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 the giant molecules that make up plastics. It might sound complex, but we'll break it down step-by-step. 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 the fuel in a bus to the molecules that make up DNA!
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. (It's like the family's key feature!)
- A similar general formula. This is an algebraic formula that works for all members of the family (e.g., $$C_nH_{2n+2}$$ for alkanes).
- Successive members that differ by a -CH₂- group.
- Similar chemical properties, because they have the same functional group.
- A gradual change (gradation) in their physical properties (like boiling point) as the molecules get bigger. Generally, the larger the molecule, the higher the boiling point.
Four Important Families to Know
Let's meet the four families you need to know for your syllabus.
Quick Review: The Four Families
1. Alkanes
Functional Group: Only carbon-carbon single bonds (C-C).
General Formula: $$C_nH_{2n+2}$$ (for n ≥ 1)
Description: These are known as saturated hydrocarbons because they have the maximum possible number of hydrogen atoms. Example: methane ($$CH_4$$), propane ($$C_3H_8$$).
2. Alkenes
Functional Group: At least one carbon-carbon double bond (C=C).
General Formula: $$C_nH_{2n}$$ (for n ≥ 2)
Description: These are unsaturated hydrocarbons because they have a double bond that could potentially bond to more atoms. Example: ethene ($$C_2H_4$$), propene ($$C_3H_6$$).
3. Alkanols (Alcohols)
Functional Group: The hydroxyl group (-OH).
General Formula: $$C_nH_{2n+1}OH$$ (for n ≥ 1)
Description: You might know ethanol, the alcohol in drinks. Alkanols are a whole family of similar compounds. Example: methanol ($$CH_3OH$$), ethanol ($$C_2H_5OH$$).
4. Alkanoic Acids (Carboxylic Acids)
Functional Group: The carboxyl group (-COOH).
General Formula: $$C_{n-1}H_{2n-1}COOH$$ (for n ≥ 1)
Description: These are weak acids. The most famous is ethanoic acid, which is the acid in vinegar. Example: methanoic acid ($$HCOOH$$), ethanoic acid ($$CH_3COOH$$).
Key Takeaway:
A homologous series is a family of organic compounds with the same functional group and similar chemical properties. We will focus on alkanes, alkenes, alkanols, and alkanoic acids.
3. Naming Carbon Compounds (Systematic Naming)
Don't worry if the names above look confusing! There's a logical system called the IUPAC system for naming every compound, so everyone in the world uses the same name for the same molecule.
A name is made of two main parts: a prefix (telling you the number of carbon atoms) and a suffix (telling you which family it belongs to).
Part 1: The Prefix (Number of Carbons)
This tells you how many carbon atoms are in the longest continuous chain.
Number of Carbons and their Prefixes
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... (The first four are tricky, the rest are like shapes: pentagon, hexagon, etc.)
Part 2: The Suffix (The Family Name)
This is determined by the homologous series (the functional group).
Alkane (C-C bonds only) → -ane
Alkenes (C=C double bond) → -ene
Alkanols (-OH group) → -anol
Alkanoic Acids (-COOH group) → -oic acid
Putting it all together: A Step-by-Step Guide
- Find the longest continuous chain of carbon atoms. This gives you the prefix.
- Identify the main functional group. This gives you the suffix.
- Number the carbon chain. Start from the end that gives the functional group the lowest possible number. (This is not needed for alkanes or for very short alkanoic acids like ethanoic acid).
- Write the full name. Combine the parts in this order: (Position of group) - (Prefix) - (Suffix).
Let's try some examples!
Example 1: Naming an Alkene
Imagine a molecule with the structure: CH₂=CH-CH₂-CH₃
- Longest chain: 4 carbons → But-
- Functional group: A C=C double bond → -ene
- Numbering: We number from the left to give the double bond the lowest number. It starts on carbon 1. (If we started from the right, it would be on carbon 3). So, the position is 1.
- Full name: But-1-ene
Example 2: Naming an Alkanol
Imagine a molecule with the structure: CH₃-CH(OH)-CH₃
- Longest chain: 3 carbons → Prop-
- Functional group: An -OH group → -anol
- Numbering: The -OH is on carbon 2, no matter which way we count. The position is 2.
- Full name: Propan-2-ol
Drawing Structural Formulae from a Name
This is just the reverse process! Let's draw pentan-1-ol.
- Prefix 'Pent-' tells us to draw a chain of 5 carbon atoms: C-C-C-C-C.
- Suffix '-anol' tells us there is an -OH group.
- Number '1' tells us the -OH is on the first carbon.
- Finally, add hydrogen atoms to every carbon so that each carbon atom has a total of 4 bonds.
Key Takeaway:
Systematic naming follows simple rules: find the longest carbon chain (prefix), identify the family (suffix), and use numbers to show the location of the functional group.
4. Addition Polymerisation: Making Big Molecules
Have you ever wondered what plastics are made of? They are polymers – giant molecules made by joining thousands of small molecules together.
Key Terms
- Monomer: The small, simple molecule that is the basic building block.
- Polymer: A very large molecule made of many repeating monomer units joined together.
- Polymerisation: The chemical process of joining monomers to form a polymer.
Analogy: Imagine you have a big box of paper clips (monomers). When you link them all together into a long chain, you've made a paper clip chain (polymer). The process of linking them is polymerisation.
What is Addition Polymerisation?
Addition polymerisation is a specific type of polymerisation. It happens with unsaturated monomers, usually alkenes, which have a C=C double bond.
Here’s the magic trick: The double bond is made of two pairs of shared electrons. During polymerisation, one of these pairs 'breaks open' and forms new single bonds with neighbouring monomers. This links them all together in one long chain.
The Process: From Ethene to Poly(ethene)
- Start with many monomers: Many ethene molecules ($$CH_2=CH_2$$).
- Reaction conditions: Under high temperature and pressure with a catalyst...
- The double bond opens: The C=C double bond in each ethene molecule breaks open.
- Monomers link up: The monomers add to each other to form a long saturated chain.
We can show this with an equation:
$$ n CH_2=CH_2 \rightarrow -[CH_2-CH_2]-_n $$
(where 'n' is a very large number)
The part inside the square brackets, -[CH₂-CH₂]-, is called the repeating unit. It's the section of the polymer chain that repeats over and over again.
Deducing Monomers and Repeating Units
- From Monomer to Polymer: To find the repeating unit, simply change the monomer's C=C double bond to a C-C single bond and show new single bonds extending out from the sides.
- From Polymer to Monomer: To find the monomer, take one repeating unit from the polymer chain, remove the bonds extending out, and turn the C-C single bond in the 'backbone' back into a C=C double bond.
Did you know?
Plastics have recycling codes (a number inside a triangle of arrows) that help identify the type of polymer they are made from. For example, PVC is code 3 and polystyrene is code 6.
Common Addition Polymers
Here are the examples you need to know, all made from different alkene monomers.
1. Poly(ethene) or Polythene (PE)
Monomer: Ethene
Repeating Unit: -[CH₂-CH₂]-
Uses: Plastic bags, cling film, bottles (it's flexible and cheap).
2. Poly(propene) or Polypropylene (PP)
Monomer: Propene
Repeating Unit: -[CH(CH₃)-CH₂]-
Uses: Ropes, carpets, food containers, chairs (it's strong and heat resistant).
3. Poly(chloroethene) or Polyvinyl Chloride (PVC)
Monomer: Chloroethene (vinyl chloride)
Repeating Unit: -[CHCl-CH₂]-
Uses: Pipes, window frames, electrical wire insulation (it's tough, rigid, and a good electrical insulator).
4. Poly(phenylethene) or Polystyrene (PS)
Monomer: Phenylethene (styrene)
Repeating Unit: -[CH(C₆H₅)-CH₂]-
Uses: Disposable cups, packaging foam, plastic cutlery (it's brittle but a great heat insulator when expanded into foam).
5. Perspex (Poly(methyl 2-methylpropenoate))
Monomer: Methyl 2-methylpropenoate
Repeating Unit: A more complex structure!
Uses: A lightweight, shatter-resistant alternative to glass, used in aircraft windows and aquariums.
Key Takeaway:
Addition polymerisation joins many unsaturated monomers (like alkenes) together by breaking their double bonds to form a long saturated chain called a polymer. You should be able to identify the monomer from a polymer structure, and vice versa.