Terminology Revision Notes

A saturated compound is an organic molecule in which every carbon-carbon bond is a single covalent bond. This means that no double or triple bonds exist between carbon atoms within the molecule. Because carbon atoms form four covalent bonds, when all carbon-carbon bonds are single, the remaining bonds are with hydrogen atoms or other atoms, resulting in the maximum number of hydrogen atoms bonded to carbon.

In saturated compounds, each carbon atom is bonded to as many hydrogen atoms as possible, making the molecule 'saturated' with hydrogen. This contrasts with unsaturated compounds, where one or more carbon-carbon double or triple bonds reduce the number of hydrogen atoms attached.

• All carbon-carbon bonds are single bonds (C–C).
• Each carbon atom forms four single covalent bonds.
• Maximum hydrogen atoms are bonded to carbon atoms.

The presence of only single bonds in saturated compounds affects their chemical behaviour. Single bonds are sigma (σ) bonds, which are strong and allow free rotation around the bond axis, giving saturated molecules flexibility in shape.

Because saturated compounds have no double or triple bonds, they do not undergo addition reactions typical of unsaturated compounds. Instead, they mainly undergo substitution or combustion reactions. This stability is important in understanding their reactivity and uses.

For example, alkanes, a class of saturated hydrocarbons, are relatively less reactive and are commonly used as fuels due to their stable single bonds.

• Single bonds (sigma bonds) allow free rotation and molecular flexibility.
• Saturated compounds are less reactive towards addition reactions.
• They commonly undergo substitution and combustion reactions.

Common examples of saturated compounds include alkanes such as methane (CH₄), ethane (C₂H₆), and propane (C₃H₈). In methane, the single carbon atom forms four single bonds with hydrogen atoms, making it fully saturated.

Ethane consists of two carbon atoms connected by a single bond, each bonded to three hydrogen atoms, maintaining saturation. Propane extends this chain with three carbon atoms, all connected by single bonds and saturated with hydrogen atoms.

These examples illustrate the general formula for alkanes: CₙH₂ₙ₊₂, where n is the number of carbon atoms, reflecting saturation with hydrogen.

• Methane (CH₄) – one carbon, four hydrogens.
• Ethane (C₂H₆) – two carbons connected by a single bond.
• Propane (C₃H₈) – three carbons in a chain, all single bonds.

Saturated compounds differ from unsaturated compounds by the type of carbon-carbon bonds present. Unsaturated compounds contain one or more double (C=C) or triple (C≡C) bonds, which reduce the number of hydrogen atoms bonded to carbon.

This difference affects physical and chemical properties. For example, unsaturated compounds can undergo addition reactions where double or triple bonds open to add atoms, whereas saturated compounds cannot.

Recognising the bond types in a molecule’s structural formula helps identify whether it is saturated or unsaturated.

• Saturated: only single C–C bonds; unsaturated: one or more double/triple bonds.
• Unsaturated compounds have fewer hydrogen atoms than saturated ones with the same carbon count.
• Bond type influences chemical reactivity and physical properties.

Understanding saturated compounds is essential because they form the basis of many organic molecules and influence chemical behaviour. Their single bonds confer stability and define their reactions and uses.

Recognising saturated compounds helps predict molecular properties and reaction types, which is fundamental in organic synthesis, fuel chemistry, and materials science.

Mastering this concept supports further study of homologous series and organic reaction mechanisms.

• Saturated compounds have only single carbon-carbon bonds.
• They are stable and less reactive than unsaturated compounds.
• Key to understanding organic molecular structure and reactions.

Unsaturated compounds are organic molecules that contain at least one carbon-carbon double bond (C=C) or triple bond (C≡C). Unlike saturated compounds, where all carbon atoms are connected by single bonds (C–C), unsaturated compounds have one or more multiple bonds between carbon atoms. These multiple bonds reduce the number of hydrogen atoms attached to the carbon chain compared to a saturated molecule with the same number of carbons.

For example, ethene (C2H4) contains a carbon-carbon double bond, making it unsaturated, whereas ethane (C2H6) has only single bonds and is saturated. The presence of double or triple bonds changes the shape and bonding properties of the molecule.

• Unsaturated compounds have one or more C=C or C≡C bonds.
• Saturated compounds have only C–C single bonds.
• Multiple bonds reduce hydrogen content compared to saturated analogues.

The two main classes of unsaturated hydrocarbons are alkenes and alkynes. Alkenes contain at least one carbon-carbon double bond, while alkynes contain at least one carbon-carbon triple bond.

For instance, propene (C3H6) is an alkene with a double bond between two carbon atoms, and ethyne (C2H2) is an alkyne with a triple bond. These multiple bonds are the defining feature that classifies these compounds as unsaturated.

• Alkenes have C=C double bonds.
• Alkynes have C≡C triple bonds.
• Examples: propene (alkene), ethyne (alkyne).

The presence of double or triple bonds in unsaturated compounds makes them more chemically reactive than saturated compounds. This is because multiple bonds contain regions of higher electron density that can participate in addition reactions, where atoms or groups add across the double or triple bond.

For example, alkenes readily undergo addition reactions with bromine water, causing the orange bromine solution to decolourise. Saturated alkanes do not react in this way under normal conditions.

• Multiple bonds are sites of higher reactivity.
• Unsaturated compounds undergo addition reactions.
• Example: bromine water decolourises with alkenes.

To determine if a compound is unsaturated, examine its molecular structure for carbon-carbon multiple bonds. Structural formulas or molecular models can help visualise these bonds.

For example, cyclohexene has a ring structure with one double bond, making it unsaturated, whereas cyclohexane has only single bonds and is saturated. Recognising these bonds is essential for predicting chemical behaviour.

• Look for C=C or C≡C bonds in structures.
• Use molecular models or formulas to identify multiple bonds.
• Unsaturation affects physical and chemical properties.

Unsaturated compounds are defined by the presence of one or more carbon-carbon double or triple bonds. This structural feature distinguishes them from saturated compounds, which contain only single bonds between carbon atoms.

The multiple bonds in unsaturated compounds influence their chemical reactivity, making them more reactive and able to undergo addition reactions. Common examples include alkenes and alkynes, which are important classes of hydrocarbons in organic chemistry.

• Unsaturated compounds contain C=C or C≡C bonds.
• They are more reactive than saturated compounds.
• Alkenes and alkynes are typical unsaturated hydrocarbons.

A homologous series is a group of organic compounds that have a similar general structure and share the same functional group. Each successive compound in the series differs from the previous one by a constant structural unit, typically a -CH2- group. This incremental change leads to a gradual variation in physical properties while maintaining similar chemical behaviour.

For example, the alkane series starts with methane (CH4), then ethane (C2H6), propane (C3H8), and so on. Each member differs by one -CH2- unit from the previous member. This pattern is a defining feature of homologous series.

• Members differ by a -CH2- group.
• All compounds have the same functional group.
• They have a general formula representing the series, e.g., CnH2n+2 for alkanes.

Compounds in a homologous series exhibit similar chemical properties because they contain the same functional group, which determines their typical reactions. For instance, all alkanes undergo combustion and substitution reactions under similar conditions.

The similarity in chemical properties allows chemists to predict the behaviour of one member based on knowledge of others in the series. This predictability is useful in organic synthesis and industrial applications.

• Same functional group leads to similar reactions.
• Chemical properties are consistent across the series.
• Differences in reactivity are usually minor and related to chain length.

Although chemical properties are similar, physical properties such as boiling point, melting point, and density change gradually as the molecular size increases. This is due to the increasing strength of intermolecular forces like van der Waals forces as the hydrocarbon chain lengthens.

For example, in the alkane series, methane is a gas at room temperature, while octane is a liquid. This trend helps in identifying and separating compounds in the laboratory or industry.

• Physical properties change gradually with molecular size.
• Boiling and melting points increase with chain length.
• Physical changes help in practical identification.

Common homologous series include: alkanes (saturated hydrocarbons with single bonds), alkenes (unsaturated hydrocarbons with one or more double bonds), and alcohols (compounds containing the -OH functional group).

Each series has a characteristic general formula and functional group, which define their chemical properties and reactions.

For example, alkenes have the general formula CnH2n and undergo addition reactions due to their double bonds.

• Alkanes: CnH2n+2, saturated, substitution reactions.
• Alkenes: CnH2n, unsaturated, addition reactions.
• Alcohols: CnH2n+1OH, contain hydroxyl group, undergo oxidation.

Understanding homologous series helps in predicting chemical behaviour, naming compounds systematically, and organising organic chemistry knowledge logically.

It also aids in recognising patterns in reactions and physical properties, which is essential for problem-solving in exams and practical work.

For example, knowing that all alcohols contain -OH allows prediction of their solubility in water and their reactions with acids.

• Predict chemical reactions based on functional groups.
• Systematic naming using general formulas.
• Organise organic compounds logically for study and application.

A homologous series is a group of organic compounds that have a similar general formula and share a common structural pattern. Each successive member differs from the previous one by a fixed unit, typically a –CH2– group. This structural similarity means that members have comparable chemical properties but differ in molecular size and mass.

• Members differ by a constant structural unit (usually –CH2–).
• All members share the same general formula (e.g., CnH2n+2 for alkanes).
• Chemical properties are similar due to similar functional groups.

The general formula represents the simplest algebraic expression of the number of atoms of each element in the compounds of the series. For example, the alkane series follows the general formula CnH2n+2, where n is the number of carbon atoms. This formula applies to all members regardless of chain length, reflecting their structural similarity.

• General formula expresses the ratio of atoms in the series.
• Applies to all members, e.g., alkanes: CnH2n+2.
• Helps predict molecular formula of any member given n.

Physical properties such as boiling point, melting point, and solubility show a gradual and predictable change as the molecular size increases within a homologous series. For instance, boiling points increase with the number of carbon atoms because larger molecules have stronger intermolecular forces, specifically van der Waals forces, requiring more energy to separate them.

• Boiling point increases with molecular size.
• Melting point generally increases but can vary with molecular shape.
• Solubility in water decreases as hydrocarbon chain length increases.

As the carbon chain lengthens, the surface area of the molecule increases, enhancing the strength of London dispersion forces (a type of van der Waals force) between molecules. Stronger intermolecular forces require more heat energy to overcome, raising boiling and melting points. Conversely, longer chains are less polar and less soluble in water, which is polar.

• Longer chains have greater surface area for intermolecular forces.
• Stronger forces lead to higher boiling and melting points.
• Increased hydrophobic character reduces water solubility.

Consider the alkane series: methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). All follow the general formula CnH2n+2. Methane boils at –161.5°C, ethane at –88.6°C, propane at –42.1°C, and butane at –0.5°C, showing a clear trend of increasing boiling points with molecular size.

• Methane (n=1): boiling point –161.5°C
• Ethane (n=2): boiling point –88.6°C
• Propane (n=3): boiling point –42.1°C
• Butane (n=4): boiling point –0.5°C