Q32 E - Organic Chemistry (Mcmurry)

Question

Question: Two infrared spectra are shown. One is the spectrum of cyclohexane,

and the other is the spectrum of cyclohexene. Identify them, and

explain your answer.

(a) 

(b) 

Step-by-Step Solution

Answer

Spectrum (b) differs from spectrum (a) in several aspects, the absorption at 1650(C=C stretch) and 3000(=C-H stretch) can be found in spectrum (b). They allow us to assign spectrum (b) to cyclohexene and spectrum (a) to cyclohexane.

Step-by-step Solution Step 1: Infrared spectrum

Step 2: identifying Compound (a)

The bands correspond to the characteristic functional groups and a bond present in a chemical substance.

The fingerprint region is used for structure elucidation by special comparison.

Thus, an IR spectrum of a compound is considered as the fingerprint for its chemical identification.

Step 2: identifying Compound (a)

(a)

This is the spectrum of cyclohexane because thethree strong absorptions corresponds to:

$$\begin{gathered} -C-H\left(Stretch\right)\to 2960c{m}^{-1}\&2850c{m}^{-1} \\ -C-C\left(Stretch\right)\to 715c{m}^{-1} \\ -C-H\left(deformation\right)\to 1140c{m}^{-1} \end{gathered}$$

Step 3: Identifying compound (b)

(b) 

This is the spectrum of cyclohexene because thethree strong absorptions corresponds to:

$$\begin{gathered} -C-H\left(Stretch\right)\to 2960c{m}^{-1}\&2850c{m}^{-1} \\ -C-C\left(Stretch\right)\to 715c{m}^{-1} \\ -C-H\left(deformation\right)\to 1140c{m}^{-1} \\ -C=C\left(Stretch\right)\to 1650c{m}^{-1} \\ =C-H\left(Stretch\right)\to 3000c{m}^{-1} \\ C{H}_2\left(Bend\right)\to 1460c{m}^{-1} \end{gathered}$$

Step 4: Distinguish between Compound (a) and (b)

Spectrum (b) differs from spectrum (a) in several aspects. Note in particular the absorption at 715 (strong), 1140 (strong) 1650 (medium), and 3000 (medium).The absorption at 1650(C=C stretch) and 3000(=C-H stretch) can be found in spectrum (b). They allow us to assign spectrum (b) to cyclohexene and spectrum (a) to cyclohexane.