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‫هبة حيدر ابراهيم‬:‫مدرس‬
Alkenes
Structre and synthesis of alkenes
2.1 Introduction
Alkenes are hydrocarbons with carbon-carbon double bonds
alkene are sometime called olefins a term derived from
olifiantgas,meaning (oil-forming gas).
CH2=CH2
Ethylene(ethane)
2.2 The orbital description of the alkene double bond
In a lewisstructure,the double of an alkene is represented by
two pairs of electrons between the carbon atoms.
2.2A The sigma bond framework
Each of the carbon-hydrogen bonds is formed by overlap of
on sp2 hybrid on carbon with the 1s orbital of a hydrogen atom.
The C-H bond length in ethylene (1.08 A0) is slightly shorter
than the C-H bond in ethane (1.09 A0) ,because the sp2 orbital
in ethylene has more s character (one-third s) than an
sp3orbital (one-fourth s). The s orbtal is closer to the nucleus
than the p orbital, contributing to shorter bond.
2.2B
The pi bond
Two more electrons must go into the carbon-carbon bonding
region .Each carbon atom still has an additional unhybradized
p orbital ,and these overlap to form a pi bonding molecular
orbital.
2.3
Elements of unsaturation
Alkenes are called unsaturated because they react with
hydrogen in the presence of
catalyst.Theproduct,analkane,iscalled saturated because it
cannot react with any more hydrogen .
CH3-CH2-CH3
CH3-CH=CH2
CH2=C(CH3)2
Propane,C3H8 propane C3H6unsaturated isobutylene saturated
2.4 Nomenclature of alkene
Simple alkenes are named much like alkanes, using the root
name of the longest chain containing the double bond.
The ending is changed from –ane to –ene. For example ,
(ethane) becomes (ethane) ,(propane) becomes (propene) and
(cyclohexane) becomes (cyclohexene) .
2.5 Commercial Importance of alkenes.
Because the carbone-carbon double bond is readily
converted to other functional groups, alkenes are important
intermediates in the synthesis of drugs, pesticides, and other
valuble chemical.
2.6A Heat of Hydrogenation
2.6B Energy differences in cis-trans isomers
The trans isomers are generally more stable than the
corresponding cis isomers. This trend seems reasonable
because the alkyl substituents are separated farther in the
trans isomers than they are in the cis isomer.
2.7 A-Boiling points and densities
The boiling points of alkene increase smothly with molecular
weight. As with alkene, increase branching leads to greater
volatility and lower boiling points. For example, isobutylene has
a boiling point of -70C ,which is lower than the boiling point of
any of the unbranchedbutenes.
name
structure
carbone
B.P.
0
C
Density(g/cm3)
Ethane
CH2=CH2
2
-104
-
Propene
CH3CH=CH2
3
-47
0.52
4
-7
0.59
CH3CH2CH=CH2 4
-6
0.59
Isobutylene (CH3)2C=CH2
1-butene
B – polarity
Like alkenes, alkene are relatively nonpolar. They are
insoluble in water but soluble in nonpolar solvents such as
hexane, gasoline, halogenated solvents, and ethers.
A alkene tend to be slightly more polar than alkanes, however,
because the more weakly held electrons in the pi bond are
more polarizable.
2.8 Alkene synthesis by Elimination of alkyl halides
A- Dehydrohalogenation by the E2 mechanism
Second-order elimination is a reliable synthetic reaction.
B- Dehydrohalogenation by the E1 mechanism
First-order dehydrohalogenation usually takes place in a
good ionizing solvent (such as an alcohol or water).
2.9 Alkene synthesis by dehydration of alcohols
Dehydration of alcohols is one of the best methods for the
synthesis of alkene. The worddehydrationliterall means the
removal of water.