che 321: advanced synthesis

1
che 321 advanced synthesis
2
Welcome

• Syllabus
• expectations
• experiments
• reports/assignments
• http//www.albion.edu/chemistry/abbethune/321_S08
  /default321.asp

3
Spectroscopy

• Need to be able to confirm synthesis, purity
• Absorption spectroscopy
• IR
• Observes vibrations of bonds and provides
  evidence of functional groups present
• MS
• Bombard molecules with electrons and break them
  apart
• Analysis of fragments gives MM, possibly
  molecular formula, and clues to structure and
  functional groups
• NMR
• Observe chemical environment of H atoms (or C, F,
  P, etc)
• Provides evidence for structure of molecule,
  functional groups
• UV
• Observe electronic transitions
• Provides info on bonding

4
Nuclear Magnetic Resonance Spectroscopy

• All nuclei have charge
• In some nuclei, this charge seems to spin on
  the nuclear axis
• Any nucleus with
• Odd mass
• Odd atomic number
• Or both
• Ex 1H, 13C, 19F (but not 12C or 16O)

5
NMR, cont

• For each nucleus with spin,
• Number of allowed possible spin states is
  quantized
• Determined by nuclear spin QN I
• I is a physical constant for each nucleus
• A nucleus will have 2I 1 allowed spin states
• Ex for 1H, I ½ and of spin states2
• For 17O, I5/2 and spin states 6
• These states are degenerate in the absence of an
  applied magnetic field (equally populated)

6
NMR, cont.

• In the presence of an applied magnetic field,
  spin states are no longer degenerate
• Spin on nucleus generates a magnetic field of its
  own

http//upload.wikimedia.org/wikibooks/en/5/5d/Spin
ningProtonMagnet.gif
7
Aligned and opposed spins
http//www.cem.msu.edu/reusch/VirtualText/Spectrp
y/nmr/Images/nucspin2.gif
8
Resonance

• In applied magnetic field, nucleus precesses
  about its own axis
• with a particular frequency (Larmor)
• Like wobble of spinning top
• Since the nucleus has a charge, this generates an
  oscillating electric field of same frequency
• If rf waves of this frequency are supplied, the
  energy can be absorbed

9
Resonance

• hn hw
• Energy of incident radiation equal to energy of
  nuclear precession
• When frequency of oscillating electric field
  component of incoming radiation (hn) equals
  electric field generated by precessing nuclei
  (hw), the two fields can couple
• energy can be transferred (causing spin change)

10
NMR instrumentation

• Continuous wave (CW)
• Scans the entire frequency range, exciting nuclei
  one at a time
• Pulse
• Short burst of energy that excites all of the
  magnetic nuclei in the molecule simultaneously
  (range of frequencies)

11
Pulse experiments

• When pulse is discontinued, excited nuclei
  relax
• Each nucleus emits EMR as it relaxes
• Simultaneously emittedfree-induction decay (FID)
  signal
• Fourier transform (mathematical method) used to
  render FID (amplitude v. time) to typical
  spectrum (amplitude v. frequency)

http//www.chemistry.nmsu.edu/Instrumentation/fids
pec.gif
12
Chemical shift (d)

• The peak or chemical shift in an NMR spectrum
  is usually given in ppm
• ppm chem shift of peak in Hz frequency
  of spectrometer in MHz
• More useful than Hz, since it does not change
  with instrument strength

13
Chemical equivalence

• Protons in chemically identical environments are
  usually chemically equivalent
• Same chemical shift
• Use symmetry
• May or may not be magnetically equivalent
  (especially locked systems)

14
Chemical environment and d

• Electronegativity effects
• Chemical shift of proton increases (shifts
  downfield) as c of attached element increases
• Hybridization effects
• sp3
• 0-2 ppm
• d increases strained ring lt 1lt2lt3
• sp2
• 4.5-7 ppm
• C atom more EN due to increased s character
• Anisotropy also responsible

15
Chemical environment and d

• Magnetic anisotropy
• Shift in ppm due to presence of unsaturated
  (p-electron) system in vicinity of proton
• Ring current
• Present in all systems with p electrons
• Characteristic shapes and directions
• sp
• 2-3 ppm due to anisotropy
• On basis of hybridization alone, would expect
  downfield of sp2!

16
Chemical environment and d

• Acidic protons
• Carboxylic acid proton very deshielded (10-12
  ppm)
• Due to resonance and EN
• Exchangeable protons
• Protons that are capable of H-bonding (-OH, -NH)
  can have extremely variable ppm
• More H-bonding more deshielded
• Function of concentration and temp

17
Integration

• Area under chemical shift
• proportional to number of chemically equivalent
  protons giving rise to the peak

18
Spin-spin splitting (coupling)

• Each type of proton senses the number of
  equivalent protons (n) on adjacent carbons
• Spin of H on adjacent carbon is sensed
• Its resonance is split into n1 peaks
• Corresponds to number of possible spin
  arrangements

19
Pascals triangle

• Gives peak height for first-order splitting
  between magnetically

Peak type neighbors Singlet 0 Doublet
1 Triplet 2 Quartet 3 Quintet 4
20
Typical splitting patterns

• X-CH-CH-Y X ? Y
• -CH2-CH-
• X-CH2-CH2-Y X ? Y
• CH3-CH-
• CH3-CH2
• CH3
• CH-
• CH3
• CH3-CH2-CH2-
• CH3-CH2-CH2-CH2-