Mechanism of dehydration of alcohols

Dehydration of alcoholsalcoholsConcentrated sulphuric acid (H2SO4) or phosphoric acid (H3PO4) : as acidic catalysts and dehydrating agents.The major product is the most stable alkene, which has greater number of alkyl group attach to C=C is follow Saytzeffs rule.

Mechanism of dehydration of alcohols3 step of mechanism

+HOHH

:

H

CH3CCH2CH3H

OH

:++OH

H:..CH3CCH2CH3OHH

:..+

Step 1 : Protonation of alcoholsWhat is protonation? reacts with acid, H+ or H30+A bond has form as a result from protonation (nucleophile attacking H atom from H30+

Why it has + charge? Oxygen has loss its pair of electron (make a bond with H atom) and it also has +1 formal chargeNucleophile (lone pair at O atom) attack electrophile (H atom form acid)Electron move to Oxygen (look at arrow movement)

Step 2 : Formation of carbocationCH3CCH2CH3OHHH

:+

Redraw the previous structure from step 1carbocation 3

2

3

HCHC+CHCH

+OH

H:..How to neutralize the + charge?Move the electron form Carbon to Oxygen (look at arrow movement). This will results a carbocation formation.30 carbocation

Step 3 : Formation of alkenes

C

CC3H+CHHHH

HHaIn order to form an alkenes, all the bond surrounded carbocation has possibilities to form a double bond(Look at arrow movement)

baORLets look at these two possibilitiesRoot aRoot b

+

Step 3 : Formation of alkenesRoot bRoot a

+OH

H:..

+OH

H:..

CCCH3HHCH3

+HOHH

:+H2O (determine from step 1) as a nucleophilealkene aalkene b

CC3H+CHHHH

HHC

CCCH2CH3HHH

b a

C+OH

H:..

CC3H+CHHHH

HH+OH

H:..

CCCH3HHCH3

+HOHH

:+CCCH2CH3HHH

+majorminorHow to determine major and minor product?Saytzeffs ruleAn elimination usually gives the most stable alkene product, commonly the most highly substituted alkene.

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Rearrangement During Dehydration of Alcohol

Example 1:

CCCH3CH3CH3CH3OHH

H2SO4 (conc.)

DCCCH2CH3CH3CH3H

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

Rearrangement During Dehydration of Alcohol

Example 1:

CCCH3CH3CH3CH3OHH

H2SO4 (conc.)

D(major product)CCCH3CH3CH3CH3

+(minor product)3CCHCH3CH2CH3CH

This is because rearrangement occurs. The actual product is.

Step 1 : Protonation of alcoholsCCCH3CH3CH3CH3OHH

:..

+HOHH

:

++OH

H:..CCCH3CH3CH3CH3OHH

:H

+

Step 2 : Formation of carbocationCCCH3CH3CH3CH3OHH

:H

+

CCCH3CH3CH3CH3H

+

CCCH3CH3CH3CH3H

+Rearrangement1,2-methy shift20 carbocationLess stable30 carbocationMore stable402010

The rearrangement occurs through the migration of an alkyl group (methyl) from the carbon atom adjacent to the one with the positive charge.

Because a group migrates from the one carbon to the next, this kind of rearrangement is often called a 1,2 shift.

Step 3 : Formation of alkenes(a)OH

H:..+

(b)

OH

H:..+CCCH3CCH3CH3H

+H

H-

H

CCHCH3CH2CH3CH3

CCCH3CH3CH3CH3

Minor productMajor product

CCCH3CH3CH3HOHH

H2SO4 (conc)CCH2CH2CH3CH3

CCHCH3CH3CH3

+

DTry this!