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  • Reactions Ionic reactions: Radical reactions:Bond breaking and bond Bond breaking and bond makingmaking take place in a take place in aheterolytic fashion homolytic fashion -R X R+ + X R X R. + X.
  • Ionic Reactions :1) substitution : Y + R X R Y + X R = aliphatic as well as aromatic2) elimination : 3) addition : R1 R1 X H R1 XH X + Y R2 Y R2 Y R2 X = O , NH
  • Nucleophilic Substitution
  • Nucleophilic Substitution Y + R X R Y + X R = aliphatic as well as aromaticNucleophile + Substrate Product + Leaving group
  • Nucleophilic Substitution S N1 S N2 S: Substitution S: Substitution N: Nucleophilic N: Nucleophilic 1: unimolecular 2: Bimolecularleaving group goes first and nucleophile comes laterY + C X C+ C Y + X nucleophile attacks and leaving group goes simultaneously Y + C X C Y + X
  • SN2 reaction and mechanism kinetics stereochemistry substrate structure nucleophiles leaving groups solvents
  • SN2 reaction and mechanism
  • Synthetic Utility of the SN2 ReactionA variety of functional groups can be prepared employing a goodnucleophile and an electrophile with a good leaving group: 9
  • SN2 reaction : Kinetics
  • The Rate Law of an SN2 ReactionObtained experimentally: Rate law includes both the alkyl halide and the nucleophile, a second- order process 12
  • SN2 reaction : Stereochemistry
  • SN2 MECHANISM nucleophilic attack R (R)-config ..H O: .. .. C : Br : .. CH3 Rattacks Hback lobe .. H O: .. CINVERSION CH3 (S)-config H
  • Stereochemistry of SN2 Walden inversion =>
  • THE INVERSION sp2 R 2p HO C BPROCESS partial bonding HO C Br activated complex CH3 H is trigonal planar (sp2 ) configuration .. R sp3 is invertedH O: .. R sp3 C : Br Ea HO : C CH3 H CH3 (R)-configuration H (S)-configuration
  • SN2 reaction : substratestructure
  • Less bulky Should stabilize the transition state
  • SN2 Reaction: substrate structureKI in Acetone at 25 krel CH3 Br 150 CH3 CH2 Br 1 CH3 CH Br 0.008 CH3 CH3 unreactive! CH3 C Br CH3
  • Relative rates of SN2 reactions of alkyl chlorides with the iodide ion The rates are given with respect to Alkyl chloride relative rate n-BuCl Me Cl 200 0.02 Cl Me 920 O Cl O Cl 1,00,000
  • O O Nu- only SN2, no SN1 Hal Nu R R R = alkyl, aryl, OR Relative rates of SN2 reactions with iodide ion O Cl 1:500 Me Cl C=O group stabilizes the T.S. by Overlap of its * orbital with full P-orbital of the C-atom under attack * of of the C-Cl + +* of the C=O Cl * of Cl H the C=O H H H Nu Nu
  • SN2 reaction : Effect of Nucleophile
  • The nucleophilicity may be correlated with the availability of the electron pairs and the ease with which it is donated
  • Trends in Nuc. Strength Increases down Periodic Table, as size and polarizability increase: I- > Br- > Cl- >F- Of a conjugate acid-base pair, the base is stronger: OH- > H2O, NH2- > NH3
  • Polarizability and nucleophilicity - increasedpolarizability makes for a better nucleophile I- > Br- > Cl- >F-
  • Effect of Nucleophile : The nucleophilicity may be correlated to its basicity as both involve the availability of the electron pairs and the ease with which it is donated rapid CH3O + H3C I CH3OCH3 + I very CH3OH + H3C I CH3OCH3 + HI slow Nucleophilicity of CH3O > A negatively charged nucleophile is CH3OH always stronger than its conjugate acid. Stronger base weaker base better nucleophile poorer nucleophile HO > H2O CH3O > CH3OH H2N > NH3The direct relationship between basicity and nucleophilicityis maintained if the reaction occurs in the gas phase
  • SN2: Nucleophilic Strength Stronger nucleophiles react faster. Strong bases are strong nucleophiles, but not all strong nucleophiles are basic. =>
  • SN2 reaction : Effect of leaving group
  • A good leaving group needs to be aStable anions that are weak baseswhich can delocalize charge 31
  • The Leaving Group
  • S