WM4 Instrumental analysis

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WM4 Instrumental analysis
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The 3 key instrumental techniques

• How do we know that salicylic acid contains OH
  and COOH groups?
• Mass spectroscopy (m.s.).
• Infrared (i.r.) spectroscopy.
• Nuclear magnetic resonance (n.m.r.) spectroscopy.

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Making use of infrared spectroscopy

• Any unidentified, new substance has its i.r.
  spectrum recorded.

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6.4 Infrared spectroscopy

• Energy in molecules is quantised.
• In i.r., molecules are exposed to radiation
  between 1014Hz 1013Hz (wavelengths 2.5µm -15
  µm). Remember c ? v
• Bonds vibrate and stretch (pull apart then push
  together again) as they absorb energy.

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Vibrational changes of CO2
Asymmetric stretch
Symmetric stretch
bending
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Infrared spectra signals stretches

• An IR spectrum hangs down from a baseline (100
  transmittance no absorbtion).
• The signals (look like icicles) on an IR
  spectra correspond to bonds absorbing a packet of
  energy and vibrating more.
• The OH and CO bonds in salicylic acid absorb
  energy at specific wavelengths (?)/µm and so
  wavenumbers (1/ ?)/ cm-1.

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Generally, particular bonds can be matched to a
particular absorption region.
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Interpreting spectra 2-ethylbut-1-ene
?
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Interpreting spectra propanone
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Regions in the IR spectrum where typical
absorptions occur
Label these regions on the previous two examples
sketch the structures and link them to the main
signals. Do this for the following examples, too.
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Examples of infrared spectra

• Butane
• Strong absorption at 2970 cm-1 characteristic of
  C-H stretching in aliphatic compounds.
• No indication of any functional groups.

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Examples of infrared spectra

• Methylbenzene
• 2 types of C-H absorption 3000 cm-1 (above C-H
  on benzene below C-H on methyl group).
• No indication of any functional groups.
• Absorption pattern 700 cm-1 is typical of a
  benzene ring with a substituted group.

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Examples of infrared spectra

• Benzoic acid
• A sharp absorption at 3580 cm-1 is due to O-H
  bond.
• A strong absorption at 1760 cm-1 is the CO
  group.
• Position of C-H absorption suggests an aromatic
  compound.

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Summary of IR spectroscopy

• An IR spectrum measures the extent to which
  electromagnetic radiation is transmitted through
  a sample of substance.
• Frequency ranges absorbed give clues about
  functional groups which are present.
• IR spectrum of salicylic acid gives evidence of
  CO and OH groups.

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Evidence from nuclear magnetic resonance (n.m.r.)
spectroscopy.

• This technique helps to determine structure, as
  it investigates the different environments in
  which (hydrogen) nuclei are situated. The n.m.r.
  spectrum for salicylic acid shows signals for the
  different environments of the 6 protons
• One proton in a COOH environment.
• One proton in a phenolic OH environment.
• Four protons attached to a benzene ring.

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n.m.r. spectrum for salicylic acid
Absorption
12 11
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The evidence so far.

• A combination of i.r. and n.m.r. spectroscopy
  shows that salicylic acid has an OH group and
  COOH group both attached to a benzene ring we
  can now rename it HYDROXYBENZOIC ACID.
• However, it could be one of 3 possible isomers
  2-hydroxybenzoic acid, 3-hydroxybenzoic acid and
  4-hydroxybenzoic acid.
• Mass spectroscopy can determine which isomer we
  have.

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The mass spectrum of salicylic acid

• Signals correspond to positively charged ions
  formed from the parent compound, and fragment
  ions.

The fragmentation pattern is characteristic of a
particular compoundthe fragment at 120 can only
come from 2-hydroxybenzoic acidcan you see why?
Parent or molecular ion
mass
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Fragmentation

• Positive ions in a mass spectrometer can break
  down into building blocks.
• Example 2-ethoxybutane.

CH3-CH2-CH-O-CH2CH3 (M87)
?
Loss of CH3
?

Loss and rearrangement of CH3CHCHCH3

?
CH3-CH2-CH-O-CH2CH3 CH3 (M102)
CH3-CH2-CH (M87) CH3

HO-CH-CH3 (M45)
Loss of CH3-CH2
?
?

CH3-CH2 (M29)
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Positively charged fragments form.Mass
difference suggests functional groups.

• For each fragmentation, one product has a
  positive charge
• M A B
• A B
• The most stable ion usually forms.

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Isotope peaks heights are in the same ratio of
abundance for particular elements.
Pairs of peaks correspond to isotopes of 35Cl and
37Cl in the ratio of 7525 ie. 31. Highlight
these.
mass
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Now its over to you!

• Do activity WM4 use accurate Mr values, isotope
  peaks and a database to lead you to the formula
  of salicylic acid.
• It shows you how chemists use fragmentation
  patterns to deduce or confirm a molecular
  structure.
• Do assignments 1 and 2 C.S. p110-111
• Do Problems for 6.5 on mass spectrometry, C.I.,
  p145-146.