Organic reactions: SAQ 2

Study Notes

Addition reactions occur only in organic compounds with double or triple bonds
Ethene (C₂H₄) is a commonly used example in addition reactions
The general equation for addition reactions involving ethene is:
- C₂H₄ + X₂ → C₂H₄X₂
Examples of addition reactions involving ethene:
- Reaction with chlorine (Cl₂): C₂H₄ + Cl₂ → C₂H₄Cl₂
- Reaction with bromine (Br₂): C₂H₄ + Br₂ → C₂H₄Br₂
- Reaction with hydrogen chloride (HCl): C₂H₄ + HCl → C₂H₅Cl
- Reaction with hydrogen (H₂): C₂H₄ + H₂ → C₂H₆
- Reaction with water (H₂O): C₂H₄ + H₂O → C₂H₅OH

Polymerisation reactions involve the addition of molecules to form a large chain
Ethene is used to produce polyethene, a common plastic
The reaction involves the combination of many ethene molecules:
- nC₂H₄ → [C₂H₄]ₙ
Polyethene is used in various products, such as plastic bags, ropes, and wrapping material

Substitution reactions involve the replacement of an atom or group of atoms with another atom or group of atoms
These reactions occur in saturated molecules (i.e., molecules with only single bonds)
The general equation for substitution reactions is:
- R-X + Y-Z → R-Y + X-Z
Example of substitution reaction: chlorination of methane (CH₄):
- CH₄ + Cl₂ → CH₃Cl + HCl

Esterification is a substitution reaction between a carboxylic acid and an alcohol to form an ester
The general equation for esterification is:
- RCOOH + R'OH → RCOOR' + H₂O
Example of esterification: reaction between ethanoic acid (CH₃COOH) and methanol (CH₃OH):
- CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O

Base hydrolysis is the reaction of an ester with a base to form a carboxylic acid and an alcohol
The general equation for base hydrolysis is:
- RCOOR' + OH⁻ → RCOO⁻ + R'OH
Example of base hydrolysis: reaction between ethylethanoate (CH₃COOC₂H₅) and sodium hydroxide (NaOH):
- CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
Soap manufacture involves the base hydrolysis of esters to form carboxylic acid salts (soaps)

Elimination reactions involve the removal of a small molecule from a larger molecule, leaving a double bond
The general equation for elimination reactions is:
- R-X → R-X + H₂O
Example of elimination reaction: dehydration of ethanol (C₂H₅OH) to form ethene (C₂H₄):
- C₂H₅OH → C₂H₄ + H₂O

Oxidation reactions involve the gain of oxygen or loss of hydrogen
Reduction reactions involve the loss of oxygen or gain of hydrogen
Examples of oxidation and reduction reactions:
- Oxidation of primary alcohols to aldehydes: RCH₂OH → RCHO + H₂O
- Reduction of aldehydes to primary alcohols: RCHO + H₂ → RCH₂OH
- Oxidation of aldehydes to carboxylic acids: RCHO → RCOOH

Breath tests involve the oxidation of primary alcohols to carboxylic acids using acidified sodium dichromate (Cr₂O₇²⁻)
The reaction is detected by a colour change from orange to green

The reaction involves the oxidation of primary alcohols to carboxylic acids using potassium permanganate (KMnO₄)
The reaction is detected by a colour change from purple to brown

The reaction involves the reduction of carboxylic acids to primary alcohols using hydrogen gas (H₂) and a nickel catalyst (Ni)
The reaction is detected by a colour change from brown to colourless### Carboxylic Acids Reactions
Carboxylic acids react by losing a H+ ion due to the inductive effect of the oxygen atom, which polarizes the O-H bond.

Synthesis is the formation of a chemical compound from simpler compounds.
Example: Aspirin is synthesized from simpler products in the lab, rather than extracted from natural sources.
Synthesis involves breaking and making of bonds, and may involve up to 3 conversions in a synthesis pathway diagram.