Organic 2

Key Words

• Homologous series

  • Family of compounds with similar chemical reactivity

  • Show trends in physical properties

  • Each member differs by CH₂

• Functional group

  • Part of a molecule (not C and H only)

  • Determines chemical reactivity

• Anaerobic

  • Occurs without oxygen

• Catalyst

  • Speeds up a reaction without being used up

  • Provides an alternative pathway with lower activation energy

• Monomer

  • Small molecule that joins to form a polymer

• Polymer

  • Long-chain molecule made from many monomers joined together

Alcohols

• General formula of alcohols: CₙH₂ₙ₊₁OH

• A functional group is a group of atoms (other than C or H) that gives a compound its characteristic reactions

• The functional group present in all alcohols is the hydroxyl group (–OH)

Alcohol Homologous Series

• Members differ by CH₂

• Similar chemical reactions

• Gradual trends in physical properties (e.g. boiling point)

Examples of Alcohols

Alcohol	No. of C atoms	Molecular formula	Structural formula	Displayed formula
methanol	1	CH₃OH	CH₃–OH
ethanol	2	C₂H₅OH	CH₃–CH₂–OH
propan-1-ol	3	C₃H₇OH	CH₃–CH₂–CH₂–OH
propan-2-ol	3	C₃H₇OH	CH₃–CH(OH)–CH₃
butan-1-ol	4	C₄H₉OH	CH₃–CH₂–CH₂–CH₂–OH
butan-2-ol	4	C₄H₉OH	CH₃–CH(OH)–CH₂–CH₃

• Alcohols with the same molecular formula but different structures are structural isomers

Ethanol

Preparation Of Ethanol (Two Methods)

1. Fermentation Of Sugars

• Carried out by yeast in the absence of oxygen (anaerobic)

• An exothermic reaction that provides energy for the yeast

• Yeast occurs naturally on fruit skins, e.g. grapes

• Ethanol concentration is limited to about 15% because yeast is killed by ethanol

• Fractional distillation is used to produce more concentrated ethanol

Equation

• Glucose → ethanol + carbon dioxide

• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Conditions

• Warm temperature (about 30–40 °C)

• No oxygen present

• Yeast as the catalyst

• Usually a batch process, though continuous-flow is possible using immobilized yeast

• Exothermic reaction, the reaction is a source of energy for yeast in the absence of oxygen

• Yeast is found naturally in the skin of many ripening fruits like grapes

• The yeast dies when the concentration of ethanol reaches approx. 15%

• The mixture is fractionally distilled to obtain higher concentrations of ethanol

2. Addition Of Steam To Ethene – Main Industrial Process For Ethanol

Equation

• Ethene + steam → ethanol

• C₂H₄(g) + H₂O(g) → C₂H₅OH(l)

Conditions

• High temperature (about 300 °C)

• High pressure (about 60–70 atm)

• Phosphoric acid catalyst on a solid support

• Continuous process with recycling of unreacted ethene

• Only a small proportion of ethene reacts each pass

• Ethanol is cooled, condensed, and collected

• Unreacted ethene is recycled, improving efficiency

Comparison Of The Two Methods

Fermentation

• Uses renewable resources such as sugar cane or sugar beet

• Batch process, so less efficient (stop–start)

• Slow rate, taking several days

• Produces impure ethanol that needs fractional distillation

• Low temperature and atmospheric pressure

• Lower energy costs and safer operating conditions

Hydration Of Ethene

• Uses finite resources from crude oil

• Continuous flow process, more efficient

• Fast reaction rate

• Produces ethanol as the only product

• Requires high temperature and high pressure

• Higher energy costs and greater safety considerations

Uses Of Ethanol

• Alcoholic beverages such as wine, beer, and spirits

• Solvent in cosmetics, toiletries, inks, and the chemical industry

• Alternative fuel; E10 fuel contains 10% ethanol

Reactions Of Ethanol

Combustion (Oxidation)

• Ethanol burns in oxygen with a clean, almost invisible flame

• Equation:
  C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

Oxidation In Air To Form Ethanoic Acid

• Occurs when wine turns into vinegar

• Ethanol is slowly oxidised by oxygen in the air

• Equation:
  C₂H₅OH(aq) + O₂(g) → CH₃COOH(aq) + H₂O(l)

Oxidation Using Potassium Dichromate(VI)

• Uses potassium dichromate(VI), K₂Cr₂O₇, and sulfuric acid

• Used in industrial chemistry

• Orange Cr₂O₇²⁻ ions are reduced to green Cr³⁺ ions

• This is a redox reaction

• [O] represents oxygen from the oxidising agent

• Equation:
  C₂H₅OH + 2[O] → CH₃COOH + H₂O

Dehydration (Removal Of Water)

• Ethanol is dehydrated to form ethene, an alkene

• Ethanol vapour is passed over hot aluminium oxide catalyst

• Ethene gas is collected, often over water

• Equation:
  C₂H₅OH(l) → C₂H₄(g) + H₂O(l)

Carboxylic Acids

• General formula (linear carboxylic acids): CₙH₂ₙO₂

• Functional group: –COOH

• Example: Vinegar is an aqueous solution of ethanoic acid.

Reactions of Carboxylic Acids

• Carboxylic acids are weak acids as they are only partially ionised to form H⁺ ions in water.

• They undergo the same reactions as mineral acids (e.g., HCl) but react more slowly.

(a) Reaction with reactive metals (e.g., magnesium)

• Carboxylic acid + metal → salt + hydrogen gas

• Example: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

(b) Reaction with a base (e.g., sodium hydroxide)

• Carboxylic acid + base → salt + water

• Example: CH₃COOH + NaOH → CH₃COONa + H₂O

(c) Reaction with a carbonate (e.g., calcium carbonate)

• Carboxylic acid + carbonate → salt + water + carbon dioxide

• Example: 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂

Reactions with Alcohols to Form Esters

• Heating a carboxylic acid with an alcohol in the presence of concentrated sulphuric acid produces an ester and water:

• All of these reactions are reversible

  • carboxylic acid + alcohol → ester + water

  • Ethanoic acid + ethanol → ethyl ethanoate + water

  • CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O

• Displayed formula example:

  

• Reaction is called esterification.

• Also a condensation reaction because water is formed when two molecules join.

Esters

Functional group: RCOOR′ (R/R′ = alkyl group, e.g., –CH₃)

Naming:

• First part from the alcohol: methanol → methyl

• Second part from the carboxylic acid (carboxylate anion): ethanoic acid → ethanoate

• Example: CH₃COOCH₃ = methyl ethanoate

Formation of esters (heat + concentrated H₂SO₄ catalyst)

Uses of esters

• Volatile compounds with distinctive smells

• Used as:

  • Solvents

  • Food flavourings

  • Perfumes

Condensation Polymerisation

• A condensation polymer forms when two types of monomer react, with the loss of a small molecule (e.g., water or HCl).

• Addition followed by elimination is called condensation.

• Monomers have different functional groups.

• Polyesters are used in textiles and clothing.

• Some polyesters are biodegradable (can be broken down in the environment).

Polyesters (contain –COO– link)

Monomers:

• The –COOH group of the acid reacts with the –OH group of the alcohol to form an ester link –COO–.

• Repeating this reaction forms a long polymer chain.

Example Reaction: Condensation polymerisation between ethanedioic acid and ethane-1,2-diol

Word equation:
Ethanedioic acid + Ethane-1,2-diol → Polyester + Water

Terylene

• Made from ethane-1,2-diol + benzene-1,4-dicarboxylic acid

• Water is released, ester linkages formed

• Properties:

  • Strong fibres, retain permanent creases (useful for trousers)

  • Low melting point due to weak intermolecular forces between chains

COMPARISON BETWEEN ADDITION AND CONDENSATION POLYMERS

Addition polymerisation:

Condensation polymerisation: