Chemistry Study Notes: Homologous Series, Naming & Addition Polymerisation
Hello! Welcome to your study notes for a really important part of Chemistry. We're going to explore the world of carbon compounds. Think of this as learning the alphabet and grammar for a whole new language – the language of organic chemistry!
In this chapter, we'll learn how to recognise and name different 'families' of carbon compounds and discover how tiny molecules can link up to form giant polymers. Let's get started!
1. The Amazing Carbon Atom
You might be wondering, "Why is there a whole topic just about carbon?" Well, carbon is a superstar element! It can form more compounds than any other element.
Why is Carbon so Special?
Carbon's uniqueness comes from its ability to form strong covalent bonds with other carbon atoms, creating long chains and rings. This property is called catenation.
- It can form strong single bonds (sharing one pair of electrons).
- It can also form double bonds (sharing two pairs of electrons).
- It can even form triple bonds (sharing three pairs of electrons).
This versatility allows for a huge number and diversity of carbon compounds, from fuels to plastics and biological macromolecules!
Key Takeaway:
Carbon is special because it can bond to itself to form long, stable chains and rings (catenation), creating an incredible variety of molecules.
2. Homologous Series: Chemical Families
With millions of carbon compounds, how do we keep them organised? We group them into 'families' called homologous series.
Analogy: Think of a family with the surname "Smith". All the Smiths are related, have similar features, and probably behave in similar ways. A homologous series is just like a chemical family.
Characteristics of a Homologous Series
All members of the same homologous series have:
- The same functional group. A functional group is a specific atom or group of atoms that gives a molecule its characteristic chemical properties.
- A similar general formula. This is an algebraic formula that works for all members of the family (e.g., \(\text{C}_n\text{H}_{2n+2}\) for alkanes).
- Successive members that differ by a -\(\text{CH}_2\)- group.
- Similar chemical properties, because they have the same functional group.
- A gradual change (gradation) in their physical properties (like boiling point and density) as the molecules get bigger due to increasing molecular size and stronger intermolecular forces.
Four Important Families to Know
Let's meet the four families required in this topic.
Quick Review: The Four Families
1. Alkanes
Functional Group: Only carbon-carbon single bonds (C-C).
General Formula: \(\text{C}_n\text{H}_{2n+2}\) (for \(n \ge 1\))
Description: These are known as saturated hydrocarbons because all carbon-carbon bonds are single bonds, containing the maximum possible number of hydrogen atoms. Example: methane (\(\text{CH}_4\)), propane (\(\text{C}_3\text{H}_8\)).
2. Alkenes
Functional Group: Carbon-carbon double bond (C=C).
General Formula: \(\text{C}_n\text{H}_{2n}\) (for \(n \ge 2\))
Description: These are unsaturated hydrocarbons because they have a double bond that can undergo addition reactions. Example: ethene (\(\text{C}_2\text{H}_4\)), propene (\(\text{C}_3\text{H}_6\)).
3. Alkanols (Alcohols)
Functional Group: Hydroxyl group (-OH).
General Formula: \(\text{C}_n\text{H}_{2n+1}\text{OH}\) (for \(n \ge 1\))
Description: Neutral organic compounds with a hydroxyl group attached to an alkyl group. Example: methanol (\(\text{CH}_3\text{OH}\)), ethanol (\(\text{C}_2\text{H}_5\text{OH}\)).
4. Alkanoic Acids (Carboxylic Acids)
Functional Group: Carboxyl group (-COOH).
General Formula: \(\text{C}_n\text{H}_{2n+1}\text{COOH}\) (for \(n \ge 0\)) or \(\text{C}_n\text{H}_{2n}\text{O}_2\) (for \(n \ge 1\))
Description: Weak acids containing the carboxyl group. Example: methanoic acid (\(\text{HCOOH}\)), ethanoic acid (\(\text{CH}_3\text{COOH}\)).
Key Takeaway:
A homologous series is a family of organic compounds with the same functional group and similar chemical properties: alkanes, alkenes, alkanols, and alkanoic acids.
3. Naming Carbon Compounds (Systematic Naming)
There is an internationally standardised system called the IUPAC system for naming organic compounds unambiguously.
A systematic name consists of a prefix / stem (telling you the number of carbon atoms in the longest continuous chain) and a suffix (indicating the principal functional group).
Part 1: The Stem / Prefix (Number of Carbons)
1 Carbon: Meth-
2 Carbons: Eth-
3 Carbons: Prop-
4 Carbons: But-
5 Carbons: Pent-
6 Carbons: Hex-
7 Carbons: Hept-
8 Carbons: Oct-
Memory Aid: Monkeys Eat Peeled Bananas (Meth-, Eth-, Prop-, But-).
Part 2: The Suffix (The Functional Group)
Alkane (C-C bonds only) → -ane
Alkene (C=C double bond) → -ene
Alkanol (-OH group) → -ol
Alkanoic Acid (-COOH group) → -oic acid
Putting It Together: Step-by-Step
- Find the longest continuous carbon chain containing the functional group. This gives the carbon stem.
- Number the carbon chain from the end that gives the functional group or double bond the lowest possible locant number.
- Identify substituents or positions (e.g., methyl groups, double bond position, hydroxyl position).
- Combine the parts: e.g., But-1-ene, Propan-2-ol, Ethanoic acid.
4. Addition Polymerisation: Making Giant Molecules
Plastics are made of polymers – macromolecules built by linking together thousands of small, repeating monomer molecules.
Key Terms
- Monomer: The small, unsaturated molecule that acts as the starting building block.
- Polymer: A macromolecule formed by joining many repeating units together.
- Polymerisation: The chemical reaction in which monomers combine to form a polymer.
- Repeating Unit: The smallest structural unit whose repetition forms the polymer chain.
How Addition Polymerisation Works
Addition polymerisation occurs when unsaturated monomers containing a carbon-carbon double bond (\(\text{C}=\text{C}\)) join together without eliminating any small molecule. The \(\text{C}=\text{C}\) double bond breaks open and forms new carbon-carbon single covalent bonds linking adjacent monomer units.
General equation for ethene:
\(n\,\text{CH}_2=\text{CH}_2 \rightarrow -[\text{CH}_2-\text{CH}_2]_n-\)
(where \(n\) is a very large integer representing the degree of polymerisation)
Core Addition Polymers in HKDSE
1. Poly(ethene) (PE / Polythene)
Monomer: Ethene (\(\text{CH}_2=\text{CH}_2\))
Repeating Unit: \(-[\text{CH}_2-\text{CH}_2]-\)
Properties & Uses: Flexible, chemically inert, waterproof; used for shopping bags, cling film, squeeze bottles.
2. Poly(propene) (PP / Polypropylene)
Monomer: Propene (\(\text{CH}_2=\text{CH}-\text{CH}_3\))
Repeating Unit: \(-[\text{CH}_2-\text{CH}(\text{CH}_3)]-\)
Properties & Uses: High tensile strength, heat resistant; used for food containers, ropes, crates.
3. Poly(chloroethene) (PVC / Polyvinyl chloride)
Monomer: Chloroethene (vinyl chloride, \(\text{CH}_2=\text{CHCl}\))
Repeating Unit: \(-[\text{CH}_2-\text{CH(Cl)}]-\)
Properties & Uses: Rigid, tough, good electrical insulator; used for drain pipes, window frames, cable insulation.
4. Poly(phenylethene) (Polystyrene / PS)
Monomer: Phenylethene (styrene, \(\text{CH}_2=\text{CH}(\text{C}_6\text{H}_5)\))
Repeating Unit: \(-[\text{CH}_2-\text{CH}(\text{C}_6\text{H}_5)]-\)
Properties & Uses: Transparent and brittle in solid form, light and thermal insulator when expanded; used for disposable foam lunch boxes and packaging.
5. Polytetrafluoroethene (PTFE / Teflon)
Monomer: Tetrafluoroethene (\(\text{CF}_2=\text{CF}_2\))
Repeating Unit: \(-[\text{CF}_2-\text{CF}_2]-\)
Properties & Uses: Extremely non-reactive, heat-resistant, low-friction non-stick surface; used for non-stick cookware coatings and waterproof membranes.
5. Environmental Impact and Treatment of Plastic Waste
While addition polymers are extremely useful, their widespread use causes major environmental problems:
Problems Caused by Plastic Waste
- Non-biodegradability: Addition polymers are resistant to bacterial attack and atmospheric weathering due to strong C-C and C-H bonds. Discarded plastics persist in the environment for hundreds of years.
- Landfill Shortage: Bulky plastic waste takes up massive landfill space.
- Harm to Wildlife: Marine organisms and terrestrial wildlife can ingest or become entangled in plastic debris.
- Toxic Fume Emission: Burning plastics releases toxic air pollutants. For example, incinerating PVC produces poisonous hydrogen chloride (\(\text{HCl}\)) gas and harmful chlorinated compounds.
Methods for Dealing with Plastic Waste
- Reduction and Reuse: Minimising single-use plastics and adopting reusable alternatives (the primary strategy in waste management).
- Recycling: Collecting, sorting (using resin identification coding), melting, and remoulding thermoplastic waste into new products.
- Incineration with Energy Recovery: High-temperature combustion in controlled incinerators to generate electricity. Scrubbers must be installed to remove toxic acidic gases such as \(\text{HCl}\).
- Development of Biodegradable Plastics: Designing polymers that degrade naturally through microbial action or sunlight.
Key Takeaway:
Addition polymers are durable and non-biodegradable, creating significant disposal challenges. Proper waste management relies on the 4Rs (Reduce, Reuse, Recycle, Replace) and safe treatment methods.