Lesson 1: Start of helll
Tuesday, August 17, 2010
What is an alkane???
1) Belong to the homologous series of saturated hydrocarbons
- Contain only single covalent bonds between atoms in molecules
- Contain only hydrogen and carbon atoms
2) Name ends with –ane
3) Can be represented by the general molecular formula CnH2n+2, n is more than or equal to 1
A saturated compound contains only single covalent bonds between atoms in the molecules.
Structural formulas of the first five alkanes:
Methane: CH4
Ethane: C2H6
Butane: C4H10
Pentane: C5H12
3:21 AM
Lesson 2: TWINS!?!?!?
Some compounds have the same molecular formula but have different structural formula, like twins! These are called Isomers.
Isomerism is the existence of two or more compounds with the same molecular formula but different structural formulas
Isomers are compounds with the same molecular formula but different structural formulas.
Alkanes with more than 3 carbon atoms in the molecules can possess isomers.
Example: Butane, C4H10
As shown, both compounds have the same molecular formula of C4H10 but different structural formulas.
- Isomers have different physical properties but similar chemical properties of varying reactivity.
3:21 AM
Lesson 3: How do alkanes act??
Physical properties
1) Alkanes have low melting and boiling points
- Made up of covalent molecules held by weak intermolecular forces, so less energy is required to overcome the forces to separate the molecules
- First 4 members are gases at r.t.p.
- Alkanes with 5 to 16 carbon atoms per molecule are liquids and those with more than 17 carbon atoms exist as solids
- Physical properties of alkanes change gradually as molecular size increases
- Alkanes become more viscous because long carbon chains are much more likely to get tangled up and makes it more difficult for the liquid to flow.
2) All members are insoluble in water but soluble in organic solvent
- Like dissolve like
3) Similar for all alkanes
- Alkanes contain the same single C-H and C-C covalent bond
4) Not very reactive
- Strong C-C and C-H bonds require a lot of energy to break.
Reactions of Alkane
§ Combustion
- Smaller alkanes burn easily in air when ignited
- If the combustion is complete, products formed are CO2 and HHasdasdaosijdoijasH2O
ð CH4(g) + 2CO2(g) à CO2(g) + 2H2O(g)
- Reaction is highly exothermic
ð Main use of alkanes as fuels for cooking; fuels for vehicles
- If the combustion is not complete, products formed are CO, C(soot) and H2O
§ Substitution reaction
- Characteristic reaction of saturated compounds
- Usually slow
- Takes place with chlorine or bromine
- Reaction is catalysed by ultra-violet light
ð Provides the energy to break the covalent bond between the molecules
- Reaction is difficult to control. A mixture of products is usually obtained
Substitution reaction is a reaction in which the hydrogen atoms of an organic molecule are replaced by atoms of other elements.
3:01 AM
Lesson 4: STUPID ALKANE NAMES!
How do we actually name these long chains of carbon?
1) Find the longest continuous carbon chain in the compound. This gives the parent name for the compound
- This means that the number appearing in the name is a smaller number.
2) Number each carbon atom in the longest chain, starting from the end nearest to the branch
3) Name the group joined to the chain and state the number of the carbon atom in which it is joined.
4) If the chain has 2 or more similar groups joined to it, prefixes like di-, tri-, tera- is used to indicate the no. of groups present
5) If a chain has 2 or more different groups joined to it, the groups are written in alphabetical order i.e. ethyl before methyl.
2:50 AM
