Alkyl Halides [BSC-I Year]
Ø Homolytic and Heterolytic Fission
Ø Introduction: Structure and Nomenclature
Ø Physical Properties
- Alkyl Halides
Bond Fission: The Force of attraction between the combining atoms
which is formed by the accommodation of a pair of electron is called bond. Bond Fission is of two types.
i) Homolysis or homolytic bond fission
ii) Heterolysis or heterolytic bond fission
i)Homolysis or homolytic bond fission:
It is a symmetrical bond fission i.e. equal no. of electron are distributed in each fragment. In homolytic bond fission, cleavage of covalent bond between two atoms takes place in such a way that each atom keep one electron of the share pair. The energy required for this is supplied as heat or light .This homolysis results in the formation of free radical. This type of fission is highly favourable in gaseous molecule.
ii) Heterolysis or heterolytic bond fission:
It is unsysmmetrical bond fission i.e. unequal no. of electron are distributed in each fragment. In heterolysis, the cleavage of covalent bond between two atoms takes place in such a way that one atom takes both electrons of the shared pair leaving none of the other. This results into two charge particles. The heterolytic fission occurs more readily (or easily) with a polar compound in polar solvent and results in the formation of ionic intermediate.
If one H- atom alkane is replaced by halogen the resulting compound is called alkyl halide and is represented by R-X.
General formula of alkyl halide is CnH2n +1 X.
R-H + X2 → R-X + HX
(alkane) (alkyl halide)
Alkyl Halide : Classification
It can be classified as primary, secondary and tertiary depending upon the no. of a carbon atoms present in the molecule.
Primary alkyl halide:The haloalkane in which the halogen attached is to the primary carbon is called primary alkyl halide.
The haloalkane in which the halogen attached to secondary and tertiary carbon is called secondary and tertiary alkyl halide respectively.
Nucleophiles and Electrophiles
Reagents which are rich in electron and give a pair of e- to the substrate are called nucleophile or nucleophilic reagent. These are also refered as nucleus loving group .
e.g. Cl- , OH-, CN- , OR- , -COOR etc. are negatively charged species or lewis base.
NH- , ROH , R-O-R, R-NH2 etc. are neutral species having lone pair of e- .
Reagents which are deficient in electron and take a pair of e- from the substrate are called electrophiles or electrophilic reagent. They are also reffered as electron loving group.
e.g. R+ , H+ , H3O, NO2+ , are positively charged species.
AlCl3 , BCl3 are neutral species. Both Al and B have a total no. of 6 electrons i.e. 2e- less than octet. therefore, they try to complete their octet and act as electrophiles.
Accepts protons and donates electrons
Donates proton and accepts electrons
Electron-rich atom, molecule, ion
Electron-deficient ion,atom or molecule
Negatively charged or neutral
Positively charged or neutral
Undergoes nucleophilic substitiution and addition
Undergoes electrophilic addition and substitution
Interchangeably called Lewis Base
Interchangeably called Lewis Acid.
A Leaving group is an atom (or a group of atoms) that is displaced as stable species taking with it the bonding electrons.A good leaving group reduces the barrier to a reaction. Stable anion that weak bases are usually excellent LG and can be localized charge.
The best leaving group are:
Electron- withdrawing (to polarize the C- atom)
Stable once it has left.
Polarizable, to stabilize the transition state.
Leaving Group reactivity
Less reactive More reactive
Alkyl Halide :Preparation
1. By the action of alcohols with halogen acid
2. By the addiction of a halogen with alkene
3. By the addiction of halogen with alkane
1. By the action of alcohols with halogen acid:
# R-OH + H-X → R- X + H2O
R-OH +SOCl2 → R-Cl + HCl + SO2
2. By the addition of a halogen acid with alkene
H H Cl
Addition of HBr with Unsymmetrical alkene
In case of an unsymmetrical alkene reaction takes place according to Morkovnikov's rule. This rule state that in the ionic addition of an acid to the C=C bond of an alkene, the halogen of an acid attached to that carbon atom which already holds the greater no. of hydrogen atom.
Another things is that secondary carbocation is more stable than primary carbocation because more e- releasing group ease a formation of carbonium ion, the more carbonium ion is formed faster and corresponding reaction is faster.
3. By the Addiction of halogen with alkane