Predicting Organic Reactions

Combustion of Hydrocarbons and Alcohols

• Every time you boil a kettle on a gas stove, you are watching a chemical oxidation reaction in real-time.
• Oxidation during combustion occurs because the carbon and hydrogen atoms in the fuel gain oxygen to form oxides.
• Complete combustion requires a plentiful supply of oxygen. The only products formed are carbon dioxide ($CO_2$) and water ($H_2O$). It produces absolutely no carbon monoxide or soot.
• Incomplete combustion happens when oxygen is limited. The fuel is not fully oxidised, producing toxic carbon monoxide ($CO$) and/or solid carbon soot ($C$) alongside water.

Flame Characteristics and Pollution:

• Alkanes and small alcohols (like methanol) typically burn with a clean, pale blue flame in excess oxygen.
• Alkenes and larger alcohols burn with a smoky, yellow flame. Because they have a higher ratio of carbon to hydrogen, they are much more likely to undergo incomplete combustion.
• The products of combustion have severe impacts: $CO_2$ is a greenhouse gas causing global warming, $CO$ restricts oxygen transport in red blood cells by binding to haemoglobin, and soot causes respiratory issues and global dimming.

Example Equations:

• Complete combustion of an alkene (Ethene): $C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O$

Addition Reactions of Alkenes

• Understanding how double bonds break explains why we can industrially convert liquid vegetable oils into solid, spreadable margarine.
• Alkenes are unsaturated hydrocarbons because they contain a carbon-carbon double bond. In an addition reaction, the $\pi$-bond (part of the double bond) breaks, allowing two new atoms to bond to the carbon atoms.
• Because no double bonds remain, the final product is always saturated.

Adding Bromine ($Br_2$):

• Alkenes react with bromine at room temperature to form dibromoalkanes.
• This reaction is the standard chemical test for unsaturation. When orange-brown bromine water is added to an alkene, the solution immediately decolourises (turns colourless).
• Alkanes do not react with bromine water, so the solution remains orange.

Adding Hydrogen ($H_2$):

• The addition of hydrogen is called hydrogenation, which turns alkenes into alkanes.
• This reaction requires specific conditions: a Nickel (Ni) catalyst and a temperature between $150^\circ C$ and $300^\circ C$.

Oxidation of Alcohols to Carboxylic Acids

• Why do some chemical reactions need to be boiled continuously without ever drying out?
• The chemical oxidation of alcohols is performed by heating the reaction mixture under reflux — a process of continuous evaporation and condensation that prevents volatile liquids from escaping.
• The oxidising agent used is acidified potassium manganate(VII) ($KMnO_4$). The mixture is acidified using dilute sulfuric acid.
• During the reaction, the hydroxyl group ($-OH$) is oxidised into a carboxyl group ($-COOH$). Water is produced as a byproduct.

Worked Example: Predicting Products

Predict the molecular formula, symbolic equation, and displayed structural change when butanol ($C_4H_9OH$) is heated with acidified potassium manganate(VII).

Step 1: Determine the reaction type and reactant carbon count.

• The reagent is an oxidising agent, so this is an oxidation reaction converting an alcohol to a carboxylic acid. Butanol has 4 carbon atoms.

Step 2: Apply the oxidation prediction rule.

• Replace two hydrogen atoms with one oxygen atom.
• $C_4H_9OH$ becomes $C_3H_7COOH$ (Butanoic acid).

Step 3: Write the balanced equation using $[O]$ to represent the oxidising agent.

• $C_4H_9OH + 2[O] \rightarrow C_3H_7COOH + H_2O$

Step 4: Predict the structure.

• The terminal $-CH_2OH$ group on the 4-carbon chain transforms. The end carbon now has a $C=O$ double bond and a $C-OH$ single bond, leaving it with exactly four bonds.