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Published on February 23, 2014

Author: adeelahmadahmad7


ASSIGNMENT ON NUCLEOPHILIC AND ELECTROPHILIC SUBSTITUTION REACTION IN ALIPHATIC AND AROMATIC COMPOUNDS: ASSIGNMENT ON NUCLEOPHILIC AND ELECTROPHILIC SUBSTITUTION REACTION IN ALIPHATIC AND AROMATIC COMPOUNDS PRSENTED TO : MAM SAMINA BANO: PRSENTED TO : MAM SAMINA BANO GROUP MEMBERS SWAIRA SHAFIQUE NEELUM HALEEMA ZAHRA ABBAS MAHAM AHMAD HINA SHAFIQUE MEMOONA NASIR : GROUP MEMBERS SWAIRA SHAFIQUE NEELUM HALEEMA ZAHRA ABBAS MAHAM AHMAD HINA SHAFIQUE MEMOONA NASIR PowerPoint Presentation: WHAT IS SUBSTITUTION REACTION “ Reaction which involve the replacement or substitution of one or more atoms or group of a compound by other atoms or groups are known as substitution reaction” These reactions may be initiated by nucleophile , electrophile , or free radical. PowerPoint Presentation: Nucleophilic substitution reaction When a substitution reaction involves the attack by nucleophile , reaction is referred to as Sn ( S stands for substitution and N for nucleophile. PowerPoint Presentation: Electrophilic substitution reaction When a substitution reactions involve the attack by electrophiles , reaction is referred to as electrophilic substitution reaction. PowerPoint Presentation: Nucleophilic substitution in aromatic compounds ( a) Under drastic conditions: The halogens of aryl halides can be displaced by OH, NH 2 or CN with appropriate nucleophilic reagents at high temperature and pressure. C 6 H 5 Cl + NaOH C 6 H 5 OH + NaCl C 6 H 5 Cl + 2NH 3 C 6 H 5 NH 2 +NH 4 Cl C 6 H 5 Br + CuCN C 6 H 5 CN + CuBr 350 C 300atm 250 C 50atm 250C pyridine PowerPoint Presentation: With strong bases such as NaNH 2 or KNH 2 -33C Liq. Aryl halides with sodium amide or potassium amide in liq. Ammonia to form respective aniline. PowerPoint Presentation: Mechanism Step1 . Elimination step A benzyne is formed through removal of H+ by NH 2 - a nd loss of Br- PowerPoint Presentation: Step2. addition step A carbanion is formed by nucleophilic addition of NH 2 - to benzyne . The carbanion abstracts a proton from solvent to give substitution product. PowerPoint Presentation: ( b) By presence of ring activating substituents Aryl halides having electron – withdrawing groups such as NO2 or CN in orthro or para position to halogens , undergoes nucleophilic substitution reactions relatively easily . For example + 160C P- Nitrochlorbenzene p - nitrophenol PowerPoint Presentation: Wurtz – Fitting reaction + + 2Na Ether heat + 2NaBr Aryl halides couple with alkyl halides when heated with sodium in ether solution to form alkyl benzene PowerPoint Presentation: ULLMANN Reaction Aryl iodides and bromides when heated with copper form biaryl compounds in which two benzene rings are bonded together 2 IODOBENZENE + 2 Cu + 2CuI heat PowerPoint Presentation: Aryl chlorides also participate in Ullmann reaction if the Cl atom is activated by the presence of NO2 group in orthro and para positions. + 2CuCl + 2 Cu 225C O- nitrochlorobenzene 2,2-dinitrobiphenyl PowerPoint Presentation: Formation of Grignard reagent Aryl iodide and aryl bromides form Grignard reagent in usual fashion. Aryl chloride do so by using tetrahydrofuran (THF) instead of ether + Mg Ether heat Bromobenzene phenyl magnesium bromide PowerPoint Presentation: Formation of Aryllithiums Aryl bromides will react with lithium in dry ether to form aryllithium . Aryllithiums gives the same reaction as Grignard reagent and more reactive. They are now frequently used in synthesis. PowerPoint Presentation: Reduction When reduced with nickel aluminium alloy in presence of sodium hydroxides , aryl halides gives corresponding arenes . + 2 [ H] Ni-Al NaOH + HBr Bromobenze benzene PowerPoint Presentation: NUCLEOPHILIC ADDITION IN ALIPHATIC COMPOUNDS The nucleophilic substitution reactions are divided into two classes: Sn1 reaction Sn2 reaction Sn2 reaction: Stands for bimolecular nucleophilic substitution reaction Rate of Sn2 reaction depend on concentration of both substrate and the nucleophile , reaction is of second order. Rate ∝ [ Substrate][Nucleophile] PowerPoint Presentation: Sn2 reaction T he energy needed for the cleavage of C-Br bond is partially provided by energy liberated by C-OH bond formation In course of reaction the configuration of carbon is inverted , rather like an umbralla blow inside out. The change in configuration is called Walden Inversion Methyl bromide transition state methyl alchol PowerPoint Presentation: Sn1 reaction: S tands for unimolecular nucleophilic substitution Rate of Sn1 depend on concentration of alkyl halides , the reaction is of first order Rate [Substrate] Consider the hydrolysis of ter -butyl bromide . The reaction consists of two steps: Step 1. The alkyl halide ionizes to give carbonium ion. This is rate determine step. PowerPoint Presentation: The carbonium ion is planar. This is because central positively charged carbon atom is sp2 hybridized. Step 2. The nucleophile can attack the planar carbonium ion from either side to give t-butyl alchol . PowerPoint Presentation: Remember that primary alkyl halides undergo hydrolysis by Sn2 mechanism. The tertiary alkyl halide undergo hydrolysis by Sn1 mechanism. This is because the attack of hydroxide ion on the crowded t-alkyl halides is quit difficult, Secondary alkyl halides may undergo hydrolysis by either Sn1 and Sn2 mechanism. HO: C X HO: C X H H H CH3 CH3 CH3 Easy attack (primary halide) Difficult attack ( tertiary halide) PowerPoint Presentation: ELECTROPHILIC SUBSTITUTION IN AROMATIC COMPOUND Benzene undergo electrophilic substitution reaction. The benzene ring with its decolorized pi electrons is an electron rich system. Where E+ is any electrophile and Nu is nucleophile. Such reaction in which hydrogen atom of aromatic ring is replaced by an electrophile are electrophilic aromatic substitution reaction. PowerPoint Presentation: General mechanism Step 1. Formation of electrophile. Step 2. Electrophile attack the aromatic ring to form a carbonium ion. E-Nu + catalysts E+ +Nu-catalyst PowerPoint Presentation: The intermediate ion is resonance stabilized. It is hybrid of three structures: Step 3. Loss of proton gives the substitution product. Nu-catalyst + H-Nu + catalyst H E E H E H PowerPoint Presentation: Halogenation: Benzene react with chlorine in presence of FeCl3 or AlCl3 at room temperature to form chlorobenzene . Mechanism . Following steps are involved: Step1 : Formation of electrophile Cl Cl + FeCl 3 Cl + + FeCl 4 - + Cl 2 FeCl3 + HCl PowerPoint Presentation: Step2 . The electrophile attacks the benzene ring to form carbonium ion. Step 3. Loss of proton gives chlorobenzene . + Cl + + H Cl + H Cl FeCl4- + HCl + FeCl3 PowerPoint Presentation: Bromine react with benzene similarly. + Br2 FeBr3 Br + HBr PowerPoint Presentation: Nitration: Benzene react with concentrated nitric acid in presence of concentrated sulphuric acid at 60C to form nitrobenzene. PowerPoint Presentation: Mechanism: Following steps are involved: Step1 . Formation of electrophile HNO 3 + 2H 2 SO 4 NO 2 + +2HSO 4 - +H 3 O+ Step2 . The electrophile attacks the benzene ring to form a carbonium ion . Step3 . Loss of proton form nitrobenzene. PowerPoint Presentation: Sulphonation : Benzene react with conc. H2SO4 acid at 120 C or fuming sulpheric acid at room temperature to gives benzenesulphonic acid. PowerPoint Presentation: Mechanism: Following steps are involved. Step1. Electrophile is formed. In this reaction electrophile is sulphur trioxide. In conc. Sulpheric acid, sulphur trioxide is produced as follows. 2H 2 SO 4 SO 3 + H 3 O+ +HSO 4 - Step2. Electrophile attacks the benzene ring to form carbonium ion. H PowerPoint Presentation: Step3 . Loss of proton + H SO 3 - + H 2 SO 4 + H 2 SO 4 + H3O+ + H2O Step 4. Addition of proton gives benzenesulphonic acid PowerPoint Presentation: Friedal -Craft alkylation: B enzene react with alkyl halides in presence of aluminium chloride to form alkylbenzene PowerPoint Presentation: Mechanism : PowerPoint Presentation: Drawbacks of Friedal -Craft alkylation: It is difficult to stop the reaction when one alkyl group has entered the ring. Di and trialkylbenzenes are formed. The alkyl group often tend to rearrange. For example when benzene is treated with n propyl chloride in presence of AlCl 3, the product is isopropyl rather than expected n propyl-benzene. This is because Friedal Craft alkylation involves the formation of carbonium ion electrophiles. These carbonium ion can undergo rearrangement before attacking the benzene ring + CH3CH2CH2Cl AlCl3 + HCl PowerPoint Presentation: Friedal -Craft acylation: Benzene react with acid chlorides in presence of aluminium chloride to give aromatic ketones + AlCl3 + HCl Acetyl chloride acetophenone PowerPoint Presentation: Mechanism Step1 CH3 C Cl + AlCl 3 CH 3 C + AlC 4 O O + Step2 + C CH3 O + + H COCH3 Step 3 + H COCH3 AlCl4- + HCl + AlCl3 PowerPoint Presentation: Electrophilic substitution in aliphatic compounds General mechanism PowerPoint Presentation: Nitrosation PowerPoint Presentation: Acetone with hydroxyl group + OH 2 + H 2 O Br Br Br + Br 2 + Br+ CH3 C C Br O H H 1 2 3 PowerPoint Presentation: Reference : BS Bahl

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