combustion, halogenation

1. Alkanes

2. Alkanes
• Alkanes are saturated hydrocarbons —each carbon holds the maximum number
of hydrogen atoms).
• Alkanes contain only carbon-carbon single bonds.
• General formula: CnH2n+2 (not rings).
• Since alkanes undergo combustion easily, they are a good source of energy (e.g.,
gasoline).
• Alkanes also provide the raw materials for the production of many other more
complex substances (plastics, etc.).

3. Some Common Alkanes
• Methane, CH₄
major component of natural gas (~85%), which is produced by bacterial decomposition of
organisms in the absence of oxygen (marsh gas, cow flatulence).
burns cleanly, so is useful for cooking.
odorless — ethanethiol (an odouriser) is added to make natural gas leaks detectable.
• Ethane, CH₃CH₃ (C₂H₆) — a minor component of natural gas (~10%).
• Propane, CH₃CH₂CH₃ (C₃H₈) — used as an industrial fuel, and in home
heating and cooking.

4. Some Common Alkanes
Butane, CH₃CH₂CH₂CH ₃(C₄H₁₀)
– cigarette lighters
– Butane is an unbranched (normal) alkane.
-There is also a branched alkane with the formula
-C₄H₁₀, having a three-carbon chain with a one carbon group connected to
the middle.
– We must give the other isomer a different name:
-CH3CH(CH3)CH3 [or CH3CH(CH3)2] is named 2-methylpropane).
• – Butane and methylpropane are structural isomers of each other.

5. Physical Properties of Alkanes
• Since alkanes are composed of relatively non polar C—C bonds and C—H
bonds, alkanes are non polar molecules.
• Because they have only weak attractions for each other, they tend to have
lower melting points and boiling points than other organic compounds of
comparable molecular weights.
• The straight chain alkanes make up a homologous series in which each
members differs from a previous member by having one additional CH2
group.

6. Physical Properties of Alkanes
• Alkanes (nonpolar) are insoluble in water (polar),
and since they are less dense than water, they float(e.g., oil
slicks).
• Alkanes and other substances that do not dissolve in water are
often referred to as being hydrophobic(“water fearing”).
• Liquid alkanes of high molecular weight serve as
emollients (skin softeners) to replace oils washed
away by bathing or swimming. Vaseline is a semisolid mixture of
alkanes.

7. Alkane Reactions
• Alkanes are the least reactive of all organic compounds. They do
not usually react with strong acids or bases, or with most oxidizing
or reducing agents.

8. Combustion
alkane + O2 CO2 + H2O + heat
• They do, however, burn very easily in combustion reactions, releasing a
great deal of energy:
• CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) H=-ve (exothermic)
• C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) H=-ve (exothermic)
• They burn in air with a clear blue flame

9. Incomplete Combustion
• In the absence of enough oxygen for complete conversion to carbon
dioxide, some common waste products are generated in the incomplete
burning of alkanes :
• 2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g)
– CO, carbon monoxide, is poisonous, colorless, and odorless. In the
exhaust train of most cars, a catalytic converter converts CO to CO2.

10. Halogenatoion
• Alkanes in which one or more hydrogen atoms are replaced by halogen atoms
(F, Cl, Br, or I).
• The speed of substitution reaction depends on
• Light intersity-greater light intensity the faster the reaction
• The reactivity of the halogen- F>Cl>Br
• The reactivity of the alkane- the smaller the alkane the faster the reaction
• This takes place in stages. One Hydrogen atom is replaced at a time
• The overall reaction is:
• Alkane + Halogen haloalkane + HCl
UV light

11. Halogenation of Methane
• Reaction takes place in the
present of UV Light
• The products produced are
the halo alkane
tetrachloroalkane CCl4 (l)
and HCl (g)

12. References
• Tindale, Ritchie et al, 2014, Chemistry for CSEC 2nd Edition,
Nelson Thornes. Chapter 14, p243-244
• https://chemlaba.wordpress.com/2011/02/18/radicals-
substitution-reactions-of-alkanes/