organic chem

• homologous series

  a group of organic compounds with the same functional group and general formula
• hydrocarbons are made up only of carbon and hydrogen atoms.
• there is a gradual change in physical properties as we go down a homologous series
• solubility of alkanes


  alkanes are insoluble in water but soluble in organic solvents
• why do melting + boiling point increase down the series?

  methane has a smaller molecular size than butane. forces of attraction is weaker compared to butane molecules. thus, little amount of energy is needed to break these attractive forces leading to a lower melting and boiling point
• functional group of alkanes

  carbon-carbon single bond
• alkanes are saturated hydrocarbons containing single carbon-carbon bonds
• general formula of alkanes


  CnH2n+2
• as we go down the alkane series, molecule size increase so more energy is needed to overcome forces of attraction. so boiling point increase
• physical properties of alkanes

  they become sticker (viscous), melting + boiling point increase
• alkanes are quite unreactive as they are saturated
• alkanes undergo 3 reactions:


  combustion, substitution and cracking
• combustion


  reaction is exothermic, releasing a lot of heat
• complete combustion of alkanes give of

  carbon dioxide and water
• alkane + oxygen -> carbon dioxide + water
• incomplete combustion


  carbon monoxide and sulfur dioxide is formed instead of carbon dioxide
• substitution reaction of alkanes

  alkanes react with halogens. this reaction takes place in the presence of UV light
• each hydrogen atom in an alkane is substituted one at a time by a halogen atom
• alkane + chlorine -> chloro-alkane + hydrogen
• cracking


  the process of breaking down larger hydrocarbons to produce smaller hydrocarbons
• cracking


  carried out by heating with a catalyst at high temperature and pressure
• cracking


  large alkane -> smaller alkane + alkenes
• purpose of cracking


  to produce hydrogen gas or short chain alkanes and alkenes that are in high industrial demand
• alkenes contain carbon carbon double bonds
• general formula for alkenes

  CnH2n
• solubility of alkenes


  alkenes are insoluble in water but soluble in organic solvents
• methene does not exist because a carbon carbon double bond requires at least 2 carbon atoms
• alkenes are quite reactive as they are unsaturated


  combustion, polymerisation, halogenation, hydrogenation
• halogenation occurs when halogens are added to double carbon carbon bonds of alkenes
• ethene + bromine = dibromoethane
• alkene + hydrogen -> alkane
• hydrogenation


  alkenes can react with hydrogen to produce alkanes in the presence of nickel catalyst at high temperature and pressure
• complete combustion of alkenes give carbon dioxide and water as products
• alkenes


  they have similar chemical properties, showing trend in reactivity but shows gradual change in physical properties
• purpose of cracking


  to produce alkenes for polymerisation to make plastics
• how to distinguish alkenes from alkane?

  use reaction with halogens
• in order to distinguish between alkanes and alkenes, test for the presence of an unsaturated compound from a saturated compound
• add liquid bromine to find out which is alkane and which is alkene
• liquid bromine


  reddish brown solution
• bubbling of an alkane into bromine water


  no visible change, bromine water remains brown. reason is alkane have saturated carbon bonds and cannot undergo addition reactions