14 - alcohols

• Alcohols

  Compounds that contain at least one hydroxyl group
• General formula of alcohols


  CnH2n+1OH
• Functional group of alcohols


  Hydroxyl group (-OH)
• Hydrogen bonding

  Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen.
• What atoms do hydrogens bonds form between in alcohols?


  - In alcohols, there are O-H bonds present in the structure.

  - Hydrogen bonds form between the δ+ hydrogen atoms (in the OH group) and lone pairs on oxygens in other molecules (e.g. water).
• Why is the hydroxyl group in alcohols polar?


  Oxygen is much more electronegative compared to hydrogen. So the oxygen has a δ- charge and the hydrogen has a δ+ charge.
• What are theonly intermolecular forces in alkanes?


  Temporary induced dipole-dipole forces (London forces).
• What is the difference in boiling points of alkanes and corresponding alcohols?


  Alcohols have a higher boiling points than alkenes.
• Why do alcohols have higher boiling points that alkanes?


  Alcohols have hydrogen bonds between molecules which are much stronger, thus require more energy to break, than the London forces between alkane molecules.
• What intermolecular forces are in alcohols?


  Hydrogen bonds and temporary induced dipole-dipole forces.
• How does boiling point change in alcohols and alkanes as chain length increases?


  Increases.
• Why does the boiling point of alcohols and alkanes increase as chain length increases?


  The longer the chain, the more surface contact there is between molecules, the stronger the London forces.
• How soluble are small alcohols in water?


  Completely soluble.
• How does the solubility of alcohols change as their hydrocarbon chain increases?


  Decreases.
• Why does solubility of alcohols decrease as their hydrocarbon chain increases?


  The hydrocarbon chains force their way between water molecules and so break the hydrogen bonds between theose water molecules.
• Primary alcohol


  
  

  An alcohol where the carbon attached to the hydroxyl group is attached to one other carbon.
• Secondary alcohol


  An alcohol in which the hydroxyl (-OH) group is attached a carbon that is attached to two other carbons.
• Tertiary alcohol


  An alcohol in which the hydroxyl (-OH) group is attached to a carbon that is in turn attached to three other carbons.
• Combustion of alcohols

  Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water.
• What type of reaction is the combustion of alcohols?


  Exomerthic, releasing a large quantity of energy in the form of heat. As the number of carbon atoms in the acohol chain increases the quantity of heat released per mole also increases.
• How can ethanol be produced sustainably as a fuel?


  Fermentation of sugars.
• What type of alcohols can be oxdisied?


  Primary and secondary alcohols.
• Oxidising agent for alcohols


  Acidified potassium dichromate (VI) solution (H⁺/K₂Cr₂O₇) or acidified sodium dichromate (VI) solutions (NaCr₂O₇).

  
  

  These are acidified with sulfuric acid (H₂SO₄).
• What happens to the colour of acidified X dichromate (VI) solution when oxidised and why?


  If the alcohol is oxidised the orange solution containing dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.

  Cr₂O₇²⁻ →Cr³⁺Dichromate(VI) ions→Chromium(III) ions
• What does partial oxidation of primary alcohols form?


  Aldehyde
• Conditions of partial oxidation of primary alcohols?


  - Distillation

  - Heat

  - K₂Cr₂O₇/H₂SO₄ or NaCr₂O₇/H₂SO₄
• Observations of partial oxidation of primary alcohols

  Orange to green solution
• By product of partial oxidation of primary alcohols

  Water
• What does complete oxidation of primary alcohols form?


  Carboxylic acid
• Conditions of complete oxidation of primary alcohols?

  - Reflux

  - Heat

  - K₂Cr₂O₇/H₂SO₄ or NaCr₂O₇/H₂SO₄
• Observations of complete oxidations of primary alcohols?

  Orange to green solution.
• By product of complete oxidation of primary alcohols

  Water
• Functional group of aldehyde


  CHO
• Functional group of carboxylic acid


  COOH
• What does complete oxidation of secondary alcohols form?


  Ketone
• Conditions of complete oxidation of secondary alcohols

  - Reflux

  - Heat

  - K₂Cr₂O₇/H₂SO₄ or NaCr₂O₇/H₂SO₄
• Observations of complete oxidation of secondary alcohols

  Orange to green solution.
• By product of complete oxidation of secondary alcohols

  Water
• Functional group of ketone


  C=O
• Oxidation of tertiary alcohols


  Tertiary alcohols do not undergo oxidation reactions.