Alkyl Halides

Sep 24, 2020 02:27 273 0

Alkyl Halides: Introduction

Alkyl Halides [BSC-I Year]

Contents

Alkyl halides

Ø Homolytic and Heterolytic Fission

Ø Introduction: Structure and Nomenclature

Ø Preparation

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.

Alkyl Halide

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 C_nH_{2n +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

Nucleophiles:

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^- .

Electrophiles:

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.

Nucleophile	Electrophile
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.

Leaving groups(LG)

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.

E.g. OH^-, NH₂, OR^- F^-, Cl^-, Br^-, I^-

^{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.