IR Spectroscopy

IR spectroscopy def

measures molecular vibrations which can be seen as

bond stretching/bending/combos of vibrational modes

IR range in the unit of (cm-1)


max to min

3,500 – 300
give max to min of fingerprint region



(wont be tested but still good to know)

given these frequencies :

2,800& 1,200

match to:



why? (explain the trend)

C-H: 2,800

C-C: 1,200


H bonded to any atom will have a higher frequency.


given these frequencies :

1,645 & 1,200

match to:



why? (explain the trend)

C=C: 1,645

C-C: 1,200


as we add more bonds between carbon atoms the frequency increases.

given these frequencies :

3,080& 1645

match to:



why? (explain the trend)


C=C: 1645

remeber any atom bonded to H will have the larger frequency

given these frequencies :

2,200& 3300

match to:



why? (explain the trend)

C=C: 2,200

C-H: 3300


again the H

Which compound would have the lower frequency in an IR spectrum:

carbonyl vs a carbonyl in conjugation with double bonds?

a carbonyl in conjugation with double bonds b/c the delocization of the pi electrons causes the  C=O bond to lose double bond character, causing to absorb near C-O frequencies (1,000-1250)

give the peak for alcohol


is it (sharp/broad)?


broad peak

compare the frequency of an ether vs alcohol

(OH) alcohol: 3300   

(ROR) ether: 1050


remember H = higher frequency

what is the frequency for the C=O (carbonyl bond)

using 1750 as a base : who will have a frequency above this value and at this value?


aldehyde vs ketone

Above 1750: aldehyde (2,700)


has a Hydrogen & 1 R grp

give an estimate  IR peak for:


carboxylix acid (2)

ketone: 1700-1750 (carbonyl)

c. acid will have 2 peaks: 1700-1750 (carbonyl)

& 3300 (alcohol)

give IR peak for amine

(how does it differ from alcohol)

3100- 3500

(same range as alcohol BUT has a sharp peak)

Will Br2 have show up on an IR?

no, for a compound to a frequency it must experience a dipole moment

(aka an atom is attached to an electronegative atom.)


Br2 is symmetrical (and CCl4) and cant be detected by IR.

give IR peak for C-H
2800-3000 cm-1
give IR peak for C-O
give IR peak for C-C

this bond is too weak for IR (

C2 just like Br2 does not have dipole moment)


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