1. Mechanisms
L.O. To know the mechanisms involved in heterolytic, electrophilic substitution of a benzene ring.
Homework: 1. Test Yourself Q11-15 inclusive P
2. Read section 12.9 and answer test yourself 16+17
Due: Wed 10th Feb
Draw out the mechanism for the reaction of 2-methylbut-2-ene with HCl. Why does it proceed as it does?

2. Electrophilic substitution reactions - nitration
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO C6H5NO H2O
nitrobenzene
Mechanism
1. Production of the electrophile (NO2+). This is called a nitronium ion or nitryl cation. It is generated in an acid-base reaction...
2H2SO HNO HSO4¯ H3O NO2+
acid base
2. Reaction of the nitronium ion with benzene

3. Electrophilic substitution reactions - halogenation
Chlorine and a halogen carrier (catalyst) = reagents
Conditions: Reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3).
Chlorine is non polar so is not a good electrophile - the halogen carrier is used to polarise the halogen
Step 1: Generation of electrophile: Cl+
Cl FeCl FeCl4¯ Cl+
Lewis Acid
Mechanism
Equation C6H Cl2 C6H5Cl HCl
P189 to see how bromine is activated by the halogen carrier

4. Friedel-crafts reactions of benzene - alkylation
Alkylation = substituting an alkyl (methyl, ethyl) group
A halogenoalkane (RX) and anhydrous aluminium chloride AlCl3 are the reagents
Conditions: Reflux with halogenoalkane and aluminium chloride (catalyst)
Step 1: Generation of the electrophile: a carbocation ion R+ (e.g. CH3+)
A catalyst is used to increase the positive nature of the electrophile and make it better at attacking benzene rings. AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.
Equation C6H C2H5Cl C6H5C2H HCl
Step 2:

5. Friedel-crafts reactions of benzene - alkylation
The anhydrous aluminium chloride acts as the catalyst. This works as follows: The Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACID.
It increases the polarisation of the C-Cl bond in the haloalkane, which makes the charge on C more positive and the following occurs
RCl AlCl AlCl4¯ R+

6. Friedel-crafts reactions of benzene - acylation
Acylation = substituting an acyl (methanoyl, ethanoyl) group
An acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3 are used as the reagents.
Conditions: Reflux at around 50°C with the acyl chloride and aluminium chloride (catalyst)
Step 1: Generation of the electrophile: RC+= O ( e.g. CH3C+O )
Step 2:
Product A carbonyl compound (aldehyde or ketone)
Equation C6H CH3COCl C6H5COCH HCl

7. Further substitution of arenes
In theory, it is possible to substitute more than one functional group onto an arene.
However, the functional group already on the ring affects the following...
• how easy it can be done • where the next substituent goes
Group ELECTRON DONATING ELECTRON WITHDRAWING
Example(s) OH, CH NO2
Electron density of ring Increases Decreases
Ease of substitution Easier Harder
Position of substitution 2,4,and and 5

8. Further substitution of arenes
Example 1: Substitution of nitrobenzene is...
• more difficult than with benzene
• produces a 1,3 disubstituted product
Example 2: Substitution of methylbenzene is…
• easier than with benzene
• produces a mixture of 1,2 and 1,4
isomeric products
Some groups (OH) make substitution so much
easier that multiple substitution takes place – remember phenol reacting with Br2?

9. Can you? Confidently draw free radical substitution mechanisms?
Confidently draw electrophilic addition mechanisms for both symmetric and asymmetric alkenes?
Draw mechanisms for electrophilic substitution reactions involving benzene?
Predict products in organic reactions based on the reactants used?
Recognise specific reactions such as Fiedel-crafts alkylation and acylation?