Alkenes

1
Alkenes
By
Dr/ Amani S. Awaad Professor of pharmacognosy,
King Saud University
2
Alkenes and Cycolalkenes
CnH2n
Ethylene
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SP2Hyperidization
Each carbon is trigonal and planar. Angle of SP2
bond 120o Bond length 1.34 Å hybridized
C--C (1.54 Å) lt CC (1.34 Å) Why?
4
Sigma (?) Bonding in Ethylene
? bond end-to-end overlap of the sp2 hybridized
orbitals
Pi (?) Bonding in Ethylene
p is weaker than d
? bond side-by-side overlap of the unhybridized
p-orbitals
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Rotate 90 degrees
SP2----- SP2 d
2 PZ are perpendicular p

• Hybridized orbitals make sigma bonds
• Unhybridized orbitals make pi bonds

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IUPAC
1) Longest continuous chain 2) Ane------ ene 3)
CC have lowest No 4) Position of CC (indicated
by lower No) 5) In Cycolalkenes between C 12
(subs. have lowest No)

H2CCH2 H3CHCCH2
Common name Ethylene
Propylene
IUPAC Ethene
propene
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CH3 CH3CH2CHCH2
CH3CHCHCH3 CH3CHCH2CHCH2
1-butene 2-butene
4-methyl-1-pentene
Cl
3-chloro-4-ethylcyclobutene
CH2-CH3
3-methylcyclohexene
CH3
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Compounds derived from ethylene and propylene
CH2CH- vinyl gp.
CH2CH-CH2- allyl gp.
CH2CH-Br
CH2CH-CH2- Cl
Common name Vinyl bromide Allyl
chloride IUPAC
Bromoethene 3-chloro-1-propene
CH CH2
Common name Vinyl cyclohexane
IUPAC Cyclohexylethene
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Geometric isomerism
A B or CD Cis A B or
C D trance
AC or BD No Cis or
trance
CH3
CH3
CH3
CH3
Cl
H
CC CC
CC
H
Cl
H
CH3
H
H
Propene 2-methyl-2-butene
1,1-dichloropropene
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Other system is E Z

• if groups of higher priority are on the same
  side, the configuration is Z (German, zusammen)
• if groups of higher priority are on opposite
  sides, the configuration is E (German, entgegen)

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- According to priority (atomic No.) -Cl gtF
C gt H Br gtH I gtC
Br
Br
F
I
CC
CC
H
CH3
Cl
H
Z-1-Bromo-2-iodopropene Br I (same)
E-1-Bromo-2-chloro-2-fluoroethene Br Cl (diff.)
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Homework 2

• Name each alkene and specify its configuration by
  the E,Z system

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Physical properties

• Solubility
• All at room temp. Non polar compounds
• C2-C4 gas C5-C17 liquids C18 and gt wax
• Like dissolves like, benzene, ether, CCl4

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Preparation of alkenes
A) Elimination reaction
General equation
-C-C- CC
AB A B (A H or halogen
B OH or halogen)
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1) Dehydration of alcohols
Mineral acids (H2SO4,
H3PO4) General equation
H
-C C- CC
H2O OH H
?
eg. CH3 CH2OH CH2 CH2
H2O
H
?
OH
H
H2O
?
H
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Saytzeff's Rule

• CH3CH2CHCH2 H2O
• 1-Butene (minor)

OH
H
CH3CH2CHCH3 2- butanol
?
CH3CHCHCH3 H2O
2- Butene (major)
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(2) Dehydrohalogenation of alkyl halides
General equation
Alcoh.
-C - C - KOH
H2CCH2 KX H2O
?
H
X
Alcoh.
i) CH3CHHCHBrCHH2 KOH
CH3CHCHCH2

CH3CH2CHCH2
?
CH3
CH3
CH3
Br
Alcoh.
KOH

?
H
3 methyl cycl.
1 methyl cycl.
Minor
Major
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(3) Vicinal dihalides
General equation
Eth. Alc.
-CX CX- Zn
-CC- Zn X2
?
acetone
i) H3CCHBrCH2Br NaI H3C-CH
CH2 I2 NaBr
Br
acetone
ii) NaI
NaBr I2
Br
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Reactions of alkenes
General equation
H2CCH2 A-B
H2A-C-C-H2B
1Addition of hydrogen
Pt / Ni
i) H2CCH2 H2
H2H-C-C-HH2
Low pr.
Pt / Ni
ii) CH3-CHCH-CH3H2
CH3CHHCHHCH3
Low pr.
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2 Addition of halogens (Halogenations)
General equation
Inerrant solvent
-CC- X2
X-C-C-X
CCl4
i) CH3CHCH2 Cl2
CH3CHClCH2Cl
Br
CCl4
ii) Br2
Br
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3 Addition of HOX (Halohydrin formation)
CH3CHCH2 Cl2, H2O
CH3CHOHCH2Cl


1-chloro-2-propanol
CH3CHCH2 Br2, H2O
CH3CHOHCH2Br

1-bromo-2-propanol
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4 Addition of acid
HA H2SO4 , (H2O / H), HCl, HBr,
General equation
-CC- HA -HC-CA-
a)Sulfuric acid ( H2SO4 H-OSO3H)
H2CCH2 H- OSO3H
H2H-C-COSO3HH2
CH3CHCH2 H2SO4
CH3CHOSO3H-CH2H
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B) Addition of water hydration (H2O, H /or
H3O)
H
CH3CHCH2HOH CH3CHOH-CHH2
C) Addition of hydrogen halides
CH3CHCHCH3 H-Cl CH3CHHCHClCH3
(one) CH3CH2CHCH2 H-Br
CH3CH2CHBrCH2H

CH3CH2CHHCH2Br
Markovinkov's Rule
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5 Ozonolysis
AlkeneO3 ozonoid H2O
Aldehyde Ketone
R
R
R
R
H2O
C C O3
CO O C
R

Aldehyd Ketone
Zn
H
H
R
CH2CH3
H
H2O
H2CCHCH2CH3
CO O C
H
Zn
H
O
O
H2O

Zn
H
H
H
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6 Oxidation
R oxidation (KMnO4 ,H2O2, OsO4)
Di-alcohol
dil.KMnO4 i)
H2CCH2
H2COHCH2OH KMnO4
cold H2O
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6Epoxidation