Alcohols

Production

Electrophilic addition of alkenes

Oxidation of alkenes

Nucleophilic substitution of halogenoalkanes

Reduction of carboxylic acids

Hydrolysis of ester

Substitution of alcohols

• In the substitution of alcohols, a hydroxy group (-OH) is replaced by a halogen to form an halogenoalkane

• The substitution of the alcohol group for a halogen can be achieved by reacting the alcohol with:

  • HX (rather than using HBr, KBr is reacted with H₂SO₄ or H₃PO₄ to make HBr that will then react with the alcohol)

  • PCl₃ and heat

  • PCl₅ at room temperature

  • SOCl₂

Reaction with Na

• When an alcohol reacts with a reactive metal such as sodium (Na), the oxygen-hydrogen bond in the hydroxy group breaks

• Though the reaction is less vigorous than sodium reacting with water, hydrogen gas is given off and a basic compound (alkoxide) is formed

  • If the excess ethanol is evaporated off after the reaction a white crystalline solid of sodium alkoxide is left

Alcohol + sodium → sodium alkoxide + hydrogen

Oxidation of alcohols

• Primary alcohols can be oxidised:

  • They form aldehydes which can undergo further oxidation to form carboxylic acids

• Secondary alcohols can be oxidised

  • The form ketones only

• Tertiary alcohols do not undergo oxidation

Oxidising agents

• The oxidising agents of alcohols include acidified K₂Cr₂O₇ or acidified KMnO₄

• This reduction requires hydrogen (H⁺) ions which are provided by the acidic medium

  • When alcohols are oxidised the orange dichromate ions (Cr₂O_7^2-) are reduced to green Cr³⁺ ions

Products

• When a warm primary alcohol is treated with an oxidising agent, it is initially oxidised to an aldehyde

  • The aldehyde has a lower boiling point than the alcohol, so it can be distilled off as it forms to prevent further oxidation

  • If it is not distilled, continued heating under reflux with excess oxidising agent will convert the aldehyde into a carboxylic acid

• In contrast, secondary alcohols are oxidised to ketones, which cannot be further oxidised, so there is no need to distil them off immediately

Dehydration of alcohols

• Alcohols can also undergo dehydration to form alkenes

  • Dehydration is a reaction in which a water molecule is removed from a larger molecule

  • A dehydration reaction is a type of elimination reaction

Test

• Tri-iodomethane (also called iodoform) is a yellow precipitate 

• The position of a secondary alcohol group in a molecule can be identified using the iodoform test with iodine in alkaline solution

  • If the –OH group is attached to a carbon atom that is next to a methyl group, the alcohol can be oxidised by the alkaline iodine to form a methyl ketone (RCOCH₃)

  • The methyl ketone then undergoes:

    • Halogenation, where the three hydrogen atoms of the methyl group are replaced by iodine atoms (–CI₃), followed by

    • Hydrolysis, forming a sodium carboxylate salt and a yellow precipitate of iodoform (CHI₃)

• A positive result (yellow precipitate) indicates that the alcohol has the structure CH₃CH(OH)R

• No precipitate means the secondary alcohol is not adjacent to a methyl group and therefore cannot form a methyl ketone intermediate