10.5 Halogenalkanes (Haloalkanes)

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10.5 Halogenalkanes (Haloalkanes)

• 10.5.1
• Describe, using equations, the substitution
  reactions of halogenoalkanes with sodium
  hydroxide.
• 10.5.2
• Explain the substitution reactions of
  halogenoalkanes with sodium hydroxide in terms of
  SN1 and SN2 mechanisms.

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Haloalkanes

• CnH2n1X (X halogen)
• Saturated compound, so are involved with
  substitution reactions
• Polar molecule, so entirely different mechanism
  for substitution.

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Polarity

• The halogen is much more electronegative than the
  carbon so it pulls the electrons in the bond
  closer to itself, gaining a partially negative
  charge, making the carbon it is bonded to
  partially positive (or electron deficient)

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Substitution Reactions

• In a substitution reaction, one atom or group of
  atoms, takes the place of another in a molecule
• Example
• (CH3)3CCl NaOH ? (CH3)3 COH NaCl

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Nucleophilic Substitution (SN)

• A nucleophile is a molecule or ion that has a
  high electron density, so its attracted to atoms
  in molecules with a lower electron density.
• It may replace another group in an organic
  molecule.
• They are attracted to the electron deficient
  carbon in the haloalkane
• Typical nucleophiles H2O, OH-, NH3, CN-

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• During reaction, the C-X bond breaks, and the
  halide (ion) is released
• Both electrons from the bond go to the halide
  (heterolytic fission)
• The group that the nucleophile replaces is called
  the leaving group

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Nucleophilic Substitution

• One covalent bond is broken as a new covalent
  bond is formed
• The general form for the reaction is
• Nu- R-X ? R-Nu X-

Nucleophile haloalkane ? Substituted
Product Leaving group/halide
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Nucleophilic Substitution

• The bond to the leaving group (C-X) is broken
• The leaving group takes both electrons that
  formed the bond with it (X-)
• The nucleophile provides the electrons to form
  the new bond ( OH-)

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Nucleophilic Substitution

• Haloalkanes commonly undergo nucleolophilic
  substitution reactions. The nucleophile
  displaces the halide leaving group from the
  haloalkane.
• There are two common mechanisms (detailed
  pathway/series of steps) for nucleophilic
  substitutions to occur. They are known as SN1
  and SN2.

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Nucleophilic Substitution Bimolecular or SN2

• A reaction is bimolecular when the rate depends
  on both the concentration of the haloalkane and
  the nucleophile.
• SN2 mechanisms occur most readily with methyl
  compounds and primary haloalkanes

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SN2 Mechanism

The general form for an SN2 mechanism is shown
above. Nu- nucleophile
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IB wants you to use curly arrows to show the
movement of electrons
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Example of SN2 bromoethane reacting with NaOH to
form ethanol and bromide. Note NaOH, only use
OH-
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Nucleophilic Substitution Unimolecular or SN1

• A unimolecular reaction occurs when the rate of
  reaction depends on the concentration of the
  haloalkane but not the nucleophile.
• A unimolecular reaction is a two step process
  since the haloalkane and the nucleophile cannot
  both appear in the rate determining step
• SN1 mechanisms occur most readily with tertiary
  haloalkanes and some secondary haloalkanes.

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SN1 Mechanism

The general form for an SN1 mechanism is shown
above. Nu- nucleophile
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SN1 Mechanism

The first step is the formation of the
carbocation. It is the slow step due to steric
hindrance (bulky groups around the carbon make it
difficult for the incoming OH- to attack the
carbon
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Resources

• Pearson Baccalaureate HL chemistry text, Brown
  and Ford
• Organic mechanisms powerpoint by L Scheffler,
  Lincoln High School
• http//www.chemguide.co.uk/mechanisms/nucsub/hydro
  xide.htmltop