Alkenes (unsaturated hydrocarbons, CnH2n)
Alkenes are similar to the alkanes, but they contain at least one C-C double bond.
When naming an alkene, the prefix stays the same, but the ending changes to ‘-ene.’
Alkenes can be found as different isomers, depending on where the double bond is situated. This means we typically need to indicate the location of the double bond within the chain. To do this, we begin numbering from the end which provides the lowest number for the double bond.
Physical Properties of Alkenes
Like the alkanes, alkenes molecules are essentially non-polar. Therefore, the physical properties of the alkenes are very similar to those of the alkanes.
Lighter alkenes are gases at RT and as chain length increases, they become liquids and solids.
Alkenes are insoluble in water, but dissolve in other liquid hydrocarbons.
They have relatively low melting points and boiling points.
Chemical Properties of the Alkenes
Combustion Reactions
Like the alkanes, the alkenes undergo combustion in which they burn in oxygen to produce carbon dioxide and water.
Addition Reactions
The double bond gives the alkenes a greater reactivity than the alkanes. An addition reaction can occur in which a molecule adds on to the alkene carbons, leaving a single bond in its wake.
Check out these videos for an explanation of how addition reactions work:
Addition of Hydrogen, Addition of Halogen, Addition of Hydrogen Chloride, Addition of Water
Alkynes (unsaturated hydrocarbons, CnH2n+2)
The
alkynes contain at least one triple
bond.
Again the prefix stays the same, but the ending changes to ‘-yne.’
Alkynes can be found as different isomers, depending on where the triple bond is situated. Like with the alkenes, we begin numbering the main chain from the end which provides the lowest number for the triple bond.
 |
| 3-heptyne |
Physical Properties of the Alkynes
The physical properties of the alkanes are similar to those of the alkanes and alkenes.
Alkynes are gases, liquids or low melting solids at room temp.
They do not dissolve in water.
They have relatively low melting points and boiling points.
Chemical Properties of the Alkynes
The chemical properties of the alkynes are similar to those of the alkenes, due to multiple bond.
They undergo combustion reactions and addition reactions with hydrogen, the halogens and hydrogen halides.
Homework:
Practice, p. 186 # 3, 4
Practice, p. 188 # 5cd
Practice, p. 190 # 6, 7
Section Questions, p. 190 # 3-5, 7, 10
Read "Functional Groups" p. 199-200 then do p. 201 # 1-6
Answers:
