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Chapter 12 1


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CARBON-CARBON BOND

STRETCHING Stronger bonds absorb at higher frequencies:

C-C 1200 cm-1

C=C 1660 cm-1

C|C 2200 cm-1 (weak or absent if internal)

Conjugation lowers the frequency:

isolated C=C 1640-1680 cm

-1

conjugated C=C 1620-1640 cm-1

aromatic C=C approx. 1600 cm-1


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CARBON-HYDROGEN

STRETCHING

Bonds with more s character absorb at a

higher frequency.

sp3 C-H, just below 3000 cm-1 (to the right)

sp2 C-H, just above 3000 cm-1 (to the left)

spC-H, at3300 cm-1


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AN ALKANE IR

SPECTRUM


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AN ALKENE IR

SPECTRUM


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AN ALKYNE IR SPECTRUM

=>


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O-H AND N-H

STRETCHING

Both of these occuraround 3300cm-1, butthey look different.

Alcohol O-H, broad with rounded tip.

Secondary amine (R2NH), broad with one

sharp spike.

Primary amine (RNH2), broad with twosharp spikes.

No signal for a tertiary amine (R3N)


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AN ALCOHOL IR

SPECTRUM

=>


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AN AMINE

IR SPECTRUM

=>


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CARBONYL STRETCHING

The C=O bond of simple ketones, aldehydes,and carboxylic acids absorb around 1710 cm-1.

Usually, its the strongest IR signal.

Carboxylic acids will have O-H also.

Aldehydes have two C-H signals around 2700and 2800 cm-1.


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A KETONE

IR SPECTRUM

=>


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AN ALDEHYDE

IR SPECTRUM

=>


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O-H STRETCH OF A

CARBOXYLIC ACIDThis O-H absorbs broadly, 2500-3500 cm-1, due to strong hydrogen bonding.


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VARIATIONS IN

C=O ABSORPTION

Conjugation ofC=O with C=C lowers the stretchingfrequency to ~1680 cm-1.

The C=O group of an amide absorbs at an even lowerfrequency, 1640-1680 cm-1.

The C=O of an ester absorbs at a higher frequency,

~1730-1740 cm-1.

Carbonyl groups in small rings (5 Cs or less) absorb at aneven higher frequency.


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AN AMIDE

IR SPECTRUM

=>


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CARBON - NITROGEN

STRETCHING

C- N absorbs around 1200 cm-1.

C=N absorbs around 1660 cm-1 and is muchstronger than the C=C absorption in thesame region.

C| N absorbs strongly just above 2200 cm-1.

The alkyneC| C

signal is much weaker andis just below 2200 cm-1 .


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A NITRILE

IR SPECTRUM

=>


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SUMMARY OF IRABSORPTIONS

=>


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STRENGTHS AND LIMITATIONS

IR alone cannot determine a structure.

Some signals may be ambiguous.

The functional group is usually indicated.

The absence of a signal is definite proof thatthe functional group is absent.

Correspondence with a known samples IRspectrum confirms the identity of the

compound.


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Chapter 1220

Out of plane flexions in

substituted benzenes Mono: 770-730 y 710-690

Di-o: 770-735

Di-m: 810-750 y 710-690 Di-p: 833-810

Tri-1,2,3: 780-760 y 745-705

Tri-1,2,4: 825-805 y 885-870

Tri-1,3,5: 865-810 y 730-675


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Chapter 1221

Out of plane flexions alkenes

Mono: 995-985 y 910-905

Di-trans: 980-965

Di-gem: 895-885 Di-cis: 690

Tri: 840-790

Tetra: None


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A

lkanes IR spectrumofdecane (Fig 12.4)


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Alkenes

IR spectrumofcyclohexene (Fig 12.5)


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Alkynes

IR spectrumof1-octyne (Fig 12.6)


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Aromatics

IR spectrumof toluene (Fig 12.7)


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Table 12.6, p.482


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Alcohols

IR spectrumof1-hexanol (Fig 12.8)

O-H (free)

O-H (H bonded)

C-O

Bond IntensityFrequency, cm-1

Medium1000 - 1250

Medium, broad3200 - 3500

3600-3650 Weak


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Ethers

IR spectrumofdibutyl ether (Fig 12.9)


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Ethers

IR spectrumofanisole (Fig 12.10)


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Ethers

Ethers have an oxygen atom bonded to twocarbon atoms

Eitheror both of the carbon atoms may be

sp3 hybridized: Dimethyl ether

sp2 hybridized: Diphenyl ether, ethyl vinyl ether.

sp hyrbridized:


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Amines

IR spectrumof1-butanamine (Fig

12.11)


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IR ofMolecules with C=O

Groups

C=O

C=O

C-H

O H

RCOH

O

RCH

O

RCR'

O

C=O

Strong1700-1725

Stretching

Carboxylic acids

Aldehydes

VibrationCarbonyl GroupFrequency

(cm-1 ) Intensity

Stretching 1630-1820 Strong

Stretching 2720 Weak

Stretching 2500-3300 Strong (broad)

KetonesStretching 1630-1820 Strong


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IR ofMolecules with

C=O Groups

C=O

sp3

C O

sp2

C O

C=O

C=O

N H

C|N

RC N

RCOCR

O O

RCOR'

O

RCNH2

O

C O Stretching 900-1300 Stro ng

Strong1735-1800Stretching

Carboxylic esters

Stre tching 1000-1100 Strong

Stre tching 1200-1250 Strong

Acid anhydrides

Stretching 1740-1760 and

1800-1850

Strong

Strong1630-1680StretchingAmides

Stretchin g 3200, 3400 Mediu m

(1 amides have two N-H stretches)(2 amides h ave one N-H stretch)

Nitriles

Stretchin g 2200-2250 Mediu m


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Aldehydes and Ketones

IR spectrumofmenthone (Fig 12.12)


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Chapter 12 35

The position ofC=O stretching vibration is sensitive to

its molecular environment

as ring size decreases and angle strain increases,

absorption shifts to a higherfrequency

conjugation shifts the C=O absorption to lowerfrequency

Carbonyl groups


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Carbonyl groups

1850cm-1

1780cm-1

1745 cm-1

1715 cm-1

O O OO

1690cm-1

1700cm-1

1717 cm-1

O OH

O


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Conjugation: shifts the C=O absorption

to lowerfrequency


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Carboxylic acids IR spectrumofpentanoic acid (Fig

12.13)


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E

sters IR ofethyl butanoate (Fig 12.14)


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E

sters: One ofthe carbons of the C-O-C group is sp2 hybridized and other sp3

hybridized.

Esters display strong C=O stretching absorption: 1750-1800cm-1


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IR Spectra ofMethylenecyclopentene ? Or 2,3-

dimethyl-2-butene?

p.490a


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IR Spectra ofMethylenecyclopentene ? Or 2,3-

dimethyl-2-butene?

p.490b


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IR Spectra of nonane ? Or 1-hexanol?

p.490c


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IR Spectra of nonane ? Or 1-hexanol?

p.491a


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IR Spectra of 2-methyl-1-butanol ? Tert-butyl methyl

ether?

p.491b


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IR Spectra of 2-methyl-1-butanol ? Tert-

butyl methyl ether?

p.491c


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Show how IR spectroscopy can be used

to distinguish between the compounds in each set.

.


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.

Show how IR spectroscopy can be used

to distinguish between the compounds in each set.

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