Halogen Compound

1
Halogen Compound
RX Alkyl halide
2
A. Preparation

• 1. Halogenation

3
A. Preparation

• 2. With HX

HX is come from NaX conc. H2SO4
4
A. Preparation

• 3. From alcohol

5
A. Preparation

• 3. From alcohol cont.
• We rarely start with PCl3, PBr3 or PI3, since
  they are easily hydrolysed by moisture in air.
  Instead, they are made in situ, using red
  phosphorus in the alcohol and halogen is put into
  the reaction flask as the reaction proceeds.

6
A. Preparation

• 3. From alcohol cont.

ROH HX ? RX H2O
(usually HBr, HI BUT RCl cannot be prepared by
this method)
Pyridine (as solvent)
ROH SOCl2 ? RCl SO2
HCl
(This product is most easily purified)
7
A. Preparation

• 3. From alcohol cont.

For aryl halide
8
A. Preparation

• 4. Diazonium coupling (for aryl halide only)
• Formation of a diazonium salt

9
A. Preparation

• 4. Diazonium coupling cont.

From diazonium salt, you can make the following
aryl halide
10
A. Preparation

• 4. Diazonium coupling cont.

From diazonium salt, you can make the following
aryl halide
11
B. Physical properties

• It has a little higher boiling point than
  corresponding alkane of comparable molecular
  mass. This is due to the dipole-dipole
  attraction between the molecules as they are
  polar.
• CH3Cl, CH3Br and C2H5Cl are gases in room
  temperature while other members are liquids.
  Chlorobenzene is colourless liquid. All alkyl and
  aryl halides are insoluble in water due to the
  inability to form extensive H-bond with water
  molecules.

12
C. Chemical Properties

• Nucleophilic Substitution
• Halogen compounds are polar compounds. The
  electron deficient carbon attach to the halogen
  is susceptible to the attack of an electron rich
  species (nucleophile) and undergo nucleophilic
  substitution.

13
C. Chemical Properties

• Nucleophilic Substitution cont.
• Alkyl halide is very reactive because the C-X
  bond is very polar while in aryl halide, the
  reaction proceeds with difficulty. It is because
  the p-orbital of X overlap with the pi-electron
  of the ring, causing a delocalization of
  electron. The C-X bond is thus less polar and
  stronger (possess double bond character).

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C. Chemical Properties
15
Chemical properties

• Lateral overlapping of the p-orbital of X with
  the pi molecular orbital of benzene
  delocalization of electron throughout the whole
  structure

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C. Chemical Properties

• a. Hydrolysis

Side product alkene (From dehydrohalogenation)
Do you remember this reaction?
17
C. Chemical Properties

• a. Hydrolysis cont.
• Side product alkene (From dehydrohalogenation)

18
C. Chemical Properties

• a. Hydrolysis cont.
• For phenol industrial process
• (not important)

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C. Chemical Properties

• Mechanism - Bimolecular Nucleophilic Substitution
  SN2

Press
Transition state (trigonal bipyramidal)
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C. Chemical Properties

• Bimolecular
• Molecularity refers to the number of species
  that are undergoing bond-making and / or
  bond-breaking process in the rate determining
  step.
• Rate k alkyl halide1 OH-1
• ? Second order reaction

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C. Chemical Properties
Transition state
22
C. Chemical Properties

• Mechanism - Unimolecular Nucleophilic
  Substitution SN1

Press
23
C. Chemical Properties

• Unimolecular
• In rate determinating step, only involve one
  molecule
• Rate k alkyl halide1 OH-0

24
C. Chemical Properties
intermediate
25
C. Chemical Properties

• Stability of carbonium ion

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C. Chemical Properties - Factors affecting choice
of mechanism

• Structure of alkyl halide

3ry 2ry 1ry CH3
Use of 3ry alkyl halide favour SN1 since

• Alkyl group is electron-donating which helps
• to stablilise the carbonium ion, thus lower the
  EA.

27
C. Chemical Properties Factors affecting choice
of mechanism

• Use of 3ry alkyl halide favour SN1 since
• Alkyl groups hinder the approach of a nucleophile
• (OR steric crowding at T.S. would destabilise a
  bimolecular transition state, thus increase the
  EA.)

is less stable than
Favour SN2
Not favour SN2
(Why not consider the steric crowding at
carbonium ion?)
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C. Chemical Properties Factors affecting choice
of mechanism

• Solvent
• Highly polar (ionising) solvent favour SN1
  (because forming ion in 1st step)
• Polar solvent aqueous, THF
• Less polar solvent alcoholic

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C. Chemical Properties Factors affecting choice
of mechanism

• Choice of nucleophile
• Strong nucleophile in high conc. favour SN2
  while weak nucleophile in dilute solution favour
  SN1.
• Strong nucleophile Weak nucleophile
• OH- H2O
• NH2- NH3
• CN- HCN
• RO- ROH
• 4. Presence of Ag ion favour SN1

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C. Chemical Properties

• Effect of halogen
• Since the electronagativity of halogen decreased
  down the group, C-Cl bond is more polar than the
  others. Hence, the carbon join to Cl is the most
  electron deficient, so the carbon in RCl in most
  susceptible to the attack of nucleophile.
• The bond strength is also important in
  determining the rate since bond strength
• decreased rapidly from C-Cl to C-I bond, the
  reaction rate decreases in the order
• R I gt R Br gt R Cl

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C. Chemical Properties Summary of SN reaction -
Press
32
C. Chemical Properties Summary of SN reaction -
33
C. Chemical Properties Summary of SN reaction -
34
C. Chemical Properties

• b. Formation of amine
• If RX is in excess, further reaction is expected
  since RNH2 is an even stronger nucleophile.

35
C. Chemical Properties

• b. Formation of amine (cont.)
• RX RNH2 ? R2NH HX
• RX R2NH ? R3N HX
• RX R3N ? R4N X-

Quarternary ammonium salt
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C. Chemical Properties

• b. Formation of amine (cont.)

This reaction is not important because under
normal condition aryl halide is very difficult to
have nucleophilic substitution rx (Why?)
37
C. Chemical Properties

• c. Formation of nitrile

38
C. Chemical Properties

• d. Formation of ether (Williamsons synthesis)

RX is usually 1ry alkyl halide should NOT be 2ry
and 3ry. (Why?)
39
C. Chemical Properties

• e. Formation of ester
• RX RCOO- Ag ? RCOOR AgX
• (Why do we use Ag compound?)

40
C. Chemical Properties

• Elimination
• Dehydrohalogenation
• That is, dehydrohalogenation reaction when alkyl
  halide is heated with a strong base using a
  relative non-polar solvent.

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C. Chemical Properties

• Elimination Dehydrohalogenation (cont.)
•  
• Note that both base and nucleophile are
  electron rich species, hence both elimination and
  nucleophilic reaction would occur at the same
  time unless the reaction conditions are carefully
  chosen.
• Normally, elimination reaction occurs at high
  temperature, alcoholic medium (relatively
  non-polar) and the alkyl halide is highly
  branched (e.g.
• tertiary or secondary).

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C. Chemical Properties

• Electrophilic substitution (Disubstitution is
  O.S.)

minor
major
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Chemical properties

• 4. Formation of Grignard Reagent

excess
44
Chemical properties

• 5. Reaction with Grignard reagent

alkane
What kind of reaction is this ?
How do you prepare
45
Chemical properties

• Wurtz Reaction
• Wurtz Fittig Reaction

46
C. Chemical Properties

• Reduction

47
D. Uses of halogen-compound

• 1. As solvents in dry-cleaning
• 2. As raw materials in the manufacture of
  poly(chloroethene) and poly(tetrafluoroethene).