Organic Chemistry Lab IR Spectroscopy

Alkanes (C-H) stretching bonds 

2850 - 2960, <3000, often overlapping 

Alkanes (C-H) bending bonds 

at 1465 or 1375 (not reliable)

Alkene (=C-H) stretching bonds 

>3000 (3010 - 3095)

Alkene (C=C) nonsymmetric stretching bonds

sharp and long 1600 - 1660, conjugation shifts red by resonance

Alkyne (C-H) stretching bonds 

3250 - 3350 

Alkyne (C-C) nonsymmetric stretching bond

sharp 2150 

Red Shift

More p character, longer bond, Low energy vibration, Lower frequency

Blue shift

Less p character, shorter bond, High energy vibrations, Higher frequency

Nitriles (C-N) stretching bonds

sharp 2250, conjugation shifts red by resonance 

Aromatic Compounds (C-H) stretch

>3000 (has a few overtones) 

Aromatic (C-C) streching bonds

In pairs at 1600 -1475, conjugation shifts red a lot by resonance (has a few overtones) 

Alcohols and Phenols (O-H) stretching 

(No H-bond sharp at 3600-3650) (Yes H-bond broad with rounded tip at 3300-3400) Samples diluted or in water show stronger O-H stretches  

Alcohols and Phenols (C-O) stretching 

1000 - 1260 (not reliable)

Amines (N-H) stretching bond

(Primary amine broad with 2 sharp spike, Secondary amine is broad with sharp spike, Tertiary amine has no hydrogen) 3300-3500

Carbonyl compounds

Resonance with (C-O bond causes more single bond character = red shift)

Aldehydes (C=O) stretching bonds

strong absorbance in 1725, (conjugation causes red shift (C-C at 1640) (C=O at 1680 -1700)) 

Aldehyde (O=C-H) stretching bonds 

Fermi pair at 2750 and 2850, causes overtone at 3500 - 3600

Ketones (C=O) stretching bonds

Strong absorbances at 1710 - 1720, weak overtone at 3400, conjugation shifts red (C-C stretch at 1620-1645) (C=O stretch at 1675 - 1700) 

Carboxylic Acids (C=O) stretching bonds

Strong absorbance at 1700 1730

Carboxylic Acids (O-H) stretching bonds

Very broad from 2400-3400, often overlapping other absorbances

Esters (C=O) stretching bonds

1735 - 1750, conjugation causes red shift at 1715 - 1740, conjugation with aromatic ring (C=C stretch 1500 - 1600)

Amides (C=O) stretching bonds

1680 - 1630, <1700

Amides (N-H) stretching bonds

Primary amides have bands at 3350 and 3180, Secondary amides have one band at 3300, and Tertiary amides have none.

Amides (N-H) bending bonds

1640 - 1500 for primary and secondary amides

Nitro group 

3 atoms and 2 bonds, 1563 asymmetric and 1383 symmetric, conjugation causes red shift