INFRARED SPECTROSCOPY

Electromagnetic Spectrum

Bond Stretching

High Frequency Stretching Low Frequency Bending

Frequency of Absorption Measured in cm -1 (Wavenumbers)

High Frequency to Low Frequency

General Trends: i) Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.) ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms. iii) Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds. (Except for bonds to hydrogen).

IR Spectrum of Octane

Decane

2,2,4-Trimethylpentane

Hybridization of Carbon Affects  of Absorption

Hexane and Hexene

1-Pentene

1-Dodecene

2-Methyl-1,3-butadiene (Isoprene)

Terminal Alkynes are Readily Identifiable with IR

1-Heptyne

Aromatics C-H str (sp 2 ) at cm-1 C-C str (breathing) 1600 & 1500 cm -1

Toluene

o-Xylene

Alcohols O-H str at cm -1 C-O str at cm -1

Cyclohexanol

O-H Stretch

2-Methyl-1-butanol

Benzyl alcohol

1 o Amines

2-Methylpentanediamine

2 o Amine

Dipropylamine

Carbonyl Compounds

Acetone

Cycloheptanone

Acetophenone

Conjugation Lowers of Absorption

Ketone and Aldehyde

Octanal

Carboxylic Acid

Hexanoic Acid

Phenyl Acetate, an Ester

Ethyl Butanoate

Methyl Benzoate

Benzoic Anhydride, an Acid Anhydride

Butanamide, an Amide