Physical Properties of the Alkanes
Since the bonds in an alkane are either C-C (ΔEneg = 0) or C-H (ΔEneg = 2.5 – 2.1 = 0.4), they are all essentially non-polar. Thus, there are no δ+ and δ- that would be attracted to each other, so alkanes have very weak forces of attraction between molecules.
As the number of C atoms in a molecule increases, so do the attractive intermolecular forces between molecules so the bp increases with chain length (gas → liquid → solid).
Due to their non-polarity, they are water-insoluble.
Chemical Properties of the Alkanes
Combustion Reaction
- Alkanes are fairly unreactive compounds.
- They are used for the manufacture of other organic compounds and fuels.
- They do undergo a complete combustion reaction with oxygen, always producing carbon dioxide and water:
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
- This reaction produces much of the energy used by society for heating and cooking.
- If a limited supply of oxygen is available, incomplete combustion occurs, which can be dangerous since carbon monoxide is one of the products.
Substitution Reactions
- Alkanes can react with the halogens during a substitution reaction.
- In the presence of heat or UV radiation, alkanes react with chlorine and bromine.
- One or more of the alkane’s hydrogens are replaced with a halogen atom.
- The reaction can stop here to produce CH3Cl, or continue to produce CH2Cl2, CHCl3 or CCl4.
See this video for a description of how this reaction proceeds.
Homework:
Practice, p. 188 # 5ab
Show the substitution reaction between (a) methane and bromine, (b) ethane and chlorine.
Read "Fractional Distillation" p. 193-195 then do p. 196 # 1-6, 10.
Answers:
