Q 42 E - Organic Chemistry (Mcmurry)

Question

Question: The mass spectrum (a) and the infrared spectrum (b) of another unknown hydrocarbon are shown. Propose as many structures as you can.

Step-by-Step Solution

Answer

Step 1: Mass spectrum

A particular compound’s mass spectrum is indicated as a bar graph comprising m/z values on the x axis and the intensity or relative abundance of the m/z values on the y axis. The tallest peak in a mass spectra is termed the base peak.

Step 2: Infrared spectrum

Wavelengths in the IR region are mentioned in micrometers and the frequencies are indicated in hertz. The useful IR region ranges from 4000 to $\mathbf{400}\mathbf{ }{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$  .The full interpretation of IR is not possible as all organic molecules have several stretching and bending motions.

Step 3: Proposing a structure from the mass and IR spectrum

The given compound shows 70 and 55 in the mass spectrum and the IR spectrum shows $1660 {\mathrm{cm}}^{-1}$ and $3000 {\mathrm{cm}}^{-1}$.

The given compound shows molecular ion peak at ${\mathrm{M}}^{+}=70$ . Hence, the molecular formula of the compound is ${\mathrm{C}}_5{\mathrm{H}}_{10}$.

The formula for identifying the degree of unsaturation is:

$$\begin{gathered} \mathrm{Degree} \mathrm{of} \mathrm{unsaturation}=\frac{2\mathrm{C}+\mathrm{N}-\mathrm{H}-\mathrm{X}}{2} \\ =\frac{2\times 5+2+0-10-0}{2} \\ =1 \end{gathered}$$

The molecule has one degree of unsaturation, hence it has one double bond or a ring. 

The molecular fragmentation is 55 indicates the cleavage of methyl group. Hence, the molecule has a terminal methyl group.

IR spectrum shows at $1660 {\mathrm{cm}}^{-1}$and $3000 {\mathrm{cm}}^{-1}$ , which shows the compound has a carbon-carbon double bond. The IR spectrum does not show peaks at $890-900 {\mathrm{cm}}^{-1}$ which shows that the compound has a terminal alkene.

The possible structure can be given as:

The correct structure is: