4.2 Alcohols, Haloalkanes & Analysis

• Alcohols


  Contain an -OH group and follow the general formula CnH2n+1OH

  
• Alcohols


  • Can be produced via two main methods of fermentation or hydration
  • Are named according to IUPAC rules and have the suffix -ol
  • Can be primary (1o), secondary (2o) or tertiary (3o), depending on the position of the hydroxyl group

  
• Polar molecules


  Alcohols are polar molecules since there is a large difference in electronegativity between carbon and oxygen
• Hydrogen bonding


  The oxygen in alcohols is very electronegative, so can hydrogen bond to water molecules
• Water soluble


  Alcohols are water soluble due to their ability to hydrogen bond
• Volatility


  Alcohols are less volatile than alkanes because they have both hydrogen bonds and van der Waals forces present between molecules
• Combustion of alcohols


  Alcohols react with oxygen to form carbon dioxide and water
• Fuels


  Alcohols make good fuels by reacting with oxygen in combustion reactions, releasing energy
• Reaction of alcohols with halogenating agents

  Nucleophilic substitution, where the -OH group is replaced by a halogen to produce a haloalkane

  
• Test for alcohols


  Reaction with PCl5 produces white steamy fumes that turn damp blue litmus paper red
• Dehydration of alcohols


  Alkenes can be formed by removing a molecule of water from alcohols, using concentrated phosphoric acid
• Oxidation of primary alcohols

  Heated with acidified potassium dichromate(VI), they are oxidised to aldehydes
• Oxidation of aldehydes


  Heated further with acidified potassium dichromate(VI) under reflux, they are oxidised to carboxylic acids

  
• Oxidation of secondary alcohols

  Heated with acidified potassium dichromate(VI), they are oxidised to ketones
• Potassium dichromate(VI)


  Used as the oxidising agent in the oxidation of alcohols, reduced as the alcohol is oxidised, causing a colour change from orange to green
• Haloalkanes


  Contain polar carbon-halogen bonds, with the halogen being more electronegative than carbon
• Haloalkanes


  • Can be classified as primary, secondary or tertiary depending on the position of the halogen

  
• Reactivity of haloalkanes


  Increases down the halogen group as the carbon-halogen bond enthalpy decreases

  
• Reaction of haloalkanes with aqueous alkali

  Nucleophilic substitution, producing alcohols

  
• Reaction of haloalkanes with ethanolic KOH

  Elimination, producing alkenes

  
• Hydrolysis of haloalkanes with aqueous silver nitrate

  Forms alcohols and precipitates of silver halides, with the colour of the precipitate identifying the halide ion
• Nucleophiles


  Electron pair donors that are attracted to δ+ regions of molecules, e.g. CN-, :NH3, -:OH
• Nucleophilic substitution mechanism - producing alcohols

  The nucleophile attacks the δ+ carbon, with the electrons transferring to the halogen
• Nucleophilic substitution mechanism - producing amines


  An intermediate with a positively charged nitrogen is formed, which then loses a hydrogen atom
• Nucleophilic substitution reactions can only occur for 1o (primary) and 2o (secondary) haloalkanes
• CFCs can produce halogen radicals under UV radiation, which catalyse the breakdown of the ozone layer
• Practical techniques


  • Heating under reflux, distillation, and separating funnels are used to prepare and purify organic compounds
• Distillation


  1. Round-bottomed flask is heated

  2. Liquid with lower boiling point evaporates first

  3. Vapour rises into attached tubing

  4. Vapour cools and condenses in condenser

  5. Condensed liquid collected in separate flask
• Condenser


  • Helps ensure vapours condense and return to flask for further heating
  • Ensures product vapours cannot escape
• Separating funnel


  Used to separate two liquids with different densities
• Separating funnel operation


  1. Mixture added to flask

  2. Flask stoppered and inverted to mix contents

  3. Liquids separate into two layers

  4. Tap opened to collect bottom, denser liquid in one flask and top, less dense liquid in another flask
• Subsequent distillations can be carried out to obtain a purer product
• Boiling point determination


  Determining the boiling point of a compound and comparing it to a databook value is a way of testing the purity of a substance
• Boiling point determination procedure

  1. Substance packed into Thiele tube with inverted capillary tube

  2. Substance heated to above boiling point and allowed to cool

  3. Temperature at which substance condenses into liquid in capillary tube is taken as boiling point
• Drying


  Compound can be dried by addition of anhydrous (contains no water) salt which will absorb moisture and water present, thus drying and purifying the compound
• Anhydrous salts used for drying

  • Magnesium sulphate
  • Calcium chloride
• Synthetic route


  The route which can be used to produce a certain product from a starting organic compound
• Multi-stage synthesis


  1. Typically involves two stages: reactant -> intermediate -> product

  2. Can cover more stages
• Ethanoic acid formation from chloroethane

  • Chloroethane -> intermediate -> ethanoic acid

  
• 1. propylamine formation from propene


  • Propene -> intermediate -> 2-propylamine