Other aliphatic hydrocarbons
Objectives
1. To learn about naming and the structures of alkenes
and alkynes and their branched chain isomers
2. To understand the effect of unsaturated bonds
3. To introduce the concept of cis and trans isomers;
4. To compare properties of saturated and unsaturated
hydrocarbons.
Rules
Alkanes have all single C-C bonds
Alkenes have at least 1 C=C bond
Alkynes have at least 1 CΞC bond
Names all follow same pattern although some other common names exist to
confuse you.
Stick to format you’ve learnt!
Names of alkenes and alkynes include numbers to indicate position of double and
treble CC bonds!
Unsaturated
The double and triple bonds in alkenes and alkynes lead to them
being referred to as unsaturated.
These bonds can broken to allow the substitution in of other
chemicals such as halogens
These bonds can be broken to allow polymerisation.
Polythene is made from long chains of ethene molecules joined by
the breaking of their double bonds
Polypropylene is made from joining propene molecules
Alkenes
I certainly learnt to call ethylene ethene and propylene propene (see slide 1)
Molecular formula
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
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



Alkanes
CnH2n+2
Alkenes
CnH2n
Alkynes?
What are the formulae of
Pentane
Hexene
Octene
Nonane
isomers
There are no isomers of ethene and propene
Butene has 4 isomers
Two of them have different spatial arrangements of their isomers and as such
are called stereoisomers: They are named cis and trans which indicate whether
subsituents are on the same or opposite sides of the double bond respectively.
Becomes very important much later on with fats!
Alkenes
Cis and trans isomers
Trans isomers are slightly more stable than cis isomers so have slightly
lower heats of hydrogenation (a measure of how much energy they give
out)
Rules for naming Alkenes and Alkynes
1. Name the longest carbon chain that contains the double or
triple bond
2. Number the longest chain from the end nearest the triple or
double bond
3. Indicate the position of the double or triple bond with the
number of the first unsaturated carbon.
4.Give the location and name of each substituent
(alphabetical order) as a prefix to the alkene/alkyne name.
Alkynes
Note: I would call acetylene ethyne
Alkynes
Cycloalkenes
These too exist. Again BPs and MPs are lower than
for straight chain isomers
They are also more stable
Nomenclature is as for alkanes.
When adding a side branch number carbons from
the first side branch anticlockwise
1-methyl cyclohexene
Plenary: Draw molecular models for
2-Butene
3-hexyne
Write the formula for each in C H format
Workout the molecular formula for alkynes
Give the possible structure of the most stable C6H12 Alkene