Halogenoalkanes, also known as haloalkanes or alkyl halides, are organic
compounds in which one or more hydrogen atoms in an alkane have been replaced by
halogen atoms, fluorine, chlorine, bromine or iodine. In carbon-halogen bond,
halogens have significantly greater electronegativities than carbon except
iodine. In result, this functional group is polarized so that the carbon is
electrophilic and the halogen is nucleophilic. Halogenoalkanes are can be
classified depending on the halogen atom position on the chain of carbon atoms.
The carbon which is attached with the halogen atom is linked up with only one
other alkyl group in primary halogenoalkanes, whereas directly linked up with
two and three other alkyl groups in secondary halogenoalkanes and tertiary
halogenoalkanes respectively. In some case, primary halogenoalkanes are counted
even though there are no alkyl groups attached to the carbon with the halogen on
it. Three characteristics provide important influences on the chemical behavior
of halogenoalkanes, these are electronegativity, covalent bond strength and the
relative stability of the corresponding halide anions. Fluoroalkanes have the
strongest of the carbon-halogen covalent bonds so that they are unreactive. This
is stronger single bond than a carbon-carbon bond. The carbon-chlorine covalent
bond is slightly weaker than a carbon-carbon bond, and the bonds to the other
halogens are weaker. The stability may be estimated from the relative acidities
of the H-X acids. All the hydrohalic acids are very strong, but with small
differences in the direction HCl < HBr < HI, with the exception of HF.
Halogenoarenes, also called haloarene, or aryl Halide, are an organic compound
in which one or more hydrogen atoms in an aromatic ring have been replaced by
halogen atoms. The Haloarenes exhibit many differences compare to haloalkanes in
the method of preparation and their chemical and phisical properties.
Haloalkanes are used in as refrigerants, solvents, blowing agents, aerosol
propellants, fire extinguishing media , and in semiconductor device fabrication.
One of big consumption of halogenoalkanes (properly speaking, halogenoalkenes)
is as a raw material to prepare plastics such as PVC [poly(chloroethene)] from
chloroethene and PTFE [poly(tetrafluoroethene)] from tetrafluoroethene.
Halogenoalkanes and halogenoarenes react with lots of compounds resulting in a
wide range of different target substances. They are useful intermediates in
making other organic compounds.
- Summary
of substitution and elimination reaction based
on alkyl group structure
Alkyl
Halides |
SN1 and E1 |
SN2 and E2 |
Primary
(RCH2X) |
Neither
SN1 nor E1
occur |
both SN2 and E2
will occur (SN2
preferred. E2 preferred when sterically
hindered base is used) |
Secondary
(R2CHX) |
will
occur slowly in high dielectric ionizing solvents
( SN1
preferred) |
both SN2 and E2
will occur (SN2 preferred
if base is weaker than acetate, pKa = 4.8,
E2 preferred if base is
strong) |
Tertiary
(R3CX) |
both SN1 and E1
will occur (SN1
preferred) |
No SN2 will occur E2 will dominate if base is strong |
Allyl
(H2C=CHCH2X)
|
will
occur slowly in high dielectric ionizing solvents |
Rapid
SN2 for
primary and secondary halides but slow for
tertiary. E2 when nucleophile is basic.
|
Benzyl
(C6H5CH2X)
|
will
occur slowly in high dielectric ionizing solvents |
Rapid
SN2 for
primary and secondary halides but slow for
tertiary. E2 when nucleophile is basic.
|
- SN1 and SN2 reaction order
based on alkyl group structure
- SN2 :
CH3X (100%)
> RCH2X > R2CHX > R3CX
- SN1
: CH3X < RCH2X < R2CHX <
R3CX (100%)
- Nucleophilicity
order
- F- < Cl- < Br- < I-
In a protic solvent
which decrease SN2 rate
due to solvating nucleophile
- F- > Cl- > Br- > I-
In a polar aprotic solvent
which increase SN2 reaction
due to no solvating nucleophile
- F- > Cl- > Br- > I-
SN2 in the gas phase
(Both protic and aprotic polar solvent enhance
SN1
due to the
stabilization of the carbocation intermediate)
- Leaving group
Among halogens: I > Br > Cl > F
Bromopentane is used as a solvent. It is used as an intermediate
to manufacture
pharmaceuticals and other organic compounds.
|