Alkyl Halides

Alkyl Halides
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.

          

 

 

 

IMG_20200921_201740.png

 

 

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.

IMG_20200921_201657.png                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

 

 

 

 

IMG_20200921_201623.png                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          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 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.

IMG_20200921_201518.png     

 

 

 

The haloalkane in which the halogen attached to secondary and tertiary carbon is called secondary and tertiary alkyl halide respectively.

IMG_20200921_201455.png 

 

 

 

 

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:

IMG_20200921_190700.png

IMG_20200921_190619.png# R-OH + H-X R- X + H2O   

 

 

 

 

 

 

 

 

 

   R-OH +SOCl2 R-Cl + HCl + SO2

IMG_20200924_103055.png

IMG_20200924_103118.png

   IMG_20200924_102939.png                       

 

 

 

 

 

 

 

2. By the addition of a halogen acid with alkene

      

 

IMG_20200924_103019.png

 

 

 

 

                         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.

                   

               

 

                                          

                     

 

IMG_20200924_102906.png

 

 

 

 

 

 

 

 

 

 

 

                                                                        

                                                                          Br     

                                                                   2- Bromopropane    

3. By the Addiction of halogen with alkane   

IMG_20200924_102702.png                  

 

 

 

 

 

 

IMG_20200924_102805.png