\text{=2}\text{.2}$

A single peak shows all protons are equivalent. The chemical formula shows an index of hydrogen deficiency of four. This, taken together with a chemical shift of $2.2$, indicates that the compound has a benzene ring, with six methyl groups attached to it. Therefore, the structure of the compound is:

Chemical formula: ${\text{C}}_{\text{3}}{\text{H}}_{\text{6}}{\text{Br}}_{\text{2}}$; Chemical shift: $\text{δ}\text{=2}\text{.6}$

A single peak shows all protons are equivalent. The higher chemical shift of $2.6$ indicates that the protons are all methyl protons, with the two bromine atoms bonded to the adjacent carbon.

Therefore, the structure of the compound is: