S Summary of mechanisms S Summary of mechanisms electrophilic addition Electrophiles: H δ in H (Ni catalyst needed), H δ in H-X; X δ in X ; H δ in H O (g) (conc H 3 PO 4 cat needed); H δ in NH 3 ; H δ in H SO 4 Nucleophilic subsitution/hydrolysis Examples: Haloalkanes with aqueous metal hydroxide (OH - is the nucleophile); ethanol solvent required as haloalkanes do not mix with water. Elimination reaction Examples: Reaction of RX with hot, ethanolic metal hydroxide: Haloalkane with cyanide ion to produce a nitrile R-CN. Useful reaction as the chain length increases by one carbon atom. Haloalkane with excess ammonia in ethanol under pressure. The product is a primary amine: R-X NH 3 R-NH NH 4 X -
Nitration of Benzene Nitration of Benzene electrophilic substitution Electrophile: NO Formation of electrophile: Concentrated nitric acid reacts with concentrated sulfuric acid catalyst at 55 o C: HNO 3 H SO 4 H NO 3 HSO 4 - H NO 3 NO H O Reduction of nitrobenzene to phenylamine: C 6 H 5 NO 6[H] C 6 H 5 NH H O NOTE: Using tin and conc HCl produces the salt C 6 H 5 NH 3 Cl -.NaOH (aq) is added to remove the salt and form phenylamine
Friedel-Crafts cylation Friedel-Crafts cylation electrophilic substitution Electrophile: RC =O Formation of electrophile: lcl 3 /FeBr 3 Halogen carrier in warm conditions (l has only 6 electrons in lcl 3 and can accept more electrons forming a co-ordinate bond with Cl - ) lcl 3 RCOCl Note: if you are asked to add an alkene to the benzene ring, you need to add HCl first to create a haloalkane. The haloalkane will then react with the carrier lcl 3 to produce an electrophile: CH =CH HCl CH 3 CH Cl CH 3 CH Cl lcl 3 CH 3 C H lcl 4 - lcl 4 - RC =O Example equation for acylation of benzene: C 6 H 6 CH 3 COCl C 6 H 5 COCH 3 HCl
Chain lengthening Chain lengthening nucleophilic addition Nucleophile: - :CN Optical activity The reaction produces a racemic mixture as the nucleophile can attack from above or below the planar >C=O group with equal likelihood: CH 3 CHO HCN CH 3 CH(OH)CN Note: HCN is highly toxic and has to be prepared in situ. It is created by reacting KCN with dilute HCl which produces HCN. Further reaction: The -hydroxypropanenitrile can be reacted with dilute HCl, which produces -hydroxypropanoic acid (lactic acid) and ammonium chloride: CH 3 CH(OH)CN HCl H O CH 3 CH(OH)COOH NH 4 Cl * * Importance or reaction: Chain length greater Optical isomers produced
Reduction of carbonyls Reduction of carbonyls Nucleophilic addition. Nucleophile: H: - Formation of alcohols: Equation: RCHO [H] RCH OH The reaction will also turn ketones into secondary alcohols. The reducing agent is NaBH 4 in warm aqueous solution or LilH 4 in dry ether which are represented as [H] in the equation. The first H is produced by the reducing agent, the second one by the solvent. Optical isomers formed? Yes if No No R & R differ
cylation cylation nucleophilic additionelimination reaction Nucleophiles: water, ammonia, alcohol, amine Ester formed Carboxylic acid formed o amide formed 1 o amide formed Naming acyl chlorides: lkanoyl chloride. E.g. ethanoyl chloride is CH 3 COCl; propanoyl chloride is CH 3 CH COCl
cid nhydrides General formula: RCOOCOR Examples: Ethanoic anhydride cid nhydrides Reasons for using acyl chlorides and acid anhydrides: Reactions are faster, produce a better yield and occur at lower temperatures. Reasons for acid anhydrides instead of acyl chlorides: nhydride group Reactions: nhydrides react in the same way as acyl chlorides. cid anhydrides are cheaper, less corrosive, less reactive with water and produce a safer by-product (which is ethanoic acid rather than hydrogen chloride). The reaction is also slower than with acyl chlorides and therefore easier to control on a large scale in industry. n important synthesis reaction using ethanoic anhydride:
mines mines General formula: Primary amine: RNH Secondary amine: RR NH Tertiary amine: RR R N Examples: Ethylamine CH 3 CH NH Ethylmethylamine CH 3 CH (CH 3 )NH Trimethylamine (CH 3 ) 3 N Forming amines: Nucleophilic substitution cid-base reaction (methylammonium salt formed) Note: Excess ammonia haloalkane mainly primary amine ammonium salt If ammonia is not used in excess, further substitution reactions will take place and a mixture of 1 o, o, 3 o and quarternary amines will be formed. The amines need to be separated by fractional distillation. Forming 1 o amines: Reduction of nitriles (Methylammonium ethanoate) 1. RX - CN R-CN X -. R-CN H R-CH NH Step 1 requires cyanide ions in aqueous ethanol Step requires a Nickel catalyst