IR Spectroscopy

Question 1

what is the functional group region of an infrared spectrum (IR)

Answer 1

4000-1400cm^-1

Question 2

what is the fingerprint region of an IR spectrum?

Answer 2

1400-600cm^-1

Question 3

what side (left or right) corresponds with high energy? low energy?

Answer 3

LEFT - high energy
RIGHT - low energy

Question 4

what is IR spectroscopy primarily used for?

Answer 4

primarily used to detect which functional groups are present

Question 5

how does IR absorption work?

Answer 5

IR absorption arises due to the stretching and bonding of covalent bonds in molecules

Question 6

what does 100% transmittance signify?
what does 0% transmittance signify?

Answer 6

100% transmittance = all light shone is transmitted, none absorbed
0% transmittance = none of the light shone is transmitted, all is absorbed

Question 7

what is the functional group (aka diagnostic region)?

Answer 7

determines the functional group present

Question 8

what is the fingerprint region?

Answer 8

used for structure elucidation by spectral comparison

Question 9

describe the relationship between wavenumber and frequency
describe the relationship between wavenumber and wavelength

Answer 9

Wavenumber is proportional to frequency (ν)
Wavenumber is inversely proportional to wavelength (λ)

Question 10

what are the x-axis and y-axis of IR spectrum?

Answer 10

x-axis: wavenumber (cm^-1)
y-axis: percent % transmittance

Question 11

wavenumber for functional group OH (aka alcohols)

Answer 11

3400 wavenumber

Question 12

wavenumber for carbonyl functional group C=O

Answer 12

1700 wavenumber

Question 13

what are the 4 regions of the IR spectrum? give characteristics and wavenumber range

Answer 13

4000-2500: bonds to hydrogen
2500-2000: triple bonds (C triple bonded to C, C triple bonded to N)
2000-1500: double bonds (C=C, C=O, C=N)
1500-400: single bonds (C-C, C-O, C-N, C-X)

Question 14

why do bonds with hydrogen have the highest wavenumber?

Answer 14

lighter atoms have higher frequency

Question 15

why are triple bonds in the higher wavenumber range of 2500-2000?

Answer 15

stronger bonds have higher frequency

Question 16

relationship between wavenumber and energy?

Answer 16

wavenumber increase = energy increase (proportional)

Question 17

the region for single bonds is also the what region?

Answer 17

fingerprint region

Question 18

what corresponds with higher frequency?

Answer 18

lighter atoms and stronger bonds (reason why it goes triple bond, double bond, single bond)

Question 19

amine IR absorptions

Answer 19

N-H stretching: ~3300 broad
1 = doublet 2 = single 3 = none
C-N stretching: ~1000-1250

Question 20

ketones IR absorption

Answer 20

C=O stretching: ~1715

Question 21

aldehydes IR absorption

Answer 21

C=O stretching: ~1725

Question 22

carboxylic acid IR absorption

Answer 22

C=O stretching: ~1700
O-H stretching: ~3000-2500

Question 23

esters IR absorption

Answer 23

C=O stretching: ~1735

Question 24

amides IR absorption

Answer 24

C=O stretching: ~1690

Question 25

acid chloride IR absorption

Answer 25

C=O stretching: ~1800

Question 26

anhydride IR absorption

Answer 26

C=O stretching: ~1820 & 1760 (two peaks)

Question 27

spectroscopically identifiable functional groups?

Answer 27

alkane, alkene, alkyne
nitrile
alkyl halide
ether
alcohol

(with benzene ring)
phenol - benzene + OH
aniline - benzene + NH2

(with C=O group)
aldehyde
ketone
carboxylic acid
ester
amide (primary, secondary, tertiary)
amine (primary, secondary, tertiary)