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Unsaturated Hydrocarbons 1
Alkenes
By:
Dr. Shatha alaqeel
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Alkenes and Cycolalkenes
Unsaturated hydrocarbons that contain one or more double
.
carbon-carbon bonds
CnH2n
Ethylene
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SP Hibridization:
Each carbon is trigonal and
planar.
Angle of SP2 bond =120o
Bond length =1.34 Å
hybridized C--C (1.54 Å)
*C=C (1.34 Å) Why?
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NOMENCLATURE OF ALKENES AND
CYCLOALKENES
IUPAC RULES
1. Determine the parent name by selecting the
longest chain that contains the double bond
and change the ending of the name of the
alkane of identical length from -ane to -ene.
• 2. Number the chain so as to include both
carbon atoms of the double bond, and begin
numbering at the end of the chain nearer the
double bond. Designate the location of the
double bond by using the number of the first
atom of the double bond as a prefix:
5. The vinyl group and the allyl group.
CH2
H2C
Common
IUPAC
CH2
Ethylene
Ethene
H3C
CH2
Propylene
Propene
H2C
Br
H2C
Vinyl bromide
Bromoethene
Cl
Allyl chloride
3-Chloro-1-propene
Vinyl cyclohexane
Cyclohexyl ethene
Dienes
• Dienes are compounds that contain 2 double
bonds.
• There are also trienes, tetraenes,
……………polyenes.
• A compound with the general molecular
formula CnH2n-2 can be: an alkyne, a cyclic
alkene or a diene.
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• When double bonds are next to each other, they are said
to be Cumulated.
• When double bonds alternate with a single bond, they are
said to be conjugated.
• When more than one single bond comes between double
bonds, it is called isolated or non-conjugated.
C=C=C
Cumulated
C=C-C=C-C=
Conjugated
C=C-C-C=
isolated
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Nomenclature of Dienes
Alkenes that contain two double bonds are called
Dienes:
• Dienes are named by the IUPAC system in
essentially the same way as alkenes except that
the suffix -adiene replaces the ending –ene of
the alkene.
• Two numbers are needed to indicate the
locations of the double bonds in the chain.
• In cyclic dienes one of the double bonds is
always assigned the number 1, and the other is
given the lowest possible number.
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1,4-Pentadiene ( non conjugated)
1,3- Cyclohexadiene (conjugated)
1,3-Hexadiene (conjugated)
1,3,5,7-Cyclooctatetraene (conjugated)
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Geometric (cis-trans) isomerism
Cis- and trans-alkenes:
The prefix cis- is used when the two arms of the
longest chain are on the same side of the
double bond.
The prefix trans- is used when they are on
opposite sides of the double bond.
THE (E)-(Z) SYSTEM FOR DESIGNATING ALKENE
• If the two groups with the higher molecular
weight are on the same side of the double
bond, that is described as the (Z)- isomer. So
you would write it is (Z)-name of compound.
• If the two groups with the higher molecular
weight are on opposite sides of the double
bond, then this is the (E)- isomer.
E
Z
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H3C
CH3
CH3CH2
Cl
Z
Cl < CH2CH3 < CH3
HOOC
CH3
H
COOH
E
COOH < CH3 < H
Physical Properties of Alkenes
 Alkenes are non polar compounds.
 Insoluble in water.
 Soluble in non polar organic solvents.
 They are less dense than water.
 Range of physical states:
 ≤ 4 C's are gases
5 - 17 C's are liquids
 ≥ 18 C's are solids
 The alkenes has a boiling point which is a small number
of degrees lower than the corresponding alkanes.
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