Presentation on theme: "Substitution and Elimination"— Presentation transcript:
1Substitution and Elimination Reaction of Alkyl Halides
2Organic compounds with an electronegative atom or an electron-withdrawing group bonded to a sp3 carbon undergo substitution or elimination reactions-SubstitutionEliminationHalide ions are good leaving groups. Substitution reaction on these compounds are easy and are used to get a wide variety of compoundsalkyl fluoridealkyl chloridealkyl bromidealkyl iodide
3Alkyl Halides in Nature Synthesized by red algaered algaeSynthesized by sea harea sea hare
5Alkyl Halides in Nature Several marine organisms, including sponges, corals, and algae, synthesize organohalides (halogen-containing organic compounds) that they use to deterpredators. For example, red algae synthesize a toxic, foultastingorganohalide that keeps predators from eating them. One predator, however, that is not deterred is a mollusk called a sea hare.
6Alkyl Halides in Nature After consuming red algae, a sea hare converts the original organohalide into a structurally similar compound it uses for its own defense. Unlike other mollusks, a sea hare doesnot have a shell. Its method of defense is to surround itself with a slimy material that contains the organohalide, thereby protecting itself from carnivorous fish.
7Substitution Reaction with Halides (1)(2)bromomethanemethanolIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (1) and (2) is doubled, the rate of the reaction quadruples.If concentration of (2) is doubled, the rate of the reaction is doubled.
8Substitution Reaction with Halides (1)(2)bromomethanemethanolRate law:rate = k [bromoethane][OH-]this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic2 stands for bimolecular
9Mechanism of SN2 Reactions Alkyl halideRelative rate1200401≈ 0The rate of reaction depends on the concentrations of both reactants.When the hydrogens of bromomethane are replaced with methyl groups the reaction rate slow down.The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of only one stereoisomer
10Mechanism of SN2 Reactions Hughes and Ingold proposed the following mechanism:Transition stateIncreasing the concentration of either of the reactant makes their collision more probable.
11Mechanism of SN2 Reactions Steric effectactivationenergy: DG2Energyactivationenergy: DG1reaction coordinatereaction coordinateInversion of configuration(R)-2-bromobutane(S)-2-butanol
12Factor Affecting SN2 Reactions The leaving grouprelative rates of reaction pKa HXHO- + RCH2I RCH2OH + IHO- + RCH2Br RCH2OH + BrHO- + RCH2Cl RCH2OH + ClHO- + RCH2F RCH2OH + FThe nucleophileIn general, for halogen substitution the strongest the base the better the nucleophile.pKaNuclephilicity
13SN2 Reactions With Alkyl Halides an alcohola thiolan ethera thioetheran aminean alkynea nitrile
14Substitution Reactions With Halides 1-bromo-1,1-dimethylethane1,1-dimethylethanolRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a SN1 reaction.S stands for substitutionN stands for nucleophilic1 stands for unimolecularIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (2) is doubled, the rate of the reaction is not doubled.
15Mechanism of SN1 Reactions Alkyl halideRelative rate≈ 0 *12The rate of reaction depends on the concentrations of the alkyl halide only.When the methyl groups of 1-bromo-1,1-dimethylethane are replaced with hydrogens the reaction rate slow down.The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of two stereoisomers* a small rate is actually observed as a result of a SN2
16Mechanism of SN1 Reactions nucleophile attacks the carbocationslowC-Br bond breaksfastProton dissociation
17Mechanism of SN1 Reactions Rate determining stepCarbocation intermediateDGR++ X-+R-OH2R-OH
18Mechanism of SN1 Reactions Inverted configuration relative the alkyl halideSame configuration as the alkyl halide
19Factor Affecting SN1 reaction Two factors affect the rate of a SN1 reaction:The ease with which the leaving group dissociate from the carbonThe stability of the carbocationThe more the substituted the carbocation is, the more stable it is and therefore the easier it is to form.As in the case of SN2, the weaker base is the leaving group, the less tightly it is bonded to the carbon and the easier it is to break the bondThe reactivity of the nucleophile has no effect on the rate of a SN1 reaction
20Comparison SN1 – SN2 SN1 SN2 A two-step mechanism A one-step mechanism A unimolecular rate-determining stepA bimolecular rate-determining stepProducts have both retained and inverted configuration relative to the reactantProduct has inverted configuration relative to the reactantReactivity order:3o > 2o > 1o > methylmethyl > 1o > 2o > 3o
21Elimination Reactions 1-bromo-1,1-dimethylethane2-methylpropeneRate law:rate = k [1-bromo-1,1-dimethylethane][OH-]this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecular
22The mechanism shows that an E2 reaction is a one-step reaction The E2 ReactionA proton is removedBr- is eliminatedThe mechanism shows that an E2 reaction is a one-step reaction
23Elimination Reactions 1-bromo-1,1-dimethylethane2-methylpropeneRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a E1 reaction.E stands for elimination1 stands for unimolecularIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (2) is doubled, the rate of the reaction is not doubled.
24The E1 Reaction The base removes a proton The alkyl halide dissociate, forming a carbocationThe mechanism shows that an E1 reaction is a two-step reaction
25Products of Elimination Reaction 30%50%80%2-butene2-bromobutane20%1-buteneThe most stable alkene is the major product of the reaction for both E1 and E2 reactionThe greater the number of alkyl substituent the more stable is the alkeneFor both E1 and E2 reactions, tertiary alkyl halides are the most reactive and primary alkyl halides are the least reactive
26Competition Between SN2/E2 and SN1/E1 rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base]SN2 and E2 are favoured by a high concentration of a good nucleophile/strong baseSN1 and E1 are favoured by a poor nucleophile/weak base, because a poor nucleophile/weak base disfavours SN2 and E2 reactions
27Competition Between Substitution and Elimination SN2/E2 conditions:In a SN2 reaction: 1o > 2o > 3oIn a E2 reaction: 3o > 2o > 1o10%90%75%25%100%
28Competition Between Substitution and Elimination SN1/E1 conditions:All alkyl halides that react under SN1/E1 conditions will give both substitution and elimination products (≈50%/50%)
29SummaryAlkyl halides undergo two kinds of nucleophilic subtitutions: SN1 and SN2, and two kinds of elimination: E1 and E2.SN2 and E2 are bimolecular one-step reactionsSN1 and E1 are unimolecular two step reactionsSN1 lead to a mixture of stereoisomersSN2 inverts the configuration od an asymmetric carbonThe major product of a elimination is the most stable alkeneSN2 are E2 are favoured by strong nucleophile/strong baseSN2 reactions are favoured by primary alkyl halidesE2 reactions are favoured by tertiary alkyl halides