Q12-49E - Organic Chemistry (Mcmurry)

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 a C-H out-of-plane bending peak near $\mathbf{880}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$. 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 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: