Q12-48E - Organic Chemistry (Mcmurry)

Question

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

Propose a structure consistent with the data.

Step-by-Step Solution

Answer

Step 1: 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.

Step 2: Infrared spectrum

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

Step 3: Proposing the structure from the data in infrared and mass spectrum

The m/z value of the compound is ${\text{M}}^{\text{ + }}\text{ = 172, 88}$. The IR spectrum shows that the compound has IR absorption at above ${\text{3000 cm}}^{\text{ - 1}}$, near ${\text{1740cm}}^{\text{ - 1}}$ and another peak near ${\text{1200 cm}}^{\text{ - 1}}$.

The value at ${\text{3000 cm}}^{\text{ - 1}}$  indicates the carbon-hydrogen stretching. The value at ${\text{1740cm}}^{\text{ - 1}}$ indicates the carbonyl group of the ester functional group. The value at ${\text{1200 cm}}^{\text{ - 1}}$ 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 ${\text{M}}^{\text{ + }}\text{ = 172}$ and it breaks to give a molecular weight ${\text{M}}^{\text{ + }}\text{ = 88}$ and is matched with alkoxide.

 

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