CHAPTER 12 – ALKENES & ALKYNES
Alkenes – hydrocarbons containing carbon-carbon double
bonds (C = C).
Alkanes have only single bonds between carbons and are
saturated with hydrogens.
Unsaturated Hydrocarbons
Alkene
C= C
Alkyne
–C=C–
Carbon atoms are not saturated with hydrogens.
Rules for Naming Alkenes
Same as alkanes plus the following rules.
1. The ending “ene” is used for a double bond. (replace
“ane” with “ene”
Example:
Ethane becomes ethene
H
H
H–C – C–H
H
H
CH3 - CH3
ethane
CH3 - CH2 -CH3
propane
H–C =C–H
H
H
CH2=CH2
ethene
CH3 - CH=CH2
2. When a carbon chain has 4 or more carbons you must
number the position of the double bond, using the lowest
possible #.
Example
CH3-CH=CH-CH3
CH2=CH-CH2-CH3
3. Branches are the same as with alkanes. (double bond
takes precedence for numbering over other groups)
H
H H
H
C = C – C – C– H
H
CH3 H
CH3
CH3
CH3-C-CH2-CH2-C=CH-CH3
H
4. The longest chain must include the double bond.
CH3- CH2 -C-CH2-CH2-CH3
C
H
H
CH3-CH2-CH2-C=CH-CH3
CH2
CH3
5. Rings containing double bonds are called cycloalkenes.
For one double bond in a ring, the carbon atoms of the
double bond are numbered from 1 to 2 in the direction
that gives the substituent the lower #. It is not necessary
to include # for double bond in name.
Example
Br
CH3
H3C
6. Compounds with 2 or more double bonds have the
ending diene or triene, etc.
CH2=CH-CH=CH2
1,3-butadiene
CH3
CH2=C-CH2CH=CH2
Cis-Trans Isomers – same molecular formula and same
order of attachment of atoms, but different arrangement of
atoms in space (different three-dimensional shape) due to
the carbon-carbon double bond.
No rotation about double bond. Ethene is planar.
Cis and Trans Isomers - two different groups must be
connected to both the double bonded carbons.
Cis: two groups are on the same side
Trans: two groups are on opposite sides
H3C
H
H3C
C=C
H
CH3
C=C
CH3
trans-2-butene
H
H
cis-2-butene
Is this a cis or trans isomer?
H3C
H
C=C
H3C
H
Is this a cis or trans isomer?
H3C
H
C=C
H
CH2CH3
Is this a cis or trans isomer?
H3C
CH2CH3
C=C
H
H
Box 12B: Cis-11-tetradecenyl acetate a pheremone
secreted by the female corn borer to attract males.
Addition Reactions of Alkenes – something is added to
the double bond.
Addition of Hydrogen:
H H
C= C
+
H2
–C–C–
Pt
Examples
H
H – C = C – H + H2
H
Pt
H
H–C – C–H
H
ethene
H
ethane
CH3-CH=CH2 +
H2
Pt
Addition of bromine and chlorine
Br Br
C= C
H
+
Br2
–C–C–
Examples
H – C = C – H + Br2
H
H
CH3-CH=CH2 +
+
Cl2
Cl2
ALKYNES
Alkynes – hydrocarbons containing carbon-carbon triple
bonds (C = C).
Rules for Naming Alkynes
Same as alkenes except:
The ending “yne” is used for a triple bond. (replace “ane”
with “yne”.
H–C = C–H
CH3 - C=CH
CH3-C=C-CH3
CH=C-CH2-CH3