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

Hybridization of Carbon Affects n of Absorption

Hexane and Hexene

1-Pentene

Terminal Alkynes are Readily Identifiable with IR

1-Heptyne

Aromatics C-H str (sp2) at 3050-3100 cm-1 C-C str (breathing) 1600 & 1500 cm-1

Toluene

Alcohols O-H str at 3600-3200 cm-1 C-O str at 1050-1150 cm-1

Cyclohexanol

1o Amines

2-Methylpentanediamine

2o Amine

Dipropylamine

Carbonyl Compounds

Cycloheptanone

Acetophenone

Conjugation Lowers n of Absorption

Ketone and Aldehyde

Carboxylic Acid

Hexanoic Acid

Phenyl Acetate, an Ester

Ethyl Butanoate

Methyl Benzoate

Butanamide, an Amide