Definition
Series concept: A homologous series is like a family of organic compounds that are very similar to each other. They all follow a pattern called a “general formula,” which means their chemical makeup is predictable. Because they have similar parts in their molecules, they also behave in similar ways during chemical reactions.
CH₂ unit difference: Each member in this series is a bit bigger than the one before it by one extra -CH₂- group (one carbon and two hydrogen atoms). This tiny difference in size creates a clear pattern in their physical properties, like boiling point and melting point.
Same functional group: All the compounds in a homologous series have the same special part in their structure, called a functional group. This part is what makes them react in similar ways with other chemicals.
Key Characteristics
General formula: A general formula is like a rule that helps us figure out what atoms are in each compound in the series. For example, alkanes follow the rule CₙH₂ₙ₊₂. This makes it easier for scientists to understand and predict how these compounds look and behave.
Physical property trend: As we go from one compound to the next in a series, physical features like boiling point and melting point slowly get higher. This is because the molecules get bigger, and the bigger the molecule, the stronger the forces between them, so more heat is needed to break them apart.
Chemical similarity: All the members of the series act in a similar way in chemical reactions. This happens because they have the same functional group, which controls how they behave during reactions.
Examples of Homologous Series
Alkanes
Formula: Alkanes use the general formula CₙH₂ₙ₊₂. This means for every carbon atom, there are two hydrogen atoms plus two more.
Functional group: They have only single bonds between the carbon atoms (C–C). These single bonds make them quite stable.
Description: Alkanes are called saturated hydrocarbons because they have as many hydrogen atoms as they can hold. They do not react easily with other substances, but they can burn (combustion) and also swap atoms in substitution reactions.
Examples: Common alkanes you might know include methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄Hₑ₀).
Boiling trend: As the number of carbon atoms goes up, the boiling point gets higher. This is because the bigger molecules stick to each other more strongly.
Alkenes
Formula: Alkenes follow the rule CₙH₂ₙ. This means there are twice as many hydrogen atoms as carbon atoms.
Functional group: They have at least one double bond between carbon atoms (C=C), which makes them more reactive.
Description: Alkenes are called unsaturated hydrocarbons because they don’t have the maximum number of hydrogen atoms. The double bond makes them more active in reactions like addition and combustion.
Examples: Some common alkenes are ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈).
Boiling trend: Just like with alkanes, the boiling point of alkenes increases as the number of carbon atoms increases.
Alkynes
Formula: Alkynes follow the formula CₙH₂ₙ₋₂. They have two fewer hydrogen atoms than alkenes.
Functional group: They include at least one triple bond between carbon atoms (C≡C), which gives them special reactivity.
Description: Alkynes are also unsaturated hydrocarbons. Their triple bond makes them reactive, and they often take part in reactions similar to alkenes.
Alcohols
Formula: Alcohols follow the general formula CₙH₂ₙ₊₁OH. That means they are like alkanes but with an -OH group attached.
Functional group: Alcohols contain a hydroxyl group (-OH), which makes them polar and able to form hydrogen bonds.
Description: Alcohols can burn (combustion), lose water molecules (dehydration), be turned into acids (oxidation), and react with acids to form sweet-smelling substances called esters.
Examples: Common alcohols include methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH).
Boiling point: Their boiling points are higher than those of alkanes because of hydrogen bonding between the -OH groups.
Carboxylic Acids
Formula: These compounds use the formula CₙH₂ₙ₊₁COOH, which means an alkyl group plus a carboxyl group.
Functional group: They contain a carboxyl group (-COOH), which makes them act like weak acids.
Description: Carboxylic acids react with bases in neutralisation and with alcohols to make esters. Though they are weak acids, they still have clear acidic behaviour.
Examples: Examples are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), and propanoic acid (C₂H₅COOH).
Boiling point: These acids have very high boiling points compared to other organic compounds because their molecules can form strong hydrogen bonds.
Esters
Formula: Their general formula is CₙH₂ₙ₊₁COOCₕH₂ₕ₊₁, which means they are formed from combining a carboxylic acid and an alcohol.
Functional group: Esters contain a carboxylate group (-COOC-), which makes them neutral and sweet-smelling.
Description: Esters are made by reacting carboxylic acids with alcohols. They don’t behave like acids or bases and are often used in perfumes and food flavourings.
Examples: Common esters include ethyl methanoate, methyl ethanoate, and ethyl ethanoate.
Functional Groups
Definition: A functional group is a special group of atoms in a molecule that decides how the molecule will react with other substances.
Series relevance: Every homologous series is defined by a certain functional group, and this is what makes all the compounds in that group behave in a similar way in reactions.
Key groups:
• C–C: Alkanes (single bonds only)
• C=C: Alkenes (contain double bonds)
• C≡C: Alkynes (contain triple bonds)
• –OH: Alcohols (hydroxyl group)
• –COOH: Carboxylic acids (carboxyl group)
• –COOC–: Esters (carboxylate group)
Impact of Chain Length on Properties
Boiling/melting point: When the number of carbon atoms in the molecule increases, the boiling and melting points go up. This is because the molecules are larger and have stronger forces pulling them together.
State of matter: The smallest compounds, which have few carbon atoms, are gases. As the chains get longer, they become liquids, and the longest ones can even be solids at room temperature.
Reactivity trend: While longer chains might change the speed of a reaction, the type of reaction stays the same since all compounds in a series have the same functional group.
Nomenclature
Two-part name: The names of organic compounds have two parts: a prefix that tells how many carbon atoms there are, and a suffix that tells the type of functional group.
Prefixes: For example, meth- means 1 carbon, eth- means 2, prop- means 3, and but- means 4.
Suffixes: These endings tell which homologous series the compound belongs to. For instance: -ane for alkanes, -ene for alkenes, -ol for alcohols, and -oic acid for carboxylic acids.
Example of isomerism: Butan-1-ol and butan-2-ol are two forms of butanol. They have the same number of atoms but the -OH group is in a different position, so they are called positional isomers.