Question

Question: The infrared spectrum of the compound with the mass spectrum shown below lacks any significant absorption above $\mathbf{3000}\mathbf{}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$. There is a prominent peak near $\mathbf{1740}\mathbf{}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$and another strong peak near $\mathbf{1200}\mathbf{}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$.Propose a structure consistent with the data.

Short answer

The m/z value of a molecular ion provides the molecular mass of a specific compound. The base peak is a peak comprising the largest abundance.It has the largest relative abundance as it is the most stable positively charged ion.

Step-by-step solution

Mass spectrum

The m/z value of a molecular ion provides the molecular mass of a specific compound. The base peak is a peak comprising the largest abundance.It has the largest relative abundance as it is the most stable positively charged ion.

Infrared spectrum

The carbonyl functional groups can be easily determined using the IR spectroscopy. They show an intense peak in the region $1670$to $1780{\mathrm{cm}}^{-1}$. Several effects like inductive, resonance and hydrogen bonding help in describing the frequencies at which carbonyl compounds absorb IR radiation.

Proposing the structure from the data in infrared and mass spectrum

The m/z value of the compound is . The IR spectrum shows that the compound has IR absorption at above$3000{\mathrm{cm}}^{-1}$ , near $1740{\mathrm{cm}}^{-1}$and another peak near$1200{\mathrm{cm}}^{-1}$ .

The value at $3000{\mathrm{cm}}^{-1}$ indicates the carbon-hydrogen stretching. The value at$1740{\mathrm{cm}}^{-1}$ indicates the carbonyl group of the ester functional group. The value at $1200{\mathrm{cm}}^{-1}$indicates the C-O stretching of ester functional group.

Therefore the molecular weight and the IR spectra can be matched with pentyl valerate.

The molecular formula of pentyl valerate is ${\mathrm{M}}^{+}=172$and it breaks to give a molecular weight ${\mathrm{M}}^{+}=88$and is matched with alkoxide.

So, the structure of the compound can be given as: