MIND MAP 5.2 ALKENE

MIND MAP
ALKENES

Prepared by: NFAG, KMPh

PREPARATION AND CHEMICAL PROPERTIES OF ALKENES

Markovnikov’s rule: Saytzeff rule:
In the addition of HX to unsymmetrically alkene, the H atom In an elimination, alkene with greater no alkyl group
added to the carbon double bond having greater no of H. at C=C is the most stable and as major product.

*Mechanism OH H OH KOH H Br
CH3CH2CCH3 CH3CHCCH3 EtOH CH3CHCCH3
Hydration
CH3 CH3 CH3
*Mechanism
Br H2O/ H+ *Mechanism Dehydration Dehydrohalogenation
Hydrohalogenation CH3CH2CCH3 Conc H2SO4 Br

Hydrohalogenation CH3 HBr H Br2 , CH2Cl2 CH3CH CCH3
(Anti Markovnikov’s) Br2 , H2O
Br HBr CH3C=CCH3 Br CH3
Hydrogenation CH3CHCHCH3 H2O2 CH3
H2 OH
CH3 Pt CH3CH CCH3

H i. O3 ii. Zn, H 2O KMnO4/OH- cold, diluted Br CH3
CH3CH2CCH3

CH3

Ozonolysis H KMnO4 , H+ OH Bromine test: Br2/ CH2Cl2 (UV/ dark place)
CH3C=O CH3CH CCH3 Bromine aqueous/water
+ O=CCH3 + MnO 2
CH3 OH CH3 The reddish brown colour of bromine is
decolourised
OH Baeyer’s test: Diluted KMnO4 , -OH/ cold
Oxidation CH3C=O + O=CCH3
CH3 The purple colour of KMnO4 is decolourised
and brown precipitate formed

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MECHANISM OF ALKENES

DEHYDRATION HYDRATION

OH conc H2SO4 H3C CH CH CH3
CH3CH2CHCH3
major product HH H2O, H + OH
STEP 1 H3C C C H CH3CHCH3
+

CH3CH2CH=CH2

major product

H +H3C CH2 CH CH3 STEP 1 HH H H slow H3C H H
+ O+ C+ CH
H3C CH2 CH +CH3 H2O H3C CC H+ H H
OH OH
+
H
OH

H 2O carbocation
H
STEP 2

H H HH
OH
H3C CH2 CH CH3 + +CH3 H2O + H H3C CC H

+ H3C CH2 C C C H+

OH H3C H + H

O

STEP 2 H H
H

STEP 3

HH H HH

H

+H fast H3C C CH
H3C CH CH CH3 H3C C CH
+CH3 OH
H3C CH + OH + OH H
H major product H
C O

H
H

H

H fast H3C CH2 CH CH2

+H3C CH2 C+ CH2 OH minor product

H

Prepared by: NFAG, KMPh

MECHANISM OF ALKENES

HYDROHALOGENATION HYDROHALOGENATION

H H Br Br H H
H3C C CH H3C C C C H
+CH3CH=CH2 HBr H +CH3CCH=CH2 HBr
Br CH3 H H
CH3

STEP 1 STEP 1

H H H H HH HH
C C C+
+ H Br slow H3C +C H slow H3C CH + H
H3C H
H H3C CH C C H H Br CC

CH3 H

20 carbocation CH3

2O carbocation

H HH rearrangement + HH
H3C C C+ C (1,2 - hydride shift) CC H
C
H H3C

CH3 H CH3 H H

2O carbocation 3O carbocation

STEP 2 STEP 2

H H H H HH Br H H
C+ C H H3C C+ fast H3C C C C H
H3C +C H Br- fast H3C C +C C H Br-
Br CH3 H H
H H CH3 H H
haloalkanes
haloalkanes 2 - bromo - 2 - methylbutane

Prepared by: NFAG, KMPh

PHYSICAL PROPERTIES OF ALKENES

BOILING POINT OF ISOMERIC ALKENES
(SIMILAR MOLECULAR WEIGHT)

Example:

Cis-molecule has higher boiling point cis-2-butene trans-2-butene
because the molecule is polar, therefore
H H H CH3
the intermolecular forces C C C C
(van der Waals forces) are relatively
H3C CH3 H3C H
stronger.
A small net dipole No net changes
In a trans isomer, the two bond dipoles
cancel each other. µ≠0 µ=0

Therefore, boiling point cis-isomer is polar molecule non – polar molecule
higher than trans-isomer.
b.p 3.7oC b.p 0.9oC

• Cis-2-butene has a higher boiling point (3.7oC)
than trans-2-butene (0.9oC).

• This difference arises because the C-C single
bond between an alkyl group and one of the
double bond carbons of an alkene is slightly
polar.

• In cis isomer, the two C-C bond dipoles reinforce
each other, yielding a small net molecular dipole.

Prepared by: NFAG, KMPh

Prepared by: NFAG, KMPh