Question

The infrared spectrum of the compound with the mass spectrum shown below has a medium-intensity peak at about $\mathbf{1650}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$. There is also aC-H out-of-plane bending peak near $\mathbf{880}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$. 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 ${\text{1780 cm}}^{\text{- 1}}$. Several effects like inductive, resonance and hydrogen bonding help in describing the frequencies at which carbonyl compounds absorb IR radiation.

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 ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$and the molecular formula is ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}$. 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.

Loss of methyl and isopropyl group

The structure of the compound can be given as: