DoAssignment.ca

DoAssignment study guide

B2.4 · Investigate and analyse organic reactions

Learn to investigate and analyse organic reactions through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

Connect observations to molecular changes and balanced equations

A reddish-brown bromine solution can lose its colour when it reacts with some organic compounds, but not with others under the same conditions. This visible change can provide evidence about the bonds in a compound. To analyse an organic reaction, connect what you observe to what changes in the molecules, then represent those changes with a balanced equation. A reaction pattern is a way to group reactions by how they change molecules.

What you will learn

  • Describe how atoms and bonds change during organic reactions.
  • Recognize addition, substitution, elimination, oxidation, esterification, and hydrolysis patterns.
  • Use balanced chemical equations to analyse organic reactions.
  • Plan a fair comparison that uses an observation to distinguish between organic compounds.

Start with what changes

A molecule is a group of atoms held together by chemical bonds. In an organic reaction, atoms are rearranged as bonds are broken and formed. Atoms are conserved, so a balanced equation has the same number of each kind of atom on both sides.
A hydrocarbon is a compound made only of carbon and hydrogen. A saturated hydrocarbon has only carbon–carbon single bonds. An unsaturated hydrocarbon has at least one carbon–carbon double bond or triple bond. This distinction helps explain a common observation: bromine solution can lose its colour when bromine reacts with an unsaturated compound.
An observation is evidence, not a complete explanation. Fading colour can support the idea that a sample contains a compound that reacts with bromine. A fair investigation also compares the sample with suitable known compounds under the same conditions.
  • Observe a change before proposing what it means.
  • Use a particle-level model to explain the observation.
  • Check that a proposed equation conserves every type of atom.

Recognize common reaction patterns

In an addition reaction, atoms from another substance join to a molecule that has a carbon–carbon double bond. The double bond becomes a single bond, and new bonds connect the added atoms. For example, ethene reacts with bromine to form 1,2-dibromoethane: CH2=CH2+Br2→CH2BrCH2Br\mathrm{CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br}.
In a substitution reaction, an atom or group in a molecule is replaced by another atom or group. For example, methane can react with chlorine to produce chloromethane and hydrogen chloride: CH4+Cl2→CH3Cl+HCl\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}.
In an elimination reaction, atoms or groups are removed from a molecule, and a double bond can form. One example is the formation of ethene and water from ethanol: CH3CH2OH→CH2=CH2+H2O\mathrm{CH_3CH_2OH \rightarrow CH_2{=}CH_2 + H_2O}.
Oxidation is a reaction in which an organic molecule gains oxygen or loses hydrogen. A primary alcohol can be oxidized to an aldehyde. In this equation, [O]\mathrm{[O]} is a simplified way to show an oxygen source: CH3CH2OH+[O]→CH3CHO+H2O\mathrm{CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O}.
Esterification is a reaction between an alcohol and a carboxylic acid that forms an ester and water. A carboxylic acid contains the group −COOH\mathrm{-COOH}. Ethanol and ethanoic acid can form ethyl ethanoate: CH3CH2OH+CH3COOH⇌CH3COOCH2CH3+H2O\mathrm{CH_3CH_2OH + CH_3COOH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O}. The double arrow indicates that the reaction can proceed in both directions under suitable conditions; it does not mean that reactant and product amounts are equal. Hydrolysis is a reaction with water. An ester can hydrolyse to form an alcohol and a carboxylic acid: CH3COOCH2CH3+H2O→CH3COOH+CH3CH2OH\mathrm{CH_3COOCH_2CH_3 + H_2O \rightarrow CH_3COOH + CH_3CH_2OH}.
  • Classify reactions by the changes in molecules, not only by the names of reactants.
  • Addition uses a double bond; elimination can form one.
  • Esterification forms an ester and water; hydrolysis uses water to split a substance.

Use equations as evidence

A structural formula shows how atoms are connected. It can make a reaction pattern easier to see than a molecular formula alone. Ethene’s structural formula shows its carbon–carbon double bond. In the product, each carbon is joined to a bromine atom instead.
To analyse an equation, compare the reactant and product structures. Ask which bonds or groups changed, then name the matching reaction pattern. Also count atoms on each side. If the counts differ, the equation is not balanced and does not correctly represent the reaction.
Add state symbols when the states are known and relevant. Do not guess them from the formula: a substance’s state depends on conditions such as temperature. A reaction equation describes a chemical change. An investigation also needs a method for observing or measuring evidence of that change.
  • Structural formulas can reveal which bonds changed.
  • Balanced equations conserve atoms.
  • An investigation needs a defined observation and a comparison.

Plan a fair comparison

A simple comparison could test whether an unknown organic sample behaves like an unsaturated or saturated compound when bromine solution is added. Decide what to record: whether the bromine colour fades. Use equal amounts of each sample and the same amount of bromine solution. Keep other conditions as similar as practical.
Include a known unsaturated compound and a known saturated compound for comparison. Do not assume the unknown’s identity from colour alone. Consider whether another factor could affect the observation, and repeat observations if appropriate. Record only what was actually seen; do not report a result before carrying out the investigation.
Follow the teacher’s instructions and school laboratory procedures for the substances and equipment used.
  • Change the sample being tested while keeping comparison conditions similar.
  • Use known samples to help interpret the unknown.
  • Separate planned observations from results that have actually been collected.

Worked example

Analyse a bromine test

A student plans to compare ethene and ethane by adding bromine solution to separate samples under the same conditions. Predict the expected difference in observation, give a particle-level explanation, and write the equation for ethene’s reaction with bromine.
  1. Predict the observation
    Ethene is unsaturated because it has a carbon–carbon double bond. Ethane is saturated because it has only carbon–carbon single bonds. Predict that bromine colour fades with ethene and does not fade with ethane under the same conditions. These are predictions, not reported results.
  2. Explain the particle change
    In ethene, bromine atoms add across the double bond. As bromine reacts, less coloured bromine remains in solution, so the colour fades. Ethane has no carbon–carbon double bond for this addition pattern.
  3. Write and check the equation
    The product is 1,2-dibromoethane. Both sides contain two carbon atoms, four hydrogen atoms, and two bromine atoms, so the equation conserves atoms.
    CH2=CH2+Br2→CH2BrCH2Br\mathrm{CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br}
  4. State the limit of the conclusion
    Fading colour supports the conclusion that a compound in the sample reacted with bromine under the test conditions. The observation alone does not identify every possible substance in an unknown sample.
Answer: The predicted evidence is fading bromine colour with ethene and no fading with ethane under the same conditions. Ethene undergoes addition with bromine to form 1,2-dibromoethane.
Check: The equation is balanced: both sides have two carbon atoms, four hydrogen atoms, and two bromine atoms.

Common mistakes and how to avoid them

Calling any colour change proof of a compound’s identity.
Correction: Treat an observation as evidence. Compare it with known samples and consider what the test can and cannot establish.
Confusing addition with substitution.
Correction: In addition, atoms join across a multiple bond. In substitution, an atom or group is replaced.
Writing an equation that changes the number of atoms.
Correction: Count each element on both sides and adjust the equation until the counts match.
Treating the double arrow in an esterification equation as meaning equal amounts.
Correction: The double arrow indicates that the reaction can proceed in both directions. It does not state that reactant and product amounts are equal.

Lesson summary

  • Organic reactions rearrange atoms and bonds while conserving atoms.
  • Addition, substitution, elimination, oxidation, esterification, and hydrolysis describe different patterns of molecular change.
  • Observations such as bromine colour fading can be connected to a particle-level explanation and a balanced equation.
  • A fair investigation uses comparisons, similar conditions, and recorded observations.

Check your understanding

Question 1

Which pattern is shown when atoms join across a carbon–carbon double bond?
  1. Addition
  2. Substitution
  3. Hydrolysis
  4. Oxidation
Show answer and explanation
Addition
Addition joins atoms to a molecule at a multiple bond. Substitution replaces an atom or group, hydrolysis uses water to split a substance, and oxidation involves gaining oxygen or losing hydrogen.

Question 2

What products form when ethyl ethanoate undergoes hydrolysis?
  1. Ethanoic acid and ethanol
  2. Ethene and water
  3. Ethanal and oxygen
  4. Ethyl ethanoate and hydrogen
Show answer and explanation
Ethanoic acid and ethanol
Hydrolysis of this ester with water forms the carboxylic acid ethanoic acid and the alcohol ethanol.

Question 3

A student tests an unknown with bromine solution. Which conclusion is best supported if the colour fades?
  1. The sample reacted with bromine under the test conditions.
  2. The sample must be pure ethene.
  3. The sample contains no carbon–carbon single bonds.
  4. The sample’s concentration is known.
Show answer and explanation
The sample reacted with bromine under the test conditions.
Colour fading is evidence that bromine reacted under the conditions. It does not, by itself, prove the sample’s identity, purity, or concentration.

Key terms

Addition reaction
A reaction in which atoms join to a molecule, often across a carbon–carbon double bond.
Substitution reaction
A reaction in which an atom or group in a molecule is replaced by another.
Elimination reaction
A reaction in which atoms or groups are removed from a molecule; a double bond can form.
Esterification
A reaction between an alcohol and a carboxylic acid that forms an ester and water.
Hydrolysis
A reaction in which water reacts with a substance, often splitting it into smaller products.
Unsaturated hydrocarbon
A hydrocarbon with at least one carbon–carbon double bond or triple bond.

Continue through SCH4U

View the complete SCH4U Ontario Grade 12 Chemistry curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question