Mechanisms L.O. To know the mechanisms involved in heterolytic, electrophilic substitution of a benzene ring. Homework: 1. Test Yourself Q11-15 inclusive P187-190 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?

Electrophilic substitution reactions - nitration Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions reflux at 55°C Equation C6H6 + HNO3 C6H5NO2 + 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... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid base 2. Reaction of the nitronium ion with benzene

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+ Cl2 + FeCl3 FeCl4¯ + Cl+ Lewis Acid Mechanism Equation C6H6 + Cl2 C6H5Cl + HCl P189 to see how bromine is activated by the halogen carrier

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 C6H6 + C2H5Cl C6H5C2H5 + HCl Step 2:

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 + AlCl3 AlCl4¯ + R+

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 C6H6 + CH3COCl C6H5COCH3 + HCl

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, CH3 NO2 Electron density of ring Increases Decreases Ease of substitution Easier Harder Position of substitution 2,4,and 6 3 and 5

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?

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?