The scale UNITS: cm-1 unit is the wavenumber and is given by 1 / (wavelength in cm). ṽ = 1/ λ

Infrared (IR) spectroscopy deals spectrum with the interaction between a molecule and radiation from the IR region of the electromagnetic (IR region = 4000 - 400 cm-1).

IR radiation causes the excitation of the vibrations of covalent bonds within that molecule ( motions between nuclei).

These vibrations include the stretching and bending modes

How to can we type a Compound by IR?

Draw four lines on your IR spectrum 3000 cm-1 2500 cm-1

2000 cm-1 1500 cm-1

Get a big picture first!

It is important the frequency of the energy absorbed by the type of bond (E= hC/) and the shape of the

peak corresponding to that absorption

AB

C

DE

F

A. broad and strong, typical of alcohols, centered around 3300cm -1, takes 80 % of the chart

B. broad and medium, typical of carboxylic acids, is not as smooth as alcohols due to other absorptions in the same region, 50% of the chart.

C. Sharp strong, typical of carbonyl group around 1700 cm -1, Usually, 98% of the chart

D. Sharp strong, typical of ΞC-H centered around 3300 cm-1, usually 75% of chart

E. Sharp medium, typical of C=C bond and others in the finger print region F. Sharp weak, typical of amines, and overtones

The IR Chart1800-1600

C=O

C=N

C=N

~3300

H-EN

~2300

CΞC

CΞN

Compound class Bond Frequency (cm-1)

U S

3000 1500

Saturated Sp3 carbonstretching

Saturated Sp3 carbonBending

FINGERPRINT

Above 3000 cm-1 [X-H (X = O, N, sp ΞC, or sp2 =C)]

3000-2500 [C-H (sp3 C –H) stretching]

2500-2000 (CΞC, CΞN triple bond stretching)

2000-1500 [C=O, appr.1700; C=C and N=O, appr. 1600]

Under 1500 (fingerprint region)

Alkanes C-H 2960-2850 (strong, stretching), 1470-1350 (bending)

Single bonds: n-hexane

U S

Double bonds/hydrocarbons

Alkenes, arenes

=C-H

Compound Bond Frequency (cm-1)

U S

3000 1500

Saturated Sp3 C-Hstretching

Saturated Sp3 carbonbending

Unsaturated Sp2 C-Hstretching

~ 1600 cm -1 C=Cstretching

Alkenes 2-hexene C=C, 1680-1640 (medium, stretching)

C-H 3090-3020 (strong, stretching),

1000-675 (bending)

C=C

Double bonds /hydrocarbons

Methyl benzene

Aromatic Rings

C=C 1600-1500 (stretching)

=C-H 3100-3000 (stretching),

2000-1600 (phenyl ring substitution overtones)

U S

overtones

Double bonds /hydrocarbons

Other Double bondsNitro Compounds N=O 1660-1500 (s, asymmetrical s1390-1260 (s, symmetrical stretching)

C=O 1760-1670 (s, stretching)FAMILY A

Aldehydes, Ketones,

FAMILY B

Carboxylic acids, EstersAmides Acyl halidesAcid Anhydrides C=O 1760-1670 (s, stretching)

Acetone: Residues of acetone used during cleaning or in contamination during GC analysis will be observed with a weak peak at 1715 cm-1

Other double bonds

1-HEXYNE

Triple bonds/hydrocarbons

Alkynes

C-H 3333-3267(sharp, strong, stretching),

700-610 (bending)

CΞC 2260-2100 (sharp, medium, stretching)

ΞC-H

CΞC

Internal Alkyne

CΞC

overtones

C=C

Triple bonds

Alkynes

C-H 3333-3267(sharp, stretching),

700-610 (bending)

CΞC 2260-2100 (sharp, strong, stretching)

Nitriles

CΞN 2260-2220 (sharp, strong stretching)

Compound Bond Frequency (cm-1)

Cyano benzene

C=C

CΞN

NitrilesCΞN 2260-2220 (sharp, strong stretching)

Alcohols, Phenols O-H (H-bond formation) 3640-3160 (s & br, stret.)

Carboxylic acids O-H (H-bond formation) 3500-2500 (b, stretching)

AminesN-H (H-bond formation)

(2 peaks for primary amine, one peak for secondary amine)

3500-3300 (m, stretching)

Compound BondFrequency (cm-1)

H-Electronegative atom

1-pentanol

Difference between a carboxylic acid and an alcohol.

Hexanoic acid

Isopropanol

Cyclobutanol

cyclohexanol

2-methyl-2-propen-1-ol, and alkene/alcohol

Benzyl alcohol

Observe C-O fingerprint regionEthyl-vinyl ether/ether- alkene

Diethyl amine

N-H

Butyl amine

N-H

H

Observe a primary amine shows two absorptions, one for each hydrogen bonded to the electronegative atom Nitrogen

Compound Bond Frequency (cm-1)

Ketones and aldehydes C=O 1760-1670 (strong, stretching)

Recall that aldehydes also show: CHO two peaks 2850 and 2780 medium.

Carbonyl family A

3-pentanone / a ketone

2-butanone- /a ketone

(Gas Phase) of Formaldehyde, H(C=O)H

2-methylpropanal: an aldehyde

butyraldhyde

CHO two peaks sharp, medium, around 2850 and 2780cm-1.

Carboxylic acids, Esters

C=O 1760-1670 (strong, stretching)

C-O 1260-1000 ( strong, stret.)

Recall that Carboxylic acids also show: O-H (H-bond formation)3500-2500 (broad, stretching)

Carbonyl/ Family B

Carboxylic acid

Benzoic acid

O-H

C=CC=O

Ester C=O and C-O

C=O C-O

Ethyl acetate

C=OC-O

amides C=O 1760-1670 (strong, stretching)

C-O 1260-1000 ( strong, stret.)

Recall amide also show:

N-H as in amines but strong and broad for H-bond formation)

One peak for secondary amide, two peaks for primary amide

Carbonyl family

Amide/acetamide

C=ON-H

H

Benzamide

C=O C=CN-H

N-H