Combustion of Alcohols: Alcohols burn completely in plentiful supply of oxygen to produce carbon
dioxide and water, like alkanes.
• Oxidising Agent: It is a solution of potassium dichromate, acidified with dilute sulfuric acid.
Acidified potassium dichromate + reflux.
• Colour Change: If primary and secondary alcohol is oxidised by an oxidising agent, the orange
solution containing dichromate (VI) ions (Cr2O7
2-
) is reduced to green solution contain chromium
(III) ions (Cr3+).
• Equation: Remember to put [O] to represent the oxidising agent. Above the arrow put K2Cr2O7/
H2SO4. In questions asking for conditions, replace ‘/ ’ with ‘and’. Below the arrow put distil or
reflux. Remember + H2O.
• AC K. Aldehyde -> carboxylic acid. Ketone.
• If doesn’t give enough information, question will allow multiple answers, but give one structure.
• If primary alcohol to carboxylic acid, remember 2[O] in equation.
• Oxidation of Primary Alcohols:
• 1) Oxidation to Aldehyde: On gentle heating of primary alcohols, an aldehyde
formed. Aldehyde is distilled out of reaction mixture to prevent further reaction and carboxylic
acid forming.
• 2) Oxidation to Carboxylic Acid: If primary alcohol heated strongly under
reflux, carboxylic acid formed. Using an excess of acidified potassium dichromate (VI) and reflux
ensures all of alcohol oxidised completely to carboxylic acid, so prevents aldehyde forming.
• 3) Oxidation of Secondary Alcohols: They are oxidised to ketones. To ensure
reaction goes to completion, heated under reflux.
Oxidation of Tertiary Alcohols: They do not undergo oxidation reactions. The acidified dichromate
(VI) ion remains orange.
• Dehydration: It is an elimination reaction in which water is removed from a saturated molecule
to form unsaturated molecule.
• 4) Dehydration of Alcohols:
– An alcohol is heated under reflux.
– In the presence of an acid catalyst- sulfuric acid H2SO4 OR H3PO4. Remember to write H2SO4
above the arrow.
– Products of reaction is an alkene and water.
– A H and the OH is removed from the alcohol to from the water.
– An elimination reaction.
• If heated just with H2SO4 or just H3PO4 (acid catalyst) it is dehydration of alcohol. And not
reactions above or below as need other reactants.
• As an alkene formed- If secondary alcohol two stereoisomers formed where double bond in
different positions- remember to draw alkenes shortened.
• 5) Sodium Halide + Alcohols:
– Example of substitution reaction.
– Forms haloalkanes.
– Alcohol is heated under reflux.
– With sulfuric acid and a sodium halide so
HBr is formed in situ. Alcohols react with
hydrogen halide.
– NaBr (s) + H2SO4 (aq) -> NaHSO4 (aq) + HBr (aq).
– HBr + alcohol -> haloalkane + water.
– The overall reaction is alcohol + NaBr + H2SO4 -> haloalkanes + NaHSO4 + H2O.
• Be careful not drawing too many carbons and hydrogens. Number of carbons stays the same- so
start off with carbons and then add on others.