Question

How will you distinguish between following compounds using IR spectroscopy:

CH3COCH3 and CH3CH2CHO​

Answer 1

Explanation:

The main differences between these molecules' IR spectra are in the

O

H

stretches and in the

C

=

O

stretches.

While the alcohol

O

H

stretch is broader, the carboxylic

O

H

stretch is less broad. While the aldehyde

C

=

O

stretch is sharp and "well-defined", the carboxylic

C

=

O

stretch is broader and more "smeared".

COMPARING THE STRUCTURES

Follow the structural formulas from left to right to determine that the structures are:

Notice how the functional groups are clearly different; the left compound has an alcohol hydroxyl and an aldehyde carbonyl group, while the right compound has a carboxylic hydroxyl and a carboxylic carbonyl group.

SIGNIFICANT VIBRATIONAL MOTIONS

To break it down further, consider the ways these compounds can stretch and bend.

It's a good rule of thumb to assume that single bonds can stretch and bend, and double bonds can stretch, but don't bend very well. Also, as an aside, alcohol bends aren't really necessary to consider because their stretches are so distinctive.

Here's what I see. I've bolded the unique motions, and put the frequencies in wavenumbers (Techniques in Organic Chemistry, Mohrig).

3-hydroxypropanal -

s

p

3

C

−

H

bend (alkane) ---

1480

~

1430

and

1395

~

1340 cm

−

1

; medium to weak; sharp

s

p

3

C

−

H

stretch (alkane) ---

2990

~

2850 cm

−

1

; medium to strong; sharp

C

=

O

stretch (aldehyde) ---

1740

~

1720

(saturated) and

1715

~

1680 cm

−

1

(conjugated); medium; sharp

C

−

H

stretch (aldehyde) ---

2900

~

2800

and

2800

~

2700 cm

−

1

; weak; sharp Fermi doublet

C

−

O

stretch (alcohol) ---

1300

~

1000 cm

−

1

; strong; sharp

O

−

H

stretch (alcohol) ---

3650

~

3550

and

3550

~

3200 cm

−

1

; medium to strong; broad

propanoic acid -

s

p

3

C

−

H

bend (alkane) ---

1480

~

1430

and

1395

~

1340 cm

−

1

; medium to weak; sharp

s

p

3

C

−

H

stretch (alkane) ---

2990

~

2850 cm

−

1

; medium to strong; sharp

C

=

O

stretch (carboxylic acid) ---

1725

~

1700

(saturated) and

1715

~

1680 cm

−

1

(conjugated); strong; sharp

C

−

O

stretch (carboxylic acid) ---

1300

~

1000 cm

−

1

; strong; sharp

O

−

H

stretch (carboxylic acid) ---

3200

~

2500 cm

−

1

; medium to weak; broad

You should eventually see that the significant differences in the frequencies given above are:

The carboxylic

C

=

O

stretch is generally stronger, slightly more upfield, and broader.

The carboxylic

C

=

O

stretch is less broad (spans a smaller range of frequencies near

3000 cm

−

1

)