Chapter 19 - Alkanes, Alkenes and Alcohols

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Alkanes

Alkanes are a family of simple hydrocarbons containing carbons and hydrogens, connected by only single covalent bonds. They are saturated because they have the maximum number of single covalent bonds with hydrogen. 

  • Methane - CH4, Ethane - C2H6, Propane - C3H8, Butane - C4H10, Pentane - C5H12
  • Homologous Series - A family of compounds with similar properties due to similar bonding.
  • Members of a homologous series have a general formula to determine the number of carbons or hydrogens in a molecule - for alkanes, the general formula is CnH2n+2.
  • The more carbons in an alkane, the higher the boiling point - methane, ethane, propane and butane are all gases at room temperature. 
  • As the molecules get bigger, the intermolecular forces increase.

Reactions of Alkanes:

  • Combustion - CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
  • Reaction with Bromine - CH4(g) + Br2(g) -> CH3Br(g) + HBr(g) [Reacted under UV light]
  • ^ A Hydrogen atom in the alkane is replaced with a bromine atom
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Alkenes

Alkenes are a different homologous series of hydrocarbons. However, unlike alkanes, alkenes have a double covalent bond between two carbon atoms, so they are not saturated.

The general formula for alkenes is CnH2n

They have similar properties to alkanes (ethene, propene and butene are gases), however, alkenes react differently to alkanes because the double bond between the carbons breaks so more atoms can be added to the molecule.

Combustion - C2H4(g) + 3O2(g) -> 2CO2(g) + 2H2O(l)

The Addition of Bromine - CH2=CH2(g) + Br2(aq) -> CH2BrCH2Br(l)

  • ^ The alkene can be written like this to represent the actual structure.
  • The double bond breaks and joins with bromine.
  • The product is a colourless liquid called 1,2-dibromoethane.
  • This reaction is used to test for a double bond because an alkane wouldn't react with it so the bromine brown colour would be visible, but with an alkene the solution turns colourless.
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Alcohols

Alcohols are a homologous series containing a -OH group that's covalently bonded onto a carbon chain. Ethanol is the most familiar alcohol found in drinks - C2H5OH or CH3CH2OH

The Production of Ethanol - Fermantation:

  • Yeast is added to sugar or starch solution and left at about 30-40 degrees for several days without air (anaerobic conditions). Enzymes in the yeast convert the sugar into ethanol and carbon dioxide. C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)
  • The yeast is killed ny the alcohol after time so it is impossible to make pure ethanol.

The Production of Ethanol - Hydration of Ethene:

  • CH2=CH2(g) + H2O(g) -> CH3CH2OH(g)
  • Temp - 300 degrees, Pressure - 60-70 atmospheres, Catalyst - phosphoric acid
  • The ethanol produces is condensed to a liquid and unreacted ethene is recycled.
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Alcohols Continued

Comparing Methods of producing Ethanol:

  • Fermentation uses renewable resources like sugar cane, whereas in hydration of ethene, ethene is needed, which comes from oil (a finite resource) so could be used up.
  • The process of fermentation is very inefficient and takes several days, whereas the hydration method is efficient and creates a continuous flow of ethanol.
  • The hydration process produces very pure ethanol that doesn't need refining but fermentation makes very impure ethanol that does need refining further.
  • Fermentation uses gentle temperatures and normal pressure unlike hydration which needs a lot of energy input to create high temperatures and high pressures.

Reactions of Alcohols:

  • Combustion - C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)
  • Dehydration of Ethanol - CH3CH2OH(g) -> CH2=CH2(g) + H2O(l)
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