Question

Which compound matches the IR spectrum best? A, B, C, D, E or F? Please give explanation too!!

Answer 1

Concepts and reason

The concept used to solve this problem is based on the identification of the IR spectrum.

IR spectrum is a technique which is used to identify the functional group present in a compound. Some compounds are IR active and give spectrum in the IR range.

Fundamentals

IR spectrum can be divided into two regions, the functional group region from about $4000 - 1500{\rm{ c}}{{\rm{m}}^{ - 1}}$ and the fingerprint region from about $1500 - 400{\rm{ c}}{{\rm{m}}^{ - 1}}$ .

Absorptions by alkane ${\rm{C}} - {\rm{H}}$ bonds, aldehyde ${\rm{C}} - {\rm{H}}$ bonds, can be found in the region $3200 - 2700{\rm{ c}}{{\rm{m}}^{ - 1}}$ . The ${\rm{s}}{{\rm{p}}^{\rm{3}}}$ hybridized carbon shows peaks are usually found slightly below ${\rm{3000 c}}{{\rm{m}}^{ - 1}}$ and the ${\rm{s}}{{\rm{p}}^{\rm{2}}}$ hybridized carbon shows peaks are located slightly above ${\rm{3000 c}}{{\rm{m}}^{ - 1}}$ . Evidence of the presence of an aldehyde is observed as two peaks around ${\rm{2700 c}}{{\rm{m}}^{ - 1}}$ and ${\rm{2900 c}}{{\rm{m}}^{ - 1}}$ . Peaks for carbonyl double bonds are usually very strong and intense in the region $1750 - 1700{\rm{ c}}{{\rm{m}}^{ - 1}}$ . Absorption peaks caused by alkene double bonds and aromatic carbon-carbon bonds are located in the region $1600 - 1500{\rm{ c}}{{\rm{m}}^{ - 1}}$ .

The structure of compound A is shown as follows:

Compound A is a substituted diene containing two double bonds and bromine substituent. Therefore, its IR spectrum should consist of a peak above ${\rm{3000 c}}{{\rm{m}}^{ - 1}}$ for ${\rm{s}}{{\rm{p}}^{\rm{2}}}$ hybridized carbon atoms.

The structure of compound B is shown as follows:

Compound B is an aromatic aldehyde. There are methyl groups substituted on the aromatic ring. The IR spectrum of this compound should consist of a peak around and for the aldehyde group.

The structure of compound C is shown as follows:

Compound C has an aldehyde functional group. The IR spectrum of this compound should consist of a peak around and for the aldehyde group.

The structure of compound D is shown as follows:

Compound D has three carbonyl functional group and six methyl groups. Its spectrum should consist of one signal for ${\rm{s}}{{\rm{p}}^{\rm{3}}}$ hybridized carbon and one for the carbonyl carbon.

The structure of the compound E is shown as follows:

Compound E has a carbonyl functional group. There are two $\left( {{\rm{C = C}}} \right)$ bonds present. Hence, the spectrum of this compound should consist of a peak for ${\rm{s}}{{\rm{p}}^2}$ hybridized carbon and one form carbonyl group.

The structure of the compound F is shown as follows:

In compound F, there are two benzene rings and two carbonyl groups present. Hence, the spectrum of this compound should consist of a peak for ${\rm{s}}{{\rm{p}}^2}$ hybridized carbon and one form carbonyl group.

Ans:

The compound D matches best with the IR spectrum.