Question

Question: Propose a structure for a compound of molecular formula ${\mathbf{C}}_{\mathbf{3}}{\mathbf{H}}_{\mathbf{8}}\mathbf{O}$ with an IR absorption at $\mathbf{3600}\mathbf{-}\mathbf{3200}\mathbf{c}{\mathbf{m}}^{\mathbf{-}\mathbf{1}}$and the following NMR spectrum:

Short answer

Spectroscopic data is used to identify unknown compounds.

The data can be used to interpret the structure and bonding of the compound.

Step-by-step solution

Spectroscopic data

Spectroscopic data is used to identify unknown compounds.

The data can be used to interpret the structure and bonding of the compound.

Interpreting the IR spectral data

It is given that the compound shows an IR absorption at $3600-3200c{m}^{-1}$.

An IR absorption at $3600-3200c{m}^{-1}$ corresponds to a hydroxyl $\left(-OH\right)$ group.

Hence, it is identified that the given compound is an alcohol.

Interpreting the NMR spectrum

In the given spectrum, the single peak (singlet) at 2.4 ppm corresponds to the OH group.

The peak (doublet) at 1.2 ppm corresponding to 6H indicates that the compound consists of six equivalent hydrogen atoms (i.e., the hydrogen atoms with the same chemical environment).

The peak at 1.2 ppm corresponds to alkyl hydrogens.

Since it is given that six hydrogens give a doublet, the carbon atom adjacent to the carbon to which the desired hydrogen is bonded consists of only one hydrogen.

According to the n+1 rule, the multiplicity of a peak depends on the hydrogen atoms present adjacent to it.

n represents the number of hydrogen atoms attached to the carbon adjacent to the concerned proton/hydrogen.

Therefore, it can be assumed that two $C{H}_3$ groups are attached to a $CH$ group.

The arrangement of methyl groups in the given compound

This is in accordance with the peak (multiplet) at 3.9 ppm corresponding to 1H.

The multiplicity of the peak corresponding to the $CH$ group, according to the n+1 rule, is:

$\begin{array}{c}n+1=6+1\\ =7\left(multiplet\right)\end{array}$

The chemical shift of 3.9 ppm indicates that the $CH$ group is directly attached to an electron-withdrawing atom/group.

On combining all points, the structure of the given compound is:

Structure of the given compound