Chap 21B - Hydroxy

Describe reaction of alcohol with Na + general reaction

• Alcohols react rapidly with sodium to form alkoxides (RO–) and H2 gas 

• Reagents and conditions: Na

• ROH + Na -> RO-Na+ + 1/2H2 

Describe distinguishing test for organic compounds with –OH group (alcohol, phenol or carboxylic acid) + observation

• Add Na metal to the unknown organic compound

• Observation:

  • If the compound contains an –OH group, effervescence is observed, H2 gas evolved extinguishes lighted splint with a ‘pop’ sound

  • If –OH group is absent from the compound, no effervescence is observed

Describe oxidation of primary alcohol to obtain aldehyde

• Use primary alcohol, K2Cr2O7(aq), H2SO4(aq) and heat with distillation

  • NOTE: Using acidified KMnO4, heat under reflux will oxidise the primary alcohol to a carboxylic acid as it is a much stronger oxidising agent (no aldehyde)

  • NOTE: NOT heat under reflux 

    • Milder reaction condition is required

    • Under the conditions of reflux -> further oxidation of the aldehyde to carboxylic acid 

    • Aldehydes have lower bp than corresponding alcohol -> the distillation process would allow the removal of the product once it is formed, shifting POE right

Describe oxidation of primary alcohol to obtain carboxylic acid + general reaction

• To obtain carboxylic acid: Use primary alcohol, K2Cr2O7(aq), H2SO4(aq) and heat under reflux

• RCH2OH + [O] → RCHO + H2O

• RCHO + [O] → RCO2H

• Overall: RCH2OH + 2[O] → RCO2H + H2O

Describe reflux

• Process of continued heating with condensation of the vapour back into the reaction mixture

• Most organic reactants are of high volatility and most organic reactions are slow and require heating

• The reflux will prevent volatile components of the reaction mixture from boiling off during the prolonged heating -> maximum yield of the desired product

Draw distillation setup

Describe oxidation of secondary alcohol

• Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), heat OR acidified KMnO4 and heat

• RCH(OH)R’ + [O] → RCOR’ + H2O

• Betanone (ketone) 

Describe oxidation of tertiary alcohol and phenols

• Compounds where an –OH group is directly bonded to a benzene ring have no hydrogen atoms on the carbon with –OH group (alpha carbon)

• CANNOT be oxidised

Describe test for primary, secondary and tertiary alcohol + observation

• Heat the alcohol with K2Cr2O7(aq) and H2SO4(aq)

• Observation:

  • If acidified K2Cr2O7 changes from orange to green, primary or secondary alcohol is present

  • If acidified K2Cr2O7 remains orange, tertiary alcohol is present

Describe dehydration of alcohol + general reaction

• Elimination of water from alcohol gives the corresponding alkene

• Reagents and conditions: concentrated H3PO4 catalyst, heat

Describe Saytzeff’s rule 

• Dehydration of secondary and tertiary alcohols containing 4 or more C atoms yields a mixture of 2 structural alkenes as products

• More substituted alkene will be predominantly formed (more alkyl groups)