Organic Chemistry

• A hydrocarbon is a compound containing only Hydrogen and Carbon eg, C₃H₈ propane; C₃H₆ propene; C₃H₄ propyne.
• Alkanes - Hydrocarbons with all single bonds (saturated);  general formula C_nH_2n+2;  quite unreactive;  will react with halogens slowly in the presence of u.v. light ® haloalkanes (substitution reactions);  undergo combustion with oxygen.  e.g. methane  CH₄, ethane C₂H₆,  propane C₃H₈ etc.
• Alkenes - Hydrocarbons with at least one carbon-carbon double bond (unsaturated); general formula (with one double bond) C_nH_2n;  they are quite reactive at the double bond site;  react rapidly and vigorously with halogens (addition reactions);  undergo combustion with oxygen.  e.g. ethene C₂H₄, propene C₃H₆ etc.

               C₃H₆    +     9/2 O₂   —›   3 CO₂    +      3 H₂O

                          C₃H₈    +        5 O₂    —›    3 CO₂    +      4 H₂O

•  Alkanes are separated from ‘crude oil’ by fractional distillation.
• The major use of the alkanes is as fuels.
• Alkenes are produced by catalytic cracking and reformation of alkanes.
• Major uses    - high octane additives to petrol

                                  - manufacture of polymers.

• Isomers are substances with the same molecular formulae but different structural formulae.

pentane	2-methylbutane
dimethylpropane (or 2,2-dimethylpropane)

      Isomers of hexene

also

2-methylpent-2-ene

3-methylpent-1-ene

3-methylpent-2-ene etc.

• Saturated - all single bonds (alkanes)
• Unsaturated - contain at least one carbon-carbon multiple bond (alkenes, alkynes)
• unsaturated hydrocarbons are more reactive;  reactive at the multiple bond.
• Polyunsaturated margarines are believed not to result in clogging of the arteries, however saturated fats are believed to cause the deposition of cholesterol into the cell walls of the arteries.
• The alcohols are a homologous series.  They have the same general formula (ROH) and the same functional group (ROH).  They have similar physical and chemical properties due to the presence of the hydroxyl group  (R OH).  There is a gradual change in physical properties as the no. of C atoms increases.eg. C₂₂H₄₆,      C₁₇H_34,           C₁₃H₂₄
• Substitution reactions - a H atom is replaced by a halogen; typical reaction of alkanes - very slow reaction in the presence of u.v. light —› haloalkanes

•  Addition reactions -  occur at a double or triple bond;  here more bonds are formed.  Reaction of alkenes or alkynes with halogens is very rapid.  This reaction can be used to distinguish between saturated and unsaturated organic compounds.

  ethene                                       1,2-dibromoethane

• Note:  Other addition reactions of alkenes include

       (i) Reaction with H₂ (hydrogenation wtih a catalyst —› alkane

                   ethene                                                ethane

• Reaction with H₂O (hydration) in the presence of an acid catalyst ® alcohol       

                   ethene                                                ethanol

• Addition polymerisation reactions in the presence of a free radical initiator or a catalyst.
•  Functional group - an atom or group of atoms that replace one or more hydrogen atoms of an alkane, thus changing the physical and chemical properties. e.g.

• Fractional distillation is a separation technique used to separate miscible liquids (liquids that dissolve in each other.)  The liquids are separated from eachother by heating until the liquid with the lower boiling point begins to boil.  this technique is used to separate the alkanes in crude oil.  As the no. of C atoms the strength of the dispersion forces also and therefore b.pt.
• Catalytic cracking - is the breaking of larger chain alkanes into smaller hydrocarbons by heating with a catalyst.

e.g.            C₁₀H₂₂        —›     C₈H₁₈         +       C₂H₄

                 decane      catalyst           octane                ethene