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!
