Organic Chemistry 1

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Organic Chemistry 1

• Autumn 2004
• Problem Set 4 - Solutions

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• C5H10O, DBE  1
• IR stretching frequency of 1715 cm-1 indicates
  carbonyl group, almost certainly an aldehyde or
  ketone.
• This suggests just 4 plausible structures

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• M  86
• m/z  71 indicates M-CH3, which implies that we
  have CH3C(O)-
• This allows us to rule out pentanal or
  3-pentanone as possible structures, and leaves us
  with

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• m/z   58, implies loss of 28 mass units, ethene.
  This arises from what is called a McLafferty
  rearrangement.

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• This means that our molecule must be 2-pentanone
• It fragments next to the carbonyl bond, so that
  the positive charge is on the fragment containing
  the CO

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• Question 2
• This also depends on the fragmentations in
  ketones being next to the carbonyl group.
• So R1C(O)R2 gives fragments R1CO and R2CO.
• These in turn lose CO to give R1 and R2

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• A ? PhCO ? Ph
• 105 77
• A is PhCOCH2CH3
•  
• B ? PhCH2CO ? PhCH2 ? Ph
• 119 91 77
• B is PhCH2COCH3

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These are more examples of the McLafferty
rearrangement
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• Probability of-
• CH2Cl35Cl35  3/4 x 3/4  9/16 mass 84
• CH2Cl35Cl37  3/4 x 1/4  3/16 mass 86
• CH2Cl37Cl35  1/4 x 3/4  3/16 mass 86
• CH2Cl37Cl37  1/4 x 1/4  1/16 mass 88
• Ratio M  848688  961

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• Probability of -
• CH2Br79Cl35  1/2 x 3/4  3/8 mass 128
• CH2Br81Cl35  1/2 x 3/4  3/8 mass 130
• CH2Br79Cl37  1/2 x 1/4  1/8 mass 130
• CH2Br81Cl37  1/2 x 1/4  1/8 mass 132
• M  128130132  341

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• Cyclohexanone, normal unstrained carbonyl,
  1720 cm-1
• Cyclohexene, normal alkene, 1640 cm-1
• The low dipole moment means that as for most
  symmetric alkenes, this is a weak band

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In cyclohexenone, however, we must consider all
relevant resonance forms

• Additional resonance form gives weaker carbonyl
  and weaker carbon-carbon double bond, therefore
  lower frequency for both. In the enone the
  polarity of the double bond is also greater, so a
  bigger change in dipole on stretching, and hence
  the absorption is stronger.

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• 6a
• Any alkyne or nitrile will be OK, RC?N, RC?CR
• 6b
• Any enone, e.g.

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• 6c
• The sp C-H stretch at 3300 cm-1 is the most
  distinctive feature of the alkyne.
• C?C and C?N can often only be distinguished by
  their intensities, and this is always a bit
  chancy
• Mass spectrometry - M for PhC?CH is 102, M for
  PhCN is 103.

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• 6d
• Anhydrides show two bands because the two
  carbonyl vibrations are coupled, and they can
  vibrate either in phase or out of phase - these
  are called the symmetric and antisymmetric
  stretches.

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• Compound E behaves as an aminoketone - the amide
  resonance is impossible as the doubly bonded
  structure would be too strained.

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• In F the lone pair on nitrogen is involved in
  amide resonance, and is therefore unavailable for
  protonation. The strong contribution from the
  amide resonance form weakens the carbon-oxygen
  double bond in F

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• Disappearance of the carbonyl stretch at
  1690 cm-1, appearance of OH peak at 3500 cm-1.

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• (CH3)3N and CH3CH2NHCH3
• Only CH3CH2NHCH3 will show N-H band at 3300 cm-1
• CH3CH2C(O)CH3 and CH3CHCHCH2OH
• CH3CH2C(O)CH3 shows CO at 1715 cm-1
• CH3CHCHCH2OH shows OH at 3500 cm-1

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• CH2CHOCH3 and CH3CH2CHO
• Only CH3CH2CHO shows CO at 1725 cm-1
• CH3CH2CH2COOH and CH3CH2COOCH3
• Only CH3CH2CH2COOH shows an OH peak, broad,
  approx. 3000 cm-1

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• CH3OCH2CH2C(O)CH3 and CH3CH2CH2COOCH3
• CH3OCH2CH2C(O)CH3, ketone carbonyl, 1715 cm-1
• CH3CH2CH2COOCH3, ester carbonyl, 1740 cm-1

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• Endocyclic nature of double bonds increases ?max