alkylhalide n.
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ALKYLHALIDE. By Mrs. Azduwin Khasri 23 rd October 2012. ELIMINATION REACTIONS OF ALKYLHALIDES. ELIMINATION REACTION. Elimination reactions involve the loss of elements from the starting material to form a new  bond in the product. The product of elimination reaction is an Alkene.

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alkylhalide

ALKYLHALIDE

By Mrs. AzduwinKhasri

23rd October 2012

elimination reaction
ELIMINATION REACTION
  • Elimination reactions involve the loss of elements from the starting material to form a new  bond in the product.
  • The product of elimination reaction is an Alkene
e1 and e2 reaction
E1 AND E2 REACTION

E2

E-Elimination

2-Bimolecular

E1

E-Elimination

1-Unimolecular

e2 reaction
E2 REACTION

Hydroxide cannot act as a nucleophile in this reaction because of the bulky tertiary halide. Rather, hydroxide acts as a base and abstracts a proton.

mechanism of e2 reaction
MECHANISM OF E2 REACTION

The removal of a proton and a halide ion is called dehydrohalogenation

slide7

MECHANISM OF E2 REACTION

An E2 reaction is also called a b-elimination or a 1,2-elimination reaction

Carbon attached to halogen

Carbon adjacent to α-carbon

slide9

The Regioselectivity of the E2 Reaction

2 product produce

base

2 structurally β-carbon

The major product of an E2 reaction is the most stable

alkene

slide10

Reaction coordinate diagram for the E2 reaction

2-Butene formed faster than 1-butene

slide11

The Zaitsev Rule

The more substituted alkeneproduct is obtained when a

proton is removed from the b-carbon that is bonded

to the fewest hydrogens

2-β-Hydrogens

3-β-Hydrogens

Monosubstituted

Disubstituted

slide13

Conjugated alkene products are preferred over the

more substituted alkene product:

Do not use Zaitsev’s rule to predict the major product

If the alkylhalide has a double bond or a benzene ring.

slide14

The anti-Zaitsev Rule (Hoffmann Product)

Zaitsev product

Hofmann product

Bulky Alkyl halide

Bulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkene

Less sterically hindered

Major product- Most stable product

slide16

Another exception to Zaitsev’s rule

When the halogen is fluorine-The major product of the E2 reaction of alkyl Fluoride is the less substituted alkene.

STRONGEST BASE-POOREST LEAVING GROUP

The Fluoride ion does not have as strong a propensity to leave as another halide ion.

slide17

Consider the elimination of 2-fluoropentane…

Major product-Less substituted alkene

A carbanion-like transition state

carbocation vs carbanion
CarbocationvsCarbanion

Carbocation stability 3° > 2° > 1°

Carbanion stability 1° > 2° > 3°

class exercise 1 e2
CLASS EXERCISE 1-(E2)

Which of the alkyl halides is more reactive in an E2 reaction?

Producing more stable alkene

Br better leaving group

Producing more stable alkene

slide20

CLASS EXERCISE 2-(E2)

Give the major elimination product obtained from an E2 reaction of the following alkyl halides with hydroxide ion:

ANSWER:

slide21

E1 REACTION

  • First order elimination reaction
  • Must have at least two step
mechanism of e1 reaction
Mechanism of E1 reaction

Alkylhalidedissociates,formingcarbocation

A Base removes a proton from β-carbon

slide23

How does a weak base like water remove a proton from

an sp3 carbon?

1) The presence of a positive charge greatly reduces the

pKa

2) Hyperconjugation weakens the C-H bond by electron

density

slide24

The Regioselectivity of the E1 Reaction

Fewest β-Hydrogen (According to Zaitsev rule)

The major product in an E1 reaction is generally the more substituted alkene

slide26

Because the first step is the rate-determining step, the rate of an

E1 reaction depends both on the ease with which the carbocation

is formed and how readily the leaving group leaves

e1 and e2 reaction1
E1 AND E2 REACTION
  • Major product generally the most stable alkene
  • Tertiary alkyl halides are the most reactive and the primary alkyl halides are the least reactive.
  • For alkyl halide with the same alkyl group, alkyl iodide are the most reactive and alkyl fluoride are the least reactive.
slide28

Because the E1 reaction forms a carbocation

intermediate, we need to consider carbocation

rearrangement

slide29

Competition Between E2

and E1 Reactions

1° Carbocation are too unstable to be formed in E1 reaction

An E2 is favored by a high concentration of strong base and an

aprotic polar solvent

An E1 is favored by a weak base and a protic polar solvent

slide30

Stereochemistry of the E2 Reaction

The bonds to the eliminated groups (H and X) must be

in the same plane

Parallel on the opposite side

Parallel on the same side

Syn elimination

Anti elimination

The anti elimination is favored over the synelimination

slide31

Consider the stereoselectivity of the E2 reaction

(E)- : the higher priority groups are on opposite sides of the double bond.

(Z)- : the higher priority groups are on the same side of the double bond.

Syn-elimination

Anti-elimination

The alkene with the bulkiest groups on opposite sides of the

double bond will be formed in greater yield, because it is the more

stablealkene

slide32

Syn-elimination

Anti-elimination

slide33

When only one hydrogen is bonded to the b-carbon, the

major product of an E2 reaction depends on the structure

of the alkene

slide34

Stereochemistry of the E1 Reaction

The major stereoisomer obtained from an E1 reaction is

the alkene in which the bulkiest substituentsare on

opposite sides of the double bond

Bothsyn and anti elimination can occur in an E1 reaction,

both E and Z formed

In contrast,E2 forms both E and Z only if the β-carbon is bonded

to 2 Hydrogen.

If β-carbon bonded to only 1 Hydrogen,E2 form only 1 product

because anti elimination favored.

slide35

Competition Between

Substitution and Elimination

Alkyl halides can undergo SN2, SN1, E2, and E1

  • 1) HO- is called nu in a substitution reaction (Attack carbon) and a base in elimination reaction (removes proton).
  • 2) Decide whether the reaction conditions favor SN2/E2 or SN1/E1
    • SN2/E2 reactions are favored by a high concentration of a good nucleophile/strong base
    • SN1/E1 reactions are favored by a poor nucleophile/weak base

3) Decide how much of the product will be the substitution product

and how much of the product will be the elimination product

slide37

A bulky alkyl halide or a sterically hindered nucleophile

encourages elimination over substitution

slide39

High temperature favors elimination over substitution:

Why? Because elimination is entropically favorable.

slide41

SN1/E1 conditions

The elimination reaction favored at higher temperatures.

Primary alkyl halides do not form carbocations; therefore

they cannot undergo SN1 and E1 reactions.