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DAT > SNS - Organic Chemistry - Spectroscopy > Flashcards

SNS - Organic Chemistry - Spectroscopy Flashcards

1
Q

Spectroscopy

A

Measures energy differences betwwen the possible states of a molecular system by determining the frequencies of elecromagnetic radioation (light) absorbed by the molecules

The possible ststes are quantized energy levels associated with different types of molecular motion, including molecular rotation, vibration of bonds and electron movement

Different types of spectroscopy measure different types of molecular motion

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2
Q

Infrared

A
  • Measures molecular vibrations including bond stretching, bending and rotation and can be symmetric or asymmetric
  • A typical spectrum is obtained by passing IR light of frequencies 4000-400 cm-1 through a sample and recording absorbtion pattern.
  • % transmittance (1/absorption) is plotted against frequency
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3
Q

Infrared

Vibrations and Regions

A
  • Useful absorptions of IR light occur in the 3000-30,000 nm region corresponding to wavenumbers 3,500-300 cm-1
  • Bond stretching involves the highest change in energy and is observed in the region 1500-4000 cm-1
  • Bending vibrations are observed at 400-1500cm-1
  • More complex combinations caudes by the motion of the whole molecule are seen at 1,500-400 cm-1 - the fingerprint region - used to identify compounds
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4
Q

Infrared

Recordable Absorptions

A
  • For an absorbtion to be recorded, it must involve a change in bond dipole moment
  • Molecules comprised of atoms with the same electronegativity or symmetrical molecules therefore won’t experience a changing dipole moment and don’t exhibit absorption
  • For example, O2 and Br2 don’t absorb, HCl and CO do
  • Symmetrical bond stretches don’t show up in spectra as they involve no change in bond dipole moment
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5
Q

Infrared

Application

A
  • Most of the useful functional group information is found between 1400 and 4000 cm-1
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6
Q

Infrared

Absorption Peaks

Alkanes

A
  • C-H = 2800-3000
  • C-C = 1200
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7
Q

Infrared

Absorption Peaks

Alkenes

A
  • =C-H = 3080 - 3140
  • C=C = 1645
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8
Q

Infrared

Absorption Peaks

Alkynes

A
  • ≡C-H = 3080-3140
  • C≡C = 1645
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9
Q

Infrared

Absorption Peaks

Aromatic

A
  • C-H = 2900-3100
  • C-C = 1475-1625
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10
Q

Infrared

Absorption Peaks

Alcohols

A
  • O-H = 3100-3500
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11
Q

Infrared

Absorption Peaks

Ethers

A
  • C-O = 1050-1150
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12
Q

Infrared

Absorption Peaks

Aldehydes

A
  • (O)C-H - 2700-2900
  • C=O - 1725-1750
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13
Q

Infrared

Absorption Peaks

Ketones

A
  • C=O - 1700-1750
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14
Q

Infrared

Absorption Peaks

Carboxylic Acids

A
  • C=O - 1700-1750
  • O-H = 2900-3300
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15
Q

Infrared

Absorption Peaks

Amines

A
  • N-H - 3100-3500
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16
Q

NMR

A
  • Certain nuclei have magnetic moments that are normally oriented at random
  • When such nuclei are placed in a magnetic field the magnetic moments tend to align either with (α state, lowest energy) or against (β state, highest energy) the direction of the applied field
  • If the nuclei are then irradiated with EM radiation, some will be excited into the α state. The absorption corresponding to this excitation occurs at different frequencies depending on an atom’s environment - nearby atoms also possess magnetic moments
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17
Q

NMR

1H NMR

A
  • Most 1H nuclei come into resonance between 0 and 10δ downstream of TMS (reference peak, designated t=0)
  • Protons attached to different groups have different magnetic environments and thus resonate at different frequencies.
  • Protons attached to the same group are magnetically equivalent due to rotation and resonate at the same frequency
  • Each distinct set of nuclei gives rise to a separate peak
18
Q

NMR

1H NMR

Position of Peaks

A
  • The position of a peak due to shielding or deshielding effects reflects chemical environment
  • Electron clouds usually screen the nuclei slightly from the magnetic field, electron withdrawing groups decrease this effect (deshield) and thus the nucleus resonates in a lower field than would otherwise
  • Electron donating groups enhance the effect (shield), causing it to resonate at a higher field
19
Q

NMR

1H NMR

Size of Peaks

A

The relative area of peaks represent the ratio of types of protons

20
Q

NMR

1H NMR

Splitting

A
  • Represents the numbe of adjacent hydrogens
  • Peaks are split into n+1 where n = the number of adjacent hydrogens
21
Q

1H NMR

Chemical Shifts

Order:

-

A
  1. RCH3
  2. -NH2
  3. -CHOH-CH2OH
  4. RCH2
  5. R3CH
  6. RCHCO-
  7. -CHCOOH / -CHCOOR
  8. -C≡CH
  9. -CHX
  10. -CHOH / -CHOR
  11. ArOH
  12. -CH≡CH
  13. Ar-H
  14. RCHO
  15. -COOH
22
Q

1H NMR

Chemical Shifts

RCH3

A

0.9

23
Q

1H NMR

Chemical Shifts

-C≡CH

A

2-3

24
Q

1H NMR

CHemical Shifts

RCH2

A

1.25

25
Q

1H NMR

CHemical Shifts

R3CH

A

1.5

26
Q

1H NMR

CHemical Shifts

-CH=CH

A

4.6-6

27
Q

1H NMR

CHemical Shifts

Ar-H

A

6-8.5

28
Q

1H NMR

CHemical Shifts

-CHX

A

2-4.5

29
Q

1H NMR

CHemical Shifts

-CHOH / -CHOR

A

3.4-4

30
Q

1H NMR

CHemical Shifts

RCHO

A

9-10

31
Q

1H NMR

CHemical Shifts

RCHCO-

A

2-2.5

32
Q

1H NMR

CHemical Shifts

-CHCOOH / -CHCOOR

A

2-2.6

33
Q

1H NMR

CHemical Shifts

-CHPH-CH2OH

A

1-5.5

34
Q

1H NMR

CHemical Shifts

ArOH

A

4-12

35
Q

1H NMR

CHemical Shifts

-COOH

A

10.5-12

36
Q

1H NMR

CHemical Shifts

-NH2

A

1-5

37
Q

NMR

13C NMR

A
  • Very similar to 1H NMR except occur 0-200δ downstream of TMS and only 1.1% C is 13C. Means that a much larger sample is needed (~50x) and coupling (splitting) between carbon atoms not observed
  • Coupling is still observed between carbon atoms and the protons directly attached to them, for example if a C is attached to two H, signal is split into a triplet
  • Can also record a spectrum without coupling of adjacent protons - spin decoupling
38
Q

Spectroscopy

UV

A
  • Obtained by passing UV light through a chemical sample (usually dissolved in an inert nonabsorbing solvent)
  • Absorbance plotted against wavelength
  • Wavelength of maximum absorbance provides info on the extent of the conjugated system and other structural or compositional info
39
Q

Spectroscopy

Mass

A
  • Not true spectroscopy - no absprbtion of EM radiation involved and is destructive
  • Most commonly, high speed beam of electrons used to ionize the sample. Then placed into a particle accelerator with magnetic field to deflect the accelerated cationic fragments (only cationic gragments show up on the spectrum). A detector records the number of particles of each mass exiting the deflector area
40
Q

Spectroscopy

Mass

Spectrum

A
  • Spectrum plots mass/charge ratio against relative abundance
  • The tallest peak belongs to the most common ion - base peak - and is assigned the relative abundance value of 100%
  • The peak with the highest m/e ratio is generally the molecular ion peak, M+ from which the molecular weight can be obtained
  • Fragmentation patterns provide indications of structure

DAT (53 decks)

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