Q 48 E - Organic Chemistry (Mcmurry)

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.

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 $1780 {\mathrm{cm}}^{-1}$. 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 . 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: