Organic Chemistry Reactions Cheatsheet | HSC Chemistry

Read the complete list of the 15 organic chemistry reactions year 12 Chemistry students must know for the HSC. Ace your HSC Chemistry exam with this free cheatsheet.

Organic Chemistry Reactions

Keeping track of organic chemistry reactions in Year 12 Chemistry can be overwhelming. Here is a comprehensive list of all the organic chemistry reactions you should know before your HSC Chemistry exam.

In this article, we cover


Addition Reactions

Addition reactions are organic reactions that occur when atoms are added across the double bond of an unsaturated molecule.



alkene + hydrogen gas → alkane

HSC 'Organic Chemistry' Reactions - Hydrogenation

Conditions: \small H_{2(g)} , metal catalyst (such as \small Pt , \small Pd , \small Ni, \small Rh ), room temperature.


Halogenation (bromination and chlorination)

alkene + bromine/chlorine → haloalkane

HSC 'Organic Chemistry' Reactions - Halogenation

Conditions: \small Br_{2(l) \text{ or } (aq)} or \small Cl_{2(l) \text{ or } (aq)}, room temperature



alkene + hydrogen halide → haloalkane

HSC 'Organic Chemistry' Reactions - Hydrohalogenation

Conditions: \small HF_{(g)} , \small HCl_{(g)} , \small HBr_{(g)} or \small HI_{(g)}, room temperature


Hydration of alkenes

alkene + water → alcohol

HSC 'Organic Chemistry' Reactions - Hydration of alkenes

Conditions: \small H_2O , dilute aqueous acid catalyst (e.g. dilute \small H_2SO_{4(aq)} ), heat


Substitution Reactions

A substitution reaction is an organic reaction that occurs when an atom or functional group in a molecule is replaced or substituted by another atom or group.


Substitution of alkanes

alkane + halogen → haloalkane + hydrogen halide

HSC 'Organic Chemistry' Reactions - Substitution of alkanes

Conditions: \small Br_2 or \small Cl_2 , UV radiation


Substitution of alkanes can only be carried out with chlorine or bromine as fluorine is too reactive (reacts explosively) and iodine is too unreactive.


Substitution of alcohols

alcohol + hydrogen halide → haloalkane + water

HSC 'Organic Chemistry' Reactions - Substitution of alcohols

Conditions: \small HCl , \small HBr , \small HI . Tertiary alcohols (3 \small \degree ) react rapidly with \small HX at room temperature while secondary (2 \small \degree ) and primary (1 \small \degree ) alcohols require higher temperatures.


Substitution of haloalkanes

haloalkane + sodium/potassium hydroxide → alcohol + metal halide

HSC 'Organic Chemistry' Reactions - Substitution of haloalkanes

Conditions: \small NaOH_{(aq)} , heat. Fluoroalkanes do not undergo substitution.


Elimination Reactions

An elimination reaction occurs when a single reactant splits into two products. In a sense, it is opposite of addition reactions.



alcohol → alkene + water

HSC 'Organic Chemistry' Reactions - Dehydration

Conditions: Concentrated acid catalyst (e.g concentrated \small H_2SO_4 or \small H_3PO_4 ) or \small Al_2O_3 catalyst, heat.

Tertiary alcohols react readily at room temperature while primary and secondary alcohols require very high temperatures.


This reaction can be performed only on alcohols that have at least one hydrogen on the adjacent carbon. For example, methanol cannot undergo dehydration.


Hydrolysis Reactions

A hydrolysis reaction is an organic chemical reaction which involves the breaking of a chemical bond by the addition of water.



ester + sodium hydroxide → sodium carboxylate + alcohol

HSC 'Organic Chemistry' Reactions - Saponification

Conditions: \small NaOH_{(aq)} , heat.


Oxidation Reactions

Oxidation reactions involve a transfer of electrons from one reactant to another. In organic chemistry, oxidation refers to loss or gain of electrons by carbon.


Oxidation of alcohols

The products of alcohol oxidation depend on the type of alcohol reacted and the reaction conditions.


Primary alcohols

Primary alcohols are oxidised stepwise to first produce aldehydes then carboxylic acids.

HSC 'Organic Chemistry' Reactions - Oxidation of primary alcohols


Conditions: Acidified permanganate ion \small ({MnO_4}^-) or dichromate ion \small ({Cr_2O_7}^{-2}) , heat.


To produce aldehydes in a controlled oxidation, milder oxidising agents like pyridinium chlorochromate (PCC) can be used.


Secondary alcohols

Secondary alcohols are oxidised to produce ketones.

HSC 'Organic Chemistry' Reactions - Oxidation of secondary alcohols

Conditions: Acidified permanganate ion \small ({MnO_4}^-) or dichromate ion \small ({Cr_2O_7}^{-2}) , heat.


Tertiary alcohols

Tertiary alcohols cannot be oxidised.


Combustion of hydrocarbons and alcohols

Complete combustion

fuel + oxygen → carbon dioxide + water

Fuel_{(s/l/g)} + O_{2(g)} \rightarrow CO_{2(g)} + H_2O_{(l)}


Incomplete combustion

fuel + oxygen → carbon soot + carbon monoxide + water

Fuel_{(s/l/g)} + O_{2(g)} \rightarrow  C_{(s)} + CO_{(g)} + H_2O_{(l)}


Condensation Reactions

A condensation reaction occurs when two or more molecules combine to form a larger molecule, with the simultaneous elimination of a small molecule such as water or methanol.


Condensation of carboxylic acid and amine

carboxylic acid + amine \rightleftharpoons amide + water

HSC 'Organic Chemistry' Reactions - Condensation of carboxylic acid and amineConditions: Heat



carboxylic acid + alcohol → ester + water

HSC 'Organic Chemistry' Reactions - Esterification

Conditions: Concentrated \small H_2SO_4  catalyst, heat


Other Organic Reactions


Fermentation of glucose

glucose → ethanol + carbon dioxide

\small C_6H_{12}O_{6(aq)}\ \ \underrightarrow{ \ zymase \ \ } \ \ 2C_2H_5OH_{(aq)}+2CO_{2(g)}


Fermentation involves following four conditions.

  1. Presence of an enzyme called zymase (found in yeast)
  2. Warm temperatures (30–40 °C)
  3. Anaerobic environment (absence of oxygen)
  4. Aqueous solution


Reaction pathway questions

Question 1

The reaction pathway below represents the synthesis of compound E. Only one organic product is formed in each step (no isomeric products possible).


Y12 Chemistry Organic Reaction Pathway


Draw the structures of compounds A to E, justifying your diagrams with reference to the conditions provided. (5 marks)

See Question 1 Solution


Question 2

The flowchart represents the reaction pathways to synthesise compound E from compounds A and B.


Y12 Chemistry Organic Reaction Pathway


Identify reagent X and draw the structures of compounds A to E, justifying your diagrams with reference to the conditions provided. (6 marks)

See Question 2 Solution


Question 3

Devise reaction pathways for the synthesis of the following compounds from the given starting material. More than one step is required. Indicate the reagents required and draw the structural formulae of any intermediate compounds.

a)Propan-2-ol from propane3 marks
b)Propyl ethanoate from propene and ethene5 marks

See Question 3 Solution


Solutions to reaction pathway questions

1Y12 Chemistry Organic Reaction Pathway Solutions
2Reagent X is acidified KMnO4

Y12 Chemistry Organic Reaction Pathway Solutions


Y12 Chemistry Organic Reaction Pathway


Y12 Chemistry Organic Reaction Pathway


Looking for more HSC Chemistry practice questions?

Now that you have mastered all the organic chemical reactions you need to know for the HSC, refine your exam skills by practicing these 20 Must Know HSC Chemistry Questions.


Written by Varisara Laosuksri

Varisara is a 2019 St George Girls High School graduate who achieved Band 6 in her HSC Chemistry and Physics.

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