|
Alkyl Halides : Nomenclature
Alkyl halides are compounds in which a
hydrogen of an alkane has been replaced by a halogen. Alkyl halides are
named as substituted alkanes (Chloromethane, CH3Cl)
Physical properties
Carbon–halogen bond
distances in alkyl halides increase in the order C-F (140
pm) C-Cl (179 pm) C-Br (197 pm) C-I (216 pm).
Carbon–halogen bonds are polar covalent bonds,
and carbon bears a partial positive charge in alkyl halides.
Nucleophilic
substitution bimolecular: SN2
SN2 is a
concerted reaction, it takes place in a single step, so no intermediates are
formed. The nucleophile attacks the carbon bearing the leaving group and
displaces the leaving group.
The nucleophile attacks
the substrate from the side opposite the bond to the leaving group. This is
called “back-side displacement,” or substitution with inversion of configuration.
Nucleophilic substitution
unimolecular: SN1
The leaving group in an
SN1 reaction departs before the nucleophile approaches, and a
carbocation intermediate is formed. In the second step, the nucleophile
reacts rapidly with the carbocation to form a product.
The SN1 reaction proceeds through the
formation of an achiral trigonal planar
carbocation intemediate, it may react with a nucleophile from either the front
side or the back side to form a racemic mixture.
Elimination bimolecular
(E2)
In an E2 reaction of an
alkyl halide, a base removes a proton from a carbon that is adjacent to the
carbon bonded to the halogen.
Elimination unimolecular
(E1)
In the absence of a
strong base, alkyl halides eliminate by the unimolecular
(E1) mechanism. The E1 mechanism involves rate-determining ionization of the
alkyl halide to a carbocation, followed by deprotonation of the carbocation.
|
