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11. Reactions of Alkyl Halides:

Nucleophilic Substitutions and

Eliminations

Organic Chemistry, 7thedition

Alkyl Halides React with

Nucleophiles and Bases

"Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic "Nucleophiles will replace the halide in C-X bonds of many alkyl halides(reaction as Lewis base) "Nucleophiles that are Br elimination

Why this Chapter?

"Nucleophilic substitution, base induced elimination are among most widely occurring and versatile reaction types in organic chemistry "Reactions will be examined closely to see: -How they occur -What their characteristics are -How they can be used

11.2 The SN2Reaction

"Reaction is with inversion at reacting center "Follows second order reaction kinetics "Ingold nomenclature to describe characteristic step: "S=substitution "N (subscript) = nucleophilic "2 = both nucleophile and substrate in characteristic step (bimolecular)

Mechanism of The SN2 Reaction

The reaction takes place in

a single step when the incoming nucleophile approaches from a direction 180˸ଉaway from the leaving halide ion .

SN2Transition State

"The transition state of an SN2 reaction has a planar arrangement of the carbon atom and the remaining three groups

11.3 Characteristics of the SN2

Reaction

"Sensitive to steric effects "Methyl halides are most reactive "Primary are next most reactive "Secondary might react "Tertiary are unreactive by this path "No reaction at C=C (vinyl halides)

Reactant and Transition State

Energy Levels Affect Rate

(a) Higher reactant energy level (red curve) = faster reaction (smaller G). (b) Higher transition state energy level (red curve) = slower reaction (larger G). Energy diagrams showing the effects of (a) substrate, (b) nucleophile, (c) leaving group And (d)solvent on Sn2 reaction rates. V = k[RX][Nu]

Reactant and Transition State

Energy Levels Affect Rate

Steric Effects on SN2 Reactions

The carbon atom in (a) bromomethane is readily accessible resulting in a fast SN2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2-bromopropane (secondary), and (d) 2-bromo-2- methylpropane (tertiary) are successively more hindered, resulting in successively slower SN2 reactions.

Order of Reactivity in SN2

"The more alkyl groups connected to the reacting carbon, the slower the reaction Vinylic halides and aryl halides are unreactive toward SN2 reaction. This lack of reactivity is probably due to steric factors.

The Nucleophile

"Neutral or negatively charged Lewis base "Reaction increases coordination at nucleophile "Neutral nucleophile acquires positive charge "Anionic nucleophile becomes neutral "See Table 11-1 for an illustrative list

Relative Reactivity of Nucleophiles

"Depends on reaction and conditions "More basic nucleophiles react faster "Better nucleophiles are lower in a column of the periodic table "Anions are usually more reactive than neutrals "What product would you expect from SN2 reaction of 1-bromobutane with each of the following? "Which substance in each of the follwoing pairs is more reactive as a nucleophile?Explain. a) (CH3)2Ǧ3)2NH ,b) (CH3)B or (CH3)B c) H2O or H2S

The Leaving Group

"In SN2 reaction the nature of the leaving group displaced by the incoming nucleophile. "A good leaving group reduces the barrier to a reaction, the best stabilize the negative charge in the transition state. "Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge such Cl-,Br-. While strong bases such as

OH-,NH2-are poor leaving groups.

Thionyl chloraide,

Phosphorus tribromid

Poor Leaving Groups

"If a group is very basic or very small, it is prevents reaction "Alkyl fluorides, alcohols, ethers, and amines do not typically undergo SN2 reactions. "Alcohol can be mede more reactive toward nucleophilic substitution by treating with para-toulenesulfonyl chloride to form tosylate. "Tosylate are more reactive reaction than halides in nucleophilic substitutions. "Rank the following compounds in order of their expected reactivity toward SN2 reaction:

CH3Br, CH3OTos, (CH3)3CCl, (CH3)2CHCl

The Solvent

"The rates of SN2 reaction are strongly affected by solvent. "Solvents that can donate hydrogen bonds (-OH or NH) slowSN2 reactions by associating with reactants "Energy is required to breakinteractions between reactant and solvent "Polar a protic solvents form weaker interactions with substrate and permit fasterreaction , Dimethyl sulfoxide,Dimethylformamide ,Acetonitril ,Hexamethylphsphoramide

11.4 The SN1 Reaction

"Tertiary alkyl halides react rapidly in protic solvents by a mechanism that involves departure of the leaving group former to addition of the nucleophile "Called an SN1 reaction occurs in two distinct steps while SN2 occurs with both events in same step

SN1 Energy Diagram

"Rate-determining step is formation of carbocation

V = k[RX]

Rate-Limiting Step

"Rate-determining step is formation of carbocation "The general rate of a reaction is controlled by the rate of the slowest step "The rate depends on the concentration of the species and the rate constant of the step "The highest energy transition state point on the diagram is that for the rate determining step.

V = k[RX]

11.5 Characteristics of the SN1

Reaction

Substrate

"Tertiary alkyl halide is most reactive by this mechanism "Controlled by stability of carbocation factor that stabilizes a high-energy intermediate stabilizes transition state

Mechanism of The SN1 Reaction

The mechanism of the

Sn1 reaction of 2-bromo-2-

Methylpropane with H2O

Involves three steps.

Allylic and Benzylic Halides

"Allylic and benzylic intermediates stabilized by delocalization of charge Primary allylic and benzylic are also more reactive in the

SN2mechanism as well as in SN1

"Rank the following substances in order of their expected SN1 reactivity: "3-Bromobut-1-ene and 1-bromobut-2-ene undergo SN1 reaction at nearly the same rate even though one is a secondary halide and the other is primary. Explain.

Effect of Leaving Group on SN1

"Seriously dependent on leaving group "Reactivity: the larger halides ions are better leaving groups "In acid, OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halide "p-Toluensulfonate (TosO-) is excellent leaving group

Nucleophiles in SN1

"Since nucleophilic addition occurs after formation of carbocation, reaction rate is not normally affected by nature or negatively charged or concentration of nucleophile (the nucleophile can't affect the reaction rate) in Sn2 nucleophile plays a major role . "the reaction occures at the same rate regardless of wether X is Cl,Br,orI

The mechanism of the Sn1

reaction of a tertiary alcohol with HBr to yield an alkyl halide.

Neutral water is the leaving

group

Nucleophiles in SN1

Solvent in SN1

"Stabilizing carbocation also stabilizes associated transition state and controls rate "Solvent effects in the SN1 reaction are due largely to stabilization or destabilization of the transition state

Polar Solvents Promote Ionization

"Polar, protic and unreactive Lewis base solvents help formation of R+

"Sn1 reaction take place much more rapidly in strongly polar solvents, such as water and methanol than in less polar solvent, such as ether and chloroform.

"Solvent polarity is measured as dielectric polarization (P) "Nonpolar solvents have low P "Polar solvents have high P values "Sn2 reactions are disfavored in protic solvents because the ground-state energy of the nucleophile is lowered by solvation. "Sn1 reactions are favored in protic solvents because the transition-state energy leading to carbocation intermediate is lowered by solvation.

11.7 Elimination Reactions of

"Elimination is an alternative pathway to substitution "Opposite of addition "Generates an alkene

Reactions

"Base-induced elimination reaction generally "In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates

Mechanisms of Elimination

Reactions

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