Electrophilic Substitution Reactions

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

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

Addition Reactions

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

Addition and elimination reactions are exactly opposite 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 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-H H-OH H-X OH-OH OH-X

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

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 Markovnikov`s rule: 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: EXAMPLE Addition of HCl to 1-Propene. 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

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.

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.

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.