Mechanism Of Dehydration Of Alcohols 23551

Concentrated sulphuric Dehydration acid of (H alcohols 2SO4) or phosphoric acid (H3PO4) : as acidic alcohols catalysts and dehydrating agents. C

C

H

OH

H2 SO4 (conc.) 

C

C

+

H2 O

The major product is the MOST STABLE ALKENE, which has greater number of alkyl group attach to C=C is follow SAYTZEFF’S RULE.

Mechanism of dehydration of alcohols

3 step of mechanism

• Protonati on of alcohols

Step 1

Step 2

• Formation of carbocation

• Formatio n of alkenes

Step 3

Step 1 : Protonation of alcohols H H has form A bond + as aOresult from H C C CH CH : H + is 3 2 What 3 protonation Why it has “+” :OH H .. protonation? (nucleophile

(nucleophile charge? reacts with acid, attacking atom OxygenHhas loss + from H 0 3 H of Electron its pair H + + move to electron (make a H or H 0 3 Nucleophile (lone pair at + H C C CH CH O HH : Oxygen (look bond with 3atom) 2 3 .. O attack + at arrow atom) and it also : OH electrophile movement) (H atom has +1 formal H acid) form charge

Step 2 : Formation of carbocation How to neutralize the H H “+” charge? + H C C CH CH Move the electron H C C CH CH form 3 2 3 2 3 3 + :OH Carbon to Oxygen (look carbocation at arrow movement). H This will results a H carbocation formation. 0 3 Redraw the + :O H carbocati previous .. on structure from step 1

Step 3 : Formation of alkenes H H H H C C+ C CH

3

H

H

b

a OR

In order to form an alkenes, all the bond surrounded carbocation has Lets look at these twopossibilities possibilitiesto form a double 1.Root a a bond 2.Root b (Look at arrow movement)

Step 3 : Formation of alkenes Root b

alkene a

Root a

H H H H H H C C+ C CH + : O H C .. 3 H C H H H2O alkene + (determine b Hfrom step 1) H asH a : Onucleophile H C C

..

H

3

+

CH2CH3

H C +

H + :O H

CH 3

H

How to determine major and minor product? H H H H Saytzeff’s rule H H + An elimination the : O H gives H C C C CH +usually C C .. 3 most stable alkene product, b H highly C CH a the most H commonly H 3 3 + + substituted alkene.major H H

H

:O H ..

H

C H

+ + :O C CH2CH3 H

minor

H

Rearrangement During Dehydration of Alcohol

Example 1: CH H HC 3

C

3

C

CH OH

CH

H SO (conc.) 2

4



3

3

You might predict the structure beside but the answer is wrong. But WHY?

CH H HC 3

C

3

CH 3

C

CH 2

Rearrangement During Dehydration of Alcohol

Example 1: CH H HC 3

C

3

C

CH OH

CH 3

H SO (conc.) 2

4



3

This is because CH CH3 3 rearrangement H C C CH CH + actual H 3C C occurs. C C H 3The 2 3 is…. C Hproduct CH 3 3 (major product) (minor product)

Step 1 : Protonation of alcohols CH H HC 3

C

3

C

CH +

CH :OH ..

3

H + :O H H

3

CH H H

:O ..

+ H

HC 3

C

3

C CH 3 + CH :OH 3

H

Step 2 : Formation of carbocation The rearrangement CH H through the occurs CH H 3 migration of an alkyl 3 H Cgroup C C CH from theH C C C+ CH (methyl) 2 1 3 3 4 + atom adjacent to 3 0 3 0 carbon 0 CH :OH CH the one 3 with the positive 2 3 H charge. carbocation Rearrangement 0

1,2-methy shift

Because group CHa H 3 the one migrates from H Cto the C next, C CH carbon this 3 + kind of3 rearrangement is 3 carbocation CH often called a 1,23 shift. More stable 0

Less stable

Step 3 : Formation of alkenes

H

(a)

+

H CH H H- C H

C +

3

C

CH HC 3

:O H ..

(b)

CH

3

Major product

3

CH

3

+

C

CH

CH

3 C CH

H :O H ..

HC 2

C CH

3 CH CH 3

3

Minor product

3

Try this! CH H 3 H C C C CH 3 3 H OH

H2SO4 (conc) 

+

CH 3 H C C CH CH 3 3

CH 3 H C C CH CH 2 2 3