Q49E - Organic Chemistry (Mcmurry)

Question

The infrared spectrum of the compound with the mass spectrum shown below has a medium-intensity peak at about 1650cm^-1. There is also a

C-H out-of-plane bending peak near 880cm^-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 1780cm^-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 mass spectrum of the compound indicates that the compound has a molecular mass of 84. The IR spectral data tells that the compound is a trisubstituted alkene. The general formula is C_nH_2n and the molecular formula is C₆H₁₂ . Hence, the compound must be 2,3-dimethyl-1-butene. The M-15 peak (69) is acquired by the loss of a methyl group and the M-43(41) peak is acquired by the loss of isopropyl group during fragmentation.

The structure of the compound can be given as: