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Alkene – has at least one carbon-carbon
double bond, C = C.
Simplest alkene, C2H4, ethene.
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Alkyne – has at least one carbon-carbon
triple bond, C  C.
Simplest alkyne, C2H2, ethyne.
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Can also have double bond with ring
structure.
Cycloalkene – a ring with one or more
carbon-carbon double bond.
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Naming an alkene.
1. Find the longest chain that includes the double
bond. Base name uses –ene suffix.
2. Number the chain so that the double bond gets
the lowest numbers (has priority over other
substituents). Only four carbons or longer will
need a number for the double bond position. Use
only the lowest number for start of double bond.
3. Number substituents based on this numbering.
4. Cycloalkenes – the double bond is ALWAYS
position #1 and #2.
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5
4
3
2
1
CH3 – CH – CH2 – CH = CH2
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CH3
Correct: 4-methyl-1-pentene
Not correct: 2-methyl-4-pentene
Correct = 3-methylcyclopentene
Not Correct = 1-methylcyclopentene
LEP #1
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The double bond in an
alkene consists of a
sigma and a pi bond.
Pi bonds do NOT allow
free-rotation.
C2H6 (Ethane) versus
C2H4 (Ethene).
The molecule around
that double bond is
planar (flat).
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The molecule 2-butene has two unique
structures.
CH3 – CH = CH – CH3
How?
LEP #2
The molecule 1-butene does not!
CH2 = CH – CH2 – CH3
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Many alkenes do not have a pair of geometric
isomers.
Requires two different sets of groups on each
side of the double bond – one large and one
small.
A
B
C=C
B
A
Opposite = trans, Same side = cis
LEP #3
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These types of molecules are found in many
places and are very important.
Ex) Bombykol – a sex attractant (pheremone)
for the silkworm moth.
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Both alkenes and alkynes can undergo an
addition reaction where the double bond is
broken and two atoms or molecule fragments
are added to each side of the double bond.
Hydrogenation, adding H2 with a catalyst.
Halogenation, adding Cl2, Br2, or I2.
Hydration, adding H2O (H-OH) with an acid
catalyst.
LEP #4
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A polymer is a long chain of repeating units
called monomers.
Analogy: train = monomers are the individual
cars.
Monomers are typically small alkenes.
Initiated by organic peroxide, R-O-O-R’,
which is split into two fragments, 2 R-O
Benzoyl
peroxide
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Each fragment has an odd electron.
Peroxide fragment then reacts with monomer
unit using one electron from double bond.
Monomer end now has odd electron, which is
used to react with a second monomer.
Process repeats thousands of times.
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CH2=CH2, ethylene makes polyethylene.
◦ Two forms – low density and high density
◦ Uses:
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CH2=CHCl, vinyl chloride makes PVC.
◦ Uses:
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CH2=CHCH3, propene makes polypropylene.
◦ Uses:
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CF2=CF2, tetrafluoroethene makes Teflon.
◦ Uses:
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CH2=CCl2, 1,1-dichloroethene makes Saran.
◦ Uses:
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CH2=CH(C6H5), phenylethene makes
polystyrene.
◦ Uses:
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Recycling – uses a series of symbols and
numbers to identify the type.
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C6H6 = Benzene
A ring structure, but very different from the
cycloalkanes.
Three double bonds alternate with every
other C-C in ring.
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Alkenes are reactive because of the double
bond.
Benzene, though, is not reactive at all.
Why?
True structure is an average of the two below.
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The six electrons from the
three double bonds are
delocalized (free to move).
Chemists often show this by
drawing the hexagon and
putting a circle inside to
represent these electrons.
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Some common mono-substituted benzene
molecules have common names that are
allowed by IUPAC rules.
OH
CH3
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Benzene as a substituent (C6H5-) is called Phenyl.
Mono-substituted benzenes do not require a
number.
Di-substituted benzenes do require numbers.
A phenol or toluene will always have the –OH or –
CH3 group as position #1.
LEP #5
Br