Other aliphatic hydrocarbons

Objectives1. To learn about naming and the structures of alkenes and alkynes and their branched chain isomers2. To understand the effect of unsaturated bonds3. To introduce the concept of cis and trans isomers;4. To compare properties of saturated and unsaturated hydrocarbons.

RulesAlkanes 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!

UnsaturatedThe 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 bondsPolypropylene is made from joining propene molecules

Alkenes

I certainly learnt to call ethylene ethene and propylene propene (see slide 1)

Molecular formula 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 isomersTwo 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 bond2. Number the longest chain from the end nearest the triple or double bond3. 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

CycloalkenesThese 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-Butene3-hexyneWrite the formula for each in C H formatWorkout the molecular formula for alkynesGive the possible structure of the most stable C6H12 Alkene