Organic Chemistry 1

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Organic Chemistry 1

• Autumn 2004
• Problem Set 8 Solutions

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Which of each of the following pairs of compounds
is the more nucleophilic?

• NH3 or NH2-
• NH2-
• Charged species are almost always more
  nucleophilic than neutral ones

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• CH3CH2OH or PhOH
• CH3CH2OH
• Because with PhOH the lone pair on oxygen
  interactions with the p-system of the benzene
  ring, and is hence less available to interact
  with an electrophile

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• (CH3)3B or (CH3)3N
• (CH3)3N
• Because N has a lone pair of electrons, and B
  does not

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• HO- or HS-
• HS-
• Because sulfur is larger and more polarizable
  than oxygen

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• Predict the product(s) of each of the following
  reactions, paying attention to the
  stereochemistry where appropriate.

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SOCl2 - thionyl chloride

• In ether HCl is not dissociated, so there is no
  free Cl
• So
• This process gives an unusual retention of
  stereochemistry - the Cl- goes straight back in,
  before the cation has an opportunity to rotate.
  the mechanism is sometimes called SNi SOCl2 -
  thionyl chloride

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• So
• This process gives an unusual retention of
  stereochemistry - the Cl- goes straight back in,
  before the cation has an opportunity to rotate.
  the mechanism is sometimes called SNi

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• However, we can change the stereochemical
  outcome, by changing the solvent to pyridine. In
  pyridine

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• Now we get a regular SN2 process, with inversion
  of stereochemistry.
• In either case the reaction is driven by the
  release of a mole of gaseous SO2

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• Predict the order of reactivity of the following
  compounds
• (a) in an SN2 reaction with EtO-/EtOH
• (b) in an SN1 solvolysis reaction with HCOOH as
  the solvent

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• Explain the following observations
• The rate of disappearance of Me3CCl in aqueous
  solution is unaffected by the presence of NaN3,
  sodium azide, in the solution, but the products
  include both the azide and the alcohol.
• SN1 reaction. Me3CCl goes to Me3C in RDS, and
  this process occurs at a rate which is not
  dependent on what else is present in solution.
  However, the carbocation, once formed, may be
  captured by any available nucleophile

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• When Me3CBr reacts with water at pH 8 to give
  Me3COH, the rate is proportional to Me3CBr, but
  for the reaction of Ph3CBr under the same
  conditions, the rate is proportional to
  Ph3CBrHO-.

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• The reaction of Me3CBr is SN1, with slow
  formation of Me3C in the RDS.
• The cation is then rapidly captured by water.
• Despite the apparent kinetics, the reaction of
  Ph3CBr cannot really be SN2.
• This time the cation, Ph3C, is so stable that it
  is formed rather readily, but is reluctant to
  react.
• Thus the RDS is now the capture of the cation by
  the nucleophile - hence the nucleophile is in the
  rate equation

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• The rate of hydrolysis of mustard gas, E, is
  first order in mustard gas, and independent of
  HO-.
• Suggest a mechanism to account for these kinetic
  data.
• Remember that primary carbocations do not usually
  form, even in very ionising solvents, and that S
  has lone pairs of electrons

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• Despite the kinetics, this cannot be an SN1
  reaction.
• However, HO- is not involved in the RDS.
• This means that the RDS is most likely to be
  intramolecular

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• Propose a mechanism for the following reactions-

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Neighboring group participation
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