Q33 E - Organic Chemistry (Mcmurry)

Question

Question: At what approximate positions might the following compounds show

IR absorptions?

(A)

(b)

(c)

(d)

(e)

Step-by-Step Solution

Answer

Absorptions with intensities for each compound are listed below:

(a) Aromatic ring C=C 1450-1600 $c{m}^{-1}$, aromatic ring C-H 3030 $c{m}^{-1}$, carboxylic acid C=O 1710-1760 $c{m}^{-1}$ , alcohol O-H 2500-3100 $c{m}^{-1}$.

(b) Aromatic ring C=C 1450-1600 $c{m}^{-1}$, aromatic ring C-H 3030 $c{m}^{-1}$and aromatic ester 1715$c{m}^{-1}$

(c)Aromatic ring C=C 1450-1600 $c{m}^{-1}$, aromatic ring C-H 3030 $c{m}^{-1}$, alcohol O-H 2500-3100$c{m}^{-1}$ and nitrile .

(d) Alkene C=C 1640-1680$c{m}^{-1}$ ,alkene =C-H 3020-3100 $c{m}^{-1}$and ketone1715 $c{m}^{-1}$

(e) Ester 1735 $c{m}^{-1}$and ketone 1715$c{m}^{-1}$

Step 1: Infrared (IR) spectroscopy

Infrared spectroscopy is the study of the interaction betweenthe sample andIR radiation which brings about change in vibrational-rotational level. Therefore, it is also called rotational-vibrational spectroscopy.

• It is a technique based on the vibrations of the atoms of a molecule.
• The waves are electromagnetic in nature and interact with polarity of the chemical bonds of the molecule.

If there is no polarity in the molecule then the infrared interaction is inactive and the molecule does not produce any IR spectrum

Step 2: IR absorption

In general, IR absorption iscaused by the interaction between the IR electric field vector and molecular dipole transition related to the molecular vibrations. Absorption is at maximum when the electric field vector and dipole transition moment are parallel to each other.

Step 3: IR (infrared) absorptions shown by following compound

(a)

(a) Aromatic ring C=C 1450-1600$c{m}^{-1}$, aromatic ring C-H 3030$c{m}^{-1}$, carboxylic acid C=O 1710-1760 $c{m}^{-1}$ , alcohol O-H 2500-3100$c{m}^{-1}$.

(b)

b) Aromatic ring C=C 1450-1600$c{m}^{-1}$, aromatic ring C-H 3030$c{m}^{-1}$, and aromatic ester 1715 $c{m^{-1}}^{}$.

(c)

(c)Aromatic ring C=C 1450-1600 $c{m}^{-1}$, aromatic ring C-H 3030$c{m}^{-1}$, alcohol O-H 2500-3100 $c{m}^{-1}$, and nitrile.

(d)

 

(e) 

(d) Alkene C=C 1640-1680 $c{m}^{-1}$,alkene =C-H 3020-3100 $c{m}^{-1}$, and ketone1715 $c{m}^{-1}$.

 

(e) Ester 1735 and ketone 1715 $c{m}^{-1}$.