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7.2 Reactions of Alkenes and Alcohols

7.2.1 Structure and formulae of alkenes

_{(stolen from spec)}
- Alkenes are hydrocarbons with a double carbon-carbon bond.
- The general formula for the homologous series of alkenes is C_nH_2n.
- Successive members of a homologous series differ by an extra carbon and two extra hydrogen atoms.
- Alkene molecules are "unsaturated" because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms.

7.2.2 Reactions of alkenes

Addition reactions

Addition reactions are where we take two reactants and combine them to make one product.
Alkenes react with hydrogen, water, and halogens by the addition of atoms across the carbon-carbon double bond.

• Hydrogenation: Adding hydrogen.
• Chlorination: Adding chlorine.
• Bromination: Adding bromine.
• Hydration: Adding water.

Hydrogenation

_{nickel catalyst, 150°C}
ethene + hydrogen → ethane
propene + hydrogen → propane

Halogenation

ethene + chlorine → 1,2-dichloroethane
propene + bromine → 1,2-dibromopropane

Hydration

ethene + water → ethanol
propene + water → propanol

7.2.3 Alcohols

- Keywords: oxidation, anaerobic, fermentation.
- Oxidation: Adding oxygen atoms; the loss of hydrogen atoms or the loss of electrons.
- Anaerobic: In the absence of oxygen.
- Fermentation: A chemical process where microorganisms like yeast break down sugars such as glucose into ethanol and carbon dioxide.
- Alcohols contain the functional group -OH.
Methanol, ethanol, propanol and butanol are the first four members of a homologous series of alcohols.
These are those alcohols.

Uses

- Alcohols are in many everyday products:

• Alcoholic drinks
• Solvents
• Perfumes, aftershaves, mouthwash
• Fuels

Complete combustion of alcohols

- 2CH₃OH (methanol) + 3O₂ → 2CO₂ + 4H₂O
- C₂H₅OH (ethanol) + 3O₂ → 2CO₂ + 3H₂O
- 2C₃H₇OH (propanol) + 9O₂ → 6CO₂ + 8H₂O

Incomplete combustion of alcohols

- 2CH₃OH (methanol) + O₂ → 4H₂O + 2C
- C₂H₅OH (ethanol) + 2O₂ → 3H₂O + 2CO
- C₃H₇OH (propanol) + 2O₂ → 4H₂O + CO + 2C (or other ways, you can balance this quite a few days)

Oxidation reactions

- Ethanol + oxygen → ethanoic acid (vinegar) + water
- C₂H₅OH + 2[O] → CH₃COOH + H₂O
- (oxygen is the oxidising agent)
- Pottassium dichromate (K₂Cr₂O₇) is an oxidising agent which can be used to oxidise alcohols.

7.2.4 Carboxylic acids

- Carboxylic acids have the functional group -COOH.
- The first four members of a homologous series of carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
- As a reminder, a strong acid is fully ionised in water (e.g. HCl).
- HCl → H+ + Cl-
- A weak acid is only partially ionised in water (e.g. ethanoic acid).
- CH3COOH ⇌ H+ + CH3COO-
- Carboxylic acids are weak acids. They give off relatively few H⁺ ions in solution and have a higher pH.
- Here is the functional group in the context of ethanoic acid:

Reactions of carboxylic acids with carbonates

- When we react carboxylici acids with a metal carbonate, it reacts as a typical acid producing salt, carbon dioxide, and water.
- ethanoic acid + sodium carbonate → sodium ethanoate + carbon dioxide + water
- 2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)
- Reactions with carboxylic acids have a lower rate of reaction because:

• There is a lower concentration of H⁺ ions
• Collisions are less frequent

Making esters

- Carboxylic acids will react with alcohols to produce esters and water.
- A catalyst is needed (in this case sulphuric acid).
- Esters are volatile compounds with distinctive smells, often fruity.
- They are used in perfumes and food flavourings.
- The one you need to know is ethyl ethanoate, made from ethanol and ethanoic acid.
- The yield is low because the reaction is reversible.
- ethanoic acid + ethanol ⇌ ethyl ethanoate + water
- CH₃COOH + C₂5H₅OH ⇌ CH₃COOC₂H₅ + H₂O
_{(sorry for how bad this image is, it was really difficult to make :c)}

Uses of esters and carboxylic acids

- Esters are used in:

• Perfumes
• Food flavourings
• Solvents for paints and glue

• Food preservatives (e.g. vinegar)
• Manufacture of soaps and detergents
• Manufacture of pharmaceuticals (e.g. aspirin)

Esterification

methanol + methanoic acid ⇌ methyl methanoate + water
CH₃OH + HCOOH ⇌ HCOOCH₃ + H₂O
- This is a condensation reaction.
propanoic acid + ethanol ⇌ ethyl propanoate + water
C₂H₅COOH + C₂H₅OH ⇌ C₂H₅COOC₂H₅ + H₂O
(and many others, but you get the idea)