Q48E - Organic Chemistry (Mcmurry)

Question

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

Propose a structure consistent with the data.

Step-by-Step Solution

Answer

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 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 M⁺ = 172 , 88. The IR spectrum shows that the compound has IR absorption at above 3000 cm^-1 , near 1740 cm^-1 and another peak near 1200 cm^-1  .

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

 

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