Alkenes and Alkynes
Chapter 1.2
Unsaturated Hydrocarbons
Alkenes and alkynes are unsaturated hydrocarbons
• An alkene is a
hydrocarbon that
contains at least one
carbon-carbon double
bond
• An alkyne is a
hydrocarbon that
contains at least one
carbon-carbon triple
bond
ethyne
ethene
Alkanes, alkenes and alkynes are all aliphatic hydrocarbons which means their strucutres
are based on straight or branched chains or rings of carbon atoms
Naming Alkenes and Alkynes
• Naming alkenes and alkynes is very similar to naming alkanes
• The parent chain must be numbered so that the double or
triple bonds are on the lowest numbered carbons possible
• The suffixes –ene and –yne are used
• If there is more than one multiple bond indicate this using
-diene or –triene
• A number may also be necessary to indicate which carbon on
the parent chain or ring the multiple bond occurs on
• For example, the molecule below would be named
3,3-dimethylbut-1-ene
Practice
Name the following
a)
b)
c)
d)
Redundancy revisited
What is wrong with the name prop-1-ene?
Stereoisomers Revisited
• Like cycloalkane ring structures, alkenes and alkynes have restricted
rotation because of the multiple bonds
• Alkenes have two groups attached to each double bonded carbon and
their position is fixed due to the lack of rotation in the double bond, thus
cis-trans isomerism is possible
• Imagine that the molecule is divided into two sides along the double bond
• The cis isomer will have two matching groups on the same side of the
double bond
• The trans isomer will have the matching groups on opposite sides of the
double bond
Practice
• Example:
• Name the following
a)
b)
c)
d)
Properties of Alkenes and Alkynes
• A functional group is a group of atoms within a
molecule that determines the properties of the
molecule
• Multiple bonds are considered to be functional
groups because they affect the properties of the
molecules that contain them
• For example, multiple bonds are less stable than
single bonds between carbon atoms, thus alkenes
and alkynes are more reactive than alkanes
Reactions of Alkenes and Alkynes
• Like alkanes, alkenes and alkynes undergo
combustion reactions
• Alkenes and alkynes also undergo addition reactions
• An addition reaction is a reaction in which the atoms
from one molecule are added to another molecule to
form a single molecule
• The four addition reactions that alkenes and alkynes
undergo are:
1) Hydrogenation
2) Halogenation
3) Hydrohalogenation
4) Hydration
Reactions of Alkenes and Alkynes
1) Addition – Hydrogenation
• Hydrogen is added to the alkene or alkyne
and reacts with the double bond to form a
more saturated product
• This reaction takes place in the presence of a
catalyst
Reactions of Alkenes and Alkynes
1) Addition – Hydrogenation
+ H2
catalyst
catalyst
+ excess H2
Reactions of Alkenes and Alkynes
2) Addition – Halogenation
• A halogen (such as bromine or chlorine) is added to
the alkene or alkyne
• The halogenation of an alkene produces an alkyl
halide
+ Br2
Reactions of Alkenes and Alkynes
2) Addition – Halogenation
• The halogenation of an alkyne produces a
halogenated alkene or, if excess halogen is
present, an alkyl halide
Reactions of Alkenes and Alkynes
3) Addition – Hydroalogenation
A hydrogen halide (such as hydrogen chloride
or hydrogen bromide) is added to an alkene or
an alkyne
But wait! There’s a PROBLEM!
Predict the products of the hydrohalogenation
of but-1-ene
+ HBr
Markovnikov’s Rule
When a hydrogen halide or water is
added to an alkene, the hydrogen atom
generally bonds to the carbon atom
within the double bond that already
has more hydrogen atoms bonded to it.
Reactions of Alkenes and Alkynes
4) Addition – Hydration
• Water is added to an alkene or an alkyne
• This reaction produces a type of organic compound
called an alcohol
• Alcohols contains a hydroxyl group (-OH)
• This reaction takes place in the presence of a catalyst
• Markovnikov’s rule may be used in hydration reactions
as well
catalyst
Reactions of Alkenes and Alkynes
A. Combustion
B. Addition
1) Hydrogenation
2) Halogenation
3) Hydrohalogenation
4) Hydration
HOMEWORK
Required Reading:
p. 18-27
Questions:
p. 21 #1-2
p. 23 #1-3
p. 26 #1-2
p. 27 #1-10
Remember to ask for extra help if you need it!!