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.
or:
HH22SOSO44
heatheat
THE NITRATION OF BENZENETHE NITRATION OF BENZENE
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 formation of the electrophile
The equation
THE HALOGENATION OF BENZENETHE 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 Friedel-Crafts Acylation of Benzene 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 FRIEDEL-CRAFTS ACYLATION OF BENZENE 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
* Sn2----- CH3-X, R-CH2X, allylic, benzylic
* Sn1----- R3-CX, allylic, benzylic
* small, strong Nu- favours Sn2----
* OH-, CH3O-, CH3CH2O
-, CN-, RS-, RCN-, Br-, I-
* weak, small Nu- favours Sn1-----
* H2O, CH3OH, CH3CH2OH, RSH, NH3, F
* Sn2---- low moderate polarity solvent as:
* acetone, NMF
* Sn1----- moderate to high polarity solvents as:
* water, methanol, ethanol
* all halogens, except F , are good leaving
group
* water is goog leaving group ( not OH)
Question: Explain WHY? then show the mechanism
R OH + HBr R-Br + H2O
R OH + HBr
H+
no reaction
16
• In an addition reaction, new groups X and Y are added to the starting material. A bond is broken and two bonds are formed.
17
• Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.
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
BA
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.
C C
H
H H
CH3
H-H H-OH H-X OH-OH OH-X
A reaction in which an atom or group of atoms is added
to a molecule. divided into:
1.Electrophlic Addition
mechanism
H2C CH2 + HX CH3CH2X
X = Cl, Br, I
a. H2C CH2 H X+ H3C CH2 + X-
b. H3C CH2 + X- H3C CH2Xfast
slow
Electrophilic addition reactions - the general picture
Why?Why?
addition to unsymmetrical alkenes
According to Markovnikov's RuleAccording 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"
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”
EXAMPLE:
Addition of HCl to 1-Propene.
It is a regioselective reaction, follow Markovnikov`s rule.
Cont.Addition of H-X
Anti-Markovnikov addition
EXAMPLE
Addition of HBr to 1-Propene in presence of peroxideperoxide.
In the presence of peroxides (chemicals containing the generalstructure ROOR'), HBr adds to a given alkene in an anti-Markovnikovfashion
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.
2.2. Nucleophilic Addtion Nucleophilic Addtion
It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)
e.g. e.g. The reaction of aldehydes and ketones with hydrogen cyanideThe reaction of aldehydes and ketones with hydrogen cyanide
hydroxynitriles.hydroxynitriles.
Form 1,1-diol (hydrate)
EXAMPLE: Formation of chloral hydrate.
29
Classify each of the following as either substitution, elimination or addition reactions.
a)OH
Br
substitution
b)
addition
c)
OH
elimination
