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I.R INTERPRETATION

Presented by:

Mr.Lokesh G patil

M.Pharm (Q.A.T)

Modern College of Pharmacy, Pune

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INFRARED SPECTRA INTERPRETATION

When analysing the spectra of unknown compound , concentrate first on determining the presence or absence of few major functional group .

The C=O , O-H , C-O , C=C , C≡C , C≡N, and NO2 peaks are the most conspicuous and give immediate structural information if they are present.

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1.Look if carbonyl group (C = O) is present?The C = O group gives strong absorption in the region 1820-1600cm-1 , the peak is strong and medium width.

2. If C=O is present , check for the presence of the following groups:

(I) Acid

(ii) Amides

(iii) Ester

(iv) Anhydrides

(v) Aldehydes

(vi) ketones

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3. If C=O is absent

i. Alcohol

ii. Phenols

iii. Amines

iv. Ether

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4. Double bond and / or aromatic ring :

(i) C=C is a weak absorption near 1650 cm-1 .

(ii) Medium to strong absorption in the region of 1600 - 1550cm-1

often imply an aromatic ring.

(iii) Confirm the double bond or aromatic ring by consulting the C-H region , if C-H stretch occurs to the left of 3000cm-1 ,then it is aromatic or vinyl . If C-H stretch occurs to right of 3000cm-1 then it is aliphatic.

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5. Triple bond :

(i) C ≡ N is a medium sharp peak near 2250cm-1

(ii) C ≡ C is a weak sharp peak near 2150 cm-1

(iii) Check also for acetylinic C-H near 3300cm-1 , which gives an idea if the triple bond is placed terminally.

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6. Nitrogroup :

Two strong absorption at 1600 - 1500cm-1 and 1390 - 1300cm-1

7. Hydrocarbon

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HYDROCARBONES:

1.ALKANES:

-They yield four stretching and bending vibration of C-H and C-C bonds.

1 .C - C BENDING VIBRATION :

2. C - H STRETCHING VIBRATION

3. CH3 BENDING VIBRATIONS:

4. CH2 BENDING VIBRATION :

ExampleDecane , cyclohexane

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2.ALKENE :

Alkenes show many more peaks than alkanes

(a) = C - H stretch for sp2 C-H occurs at region slightly greater than 3000cm-1

(b) = C - H out of plane (oop) bending occurs in ranges of 1000-650cm-1.

Example:1-hexene, cyclohexene, cis-2-pentene.

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3.ALKYNES:

Terminali alkyne show prominent peak at about 3300cm-1 for SP hybridized C-H.

≡C – H stretching vibration occurs at 3300cm-1

C≡C stretch occurs near 2150cm-1 this is only for terminal alkynes (absent or very weak for non terminal alkynes)

Example:1-octyne,4-octyne

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4.AROMATIC HYDROCOMPOUND:

The C-H stretching for SP2 carbon aapears at value greater than 3000cm-1. since the C-H stretch for alkenes appears in the same range ,it may be difficult to use the C - H stretching bands to differentiate between alkenes and aromatic compounds.

Example: Toluene, o-diethylbenzene ,m-diethylbenzene, p-diethylbenzene, and styrene.

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5.ALCOHOL AND PHENOL:

Alcohol and phenol show strong and broad hydrogen bonded O-H stretching bands centering between 3400cm-1 and 3300cm-1. In solution it will also be possible to observe a free O-H stretching band at 3600cm-1.

Example: 1-hexanol,2-butanol,para cresol.

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6.ETHERS:

Ether show prominent C-O stretching band at 1300 to 1000cm-1.

Absence of C=O and O-H is required to ensure that C-O stretch is not due to an ester and phenol .phenyl alkyl ether gives two strong bands at 1250cm-1 and 1040cm-1.

Example: dibutylether, anisole

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7.ALDEHYDE:

Aldehydes show a very strong band for C=O that appears in the range of 1740 to 1725cm-1 ,for simple aliphatic aldehydes. This band is shifted to lower frequency with conjugation to a C=C or phenyl group .

EXAMPLE : Nonanal , crotonaldehyde , benzaldehyde.

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8.KETONE:

ketone show a very strong band for c=o group that appears in the range of 1720 to 1708cm-1 for simple aliphatic ketones .this bond is shifted to lower frequency with conjugation to a c=c or phenyl group .ring strain moves the absorption to a higher frequency in cyclic ketones .

An overtones is seen at 3430cm-1

Ex:3-methyl-2-butanone,mesityl oxides,acetophenone,cyclopentanone,2,4-pentanedione.

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9.Carboxylic acid:

It shows very strong bands that appears in the range of 1730-1700cm-1 for simple aliphatic carboxylic acids, this band is shifted to lower frequency conjugation to a C=C or phenyl group.

Example : isobutyric acid

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10.ESTER

Esters show a vey strong band for the C=O group that appears in the range of 1750-1735cm-1 for simple aliphatic esters. The C=O band is shifted to lower frequency when it is conjugated to C=C or phenyl group.

Ex: ethyl butyrate, methyl methacrylate, vinyl acetate, methyl benzoate, methyl salicylate.

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13.AMIDES

Amides show a very strong band for the C=O group that appears in the range of 1680 to 1630cm-1.

N-H stretch in primary amides (-NH2) gives two bands near 3350cm-1 (asymmetric), and 3180cm-1 (symmetric) secondary amides (-NH) have one band at 3300cm-1. Tertiary amides will not show an N-H stretch because they do not contain N-H.

EX: Propionamide, N-Methylacetamide

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11.ANHYDRIDES

EX: Propionic anhydride.

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12.AMINES

N-H stretch occurs in the range of 3500-3300cm-1.

Ex: butylamine, dibutylamine, tributylamine, n-methylaniline.

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Referances

1. Pharmaceutical drug analysis Ashutosh kar , New age international publishers page 330-335. Spectroscopy

2. B.K Sharma, Goel Publishing house, page 193-333.

3. Elementary organic spectroscopy .Y.R Sharma.S.Chand and company Ltd, page no 90.

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