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Elimination ReactionsJust as there are two mechanisms of substitution (S N

2 and S

N

1), there are two

mechanisms of elimination (E2 and E1).

E2 mechanism

b imolecula r elimination

E1 mechanism - unimolecular elimination

The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation,analogous to the S N 2an d S N

1 mechanisms.

E2 and S

N

2 reactions have some features in common, as do E1 and S

N

1 reactions.Base removes a

p roton from theȕ-carbon atom, while the halogen atom leaves from theɲ-carbon resulting in the formation of aʌ-bond. Sucheliminations are also called elimination eliminations are also called elimination reactions

The Zaitsev (Saytseff) Rule

When alkyl halides have two o

r more differentȕcarbons, more than one alkene p roduct is formed. In such cases, the major product is the more stable product - the one with the more substitute d double b on d . This p henomenon is calle d the Zaitsev rule.

The Zaitsev product or the more substituted alkene product is more stable than the lesssubstituted product. The stability of the more substituted alkene is a result of number of

different contributing factors, including hyperconjugation.Each alkyl group that can involve in hyperconjugation with the double bond stabilizesit by approximately 6 kcal/mol

The E2 Mechanism

The most common mechanism for dehydrohalogenation is the E2 mechanism.

It exhibits secon

d -orde r kinetics an d b oth the alk y l halide and the b ase a pp ea r in the y pp rate equation rate = k[(CH 3 3

CBr][HO

The reaction is concerte

d all b onds are b roken an d forme d in a sin g le ste p g p E2 reactions are regioselectiveand favor the formation of Zaitsev products.

Energy Profile for an E2 Reaction

CH 2 H B H 3 C H 3 C HO B r (CH 3 3

CBr + OH

CH 3 2 C=CH 2 + H 2

O + Br

3 2 2 2

Factors Affectin

g the Rate of an E2 Reaction g

There are close parallels between E2 and S

N

2 mechanisms in how the identity of the

base, the leaving group and the solvent affect the rate. The base appears in the rate equation, so the rate of the E2 reaction increases

as the strength of the base increases.E2 reactions are generally run with strong negatively charged bases like OH

E2 reactions are generally run with strong negatively charged bases like OH and OR Polar aprotic solvents increase the rate of E2 reactions There is a partial breaking of the bond to the leaving group in the transition state. So, the better the leaving group the faster the E2 reaction.

Rate of reaction follows the orderRate

of reaction follows the order

RI > RBr > RCl > RF

Factors Affectin

g the Rate of an E2 Reaction g The S N

2 and E2 mechanisms differ in how the R group affects the reaction rate.

As the number of R groups on the carbon with the leaving group increases, the rate of

the E2 reaction increases.The increase in E2 reaction rate with increasing alkyl substitution can be rationalized interms of transition state stability.

In the transition state, the double bond is partially formed. Thus, the transition state fora more substituted alkene is lower in energy, reducing the activation energy for the

reaction and making the reaction faster.

Characteristics of an E2 Reaction

Kinetics - Second orderMechanism

Single step

Identity of R group - More substituted halides react faster

Rate: R

3

CX > R

2

CHX > RCH

2 X Strength of the base - Stronger bases favor the reaction Leaving group - Better leaving group leads to faster reaction ratesreaction rates Type of solvent - Favored by polar aprotic solvents E2 reactions are stereoselective, resulting in the formation of trans-double bonds preferably.

The E1 Mechanism

The E1 reaction proceeds via a two-step mechanism: the bond to the leaving groupbk fi t bf th bd i fd Th l t i ill ili l b rea k s fi rs t b e f ore th eʌ b on d i s f orme d Th es l ow s t ep i sun i mo l ecu l ar, i nvo l v i ng on l y the alkyl halide.

It exhibits first-order kinetics,

tk[( CH CCl] ra t e = k[( CH 3 3 CCl] E1 reactions also are regioselective and follow Zaitsev rule E1 reactions also are regioselective and follow

Zaitsev

rule

Energy Profile for an E1 Reaction

(CH 3 3

C---Xįį+

(CH 3 3 C (CH 3 3 CX (CH 3 2

C=CH2 + HX

Factors Affectin

g the Rate of an E1 Reaction g The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases. I n c r e a s i n g r a t e o f E 1 r e a c t i o n RCH 2 XR 2 CH XR 3 C X 1 2 3 I n c r e a s i n g r a t e o f E 1 r e a c t i o n 1 2 3 I i b i l i f b i R CH 2 R 2 CH R 3 C

1° 2°3°

I nc r eas i ng sta b i l i ty o f ca r b ocat i ons The strength of the base usually determines whether a reaction follows the E1 or E2 mechanism. Stron g b ases like OH an d OR favo r

E2 reactions

whereas weake r b ases g like H 2

O and ROH favor E1 reactions.

Characteristics of an E1 Reaction

Kinetics - First orderMechanism - Two steps

Identity of R group - More substituted halides react faster

Rate: R

3

CX > R

2

CHX > RCH

2 X Strength of the base - Favored by weaker bases such as H 2

O and ROH

Leaving group - Better leaving group leads to faster reaction rates. Just as in S 1 reactions the rate determining step Just as in S Nquotesdbs_dbs9.pdfusesText_15

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