Alcohols

Alcohols

• Functional group - OH

• General formula - CnH2n+1OH

Physical properties

• they have a higher MPT and BPT than their corresponding alkane

• soluble in water

• alcohols can form hydrogen bonds

Combustion

• observation = clean blue/pale yellow flame

• C2H5OH + 302 → 2C02 + 3H20

Halogenation

Chlorination:

i) Add PCl5 to alcohol and mix at room temperature in a fume hood

e.g. CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl (steamy white fumes that turn damp blue litmus paper red (acidic)

ii) conc. HCl - reflux then distil

Bromination:

KBr + 50% H2S04 (makes HBr so 50% avoids HBr oxidising to bromine) and heat under reflux

Iodination:

• Damp red phosphorous + iodine (moisture allows solids to come into contact and react)

• 2P + 3i2 → 2Pi3

Oxidation

• alcohols are oxidised by K2Cr2O7 / H2SO4

• K2Cr2O7 is an oxidising agent so is reduced itself

• Cr (VI) = orange

• Cr(III) = green

Primary OH:

• 1 OH → aldehyde = distillation

• 1 OH → COOH = heat under reflux

Secondary OH:

• 2 OH → ketone (cannot oxidise further) = reflux

Tertiary OH:

• no reaction

Elimination

• Alcohol → alkene + water

• method 1 - pass the alcohol over a hot aluminium oxide catalyst

• method 2 - mix the alcohol with H3P03 and warm

Testing for alcohols

1. testing for OH → add PCl5

   Positive result = steamy white fumes which turns damp blue litmus red

   But this could be OH in COOH

2. Testing for 3 OH → add acidified K2Cr2O7 and warm

   Positive result = stays orange

   If 1 or 2, turns green

3. Testing for 1 → oxidise first and distil off product, then add benedict’s solution

   Positive result → blue solution changes to/forms brick red ppt. = aldehyde

   Negative result → stays blue = ketone