Alcohols and Some of Their Reactions

Alcohols

What is an alcohol?

An organic compound containing a hydroxyl (–OH) functional group.

What is the functional group of an alcohol?

The hydroxyl group, –OH.

What is the general formula of a monohydric alcohol?

CₙH₂ₙ₊₁OH.

What does R represent in the formula ROH?

An alkyl group.

How are alcohols classified as primary, secondary or tertiary?

By the number of alkyl groups attached to the carbon atom bonded to the –OH group.

What is a primary alcohol?

An alcohol where the carbon bonded to –OH is attached to one alkyl group.

What is the abbreviated formula for a primary alcohol?

RCH₂OH.

What is a secondary alcohol?

An alcohol where the carbon bonded to –OH is attached to two alkyl groups.

What is the abbreviated formula for a secondary alcohol?

R₂CHOH.

What is a tertiary alcohol?

An alcohol where the carbon bonded to –OH is attached to three alkyl groups.

What is the abbreviated formula for a tertiary alcohol?

R₃COH.

What three reactions of alcohols are covered in this topic?

Combustion, conversion to halogenoalkanes and dehydration to alkenes.

What are the products of complete combustion of an alcohol?

Carbon dioxide and water.

What is the equation for the complete combustion of ethanol?

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

What is halogenation of an alcohol?

A reaction in which the hydroxyl group of an alcohol is replaced by a halogen atom.

What type of compound is produced when the –OH group of an alcohol is replaced by a halogen?

A halogenoalkane.

How can an alcohol be converted into a chloroalkane?

React it with phosphorus(V) chloride, PCl₅.

What are the conditions for reacting an alcohol with PCl₅?

The reaction occurs vigorously at room temperature.

What is the general change when an alcohol reacts with PCl₅?

ROH → RCl.

What are the inorganic products when an alcohol reacts with PCl₅?

POCl₃ and HCl.

What is an example equation for converting an alcohol into a chloroalkane using PCl₅?

CH₃CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂Cl + POCl₃ + HCl.

How can a tertiary alcohol be converted into a chloroalkane?

Mix it with concentrated hydrochloric acid at room temperature.

Why is concentrated HCl mainly used with tertiary alcohols?

The method works well for tertiary alcohols but not well for primary and secondary alcohols.

How can an alcohol be converted into a bromoalkane?

Heat it with potassium bromide and about 50% concentrated sulfuric acid.

Why are KBr and H₂SO₄ used when making a bromoalkane?

They react together to produce HBr, which then reacts with the alcohol.

What equation shows the formation of HBr from KBr and H₂SO₄?

KBr + H₂SO₄ → KHSO₄ + HBr.

What happens when an alcohol reacts with HBr?

The –OH group is replaced by Br, producing a bromoalkane and water.

What is the general equation for an alcohol reacting with HBr?

ROH + HBr → RBr + H₂O.

Why should sulfuric acid used to prepare a bromoalkane be no more than about 50% concentrated?

More concentrated sulfuric acid oxidises bromide ions to bromine, producing different products.

How can an alcohol be converted into an iodoalkane?

Heat the alcohol under reflux with a mixture of red phosphorus and iodine.

What substance is formed from red phosphorus and iodine during preparation of an iodoalkane?

Phosphorus(III) iodide, PI₃.

What is the equation for forming phosphorus(III) iodide?

2P + 3I₂ → 2PI₃.

What is the general reaction between an alcohol and PI₃?

3ROH + PI₃ → 3RI + H₃PO₃.

What is dehydration of an alcohol?

A reaction in which the –OH group and a hydrogen atom from an adjacent carbon are removed, forming a C=C double bond.

What organic product is formed by dehydration of an alcohol?

An alkene.

What are the conditions for dehydrating an alcohol?

Heat with concentrated phosphoric acid, H₃PO₄.

What role does concentrated phosphoric acid have in alcohol dehydration?

It acts as an acid catalyst.

What is removed from an alcohol during dehydration?

The –OH group and a hydrogen atom from an adjacent carbon atom.

What other product is formed during dehydration of an alcohol?

Water.

What is the general change during dehydration?

Alcohol → alkene + water.

What products can form when butan-2-ol is dehydrated?

But-1-ene, but-2-ene and water.

Why can butan-2-ol form more than one alkene when dehydrated?

Hydrogen can be removed from either adjacent carbon atom, placing the C=C bond in different positions.

What equation shows butan-2-ol forming but-1-ene?

CH₃CH(OH)CH₂CH₃ → CH₂=CHCH₂CH₃ + H₂O.

What equation shows butan-2-ol forming but-2-ene?

CH₃CH(OH)CH₂CH₃ → CH₃CH=CHCH₃ + H₂O.

Why can dehydration of butan-2-ol give three isomeric alkene products?

But-1-ene forms as one product, while but-2-ene exists as two E/Z stereoisomers.

What happens to the phosphoric acid during dehydration?

It is not used up because it acts as a catalyst.

Why does H₃PO₄ not appear in the dehydration equation?

It is a catalyst and is regenerated during the reaction.

What should you remember when balancing combustion equations for alcohols?

The alcohol molecule already contains an oxygen atom, which must be included when balancing the oxygen atoms.