Study Notes: Homologous Series in Organic Chemistry
Hello! Welcome to the fascinating world of organic chemistry. Think of it like learning about different families. In chemistry, these families are called homologous series. Each family has a unique "surname" (its functional group) and its members share a strong family resemblance (similar chemical properties).
In these notes, we'll explore the main families of carbon compounds. We'll learn how to identify them, give them their proper names, and understand why they behave the way they do. Don't worry if the names look long and complicated at first – we'll break it all down into simple, easy-to-follow steps. Let's get started!
The Basics: What are Homologous Series?
So, what exactly is a Homologous Series?
A homologous series is a group or 'family' of organic compounds that follow a set of simple rules. To be in the same family, all members must have:
The same functional group. This is the special part of the molecule that gives the family its chemical character.
The same general formula. This is like a mathematical formula that works for every member of the family.
Each member differs from the next one by a -CH₂- group.
Similar chemical properties. Because they have the same functional group, they react in very similar ways.
A gradual change in physical properties (like boiling point) as the molecules get bigger.
What is a Functional Group?
The functional group is the most important part of an organic molecule. It's an atom or a group of atoms that is responsible for the characteristic chemical reactions of a compound.
Analogy: Think of a car model, like a Toyota Corolla. All Corollas have the same engine and perform similarly. The engine is like the functional group. The size of the car might change slightly (a bigger chassis), but the engine defines what it is and how it runs.
Key Takeaway
A homologous series is a family of compounds with the same functional group and similar chemistry. Physical properties change gradually as the molecules get bigger.
Naming Organic Compounds: The IUPAC System
To avoid confusion, chemists worldwide use a standard set of rules for naming compounds, called the IUPAC (International Union of Pure and Applied Chemistry) system. It's like a grammar for chemistry!
The 3-Step Guide to Naming
Step 1: Find the 'Stem' (the parent chain)
Count the number of carbon atoms in the longest continuous chain. This gives you the first part of the name (the stem or prefix).
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: "My Elephant Prefers Bananas" can help you remember the first four!
Step 2: Find the 'Suffix' (the family name)
Look at the functional group to determine which family the compound belongs to. This gives you the ending of the name (the suffix).
Examples: -ane for alkanes, -ene for alkenes, -ol for alcohols. We'll cover all of these below!
Step 3: Number the Chain
Sometimes, you need to say *where* the functional group is located on the carbon chain. We do this by numbering the carbon atoms, starting from the end that gives the functional group the lowest possible number.
For example, in butan-2-ol, the "-2-" tells us the -OH group is on the second carbon atom of a four-carbon chain.
Key Takeaway
Naming is logical! Just identify the Stem (carbon count), the Suffix (family/functional group), and the Number (location of the functional group).
The Homologous Series Families
1. The Alkanes (Saturated Hydrocarbons)
These are the simplest organic compounds, made only of carbon and hydrogen atoms connected by single bonds. They are called saturated because they hold the maximum possible number of hydrogen atoms.
Functional Group:
None (only C-C and C-H single bonds)
General Formula:
$$C_nH_{2n+2}$$
Naming (Suffix):
Ends in -ane.
Examples:
Propane (n=3, C₃H₈)
Structural Formula: CH₃-CH₂-CH₃
Used in: LPG for cooking and heating.
Physical Properties:
Boiling points increase as the carbon chain gets longer. This is because the intermolecular forces (van der Waals' forces) become stronger for larger molecules.
Chemical Properties:
Generally unreactive, but they undergo combustion (burning) and substitution reactions (e.g., with chlorine in UV light).
2. The Alkenes (Unsaturated Hydrocarbons)
Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). They are called unsaturated because they could hold more hydrogen atoms if the double bond were broken.
Functional Group:
Carbon-carbon double bond (>C=C<)
General Formula:
$$C_nH_{2n}$$ (for one double bond)
Naming (Suffix):
Ends in -ene. You need to number the chain to show the position of the double bond (e.g., but-1-ene vs. but-2-ene).
Examples:
Ethene (n=2, C₂H₄)
Structural Formula: CH₂=CH₂
Used in: Making poly(ethene) plastic and ripening fruits.
Chemical Properties:
The double bond is a site of high reactivity. Alkenes undergo addition reactions, where the double bond breaks and atoms are added to the molecule.
3. The Haloalkanes
These are alkanes where one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br, or I).
Functional Group:
A halogen atom (-X, where X = F, Cl, Br, I)
General Formula:
$$C_nH_{2n+1}X$$
Naming (Prefix):
Uses a prefix: fluoro-, chloro-, bromo-, iodo-. You must number the carbon atom the halogen is attached to.
Examples:
1-chloropropane (C₃H₇Cl)
Structural Formula: CH₃-CH₂-CH₂-Cl
Common Name: Chloroform is the trivial name for trichloromethane.
Chemical Properties:
The C-X bond is polar, making haloalkanes more reactive than alkanes. They undergo substitution reactions.
4. The Alcohols (Alkanols)
Alcohols contain a hydroxyl functional group. They are very common and important compounds.
Functional Group:
Hydroxyl group (-OH)
General Formula:
$$C_nH_{2n+1}OH$$
Naming (Suffix):
Ends in -ol. Number the chain to show the position of the -OH group.
Examples:
Ethanol (C₂H₅OH)
Structural Formula: CH₃-CH₂-OH
Used in: Alcoholic drinks, disinfectants, and as a fuel.
Physical Properties:
Alcohols have much higher boiling points than alkanes of similar size because the -OH group allows for hydrogen bonding between molecules. Small alcohols are soluble in water.
Chemical Properties:
They can be oxidised (primary and secondary alcohols), dehydrated to form alkenes, and undergo substitution.
Did you know? The "proof" of an alcoholic beverage is simply double its percentage of ethanol by volume. So, an 80-proof vodka is 40% ethanol.
5. The Aldehydes
Aldehydes have a carbonyl group (>C=O) at the end of a carbon chain.
Functional Group:
The aldehyde group (-CHO)
Naming (Suffix):
Ends in -al. No number is needed because the -CHO group is always at carbon #1.
Examples:
Propanal (C₃H₆O)
Structural Formula: CH₃-CH₂-CHO
Common Name: Formaldehyde is the trivial name for methanal.
Chemical Properties:
Aldehydes are easily oxidised to form carboxylic acids. They can also be reduced back to primary alcohols.
6. The Ketones
Ketones also have a carbonyl group (>C=O), but it is located within the carbon chain, not at the end.
Functional Group:
The carbonyl group (>C=O) between two carbon atoms.
Naming (Suffix):
Ends in -one. For chains of 5 carbons or more, you must number the position of the C=O group.
Examples:
Propanone (C₃H₆O)
Structural Formula: CH₃-CO-CH₃
Common Name: Acetone. Used as a solvent and in nail polish remover.
Chemical Properties:
Ketones are more difficult to oxidise than aldehydes. They can be reduced to form secondary alcohols.
Quick Review: Aldehydes vs. Ketones
Both have a C=O group. The key is its location!
- Aldehyde: C=O is at the END of the chain (has an H attached: -CHO).
- Ketone: C=O is in the MIDDLE of the chain (has carbons on both sides: -CO-).
7. The Carboxylic Acids
These are the acids of organic chemistry. They are responsible for the sour taste of many foods.
Functional Group:
The carboxyl group (-COOH)
Naming (Suffix):
Ends in -oic acid. The carboxyl carbon is always carbon #1.
Examples:
Ethanoic acid (CH₃COOH)
Structural Formula: CH₃-COOH
Common Name: Acetic acid. This is the acid in vinegar.
Physical Properties:
Have high boiling points due to strong hydrogen bonding (they can form stable pairs called dimers). They have characteristic sharp or sour smells.
Chemical Properties:
They are weak acids. They react with bases (neutralisation), and react with alcohols to form esters (esterification).
8. The Esters
Esters are known for their pleasant, often fruity, smells. They are formed from a reaction between a carboxylic acid and an alcohol.
Functional Group:
The ester link (-COO-)
Naming (Two parts):
This is a bit different! The name has two parts:
The first part comes from the alcohol, ending in -yl. (e.g., methanol becomes methyl)
The second part comes from the carboxylic acid, ending in -oate. (e.g., ethanoic acid becomes ethanoate)
So, the ester made from methanol and ethanoic acid is called methyl ethanoate.
Examples:
Ethyl propanoate
Made from: Ethanol and Propanoic acid
Smell: Pineapple
Chemical Properties:
Esters can be broken down by heating with water, an acid or an alkali. This reaction is called hydrolysis.
9. The Amides (Unsubstituted)
Amides can be thought of as derived from carboxylic acids where the -OH group is replaced by an -NH₂ group.
Functional Group:
The amide group (-CONH₂)
Naming (Suffix):
Ends in -amide.
Examples:
Ethanamide (CH₃CONH₂)
Structural Formula: CH₃-CO-NH₂
Chemical Properties:
They can undergo hydrolysis to form a carboxylic acid and ammonia. The links in nylon are amide links, making it a polyamide.
10. The Amines (Primary)
Amines are organic derivatives of ammonia (NH₃). Primary amines have one hydrogen in ammonia replaced by a carbon group.
Functional Group:
The amino group (-NH₂)
General Formula:
$$C_nH_{2n+1}NH_2$$
Naming (Suffix):
Ends in -amine.
Examples:
Ethanamine (C₂H₅NH₂)
Structural Formula: CH₃-CH₂-NH₂
Chemical Properties:
Amines are weak bases, similar to ammonia. They are known for their strong, often fishy, smells.
Summary of Homologous Series
Here is a quick reference table to help you remember the key families.
Series: Alkanes
Functional Group: C-C single bonds
Suffix: -ane
Example: Ethane
Series: Alkenes
Functional Group: >C=C<
Suffix: -ene
Example: Ethene
Series: Haloalkanes
Functional Group: -X (e.g., -Cl)
Prefix: chloro-
Example: Chloroethane
Series: Alcohols
Functional Group: -OH
Suffix: -ol
Example: Ethanol
Series: Aldehydes
Functional Group: -CHO
Suffix: -al
Example: Ethanal
Series: Ketones
Functional Group: >C=O
Suffix: -one
Example: Propanone
Series: Carboxylic Acids
Functional Group: -COOH
Suffix: -oic acid
Example: Ethanoic acid
Series: Esters
Functional Group: -COO-
Suffix: -yl -oate
Example: Methyl ethanoate
Series: Amides
Functional Group: -CONH₂
Suffix: -amide
Example: Ethanamide
Series: Amines
Functional Group: -NH₂
Suffix: -amine
Example: Ethanamine
Well done for working through all these families! The key to mastering organic chemistry is practice. Try drawing out the molecules and naming them. The more you do it, the more familiar the patterns will become. You've got this!