Kinetic information tells us that the rate is doubled when the [CH3l]is doubled,and also doubled when the [HO]is doubled. The rate is first order w.r.t.both reactants and is therefore 2dorder overall. Rate=k,[CH3I][HO] The rate and mechanism are consistent since they the mechanism requires a collision between the hydroxide ion and methyl iodide.Both species are present in the transition state,and the frequency of collisions is proportional to the concentrations of the reactants SN2 substitution,nucleophilic,bimolecular. Bimolecular means that the transitions state of the R.D.S.involves the collision of two molecules.(Bimolecular reactions generally have 2d order overall rate equations). Ch06 Alkyl Halides (landscape) Page 11
Ch06 Alkyl Halides (landscape) Page 11 Kinetic information tells us that the rate is doubled when the [CH3I] is doubled, and also doubled when the [HO- ] is doubled. The rate is first order w.r.t. both reactants and is therefore 2nd order overall. Rate = kr [CH3I] [HO- ] The rate and mechanism are consistent since they the mechanism requires a collision between the hydroxide ion and methyl iodide. Both species are present in the transition state, and the frequency of collisions is proportional to the concentrations of the reactants. SN2 = substitution, nucleophilic, bimolecular. Bimolecular means that the transitions state of the R.D.S. involves the collision of two molecules. (Bimolecular reactions generally have 2nd order overall rate equations)
Versatility of the S2 mechanism The SN2 mechanism is a common reaction mechanism and can cover a variety of functional group transformations of alkyl halides All of the type: Nuc:R-X Nuc-R X Nucleophile Product Class of Product R一X+-i: R-I: alkyl halide R-X+-:OH R—OH alcohol R X+-:OR R-OR' ether R-X +-:SH R-SH thiol(mercaptan) R-X +:SR' R-SR' thioether (sulfide) R一X+:NH3 → R-NH时X amine salt R-X+N-N-N:- → R-N-N-N: azide R-X+-:C=C一R R—C=C-R alkyne R-X+-:C=N: R—C=N: nitrile ℃ R-X+0-C-R' R-Q-C-R ester R一X+:PPh3 [R-PPh3]+-X phosphonium salt Ch06 Alkyl Halides (landscape) Page 12
Ch06 Alkyl Halides (landscape) Page 12 Versatility of the SN2 mechanism The SN2 mechanism is a common reaction mechanism and can cover a variety of functional group transformations of alkyl halides. All of the type:
Halogen exchange reactions are normally used to prepare either iodo-of fluoro-compounds from other alkyl halides since direct iodination is too slow and direct fluorination is too violent. H2C=CH-CH2CI Nal H2C=CH-CH2l NaCl HaC-CH2CI +KF18-crown-HC-CH2F +KCI Nucleophile Strength The rate of the SN2 reaction strongly depends on the nature of the nucleophile-a good nucleophile gives faster rates than a worse nucleophile. Consider methanol(CH,OH)and methoxide(CH,O)reacting with CHI. It is found that methoxide reacts about a million times faster in SN2 reactions than methanol Generally,negatively charged species are much better nucleophiles than analogous neutral species. The two transition states are different energetically. Ch06 Alkyl Halides (landscape) Page 13
Ch06 Alkyl Halides (landscape) Page 13 Halogen exchange reactions are normally used to prepare either iodo- of fluoro- compounds from other alkyl halides since direct iodination is too slow and direct fluorination is too violent. Nucleophile Strength The rate of the SN2 reaction strongly depends on the nature of the nucleophile – a good nucleophile gives faster rates than a worse nucleophile. Consider methanol (CH3OH) and methoxide (CH3O - ) reacting with CH3I. It is found that methoxide reacts about a million times faster in SN2 reactions than methanol. Generally, negatively charged species are much better nucleophiles than analogous neutral species. The two transition states are different energetically
The two transition states are different energetically The T.S.with methoxide has the negative charge shared over the oxygen atom and the leaving halide CH,-0 CH,一 H conjugate base lower E (stronger nucleophile) CH,-0: H conjugate acid higher E. (weaker nucleophile) ◆2013 Parson Edcon ine In the methanol case,there is no negative charge.The halide has a partial negative charge and the oxygen has a partial positive charge.This is of higher energy. Ch06 Alkyl Halides (landscape) Page 14
Ch06 Alkyl Halides (landscape) Page 14 The two transition states are different energetically. The T.S. with methoxide has the negative charge shared over the oxygen atom and the leaving halide. In the methanol case, there is no negative charge. The halide has a partial negative charge and the oxygen has a partial positive charge. This is of higher energy
Basicity and Nucleophilicity Basicity is defined by the equilibrium constant for abstracting a proton. Basicity B-H+A: Nucleophilicity B-c+x: Nucleophilicity is defined by the rate of attack on an electrophilic carbon atom Trends in Nucleophilicity (there are three) 1)Species with a negative charge are stronger nucleophiles than analogous species without a negative charge. (Bases are always stronger nucleophiles than their conjugate acids). OH>H,O SH>HS NH,>NH 2)Nucleophilicity decreases from left to right across the periodic table. (The more electronegative elements hold on more tightly to their non-bonding electrons). NH2>OH>F NH3>H2O (CH;CH2)3P>(CH3CH2)2S Ch06 Alkyl Halides (landscape) Page 15
Ch06 Alkyl Halides (landscape) Page 15 Basicity and Nucleophilicity Basicity is defined by the equilibrium constant for abstracting a proton. Nucleophilicity is defined by the rate of attack on an electrophilic carbon atom. Trends in Nucleophilicity (there are three) 1) Species with a negative charge are stronger nucleophiles than analogous species without a negative charge. (Bases are always stronger nucleophiles than their conjugate acids). -OH > H2O - SH > H2S -NH2 > NH3 2) Nucleophilicity decreases from left to right across the periodic table. (The more electronegative elements hold on more tightly to their non-bonding electrons). -NH2 > -OH > F- NH3 > H2O (CH3CH2)3P > (CH3CH2)2S