Chapter 3.9, Problem 3.48P

Interpretation Introduction

Interpretation:

The structure for a compound with molecular formula $C_8{H}_{10}O$ that is consistent with the following proton NMR has to be proposed.

Concept Introduction:

Determination of structure of a compound from its proton NMR spectrum:

There are several steps to be followed for the determination of the structure as given below.

Step $1$:

Calculation of hydrogen deficiency index (HDI) from the molecular formula.

It can be formulated as given below.

  $HDI=\frac{2C+2+N-H-X}{2}$

Where,

$C$ is number of carbon atoms.

$N$ is number of nitrogen atoms.

$H$ is total number of hydrogen atoms present in the molecule.

$X$ is number of halogens.

If $HDIis1$, a compound possess one double bond or one ring.

If $HDIis2$, a compound possess two double bonds or two rings, one double bond and one ring, or one triple bond.

If $HDIis5$, a compound may possess benzene ring, one double bond or one ring.

Step $2$:

Consider the number of signals and integration of each signal.  This gives the clues about the symmetry of the compound.

In order to convert the integration values into useful information, a smallest number should be chosen and then all integration values should be divided by this number.  If the numerical values are not whole numbers, then in order to arrive at whole numbers all the numbers should be multiplied by a suitable integer.

Step $3$:

Analyze each signal and then draw fragments consistent with each signal.  Consider chemical sift value, integration and multiplicity.  If $n$ is the number of neighboring protons, then the multiplicity will be $n+1.$

Step $4$:

Assemble the fragments into a molecular structure.

Important chemical shift values and splitting patterns:

An ethyl group is characterized by a triplet of $3$ protons and a quartet of $2$ protons.  An isopropyl group is characterized by a doublet of $6$ protons and a quartet of one proton.  A tert-butyl group is characterized by a singlet of $9$ protons.

Type of proton	Chemical shift $\text{(}\delta \text{)}$
Methyl group	$~0.9$
Methylene group	$~1.2$
Methine group	$~1.7$
Allylic	$~2$
Alkynyl	$\text{~}\text{2}\text{.5}$
Aromatic methyl	$\text{~}\text{2}\text{.5}$
Alkyl halide	$\text{2-4}$
Alcohol	$\text{2-5}$
Vinylic	$\text{4}\text{.5-6}\text{.5}$
Aryl	$\text{6}\text{.5-8}$
Aldehyde	$\text{~10}$
Carboxylic acid	$\text{~12}$

Key points:

• Methyl protons, methylene protons and methine protons attached to a carbonyl group give a chemical shift value of $~2-2.5ppm.$
• The chemical shift value for $-OC{H}_{3}or-O-C{H}_2-R$ comes around $3.5-4.5ppm.$