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O-H 3600 N-H 3400 C-H 3000 C=N 2250 C=C 2150 C=O 1715 C=C 1650 C-O 1100. =. =. HYDROCARBONS (C-H ABSORPTIONS) ALCOHOLS ACIDS (O-H ABSORPTIONS) AMINES (N-H ABSORPTIONS). SURVEY OF SPECTRA. 2.5 . 4. 5. 5.5. 6.1. 6.5. 15.4. 4000. 2500. 2000. 1800.

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slide1

O-H 3600

N-H 3400

C-H 3000

C=N 2250

C=C 2150

C=O 1715

C=C 1650

C-O 1100

=

=

HYDROCARBONS

(C-H ABSORPTIONS)

ALCOHOLS

ACIDS

(O-H ABSORPTIONS)

AMINES

(N-H ABSORPTIONS)

SURVEY OF SPECTRA

slide2

2.5

4

5

5.5

6.1

6.5

15.4

4000

2500

2000

1800

1650

1550

650

Typical Infrared AbsorptionRegions

WAVELENGTH (mm)

C-Cl

C=O

C=N

O-H

C-H

C N

Very

few

bands

C-O

C=C

N-H

C C

C-N

X=C=Y

C-C

*

N=O N=O

(C,O,N,S)

FREQUENCY (cm-1)

slide3

BASE VALUES

Guideposts for you to memorize.

slide4

BASE VALUES

These are

the minimum

number of

values to

memorize.

(+/- 10 cm-1)

O-H 3600

N-H 3400

C-H 3000

C N 2250

C C 2150

C=O 1715

C=C 1650

large range

C O ~1100

slide6

2.5

4

5

5.5

6.1

6.5

15.4

4000

2500

2000

1800

1650

1550

650

Typical Infrared AbsorptionRegions

WAVELENGTH (mm)

C-H

C-Cl

C=O

C=N

O-H

C-H

C N

Very

few

bands

C-O

C=C

N-H

C C

C-N

X=C=Y

C-C

*

N=O N=O

(C,O,N,S)

FREQUENCY (cm-1)

We will look at

this area first

slide7

The C-H stretching region

BASE VALUE = 3000 cm-1

  • C-H spstretch ~ 3300 cm-1

UNSATURATED

  • C-H sp2 stretch > 3000 cm-1

3000 divides

  • C-H sp3 stretch < 3000 cm-1

SATURATED

  • C-H aldehyde, two peaks (both weak)
  • ~ 2850 and 2750 cm-1
slide8

STRONGER BONDS HAVE LARGER FORCE CONSTANTS

AND ABSORB AT HIGHER FREQUENCIES

increasing frequency (cm-1)

3300

3100

3000

2900

2850 2750

=

=C-H

=C-H

-C-H

-CH=O

(weak)

sp-1s

sp2-1s

sp3-1s

aldehyde

increasing s character in bond

increasing CH Bond Strength

increasing force constant K

CH BASE VALUE = 3000 cm-1

slide9

METHYLENE GROUP STRETCHING VIBRATIONS

Two C-H bonds share a central carbon

(hydrogens attached to the same carbon)

in-phase

Symmetric Stretch

~2853 cm-1

out-of-phase

Asymmetric Stretch

~2926 cm-1

Any time you have two or more of the same kind of bond sharing

a central atom you will have symmetric and asymmetric modes.

slide10

METHYL GROUP STRETCHING VIBRATIONS

Three C-H bonds share a central carbon

(hydrogens attached to the same carbon)

in-phase

Symmetric Stretch

~2872 cm-1

out-of-phase

Asymmetric Stretch

~2962 cm-1

hexane

ALKANE

Hexane

CH bending vibrations

discussed shortly

CH

stretching

vibrations

includes

CH3 sym and asym

CH2 sym and asym

slide13

THE C-H BENDING REGION

  • CH2 bending ~ 1465 cm-1
  • CH3 bending (asym) appears near

the CH2 value ~ 1460 cm-1

  • CH3 bending (sym) ~ 1375 cm-1
slide14

METHYLENE GROUP BENDING VIBRATIONS

Scissoring

Wagging

~1465 cm-1

~1250 cm-1

~720 cm-1

~1250 cm-1

Rocking

Twisting

in-plane

out-of-plane

Bending

Vibrations

slide15

METHYLENE AND METHYL BENDING VIBRATIONS

CH2

CH3

C-H Bending, look near

1465 and 1375 cm-1

asym

sym

1465

1460

1375

these two peaks

frequently overlap

and are not resolved

slide16

METHYLENE AND METHYL BENDING VIBRATIONS

ADDITIONAL DETAILS FOR SYM CH3

The sym methyl peak

splits when you have

more than one CH3

attached to a carbon.

CH2

CH3

asym

sym

one peak

1465

1460

1375

1380

1370

two peaks

geminal dimethyl

(isopropyl)

1390

1370

t-butyl

two peaks

hexane1

ALKANE

Hexane

CH2

rocking

> 4C

CH3

bend

CH2

bend

CH

stretch

1 hexene

ALKENE

1-Hexene

=CH

C=C

CH2

CH3

bend

CH oops

CH

toluene

AROMATIC

Toluene

CH3

Ar-H

C=C

benzene

Ar-H oops

1 hexyne

ALKYNE

1-Hexyne

=

C=C

CH2, CH3

C-H

=

=C-H

slide22

2.5

4

5

5.5

6.1

6.5

15.4

4000

2500

2000

1800

1650

1550

650

Typical Infrared AbsorptionRegions

O-H

WAVELENGTH (mm)

C-Cl

C=O

C=N

O-H

C-H

C N

Very

few

bands

C-O

C=C

N-H

C C

C-N

X=C=Y

C-C

*

N=O N=O

(C,O,N,S)

FREQUENCY (cm-1)

the o h stretching region

broadens

shifts

FREE

H-BONDED

3600

3300

The O-H stretching region
  • O-H 3600 cm-1 (alcohol, free)
  • O-H 3300 cm-1 (alcohols & acids,

H-bonding)

effect of hydrogen bonding on o h stretching

Free

OH

Free

OH

C-H

H-bonded

OH

H-bonded

OH

C-H

C-H

(a)

Pure Liquid

(b)

Dilute Solution

(c)

Very Dilute Solution

Effect of Hydrogen-Bondingon O-H Stretching

4000 3600 3200 2800

4000 3600 3200 2800

4000 3600 3200 2800

“neat”

1-Butanol

slide25

HYDROGEN-BONDED HYDROXYL

Many kinds of OH

bonds of different

lengths and strengths

This leads to a broad

absorption.

“Neat” solution.

Longer bonds are weaker and lead to

lower frequency.

Hydrogen bonding occurs in concentrated solutions

( for instance, undiluted alcohol ).

slide26

“FREE” HYDROXYL

The “free” hydroxyl vibrates without interference from any

other molecule.

Distinct bond has a

well-defined length

and strength.

Solvent molecules

surround but do not

hydrogen bond.

Occurs in dilute solutions of alcohol in an “inert” solvent like CCl4.

cyclohexanol

ALCOHOL

Cyclohexanol

neat solution

O-H

H-bond

CH2

C-O

C-H

butanoic acid

CARBOXYLIC ACID

Butanoic Acid

neat solution

O-H

H-bond

C-O

CH2

C-H

C=O

slide29

CARBOXYLIC ACID DIMER

Strong hydrogen bonding in the dimer weakens the OH

bond and leads to a broad peak at lower frequency.

slide31

2.5

4

5

5.5

6.1

6.5

15.4

4000

2500

2000

1800

1650

1550

650

Typical Infrared AbsorptionRegions

N-H

WAVELENGTH (mm)

C-Cl

C=O

C=N

O-H

C-H

C N

Very

few

bands

C-O

C=C

N-H

C C

C-N

X=C=Y

C-C

*

N=O N=O

(C,O,N,S)

FREQUENCY (cm-1)

the n h stretching region
The N-H stretching region

N-H 3300 - 3400 cm-1

  • Primary amines give two peaks
  • Secondary amines give one peak
  • Tertiary amines give no peak

symmetric

asymmetric

slide33

PRIMARY AMINE

aliphatic

1-Butanamine

NH2

scissor

CH3

CH2

NH2

slide34

PRIMARY AMINE

aromatic

3-Methylbenzenamine

-CH3

Ar-H

NH2

benzene

Ar-H

oops

slide35

SECONDARY AMINE

N -Ethylbenzenamine

NH

CH3

Ar-H

oops

benzene

slide36

TERTIARY AMINE

N,N -Dimethylaniline

Ar-H

-CH3

no N-H

CH3

benzene

Ar-H

oops

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