Organic Chemistry
Organic Reactions
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to define and identify addition, substitution, and elimination reactions.
- ✓By the end of this lesson students will be able to describe the mechanism of free radical halogenation of alkanes, including initiation, propagation, and termination steps.
- ✓By the end of this lesson students will be able to write balanced chemical equations for common organic reactions, predicting major products where applicable.
- ✓By the end of this lesson students will be able to explain and differentiate between addition polymerisation and condensation polymerisation, providing examples of each.
Key concepts
Addition reactions occur when two or more molecules combine to form a larger single molecule. These reactions are characteristic of unsaturated compounds (alkenes and alkynes) where a multiple bond (double or triple) is broken, and new single bonds are formed with the atoms of the adding molecule. The carbon atoms of the multiple bond become saturated.
Substitution reactions involve the replacement of one atom or group of atoms in a molecule by another atom or group of atoms. These reactions are characteristic of saturated compounds (alkanes) and aromatic compounds, as well as alcohols.
Elimination reactions involve the removal of atoms or groups of atoms from adjacent carbon atoms in a molecule, leading to the formation of a multiple bond (double or triple) and a smaller molecule. These reactions are often the reverse of addition reactions.
Free radical halogenation is a type of substitution reaction where a hydrogen atom in an alkane is replaced by a halogen atom (e.g., chlorine or bromine) via a free radical mechanism. This reaction requires UV light or high temperatures to initiate and proceeds through three main steps: initiation, propagation, and termination.
Addition polymerisation is a process where monomer units (typically unsaturated alkenes) add to one another in a chain reaction without the loss of any atoms. The double bond in each monomer breaks, allowing them to link together to form a long chain polymer. Examples include poly(ethene) from ethene monomers.
Condensation polymerisation is a process where monomers join together to form a polymer with the simultaneous elimination of a small molecule, such as water (H₂O), methanol (CH₃OH), or hydrogen chloride (HCl). This typically occurs between monomers with two different functional groups that can react with each other. Examples include polyesters and polyamides.
Key facts to remember
- 1Addition reactions are characteristic of unsaturated compounds (alkenes and alkynes).
- 2Substitution reactions are characteristic of saturated compounds (alkanes) and aromatic compounds.
- 3Elimination reactions typically form a multiple bond and a small molecule.
- 4Free radical halogenation of alkanes requires UV light and proceeds via initiation, propagation, and termination steps.
- 5Markovnikov's Rule predicts the major product in addition reactions of unsymmetrical alkenes with unsymmetrical reagents (H adds to the carbon with more H's).
- 6Addition polymerisation involves monomers adding without loss of atoms, usually from unsaturated compounds.
- 7Condensation polymerisation involves monomers joining with the loss of a small molecule (e.g., H₂O, HCl).
- 8Polyesters and polyamides are common examples of condensation polymers.
Worked examples
Example 1
Identify the type of reaction and predict the major organic product when propene reacts with hydrogen bromide (HBr).
Answer
Reaction Type: Addition reaction. Product: 2-bromopropane (CH₃-CHBr-CH₃).
Markovnikov's Rule is crucial for predicting the major product in addition reactions involving unsymmetrical alkenes and unsymmetrical reagents.
Example 2
Describe the free radical halogenation of methane with chlorine (Cl₂) in the presence of UV light, showing the initiation, propagation, and termination steps.
Answer
Initiation: Cl₂ → 2Cl• Propagation: CH₄ + Cl• → •CH₃ + HCl •CH₃ + Cl₂ → CH₃Cl + Cl• Termination: Cl• + Cl• → Cl₂ •CH₃ + •CH₃ → CH₃CH₃ •CH₃ + Cl• → CH₃Cl
The propagation steps are the most important for the overall reaction, as they produce the main product and regenerate the catalyst (free radical).
Example 3
Draw the structure of the monomer(s) and the repeating unit of the polymer formed from the reaction of ethene (CH₂=CH₂). State the type of polymerisation.
Answer
Type of Polymerisation: Addition Polymerisation Monomer: CH₂=CH₂ (Ethene) Repeating Unit: [-CH₂-CH₂-]n
Remember to show the bonds extending outside the brackets in the repeating unit to indicate continuation of the polymer chain.
Common mistakes
- ✗Confusing addition and substitution reactions, especially when dealing with different types of organic compounds.
- ✗Incorrectly applying Markovnikov's Rule or forgetting to apply it for unsymmetrical additions.
- ✗Omitting the conditions (e.g., UV light for free radical halogenation) when writing reaction equations or mechanisms.
- ✗Not showing the correct bond breaking and forming in free radical mechanisms, particularly in propagation steps.
- ✗Failing to identify the small molecule eliminated in condensation polymerisation or drawing the repeating unit incorrectly for polymers.
Exam tips
- ★Always identify the functional groups present in the reactants to help determine the likely type of reaction.
- ★For free radical halogenation, clearly label each step (initiation, propagation, termination) and ensure all species are shown with correct radical notation (•).
- ★Practice drawing monomers and repeating units for various addition and condensation polymers, paying attention to the bonds extending from the repeating unit.
- ★When asked to predict products, consider all relevant rules (e.g., Markovnikov's Rule for additions, Saytzeff's Rule for eliminations, though less emphasised at Leaving Cert) and state the major product clearly.
Ready to practise?
Try a problem on this topic
Snap a photo or type a question — get step-by-step working instantly.