Why?

Why?

Summary

Mechanism for dehyrdration of 2 and 3 via E1 mechanism

QuestionMechanism for dehydration of 1 via E2 mechanism How to determine the major product ? Dehalogenation of alkyl halide

Dehydration of alcohol

Synthesis of alkene

C= CWhat is about dehydration of alcohol? How to determine the major product using Zaitsev rule?

What is about?

The arrangement of carbocation

E2 mechanism

E1 mechanism

Characteristics

Characteristics

Dehalogenation of vicinal dibromide When is mechanism E1 or E2?

At the end of this lesson.. 1) Student should be able to differentiate between reaction and synthesis of alkene. 2) Student shoud know the mechanism that involve in dehydration of alcohol and dehalogenation of alkyl halide.

3) Student should be able to apply the Zaitsev rule in order to determine the product for synthesis of alkene.

What can you see from the pictures below.

Discuss..

From the pictures, it shows that how to prepare or make soy milk by using the soy been. In this lesson, the analogy of producing soy milk are use to imagine the synthesis of alkene

Hmm.. What are the meaning by the synthesis of akene actually.. Lets go and ask Mr. Edwin for the meaning of synthesis..

What are you understand by the word of synthesis?? Synthesis in chemistry refers to the process of creating a chemical compound by the union of simpler chemical compounds or elements. So synthesis of alkene is the process of creating C=C Mr. Edwin

So how the synthesis of alkene occur? What are the reaction occur? What are the reagent involve in the synthesis of alkene?

Hello Mrs. Etene, can you help me? I want alkene but I dont know how to synthesis the alkene??

Of course I can help you Mr. Butene. There are 3 common ways how to synthesis of alkene.First: By dehydration of alcohol (the loss of water from an alcohol. Second: By dehydrohalogenation (the loss of HX from an alkyl halide) Third: By dehalogenation of vicinal dibromides. But you must know that the most common alkenes forming elimination reaction are dehydration and dehydrohalogenation

Lets look the first part Dehydration of alcohol.

Reaction is carried out by treatment of an alcohol with strong acid H2SO4, H3PO4. Product yield follow Zaitsev rule. (More highly substituted alkene product predominate.

Oh what is mean by the Zaitsev rule.. Confuse.

If more than one alkyl group can be formed during dehydrohalogenation by an elimination reaction. The most stable alkene group will be the major product.

The most stable alkene which contain more highly substitute at c=c double bond

Zaitsev rule

H3C H3C -C -CH2CH3 OH CH3 H2SO4 H3C C=C CH3 H H CH3

+H

C=CCH2CH3 Minor product Disubstituted alkene

Major product Trisubstituted alkene

Alcohol can be classified into

secondary alcohol (2)

Tertiary alcohol (3)

Primary alcohol (1)

Undergo E1 mechanism

Undergo E1 mechanism

Undergo E2 mechanism

Why secondary and tertiary alcohol undergo E1 mechanism, Mr Elbert?

Secondary and tertiary alcohol are more reactive ad react faster than primary alcohol.

To show the mechanism of dehydration of secondary alcohol and tertiary alcohol via E1 mechanism Let follow the step below..

E1 MECHANISM FOR ALCOHOLS Step 1: An acid/base reaction. Protonation of the alcoholic oxygen to make a better leaving group. This step is very fast and reversible. The lone pairs on the oxygen make it a Lewis base.

Step 2: Cleavage of the C-O bond allows the loss of the goodleaving group, a neutral water molecule, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)Step 3: An acid/base reaction. Deprotonation by a base (a water molecule) from a C atom adjacent to the carbocation center leads to the creation of the C=C

Remind again!! Primary alcohols cannot undergo E1 dehydration because of the instability of the carbocation-like transition state in the 2nd step

Primary Alcohols via E2 mechanism First step: Protonation of the hydroxyl to yield the good leaving group water.

Step 2: The C-H and C-O bond are broken and the bond is formed

Click the link below for more details..http://www.yteach.com/index.php/resources/alken_reaction_electrophilic_addition_stability_carboc ation_Markovnikov_addition_polymerisation_t_page_28.html

Mrs Bee,do you know that the arrangement of carbocation can be occur if a more stable carbocation can be obtained? Oh sure Mr. Crab.. For example rearrangements of secondary carbocations to tertiary carbocation. Look at the example below.

There are two common rearrangement of carbocation which relocates the carbocation to a more stable position either secondary or tertiary. First: 1,2- hydride shift Second: 1,2- alkyl shift What are mean by the 1,2 shift Mr.Bee?

Oh.. It because they involve migration of an alkyl group or hydrogen atom from carbon to an adjacent carbon atom. For more details lets look for the mechanism below..

This is the mechanism of rearrangement of secondary carbocation to tertiary carbocation

The first two steps are same as any E1 dehydration

In the third step the less stable 2o carbocation rearranges by shift of a methyl group

This is called 1,2- shift

The removal of a proton to form the alkene occurs to give the Zaitsev (most substituted) product as the major one

Similarly when there is no alkyl group are available for alkyl shifting, a hydride can take it bonding

The second part dehalogenation of vicinal dibromide. Look at example below..

Na+ I- +

H3C-C-C-CH3 H Br

acetone CH3 C=C

H

-Vicinal dibromide can be converted to alkene by reduction with either iodide ion or zinc in acetic acid. - The reaction proceeds via an E2 mechanism taking place through an anti-coplanar transition state.

H + IBr +NaBr

Br H

CH3

The third part is dehydrohalogenation.

-Removal of element HX (X= CI, Br) to introduce the bond and prepare an alkene. - Tertiary alkyl halide (3 )more stable than 2 alkyl halide. Primary alkyl halide 1 is the less stable.

Look at example below. H3C H H3C-C-C-H Br H-OH

H3C H H3C-C=C H + H2O + Br -

The most common mechanism for dehydrohalogenation is the E2 mechanism.Click the link below for more detailshttp://www.yteach.co.uk/index.php/resources/alken_reaction_electrophilic_addition_stability _carbocation_Markovnikov_addition_polymerisation_t_page_27.html

RCH2 X (1)

R2CH X (2 )

R3C-X (3)

Increasing rate of an E2 reaction

The rate of an E2 reaction increase as the number of the carbon with the leaving group increase.

Zaitsev rule also apply when more product can be form. Major product is the more stable which consist of more substituted group at carbon-carbon double bond C=C

CH3 I

OH-

CH3

CH2 Minor disubstituted alkene

Major trisubstituted alkene

The E2 reaction is regioselective because more substituted alkene predominates. A reaction is regioselective when it yield predominantly one constitutional isomer when more than one is possible.

Lets go for the next example

H

Br

CH

CH H

HC C

Trans alkene (Major) When a mixture of stereoisomer is possible from dehydrohalogenation, the major product is the more stable stereoisomer. Trans alkene is more stable than cis alkene.

C

C

HCis alkene (Minor)

H

A reaction is stereoselective when it form predominantly one stereoisomer when two or more are possible. The E2 reaction is stereoselective because one stereoisomer is formed preferentially.

E1 mechanism also occur in dehydrohalogenation. For example the dehydrohalogenation of (CH3)3 CI with H2O to form (CH3)2 C=CH2

Step 1: The C-Br bond broken

CH3slow, unimolecular CH3

CH3

C: Br : ..

CH2CH3

CH3

C +

CH2CH3

: Br : ..Intermediate carbocation

..

CH3

Step 2: The C-H bond is cleaved and the bond is formed CH2CH3

CH3

C +

H+

CH2 + C CH3 CH2CH3 CH3

CH3C CHCH3

What is the Characteristic of E1 reaction??RCH2X (1) R2CH-X (2 )

R3C-X (3)

Increasing rate of an E1 reaction

Strong base like OH- and R OR favor E2 reactions whereas weaker base like H2O and ROH favor E1 mecahanism

When is the mechanism of E1 or E2???

Because the rate of both E1 and E2 reaction increase as the number of R group on the carbon with the leaving group increase, identity of alkyl halide cannot be use to decide which elimination mechanism occur.

The strength of the base is the most important factor in determining the mechanism for elimination. Strong base favor the E2 mechanism. Weak bases favor the E1 mechanism

Exercise 1

Lets try to do the exercise below good luck!!

H2SO4 OH Which one the products below is not correct for above reaction?

A.

B.

C.

Try for the next question

Br

Chose the best product for the above reaction.

A.

B.

Exercise 2

Solve the puzzle given by answering all the question below..Across

1. What is the rule that are considered in order to determine the major product in dehydration of alcohol and dehalogenation of alkyl halide?3. What is the type f reaction occur in synthesis of alkene?

5. Which carbocation react faster in dehydration of alcohol? 7. What is the product for dehalogenation of 2-bromo-2-methypropane? 8. What is the dehalogenation reaction other than dehalogenation of halide that form alkene? alkyl

Down. 2. Give one reagent that are used for dehydration of alcohol. 4. Primary alcohol favor which mechanism? E1 or E2 mechanism? 6. What is the product form other than alkene in dehydration of acohol.

6

I

2

7

4 3

5

8

Check your answer

Exercise 3

Complete the mechanism for the reaction of dehydration of tbutyl alcohol by using sulphuric acid as reagentStep 1

Step 2

Step 3Check your answer

Exercise 4 Zafri is lost. Please help him find his way back to home.. Chose one road and answer the question in The correct answer is the clue for him to find his home.. Good luck!

1

2

3

Check your answer

Question 1

OH

H2SO4

C

+ H2O

What is C?

A.

B.

Question 2

D

KOH Ethanol

+ KBr + H2O

What is D?

A.

B.

Br

Br

Question 3 What is the best product for the synthesis below?

CINaOH Ethanol C + NaCI + H2O

A.

B.

SummarySynthesis of alkene can occur in three ways. 1) Dehalogenation of alkyl halide. 2) Dehydration of alcohol. 3) Dehalogenation of vicinal dibromide For dehalogenation of alkyl halide and dehydration of alcohol the Zaitsev rule must be applied in order to determine the major product.

Primary alcohol favor E2 mechanism whereas secondary and and tertiary alcohol favor E1 mechanism.In dehalogenation of alkyl halide, rate of both E1 and E2 reaction increase as the number of R group on the carbon with leaving group increase , so the strength of base is the most important in determining the mechanism for elimination. Strong base favor the E2 mechanism. Weak base favor the E1 mechanism.

YES, YOURE RIGHT!!!

SORRY, YOURE WRONG!!!

YES, YOURE RIGHT!!!

SORRY, YOURE WRONG!!!

YES, YOURE RIGHT!!!

SORRY, YOURE WRONG!!!

YES, YOURE RIGHT!!!

SORRY, YOURE WRONG!!!

YES, YOURE RIGHT!!!

SORRY, YOURE WRONG!!!

W A Z A I T E R 2 M E T S U L E V

This is the answers for Exercise 2

P HU R

Y

L

P

R

O

P

E

N

E

E2 T E R

L

I

M

I CA

N

A

T

I

O

N

T

I

R

Y

C IV I C I N A L D I B R O M I D E

Answer for the exercise 3 Step 2

Step 3

Answer for exercise 4

1

Zafri is lost. Please help him find his way back to home.. Chose one road and answer the question in The correct answer is the clue for him to find his home.. Good luck!

2

3

SYNTHESIS OF ALKENE

Group members: Siti Suriani Bt. Ismail D20091035081 Nurul Hidayah Bt. Hamzah D20091035090