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or more hydrogen atoms in an alkane have been replaced by halogen atoms,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more hydrogen atoms in an aromatic ring have been replaced by halogen atoms.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
1-Bromododecane, fatty bromide, is used as a solvent.
It is used as an intermediate to manufacture surfactant, pharmaceuticals and
other organic compounds. |
