FORMATION OF GRIGINARD’S REAGENT

FORMATION OF GRIGINARD’S REAGENT
GRIGINARD’S REAGENT
In the preparation of a griginard’s reagent great care must be taken that the reactants are absolutely dry and pure. The other experimental details are given below: Magnesium: magnesium ribbon is washed with ether and dried in desicater. Ether: it is distilled over sodium metal and P2O5. Ethyl bromide: it is purified and then distilled over P2O5. Take dry ether in a round bottom flask and add pieces of Mg ribbon in it. Fit it with water condenser and add ethyl bromide gradually through the condenser. Heat a little so that magnesium dissolves. Cool if necessary.A clear solution of griginard’s reagent – ethyl magnesium bromide, in ether appears. This ethereal solution is used as griginard’s reagent.
C2H5--Br + Mg à C2H5--Mg--Br
BIMOLECULAR REACTION
A chemical reaction in which two molecules react together in rate determining step is called bimolecular reaction. A bimolecular reaction may be SN2 or E2. Rate of reaction of A bimolecular reaction is expressed as:
Rate = K[R--X][Nu]
UNIMOLECULAR REACTION
A chemical reaction in which only one molecule takes part in rate determining step.is called a unimolecular reaction. A unimolecular reaction may be SN1 or E1. Rate of reaction of a unimolecular reaction is expressed as:
Rate = K[R--X]

SN2-REACTIONS

SN2-REACTIONS
SN2-REACTIONS
In SN2-REACTIONS, the formation of the carbon-nucleophile bond [C--Nu] and the cleavage of the carbon- halogen bond [C--X] occur simultaneously. These are nucleophilic substitution reactions which occur in one step .
MECHANISM
The reaction proceeds in one step. The attack of nucleophile and release of halide group take place at the same time.i.e
Nu- + R-X ® [Nu…R…X] ® R--Nu + X- Transition state
Reactions of secondary alkyl halide may be SN1 or SN2 depending upon the nature of solvent.
RATE OF SN1-REACTIONS
The rate of an SN2 reactions depend on the concentrations of nucleophile and the substrate. i.e Rate of reaction = k[substrate][Nu] Since both substrate and nucleophile are present in rate expression therefore ifst is a bimolecular reaction. It may be defined as "a bimolecular nucleophilic substitution reaction which occur in one step .
FACTORS AFFECTING THE NATURE Of SN REACTIONS
The mechanism of SN-reactions depend upon : The structure of alkyl group Nature of the solvent Structure Of Alkyl Group Reactions of primary alkyl halides take place in SN2 mechanism. Reactions of tertiary alkyl halides take place in SN1 mechanism. Reactions of secondary alkyl halides take place both in SN1 and SN2 mechanism but it depends upon the nature of solvent. The order of stability of carbonium ion is as under: R3C+[tertiary carbonium ion]>R2C+H[secondary carbonium ion]>RC+H2[primary carbonium ion] Nature Of Solvent:A polar solvent favours SN1 mechanism due to capability of ionization where as a non- polar solvent favours SN2 mechanism.

NUCLEOPHILIC SUBSTITUTION REACTIONS

NUCLEOPHILIC SUBSTITUTION REACTIONS
DEFINITION
" A reaction in which a nucleophile displaces another nucleophile and takes its position is called a nucleophilic substitution reaction. "
Nucleophilic substitution reaction are represented by the symbol "SN".
EXPLANATION
In alkyl halides, the C-X bond is polar because C-atom is attached to a highly electro-negative halogen atom.Therefore in alkyl halides carbon atom is electrophilic and halogen has a nucleophilic character.
For example in iso propyl bromide , C-atom has partial positive charge and Br-atom has partial negative charge. The C-atom has capability to accept a pair of electrons to form a new bond. In doing so the C-Br bond breakes and Br- leaves the molecules. The net result is that one nucleophile displaces another nucleophile. Such displacement reactions are called nucleophilic substitution reactions.
Following examples illustrate the nature of "SN" reactions: OH- + CH3+-I- à OH – CH3 + I- In this example hydoxyl ion is an attacking nucleophile while iodide-ion is called leaving group. CH3-CH2-I + OH- àCH3-CH2-OH +I- CH3-I + -OCH3 àCH3-O-CH3 +I- CH3-Br + CH3COO- àCH3COOCH3 + Br-
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GENERAL EXPRESSION
A nucleophilic substitution reaction (SN). Nu- + Rd+-Xd- à Nu-R + X- In general it can be written as: Alkyl halide + nucleophile à new compound + halide ion
MECHANISM OF SN REACTIONS
There are two classes of SN-reactions: SN1-reactions SN2-reactions
SN1-REACTIONS
Nucleophilic substitution reactions which take place in two steps are referred to as "SN1-reactions.
MECHANISM
SN1 mechanism occurs in two steps.In the first step, the substrate ionizes slowly and reversibly to produce carbonium ion. R-X àR+ + X- In the second step, nucleophile reacts with carbonium ion to produce a new compound. R+ + Nu- à R-Nu
RATE OF SN1-REACTIONS
The rate of an SN1 reaction depends upon the concentration of substrate (R-X) only and not on the nucleophile.i.e. Rate = K[R-X] This means that it is a unimolecular reaction. Therefore, SN1 is a unimolecular nucleophilic substitution reaction.

PREPARATION OF ALKYL HALIDES

PREPARATION OF ALKYL HALIDES
Mono haloalkanes or Alkyl halides can be prepared by a number of methods.Some important synthetic methods are as follows:
METHOD No. 1
BY USING ALCOHOL AS A MAIN REAGENT
A number of reagents such as halogen acids, PCl5, PCl3, SOCl2 etc. react with aliphatic alcohols to produce corresponding alkyl halides.
FROM HALOGEN ACID
Alcohols react with halogen acids (HCl ,HBr or HI) to produce alkyl halides.
R-OH + HX ® R-X + H2O CH3-OH + HCl® CH3-Cl + H2O (in the presence of ZnCl2/H2SO4) C2H5OH + HI® C2H5I + H2O C2H5OH + HBr® C2H5Br + H2O CH3-CH2-CH2-OH + HCl ® CH3-CH2-CH2-Cl + H2O
FROM PHOSPHORUS PENTA CHLORIDE (PCl5)
When alcohols are treated with phosphorus penta chloride, corresponding alkyl chlorides are obtained.
CH3-CH2-OH + PCl5 ® CH3-CH2-Cl + POCl3 + HCl
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FROM THIONYL CHLORIDE(SOCl2)
CH3-CH2-OH +SOCl2 ® CH3-CH2-Cl + SO2 + HCl
Above reaction is carried out in the presence of Pyridine. Pyridine absorbs HCl as it is formed.
FROM PHOSPHORUS TRI BROMIDE (PBr3)
3C2H5OH + PBr3 ® 3C2H5Br + H3PO3
FROM PHOSPHORUS TRI IODIDE (PI3)
3CH3OH + PI3 ® 3CH3I + H3PO3
METHOD No. 2
BY THE HALOGENATION OF ALKANES
Alkanes react with chlorine and bromine in presence of sunlight and undergo photo chemical series of substitution reactions to yield alkyl halides.
CH4 + (Cl2) ® CH3Cl + CH2Cl2 + CHCl3 + CCl4
REMEMBER: THIS IS NOT AN APPROPRIATE METHOD FOR THE LABORATORY PREPARATION OF ALKYL HALIDES BECAUSE THE RESULTANT MIXTURE CONTAINS POLYHALOGEN DERIVATIVES WHICH ARE DIFFICULT TO SEPARATE.
METHOD No. 3
FROM ALKENE
we know that alkenes are unsaturated hydrocarbons.Therefore when alkenes are treated with halogen acid, addition reaction takes place and alkyl halides are formed. Additionof HX takes place according to MARKONIKOV'S PRINCIPLE.
CH2 = CH2 + HCl ® CH3 – CH2 – Cl
CH3-CH=CH2 + HBr ® CH3-CHBr-CH3
The order of reactivity of halogen acid is as follows:
HI > HBr > HCl

CHEMISTRY OF ALKYL HALIDES

ALKYL HALIDES
ALKYL HALIDES
Organic compound containing halogen atom as a functional group are called alkyl halides. Alkyl halides are very reactive organic compounds . The general formula of alkyl halide is CnH2n+1-- X, where X= Cl, Br, I. Example: CH3-Cl (Methyl Chloride),CH3-Br (Methyl bromide) ,C2H5-Br (Ethyl Bromide) ,C3H7-Br (Propyl bromide) etc. etc.
CLASSIFICATION OF ALKYL HALIDES
Alkyl halides are classified into three classes: PRIMARY ALKYL HALIDE: Alkyl halides containing primary C-atom are called primary alkyl halides.
For example: CH3-CH2-I (Ethyl iodide) ,
SECONDARY ALKYL HALIDE: Alkyl halides containing secondary c-atom are called secondary alkyl halides. For example:
TERTIARY ALKYL HALIDE: Alkyl halides containing tertiary c-atom are called tertiary alkyl halide. For example: