Transcript lec-2- 211(ES +Add)

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THE NITRATION OF BENZENE
Benzene is treated with a mixture of concentrated nitric acid
and concentrated sulphuric acid at a temperature not
exceeding 50°C. As temperature increases there is a greater
chance of getting more than one nitro group, -NO2,
substituted onto the ring.
Nitrobenzene is formed.
H2SO4
or:
heat
The formation of the electrophile
If you are going to substitute an -NO2 group into the ring,
then the electrophile must be NO2+. This is called the
"nitronium ion" or the "nitryl cation", and is formed by
reaction between the nitric acid and sulphuric acid
The equation
THE HALOGENATION OF BENZENE
Benzene reacts with chlorine or bromine in an electrophilic
substitution reaction, but only in the presence of a catalyst.
The catalyst is either aluminium or ferric chloride (or
aluminium (ferric) bromide if you are reacting benzene with
bromine) or iron.
FeCl3
FeBr3
The formation of the electrophile
As a chlorine molecule approaches the benzene ring, the
delocalised electrons in the ring repel electrons in the
chlorine-chlorine bond
it is the slightly positive end of the chlorine molecule
which acts as the electrophile. The presence of the
aluminium chloride helps this polarisation.
* Friedel-Crafts Acylation of Benzene
Named after Friedel and Crafts who discovered the reaction.
 Reagent : normally the acyl halide (e.g. usually RCOCl) with
aluminum trichloride, AlCl3, a Lewis acid catalyst.
 The AlCl3 enhances the electrophilicity of the acyl halide by
complexing with the halide.
FRIEDEL-CRAFTS ACYLATION OF BENZENE
 Electrophilic species : the acyl cation or acylium ion (i.e. RCO +
) formed by the "removal" of the halide by the Lewis acid catalyst,
which is stabilised by resonance as shown below.
Other sources of acylium can also be used such as acid
anhydrides with AlCl3
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• In an addition reaction, new groups X and Y are
added to the starting material. A  bond is broken
and two  bonds are formed.
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• Addition and elimination reactions are exactly
opposite. A  bond is formed in elimination
reactions, whereas a  bond is broken in addition
reactions.
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The double bond dissolves back to single bond and new
bonds reach out to A and B whose bond is also dissolving
C
C
C
C
A
B
C
C
A
B
A-B can be :
H-H
H-OH
H-X
OH-OH
OH-X
Draw the product of each of these examples
of A-B when they add to 1-propene.
H
CH3
C
H
H-H
H-OH
H-X
C
H
OH-OH
OH-X
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A reaction in which an atom or group of atoms is added
to a molecule. divided into:
1. Electrophlic Addition
H2C
CH3CH2X
+ HX
CH2
X = Cl, Br, I
mechanism
a. H2C
b. H3C
CH2
CH2
slow
+
H
+
-
X
X
fast
H3C
CH2
H3C
CH2X
+
X-
Electrophilic addition
reactions - the general picture
addition to unsymmetrical alkenes
Why?
According to Markovnikov's Rule
•Which is an empirical rule based on Markovnikov's experimental observations
on the addition of hydrogen halides to alkenes.
•The rule states that :
"when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl
halide, the hydrogen adds to the carbon of the alkene that has the greater
number of hydrogen substituents, and the halogen to the carbon of the alkene
with the fewer number of hydrogen substituents"
* Addition of H-X
 Reactivity rank: HI > HBr > HCl > HF.
 It is an electrophilic addition reaction.
 It Follows Markovnikov`s rule.
 Markovnikov`s rule:

“In addition of HX to asymmetrical alkenes,
the H+ of HX goes to the double-bonded carbon
that already has the greatest number of
hydrogens”
Cont.
Addition of H-X
EXAMPLE:
 Addition
 It
of HCl to 1-Propene.
is a regioselective reaction, follow Markovnikov`s rule.
 Anti-Markovnikov addition
EXAMPLE

Addition of HBr to 1-Propene in presence of peroxide.
In the presence of peroxides (chemicals containing the
generalstructure ROOR'), HBr adds to a given alkene in an antiMarkovnikovfashion
* ADDITION OF H2O
HBr and HCl easily add to alkenes. Since water also is a molecule of the type
HX which can donate a proton, H2O should be able to add to alkenes in the
same way as HBr, for example, resulting in the hydration of an alkene.
However, for the addition of H2O to alkenes to occur acid catalysts are
required.
2. Nucleophilic Addtion
It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)
e.g. The reaction of aldehydes and ketones with hydrogen cyanide
hydroxynitriles.
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Some Examples of Nucleophilic
Addition Reaction
Addition reaction of aldehydes
and ketones
 The carbon-oxygen double bond is polar.
 A carbonyl compound may be attacked either by a
nucleophile or by a electrophile.
 Reactivity rank: ???
* ADDITION OF H2O

Form 1,1-diol (hydrate)
EXAMPLE:

Formation of chloral hydrate.
 Addition of Grignard reagents: formation of
alcohols.
 Addition of hydrogen cyanide: formation of
cyanohydrins.
 Addition of alcohols: formation of hemiacetals
and acetals.
Classify each of the following as either substitution,
elimination or addition reactions.
OH
a)
Br
substitution
b)
addition
c)
elimination
OH
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