16.1 alcohols

State the ways in which alcohols can be produced

• Hydration of alkenes

• Oxidation of alkenes

• Substation of halogenoalkane

• Reduction of aldehyde or ketones

• Reduction of carboxylic acid

• Hydrolysis of ester

Hydration of alkene

React hot steam with alkene,

Concentrated (H₃PO₄)as a catalyst,

electrophilic addition 

Oxidation of alkene

• Cold, dilute KMnO₄ is a mild oxidising agent and oxidises alkenes

• The C-C double bond is not fully broken and a diol is formed

Substitution of halogenoalkane

halide atom in halogenoalkanes can be substituted when heated with aqueous NaOH in a nucleophilic substitution reaction

Reduction of aldehyde or ketone

• Reduced by reducing agents such as NaBH₄ or LiAlH₄ 

• Aldehydes are reduced to primary alcohols

• Ketones are reduced to secondary alcohols

Reduction of carboxylic acid

• carboxylic acids are reduced by LiAlH₄ to primary alcohols

• NaBH₄ is not used as it is not a strong enough reducing agent

Hydrolysis of ester

• When an ester is heated with dilute acid or alkali, hydrolysis will take place and the carboxylic acid and alcohol will be reformed

State the ways in which alcohols can react

1. Combustion

2. Substitution to form halogenoalkanes

3. Reaction with Na

4. Oxidation

5. Dehydration to an alkenes

6. Esterification

Combustion of alcohols

react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water

Substitution of alcohols

a hydroxy group (-OH) is replaced by a halogen to form an halogenoalkane

• HX (rather than using HBr, KBr is reacted with H₂SO₄ or H₃PO₄ to make HBr in situ)

• PCl₃ and heat

• PCl₅ at room temperature

• SOCl₂

Alcohol + PCl3 →

3 halogenoalkane

• H3PO3

(Heat)

Alcohols + PCl5 →

RTP

Halogenoalkane + HCl + POCl3

Alcohol + SOCl2 →

Halogenoalkane + HCl + SO2

Reaction of alcohol with a reactive metal (Na)

• 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 primary alcohols.

With acidified KMnO4 to form aldehydes which can be further oxidised to carboxylic acids

Oxidation of secondary alcohols

With acidified KMnO4 to form ketones

Do tertiary alcohols undergo oxidation

No

Dehydration of alcohols

Alcohol vapour is passed over a hot catalyst of Al₂O₃ powder to form alkene and water

Esterification of alcohols

• condensation reaction between a carboxylic acid and an alcohol to form an ester and a water molecule

• Carboxylic acid and alcohol are heated under reflux with a strong acid catalyst

How to classify alcohols

Use acidified potassium dichromate

Primary and secondary go green

Tertiary remains orange

Iodoform can test for

The position of secondary alcohol group

If the OH group is attached to a c atom that is next to a methyl group, the alcohol can be oxidised by the alkaline iodine to form methyl ketone

Describe the dissociation of water

ROH (aq)  ←> RO- + H+

Low degree of dissociation

The position of eq lies to the left so far more alcohol molecules than ions

Compare the acidity of alcohols to water

• When water dissociates, the position of the equilibrium still lies to the left, but there are more H⁺ ions compared to the dissociation of alcohols

• As alcohols have lower [H+] in solution compared to water, alcohols are weaker acids than water

Explain the inductive effect in alcohols

the oxygen atom in the alkoxide ion is bonded to an electron-donating alkyl group

Therefore oxygen has high electron density

Therefore the alkoxide ion is more likely to accept the hydrogen ion and form the alcohol again

There are no electron donating groups in the hydroxide ion, so O- is less likely to accept a proton to reform water