Alkanes and their reactions

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: C n H 2n +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 , CH ₄ 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 , CH ₃ CH ₃ ( C ₂ H ₆ ) — a minor component of natural gas (~10 %). Propane , CH ₃ CH ₂ CH ₃ ( C ₃ H ₈ ) — used as an industrial fuel, and in home heating and cooking.

4. Some Common Alkanes Butane , CH ₃ CH ₂ CH ₂ CH ₃ (C ₄ H ₁₀ ) – cigarette lighters – Butane is an unbranched (normal) alkane . -There is also a branched alkane with the formula -C ₄ H ₁₀ , 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 CH 2 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 + O 2 CO 2 + H 2 O + heat They do, however, burn very easily in combustion reactions, releasing a great deal of energy: CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(g) H=- ve (exothermic) C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O(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 : 2CH 4 (g ) + 3O 2 (g ) → 2CO(g ) + 4H 2 O(g) – CO, carbon monoxide, is poisonous, colorless, and odorless. In the exhaust train of most cars, a catalytic converter converts CO to CO 2 .

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 CCl 4 (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/