![SN1 : Unimolecular Nucleophilic Substitution
• In this reaction, bond breaking between carbon and leaving
group is complete before bond formation with nucleophile.
• This type of reaction is classified as unimolecular because
only the alkyl halide is involved in rate determing step.
• Rate: k[alkyl halide]
14Mr. Mote G.D
C
CH3
H3C
CH3
Br
Nucleophilic
substitution C
CH3
H3C
CH3
OCH3
Methanol
CH3OH
HBr
2-methoxy-2-methylpropane2-bromo-2- methylpropane](https://image.slidesharecdn.com/alkylhalide-190214054224/75/Alkyl-halide-14-2048.jpg)
![SN2 : bimolecular Nucleophilic Substitution
• It is concerted process, bond breaking and bond formation
takes place simultaneously
• This type of reaction is classified as bimolecular because the
alkyl halide and nucleophile are involved in rate determining
step.
• Rate: k[alkyl halide][Nucleophile]
17Mr. Mote G.D
C
H
H
H
Br
Nucleophilic
substitution C
H
H
H
OH
NaOH
NaBr
methanolmethyl bromide](https://image.slidesharecdn.com/alkylhalide-190214054224/75/Alkyl-halide-17-2048.jpg)
Uploaded byGanesh Mote
Alkyl halide
Alkyl halides are derivatives of alkanes where one or more hydrogen atoms are replaced by halogen atoms such as fluorine, chlorine, bromine, or iodine. They are represented by R-X, where R is an alkyl group and X is a halogen. Common methods for preparing alkyl halides include direct halogenation of alkanes, addition of hydrogen halides to alkenes and alkynes, and reactions of hydrogen halides, phosphorus halides, or thionyl chloride with alcohols. Alkyl halides undergo nucleophilic substitution and elimination reactions. They can be reduced to alkanes or used to form Grignard reagents. Common uses
More Related Content
Alkyl halide
- 1. Alkyl halide Mr. MoteG.D. ADCBP, Ashta Dist: Sangli Maharashtra
- 2. Alkyl halides • Alkylhalides are the derivatives of alkanes in which hydrogen atom is replaced by a halogen atom F, Cl, Br or I • Alkyl halides are represented by R-X, R-alkyl group, X- halogen like chloro, Fluoro, Bromo, and Iodo group. C H H H Cl C H H H C C H H H C H H Cl H H C H H Cl Methyl chloride ethtyl chloride propyl chloride
- 3. Preparation methods ofalkyl halides 1. Direct Halogenations of alkanes. 2. Addition of hydrogen halides to alkenes and alkynes. 3. Action of Hydrogen halides on alcohol. 4. Action of Phosphorus halides on alcohols. 5. Action of Thionyl chloride on alcohols. 6. Halogen Exchange. 7. The Hunsdiecker Reaction.
- 4. 1. Halogenation ofalkane • It involves the substitution of H-atoms of alkanes by as many halogen atoms i.e., by chlorine (chlorination) ; by bromine (bromination) by iodine (iodination) or by fluorine (fluorination) • Methane reacts with chlorine in the presence of ultraviolet light or at high temperature (300°C) to yield methyl chloride or chloromethane and hydrogen chloride. CH4 Cl2 uv light CH3Cl HCl Methane chlorine methyl chloride 4Mr. Mote G.D
- 5. 2. Addition ofhydrogen halides 1. Alkenes react with hydrogen halides (HCl, HBr or HI) to form alkyl halides. H2C CH2 HBr CH3 CH2 Br ethyl bromide ethylene C H C H HBr 2-Bromo butane2-Butene H3C CH3 C H C H H3C CH3 BrH 5Mr. Mote G.D
- 6. 3. Action ofhalogen acids on alcohol • Alcohols react with HBr or HI to produce alkyl bromides or alkyl halides. 6Mr. Mote G.D H3C H2 C HBr CH3 CH2 Br ethyl bromide OH H2O ethyl alcohol Mechanism: HBr H+ + Br- H3C H2 C OH CH3 CH2 Br ethyl bromide H2O
- 7. 4. Action ofPhosphorus halides on alcohol • Alcohols react with phosphorus halides to produce alkyl halides. 7Mr. Mote G.D H3C H2 C PCl5 CH3 CH2 Cl ethyl bromide OH HCl ethyl alcohol POCl3 P Cl Cl Cl Cl Cl H3C H2 C OH ethyl alcohol + -HCl H3C H2 C O- P Cl Cl Cl Cl+ CH3 CH2 Cl ethyl bromide POCl3 Mechanism
- 8. 5. Action ofthionyl chloride on alcohol • Alcohols react with thionyl chloride in the presence of pyridine to produce alkyl halides. 8Mr. Mote G.D H3C H2 C SOCl2 CH3 CH2 Cl ethyl chloride OH SO2 ethyl alcohol HClpyridine S OCl Cl H3C H2 C OH ethyl alcohol -HCl CH3 CH2 O- + S O Cl CH3 CH2 Cl ethyl chloride SO2+ Mechanism
- 9. 6. Halogen exchangereaction • The alkyl bromide is heated with concentrated solution of sodium iodide in acetone to form alkyl iodide. 9Mr. Mote G.D H3C H2 C NaI CH3 CH2 I ethyl iodide Br NaBr ethyl bromide pyridine
- 10. 7. Hunsdiekar reaction •Silver salt of carboxylic acid react with halogen to give unstable intermediate which is decarboxylated to form alkyl halides. 10Mr. Mote G.D CH3 C O OAg Silver acetate Br2 CCl4 CH3 Br + AgBr + CO2 Methyl bromide Mechanism CH3 C O OAg Silver acetate Br2 -AgBr CH3 C O OBr CH3 C O O + Br -CO2 CH3 + Br CH3 Br Methyl bromide
- 11. Reactions of alkylhalides 1. Nucleophilic substitution reactions a) Substitution by hydroxyl group b) Substitution by amino group c) Substitution by alkynyl group d) Substitution by alkoxy group e) Substitution by thiol group (SH) f) Substitution by thioether group (—S—). g) Substitution by an ester group. (RCOO—). h) Substitution by a Cyano or Isocyano group 2. Reduction: Reduction of Alkyl halides 3. Elimination reactions 4. Wurtz reaction 5. Reactions with Metals.
- 12. Nucleophilic substitution reaction •Nucleophile: any reagent that donates an unshared pair of electrons to form a new covalent bond. • Nucleophilic substitution reaction in which one nucleophile is substituted for another. • Two types of nucleophilic substitution reactions 1) SN1 : Unimolecular Nucleophilic Substitution 2) SN2 : Bimolecular Nucleophilic Substitution 12Mr. Mote G.D Nu: C X Nucleophilic substitution C Nu :X Nucleophile Alkyl halide
- 13. I. Substitution byhydroxyl group • Alkyl halides react with aqueous potassium hydroxide to form alcohol. 13Mr. Mote G.D H3C H2 C NaOH CH3 CH2 OH Ethanol Br NaBr ethyl bromide H2O
- 14. SN1 : UnimolecularNucleophilic Substitution • In this reaction, bond breaking between carbon and leaving group is complete before bond formation with nucleophile. • This type of reaction is classified as unimolecular because only the alkyl halide is involved in rate determing step. • Rate: k[alkyl halide] 14Mr. Mote G.D C CH3 H3C CH3 Br Nucleophilic substitution C CH3 H3C CH3 OCH3 Methanol CH3OH HBr 2-methoxy-2-methylpropane2-bromo-2- methylpropane
- 15. I. Mechanism ofSN1 1. Ionization of a C-X bond gives a 3° carbocation intermediate 2. Reaction of methanol from either face of the planra carbocation intermediate gives an oxonium ion. 3. Proton transfer to give tert- butyl methyl ether 15Mr. Mote G.D C CH3 H3C CH3 Br slow, rate determining C CH3 H3C CH3 Br- C CH3 H3C CH3 fast HOCH3 C CH3 H3C CH3 OCH3 H C CH3 H3C CH3 OCH3 H Br- C CH3 H3C CH3 OCH3 HBr 2-methoxy, 2-methyl propaneoxonium ion oxonium ionCarbocation ter butyl carbocation ter butyl bromide
- 16. Energy profile diagramof SN1 Two peak will arises 1. One is formation transition when leaving group is removing. 2. Second peak arises when nucleophile forming pair with carbocation. 3. Carbocation lies in between two peak. 4. Product has lower energy 16Mr. Mote G.D
- 17. SN2 : bimolecularNucleophilic Substitution • It is concerted process, bond breaking and bond formation takes place simultaneously • This type of reaction is classified as bimolecular because the alkyl halide and nucleophile are involved in rate determining step. • Rate: k[alkyl halide][Nucleophile] 17Mr. Mote G.D C H H H Br Nucleophilic substitution C H H H OH NaOH NaBr methanolmethyl bromide
- 18. Mechanism of SN2 1.The nucleophile attacks the reactive centre from the opposite side of leaving group, Backside attack by nucleophile. 2. SN2 reaction is driven by the attraction between negative charge of nucleophile and positive charge on leaving group. 18Mr. Mote G.D NaOH OHNa C H H Na H BrOH- C H H H OH NaBr C H H H BrHO
- 19. energy profile ofSN2 1. Only one rate determining step because one transition step is generated 2. Initially reation is slow and after transition reaction will faster 19Mr. Mote G.D
- 20. SN1 versus SN2 20Mr.Mote G.D Sr.No SN1 SN2 1 Unimolecular Bimolecular 2 Most favorable alkyl halide 3°>2°>1 Most favorable alkyl halide 1°>2°>3° 3 Reacting nucleophile weak Reacting nucleophile strong base 4 Solvent is protic like alcohol Solvent is aprotic DMSO and acetone 5 Stereochemistry is mixed retention as well as inversion Stereochemistry is inversion
- 21. Factor affecting onSN1 and SN2 21Mr. Mote G.D Sr. no SN1 SN2 1 Good leaving group- alcohol Better leaving groups faster the reaction 2 Reactivity order: 3°>2°>1° Reactivity order: 3°>2°>1° 3 Stable carbocation-tertiary carbocation 4 Weak bases-Water or Alcohol Favored by strong base 5 Polar protic solvents Polar aprotic solvents 6 Only substrate concentration fasters the reaction substrate concentration as well as strong base fasters the reaction 7 First order kinetics Second order kinetics
- 22. II. Substitution byamino group • Alkyl halides react with alcoholic solution of ammonia to form amines. 22Mr. Mote G.D NaBr NH3 NH2 + H+ H3C H2 C Br δ+ δ− CH3 CH2 NH2 HBr Mechanism H3C H2 C NH3 CH3 CH2 NH2 ethyl amine Br HBr ethyl bromide Ethanol ethyl amine ethyl bromide
- 23. III. Substitution byalkynyl group • Alkyl halides react with sodium acetylides to form higher alkynes. 23Mr. Mote G.D H3C H2 C C CH3 CH2 CBr NaBr ethyl bromide Ethanol CHNa CH but-1-yne CH + Na+ H3C H2 C Br δ+ δ− CH3 CH2 C Mechanism ethyl bromide CH C NaBr CH CNa but-1-yne
- 24. IV. Substitution byalkoxy group • Alkyl halides react with sodium methoxide to form ethers. • Sodium alkoxide is prepared by dissolving sodium in alcohol. • This method of making alcohol is called as williamson ether synthesis 24Mr. Mote G.D H3C H2 C CH3 CH2 OBr NaBr ethyl bromide CH3ONa CH3 methoxyethanesodium methoxide CH3ONa CH3O + Na+ H3C H2 C Br δ+ δ− CH3 CH2 O CH3 NaBr Mechanism
- 25. V. Substitution bythiol group • Alkyl halides react with potassium hydrosulfide to form thiols. 25Mr. Mote G.D + K+ H3C H2 C I δ+ δ− CH3 CH2 SH Mechanism H3C H2 C KSH CH3 CH2 SH ethanethiol I KI ethyl iodide Ethanol SH KI ethanethiol KSH
- 26. VI. Reaction withpotassium sulphide • Alkyl halides react with potassium sulphide to form dialkyl sulfides 26Mr. Mote G.D K2S ethyl iodide CH3 CH2 I S CH2CH3H3CH2C diethylsulphide 2 2KI+ CH3 CH2 I SK K+ + I CH2 CH3 S CH2CH3H3CH2C diethylsulphide 2KI+ Mechanism
- 27. VI. Reaction withsilver salt of carboxylic acid • Alkyl halides react with silver salt of carboxylic acid to form ester 27Mr. Mote G.D CH3COOAg ethyl bromide CH3 CH2 Br silver acetate CH3 CH2 COOCH3 AgBr ethyl acetate ethanol CH3COOAg CH3COO + Ag+ H3C H2 C Br δ+ δ− AgBr Mechanism CH3 CH2 COOCH3 ethyl acetate
- 28. VII. Reaction withsodium cyanide • Alkyl halides react with sodium cyanide in ethanol to form alkyl cyanides. 28Mr. Mote G.D NaCN ethyl iodide CH3 CH2 I ethanol CH3 CH2 CN ethyl cyanide NaI NaCN + Na+ H3C H2 C I δ+ δ− NaI Mechanism CH3 CH2 CN ethyl cyanide CN-
- 29. 2. Reduction ofalkyl halides • Alkyl halide undergo reduction with nascent hydrogen in presence of reducing agent like Zn/HCl to form alkanes. R X Alkyl halide H2 R H HX Alkane Zn/HCl H3C I Methyl iodide H2 H3C H HI methane Zn/HCl 29Mr. Mote G.D
- 30. 3. Elimination reaction:dehydrohalogenation of alkyl halides • When alkyl halide is heated with alcoholic solution of sodium hydroxide to form alkene and hydrogen halide. R H C H CH2 x NaOH in alcohol ∆ R H C CH2 alkene H3C H C H CH2 Cl NaOH in alcohol ∆ H3C H C CH2 Propene alkyl halide Propyl chloride H2O H2O HX HCl 30Mr. Mote G.D
- 31. 4. Wurtz reaction •Higher alkanes are produced by heating an alkyl halide with sodium metal in dry ether. Two molecules of alkyl halide lose their halogen atoms as NaX. These net result is the joining of two alkyl group to yield symmetrical alkane having even number of carbon atoms. R X 2Na R X Alkyl halide ether R R Symmetrical alkane 2NaX H3C Br 2Na H3C Br Methyl halide ether H3C CH3 ethane 2NaBr 31Mr. Mote G.D
- 32. 5. Reaction withmetal: formation of Grignard reagents • Alkyl magnesium halides(Grignard reagent) are obtained by treating alkyl halides with magnesium in anhydrous ether to give alkanes. RX Mg ether RMgX Alkyl halide Alkyl magnesium halide RMgX H OH R H MgX(OH) Alkane CH3MgI H OH 3HC H MgI(OH) Methane Methyl magnesium iodide 32Mr. Mote G.D
- 33. Uses of alkylhalides Sr. No Name Uses 1 Ethyl chloride Used to prevent pain caused by injection, muscle pain, injury 2 Chloroform Solvent for fats, waxes and rubber, preparation of chloropicrin and chloretone. 3 Trichloroethylene To remove grease from metal parts, solvent 4 tetrachloroethylene To remove grease from metal parts, solvent Dry cleaning 5 Dichloro methane Strong solvent, flammability suppressant, vapour pressure depressant, Viscosity thinner 6 tetra chloromethane Propellants for aerosol, dry cleaning, degreasing, fire extinguisher, pesticide 7 iodoform Disinfectant, antiseptic, CH3 CH2 Cl ethyl chloride CHCl3 chloroform C C Cl Cl H Cl trichloro ethylene C C Cl Cl Cl Cl tetrachloroethylene CH2 Cl Cl dichloro methane C Cl ClCl Cl tetrachloro methane CHI3 iodoform