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Electrophilic Substitution Reactions

Electrophilic Substitution Reactions. THE NITRATION OF BENZENE.

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Electrophilic Substitution Reactions

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  1. Electrophilic Substitution Reactions

  2. 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. or: H2SO4 heat

  3. 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

  4. 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

  5. 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.

  6. 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

  7. 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

  8. Factors affecting Nuecleophilicsubistitution reaction

  9. Sn2----- CH3-X, R-CH2X, allylic, benzylic • Sn1----- R3-CX, allylic, benzylic Sterric nature of the reactant

  10. small, strong Nu- favours Sn2---- • OH-, CH3O-, CH3CH2O-, CN-, RS-, RCN-, Br-, I- • weak, small Nu- favours Sn1----- • H2O, CH3OH, CH3CH2OH, RSH, NH3, F Nature of nucleophile

  11. Sn2---- low moderate polarity solvent as: • acetone, NMF • Sn1----- moderate to high polarity solvents as: • water, methanol, ethanol Solvent used

  12. all halogens, except F , are good leaving group • water is goog leaving group ( not OH) Leaving group

  13. Question: Explain WHY? then show the mechanism

  14. Addition Reactions

  15. In an addition reaction, new groups X and Y are added to the starting material. A  bond is broken and two  bonds are formed.

  16. Addition and elimination reactions are exactly opposite. A  bond is formed in elimination reactions, whereas a  bond is broken in addition reactions.

  17. The double bond dissolves back to single bond and new bonds reach out to A and B whose bond is also dissolving A-B can be : H-H H-OH H-X OH-OH OH-X

  18. Draw the product of each of these examples of A-B when they add to 1-propene. H-H H-OH H-X OH-OH OH-X

  19. A reaction in which an atom or group of atoms is added • to a molecule. divided into: • Electrophlic Addition • mechanism Addition reactions

  20. Electrophilic addition reactions - the general picture

  21. addition to unsymmetrical alkenes Why? According to Markovnikov's Rule

  22. 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” Addition of H-X

  23. Cont.Addition of H-X EXAMPLE: • Addition of HCl to 1-Propene. • It 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 anti-Markovnikovfashion

  24. 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. ADDITION OF H2O

  25. NucleophilicAddtion • It is the most common reaction of aldehydes (RCHO) and ketones (RCOR) e.g. The reaction of aldehydes and ketones with hydrogen cyanide hydroxynitriles.

  26. Some Examples of Nucleophilic Addition Reaction

  27. Form 1,1-diol (hydrate) EXAMPLE: • Formation of chloral hydrate. ADDITION OF H2O

  28. Classify each of the following as either substitution, elimination or addition reactions. a) substitution b) addition c) elimination

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