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ALKYL HALIDES for AIEEE and related exams

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PostSubject: ALKYL HALIDES for AIEEE and related exams Thu Apr 15, 2010 11:56 pm

ALKYL HALIDES


Preparation

1. From Alcohols (Replacement of OH by X)

ROH R-X.

2. Halogenation of Hydrocarbons

R-H R-X + HX


3. Addition of Hydrogen Halides to Alkenes (Discussed
under alkenes)






4. Addition of Halogens to Alkenes and Alkynes



5. Halide Exchange


R-X + I- RI + X-

Nucleophilic Substitution







The order of reactivity is RI>RBr>RCl>RF.

RX + -OH ROH + X- Alcohol
RX + H2O ROH Alcohol
RX + -OR' R OR' Ether
(Williamson synthesis)

RX + -C CR' R - C CR' Alkyne
RX + I- RI Alkyl iodide
RX + -CN RCN Nitrile

RX + R'COO- Ró - - OR Ester

RX + :NH3 RNH2 Primary amine

RX + :NH2R' RNHR' Secondary amine

RX + :NH R'R" RNR'R'' Tertiary amine

RX + SH- RSH Thiol (mercaptan)

RX + :SR' RSR' Thioether (sulfide)

RX + ArH + AlCl3 Ar R Alkyl benzene
(Friedel Craft reaction)

Nucleophilic Displacement by SN1 and SN2
Mechanisms


Sn2 Reaction

1. Mechanism:




2. Kinetics:

The reaction between methyl bromide and hydroxide ion
to yield methanol follows second order kinetics; that is, the rate
depends upon the concentrations of both reactants :

CH3Br +-OH CH3OH + Br-

rate = K [CH3Br] [OH-]


3. Stereochemistry:
A reaction that yields a product whose configuration is opposite to
that of the reactant is said to proceed with inversion of configuration.







4. Reactivity:

In SN2 reactions the order of reactivity of RX is CH3X>1o>2o>3o.


Sn1 Reaction

Mechanism and Kinetics
The reaction between tert-butyl bromide and hydroxide ion to yield
tert-butyl alcohol follows first order kinetics; i.e., the rate depends
upon the concentration of only one reactant,
tert-butyl bromide.





SN1 reaction follows first order kinetics.

Nucleophilic Displacement By SN1 And SN2
Mechanisms





SN1

SN2

Steps

Two : (1) R:XR+ + X-


(2) R+ + Nu- RNu or
R+ + :Nu RNu+
One : R:X + Nu- RNu + X-
or R:X + Nu RNu+ X-
Rate
=K [RX] (1st order)
=K[RX] [:Nu] (2nd order)


TS of slow step
|
Cd+......Xd-


Stereochemistry
Inversion and racemization
Inversion (backside attack)
Molecularity

Unimolecular
Bimolecular
Reactivity


structure of R
Determining
factor
Nature of X
Solvent effect on rate


3o> 2o> 1o> CH3
Stability of R+
;
RI> RBr> RCl> RF
Rate increases in polar solvent

CH3> 1o> 2o> 3o
Steric hindrance in R group

RI> RBr> RCl> RF
with Nu- there is a large rate increase in polar
aprotic solvents.
Effect
of nucleophile


Rate depends on nucleophilicity
I- > Br- > Cl- ; RS-
> RO-
Catalysis
Lewis acid, eg. Ag+, AlCl3, ZnCl2
None
Competition
reaction

Elimination, rearrangement
Elimination


Stereochemistry


When (-)-2-bromo octane is converted into the alcohol under conditions
where first-order kinetics are followed, partial racemization
is observed.

The optically active bromide ionizes to form bromide ion and the flat
carbocation. The nucleophilic reagent then attaches
itself to carbonium ion from either face of the flat ion.

If the attack were purely random, we would expect equal amounts of two
isomers; i.e. we would expect only the racemic modification. But the
product is not completely racemized, for the inverted product exceeds
its enantiomer.


We can say in contrast to SN2 reaction, which proceeds with
complete inversion; an SN1 reaction proceeds with
racemization though may not be complete.






SN1 reaction follows first order kinetics.

Nucleophilic Displacement By SN1 And SN2
Mechanisms



SN1

SN2


Carbonium ion


Steps

Two : (1) R:XR+ + X-


(2) R+ + Nu- RNu or
R+ + :Nu RNu+
One : R:X + Nu- RNu + X-
or R:X + Nu RNu+ X-
Rate
=K [RX] (1st order)
=K[RX] [:Nu] (2nd order)


TS of slow step
|
Cd+......Xd-


Stereochemistry
Inversion and racemization
Inversion (backside attack)
Molecularity

Unimolecular
Bimolecular
Reactivity

structure of R
Determining
factor
Nature of X
Solvent effect on rate

3o> 2o> 1o> CH3
Stability of R+

RI> RBr> RCl> RF
Rate increases in polar solvent

CH3> 1o> 2o> 3o
Steric hindrance in R group

RI> RBr> RCl> RF
with Nu- there is a large rate increase in polar
aprotic solvents.
Effect
of nucleophile


Rate depends on nucleophilicity
I- > Br- > Cl- ; RS-
> RO-
Catalysis
Lewis acid, eg. Ag+, AlCl3, ZnCl2
None
Competition
reaction

Elimination, rearrangement
Elimination


Stereochemistry

When (-)-2-bromo octane is converted into the alcohol under conditions
where first-order kinetics are followed, partial racemization
is observed.

The optically active bromide ionizes to form bromide ion and the flat
carbocation. The nucleophilic reagent then attaches
itself to carbonium ion from either face of the flat ion.
If the attack were purely random, we would expect equal amounts of two
isomers; i.e. we would expect only the racemic modification. But the
product is not completely racemized, for the inverted product exceeds
its enantiomer.

We can say in contrast to SN2 reaction, which proceeds with
complete inversion; an SN1 reaction proceeds with
racemization though may not be complete.



r.d.s formation of carbonium ion.

Reactivity of an alkyl halide depends chiefly upon how stable a
carbonium ion it can form.


In SN1 reactions the order of reactivity of alkyl halides is
Allyl,benzyl>3o>2o>1o>CH3
X.
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