Question

Question: What functional groups are responsible for the absorptions above 1500 ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$in the IR spectra for compounds A and B?

Short answer

Answer

a. ${\mathrm{sp}}^2$hybridized C–H bond, ${\mathrm{sp}}^3$hybridized C–H bond, and C=C double bond

b. ${\mathrm{sp}}^3$ hybridized C–H bonds.

Step-by-step solution

IR spectrum of organic molecules

IR spectrum can be analyzed to study the structure of a compound.

Functional groups absorb IR radiation of specific frequency to produce peaks in the IR spectrum at the same place in every compound.

Therefore, the peaks in the IR spectrum of the compound can be used to study the functional groups present in the compound.

Predicting functional groups from IR spectrum

The standard values of absorption by different functional groups are found out by plotting the IR spectrum of known compounds.

These values will be the same for the particular functional group and are independent of the compound.

The peaks in the IR spectrum of the unknown compound are compared with standard values to find the functional groups in the compound.

Identifying functional groups in IR of given compounds

(a) Compound A has three peaks above 1500 ${\mathrm{cm}}^{-1}$.

The first peak is at 1650 ${\mathrm{cm}}^{-1}$which corresponds to double bond.

The second peak is at 3000-2850 ${\mathrm{cm}}^{-1}$which corresponds to ${\mathrm{sp}}^3$hybridized C–H bond.

The third peak is at approximately 3150 ${\mathrm{cm}}^{-1}$which corresponds to ${\mathrm{sp}}^2$hybridized C–H bond.

Hence, Compound A has a double bond with bonds${\mathrm{C}}_{{\mathrm{sp}}^3}-\mathrm{H}$ and ${\mathrm{C}}_{{\mathrm{sp}}^2}-\mathrm{H}$ bonds.

(b) Compound B has only one peak at 3000-2850 ${\mathrm{cm}}^{-1}$which corresponds to ${\mathrm{sp}}^3$hybridized C–H bond.

Hence, compound B has only single bonds with all carbon atoms in ${\mathrm{sp}}^3$ hybridization