Formulae, functional groups and terminology Revision Notes

A displayed formula is a detailed representation of a molecule that shows every atom and every bond between atoms explicitly. Unlike molecular or empirical formulas, which only indicate the number and type of atoms, displayed formulas reveal the exact connectivity and arrangement of atoms within the molecule.

Displayed formulas are particularly useful in organic chemistry to visualise the structure of molecules, identify functional groups, and understand how atoms are bonded together. Each atom is represented by its chemical symbol (e.g., C for carbon, H for hydrogen, O for oxygen), and bonds are shown as lines connecting the atoms. A single line represents a single covalent bond (sharing one pair of electrons), while a double line represents a double bond (sharing two pairs of electrons).

• Shows all atoms and bonds explicitly.
• Uses chemical symbols for atoms.
• Single lines represent single covalent bonds.
• Double lines represent double covalent bonds.

To draw a displayed formula, follow these steps: identify all atoms in the molecule, determine how they are connected, and represent each bond with a line. Carbon atoms typically form four bonds, hydrogen one bond, oxygen two bonds, and so on, following their valency.

For example, consider ethanol (C2H5OH). It contains two carbon atoms, six hydrogen atoms, and one oxygen atom. The displayed formula shows the two carbon atoms connected by a single bond, with hydrogen atoms attached to each carbon to satisfy their four bonds, and the oxygen atom bonded to one carbon and one hydrogen (the hydroxyl group).

• Identify all atoms and their number.
• Determine bonding between atoms.
• Draw atoms using chemical symbols.
• Connect atoms with lines representing bonds.
• Ensure valency rules are satisfied for each atom.

Analysing a displayed formula involves recognising the types of atoms, the bonds between them, and the overall molecular structure. This helps in identifying functional groups and understanding chemical properties.

For example, in the displayed formula of ethene (C2H4), the two carbon atoms are connected by a double bond, and each carbon is bonded to two hydrogen atoms. The double bond indicates unsaturation, which affects the molecule's reactivity.

By counting bonds, you can confirm the valency of each atom. Carbon forms four bonds, oxygen two, and hydrogen one. Any deviation suggests an error in the formula.

• Identify atoms and count bonds to check valency.
• Recognise single and double bonds.
• Locate functional groups (e.g., -OH, C=O).
• Understand molecular shape and connectivity.

Propanoic acid has the molecular formula C3H6O2. To draw its displayed formula:

1. Identify the atoms: 3 carbons, 6 hydrogens, 2 oxygens.

2. Recognise the functional group: carboxylic acid (-COOH). This consists of a carbon double bonded to one oxygen and single bonded to an -OH group, attached to the carbon chain.

• Draw a chain of three carbon atoms connected by single bonds.
• Attach the carboxyl group (-COOH) to the end carbon: one oxygen double bonded, one oxygen single bonded to hydrogen.
• Add hydrogen atoms to satisfy carbon valency (each carbon must have four bonds).
• The displayed formula shows all atoms and bonds explicitly.

Always show all atoms, including hydrogens, in displayed formulas unless otherwise instructed.

Use single lines for single bonds and double lines for double bonds; triple bonds are shown with three lines.

Check that each atom satisfies its valency: carbon (4 bonds), hydrogen (1 bond), oxygen (2 bonds). Incorrect valency leads to loss of marks in exams.

• Label all atoms clearly.
• Represent all bonds accurately.
• Verify valency for each atom.
• Use displayed formulas to identify functional groups.
• Practice drawing displayed formulas for common organic molecules.

A homologous series is a group of organic compounds that share the same functional group and have similar chemical properties. Each successive member differs by a -CH2- unit, which affects the molecular formula predictably. The general formula expresses the relationship between the number of carbon atoms (n) and hydrogen or other atoms in the molecule.

Understanding general formulae allows chemists to write molecular formulae for any member of the series and predict properties and reactivity. Four common homologous series are alkanes, alkenes, alcohols, and carboxylic acids, each with a distinct general formula and functional group.

• Homologous series members differ by -CH2- units.
• General formula relates carbon number (n) to hydrogen and functional groups.
• Functional groups determine chemical behaviour and naming.

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. Their general formula is CnH2n+2, where n is the number of carbon atoms.

Because all bonds are single, alkanes are saturated, meaning they contain the maximum number of hydrogen atoms per carbon. The simplest alkane is methane (CH4, n=1).

Example: For butane (n=4), molecular formula is C4H(2×4+2) = C4H10.

• General formula: CnH2n+2
• Saturated hydrocarbons with single C–C bonds
• Example: Methane (CH4), Ethane (C2H6), Propane (C3H8)
• Used as fuels and solvents

Alkenes contain at least one carbon-carbon double bond and are unsaturated hydrocarbons. Their general formula is CnH2n.

The presence of a double bond reduces the number of hydrogen atoms compared to alkanes with the same number of carbons.

Example: Ethene (n=2) has formula C2H4. The double bond affects reactivity, making alkenes more reactive than alkanes.

• General formula: CnH2n
• Contain at least one C=C double bond
• Unsaturated hydrocarbons
• Example: Ethene (C2H4), Propene (C3H6)
• Used in polymer production

Alcohols are organic compounds containing a hydroxyl (-OH) functional group attached to a saturated carbon atom. Their general formula is CnH2n+1OH.

The -OH group replaces one hydrogen atom in the corresponding alkane, increasing the oxygen content.

Example: Methanol (n=1) has formula CH3OH; ethanol (n=2) is C2H5OH. The presence of the hydroxyl group affects physical properties such as boiling point and solubility.

• General formula: CnH2n+1OH
• Contain hydroxyl (-OH) functional group
• Derived from alkanes by replacing one H with -OH
• Example: Methanol (CH3OH), Ethanol (C2H5OH)
• Used as solvents and fuels

Carboxylic acids contain the carboxyl (-COOH) functional group, which imparts acidic properties. Their general formula is CnH2n+1COOH.

The carboxyl group consists of a carbonyl (C=O) and hydroxyl (-OH) group attached to the same carbon atom.

Example: Ethanoic acid (acetic acid, n=1) has formula CH3COOH; propanoic acid (n=2) is C2H5COOH. These acids are weak acids and react with bases to form salts.

• General formula: CnH2n+1COOH
• Contain carboxyl (-COOH) functional group
• Acidic organic compounds
• Example: Ethanoic acid (CH3COOH), Propanoic acid (C2H5COOH)
• Used in food preservation and chemical synthesis

A functional group is a specific atom or group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. It determines the chemical behaviour and properties of the compound.

In organic chemistry, compounds that share the same functional group belong to the same homologous series and exhibit similar chemical properties.

The functional group is the reactive part of the molecule, while the rest of the molecule (the hydrocarbon chain or skeleton) mainly affects physical properties such as boiling point or solubility.

• Functional group = atom/group of atoms defining chemical characteristics.
• Determines reactivity and chemical properties.
• Commonly found in organic compounds.

Several functional groups are important in organic chemistry. Each has a characteristic formula and influences the compound's chemical reactions.

Examples include: hydroxyl group (-OH) in alcohols, carboxyl group (-COOH) in carboxylic acids, and amino group (-NH2) in amines.

Structural formulae show the arrangement of atoms in the functional group and how it connects to the rest of the molecule.

• Hydroxyl group (-OH): found in alcohols, e.g., ethanol CH3CH2OH.
• Carboxyl group (-COOH): found in carboxylic acids, e.g., ethanoic acid CH3COOH.
• Amino group (-NH2): found in amines, e.g., methylamine CH3NH2.

The presence of a functional group determines how a compound reacts chemically. For example, alcohols react with sodium to release hydrogen gas due to the hydroxyl group.

Carboxylic acids are acidic because of the carboxyl group, reacting with bases to form salts and water.

The amino group in amines makes them basic, allowing them to react with acids to form salts.

• Functional groups dictate reactivity patterns.
• Example: Alcohol + sodium → sodium alkoxide + hydrogen gas.
• Example: Carboxylic acid + base → salt + water.
• Example: Amine + acid → ammonium salt.

A homologous series is a group of organic compounds that share the same functional group and have similar chemical properties.

Members of a homologous series differ by a -CH2- unit but retain the same functional group, which causes their chemical behaviour to be similar.

For example, the alcohol series (methanol, ethanol, propanol, etc.) all contain the hydroxyl group (-OH) and undergo similar reactions such as combustion and reaction with sodium.

• Same functional group defines a homologous series.
• Chemical properties are similar within the series.
• Members differ by -CH2- units but share functional group.
• Example: Alcohols all contain -OH group.

Consider the molecule propanoic acid, CH3CH2COOH. The functional group is the carboxyl group (-COOH), which defines it as a carboxylic acid.

This functional group causes propanoic acid to behave as an acid, reacting with bases and metals to produce salts and hydrogen gas.

Recognising the -COOH group allows prediction of chemical reactions and classification into the carboxylic acid homologous series.

• Molecule: CH3CH2COOH (propanoic acid).
• Functional group: carboxyl (-COOH).
• Chemical behaviour: acidic reactions, salt formation.
• Classification: carboxylic acid homologous series.

A homologous series is a set of organic compounds that have the same functional group and similar chemical properties. Members of the series differ from each other by a constant unit, typically a -CH2- group. This structural similarity means they react in comparable ways chemically, although their physical properties may vary systematically.

• Same functional group defines the series.
• Each member differs by one -CH2- unit.
• Similar chemical properties across the series.

The functional group is a specific atom or group of atoms within a molecule responsible for its characteristic chemical reactions. Because all members of a homologous series contain the same functional group, they undergo similar types of chemical reactions. For example, all alcohols contain the -OH group, which imparts similar reactivity such as undergoing oxidation or substitution reactions.

• Functional group determines chemical behaviour.
• Identical functional groups cause similar reactions.
• Examples include -OH in alcohols, -COOH in carboxylic acids.

Each successive compound in a homologous series differs by one methylene (-CH2-) group. This incremental change increases the carbon chain length by one, affecting physical properties like boiling point and solubility but not the fundamental chemical reactivity dictated by the functional group. For example, the alkane series starts with methane (CH4), then ethane (C2H6), propane (C3H8), and so on.

• Difference of one -CH2- unit between members.
• Increases molecular size and mass.
• Physical properties change gradually, chemical properties remain similar.

Common homologous series include alkanes, alkenes, alcohols, and carboxylic acids. Each series has a general formula and a characteristic functional group. For instance, alkanes have the general formula CnH2n+2 and contain only single bonds, while alcohols have the formula CnH2n+1OH and contain the hydroxyl (-OH) group.

• Alkanes: CnH2n+2, functional group: none (saturated hydrocarbons).
• Alkenes: CnH2n, functional group: carbon-carbon double bond (C=C).
• Alcohols: CnH2n+1OH, functional group: hydroxyl (-OH).
• Carboxylic acids: CnH2n+1COOH, functional group: carboxyl (-COOH).

Because the functional group is the reactive part of the molecule, compounds in the same homologous series undergo similar chemical reactions. For example, all alcohols can be oxidised to form aldehydes or carboxylic acids under suitable conditions. The presence of the same functional group means that reaction mechanisms and types of products formed are comparable across the series.

• Functional group controls chemical reactions.
• Similar reaction types within a series.
• Example: oxidation of alcohols to aldehydes or acids.

A saturated compound is an organic molecule in which all carbon-carbon bonds are single bonds. This means that each carbon atom forms four single covalent bonds, either with other carbon atoms or with hydrogen atoms, resulting in the maximum number of hydrogen atoms bonded to the carbon skeleton.

In contrast to unsaturated compounds, saturated compounds do not contain any double or triple bonds between carbon atoms. The absence of multiple bonds means the molecule is 'saturated' with hydrogen atoms, as no more can be added without breaking existing bonds.

• Saturated compounds contain only single C–C bonds.
• Each carbon atom is bonded to as many hydrogen atoms as possible.
• No carbon-carbon double (C=C) or triple (C≡C) bonds are present.

Saturated compounds are commonly represented by their molecular formula, structural formula, or displayed formula. The simplest saturated compounds belong to the alkane homologous series, which have the general formula CnH2n+2.

For example: Methane (CH4) is the simplest saturated hydrocarbon, with one carbon atom bonded to four hydrogens. Ethane (C2H6) has two carbon atoms connected by a single bond, each bonded to three hydrogens.

Structural formula of ethane: CH3–CH3, showing only single bonds between carbons and hydrogens.

• Alkanes are saturated hydrocarbons with formula CnH2n+2.
• Methane (CH4) and ethane (C2H6) are examples of saturated compounds.
• Structural formulae clearly show single bonds only.

The presence of only single bonds in saturated compounds makes them relatively chemically stable compared to unsaturated compounds. Single bonds are sigma (σ) bonds, which are strong and less reactive than pi (π) bonds found in double or triple bonds.

Because saturated compounds have no double or triple bonds, they do not undergo addition reactions typical of unsaturated compounds. Instead, they mainly undergo substitution and combustion reactions.

For example, alkanes combust in oxygen to produce carbon dioxide and water:

• Single C–C bonds (sigma bonds) confer stability.
• Saturated compounds do not undergo addition reactions.
• Typical reactions include substitution and combustion.

Ethane (C2H6) burns in oxygen to form carbon dioxide and water. The balanced chemical equation is:

\[ 2C_2H_6 (g) + 7O_2 (g) \rightarrow 4CO_2 (g) + 6H_2O (g) \]

This reaction demonstrates the complete combustion of a saturated hydrocarbon, producing carbon dioxide and water vapour as products.

• Combustion of ethane: 2C2H6 + 7O2 → 4CO2 + 6H2O
• Reaction requires sufficient oxygen and ignition energy.
• Produces carbon dioxide and water vapour.

Unlike saturated compounds, unsaturated compounds contain one or more carbon-carbon double or triple bonds. These multiple bonds reduce the number of hydrogen atoms bonded to the carbon skeleton.

Unsaturated compounds can undergo addition reactions where the double or triple bonds open up to add atoms, such as hydrogenation or halogenation.

Recognising the presence or absence of multiple bonds is key to identifying saturation in organic molecules.

• Saturated: only single C–C bonds; maximum hydrogens.
• Unsaturated: one or more C=C or C≡C bonds; fewer hydrogens.
• Saturation affects chemical reactivity and types of reactions.

An unsaturated compound is an organic molecule that contains at least one carbon-carbon multiple bond, which means one or more carbon-carbon bonds are double (C=C) or triple (C≡C) bonds rather than single (C–C) bonds.

This contrasts with saturated compounds, where all carbon-carbon bonds are single bonds only.

The presence of these multiple bonds affects the molecule's chemical properties and reactivity.

• Unsaturated compounds have one or more C=C or C≡C bonds.
• Saturated compounds have only C–C single bonds.

Unsaturated compounds contain carbon-carbon double bonds (alkenes) or triple bonds (alkynes).

A double bond consists of one sigma (σ) bond and one pi (π) bond, while a triple bond consists of one sigma bond and two pi bonds.

These multiple bonds are regions of higher electron density and are more reactive than single bonds.

• Double bond: C=C (alkenes)
• Triple bond: C≡C (alkynes)

The general formula for alkenes (unsaturated hydrocarbons with one double bond) is CnH2n.

For example, ethene (C2H4) has the structural formula CH2=CH2, showing the double bond between the two carbon atoms.

Alkynes have the general formula CnH2n-2, for example, ethyne (acetylene) C2H2 with the structural formula HC≡CH.

• Ethene: CH2=CH2 (alkene)
• Ethyne: HC≡CH (alkyne)

The presence of carbon-carbon multiple bonds allows unsaturated compounds to undergo addition reactions, where atoms add across the double or triple bond, converting it into single bonds.

For example, ethene reacts with bromine (Br2) in an addition reaction:

CH2=CH2 + Br2 → BrCH2–CH2Br (1,2-dibromoethane) (balanced equation)

• Addition reactions occur at C=C or C≡C bonds.
• Unsaturated compounds decolourise bromine water (orange to colourless).

Saturated compounds contain only single bonds between carbon atoms and do not readily undergo addition reactions.

Unsaturated compounds contain double or triple bonds and react with reagents such as bromine water, which is a common test for unsaturation.

The bromine test involves adding bromine water to the compound; a colour change from orange to colourless indicates unsaturation.

• Saturated: only single C–C bonds, no bromine decolourisation.
• Unsaturated: C=C or C≡C bonds, bromine water decolourises.

A structural formula is a graphical representation that shows how atoms are connected and arranged within a molecule. Unlike molecular formulas, which only indicate the number and type of atoms, structural formulas reveal the bonding pattern and the position of atoms, including single, double, or triple bonds.

This detailed depiction is essential for understanding the chemical behaviour and properties of molecules, as the arrangement of atoms influences reactivity and physical characteristics.

• Shows connectivity between atoms.
• Indicates types of bonds (single, double, triple).
• Clarifies molecular shape and functional groups.

Ethene is an unsaturated hydrocarbon with the molecular formula C2H4. Its structural formula is written as CH2=CH2, which shows a double bond between the two carbon atoms.

Each carbon atom is bonded to two hydrogen atoms and shares a double bond with the other carbon atom. The double bond consists of one sigma (σ) bond and one pi (π) bond, which restricts rotation and affects the molecule's reactivity, such as undergoing addition reactions.

The structural formula clearly distinguishes ethene from ethane (C2H6), which has only single bonds.

• Double bond between carbon atoms shown as '='.
• Each carbon bonded to two hydrogens.
• Indicates unsaturation and reactivity.

Ethanol is an alcohol with the molecular formula C2H6O. Its structural formula, CH3CH2OH, shows the arrangement of atoms: a methyl group (CH3) bonded to a methylene group (CH2), which is bonded to a hydroxyl group (–OH).

The –OH group is the functional group characteristic of alcohols and is responsible for ethanol's chemical properties, such as its ability to form hydrogen bonds and its solubility in water.

The structural formula helps distinguish ethanol from other isomers like dimethyl ether (CH3OCH3), which has the same molecular formula but a different arrangement of atoms.

• Shows –OH functional group attached to carbon chain.
• Indicates single bonds between atoms.
• Highlights alcohol functional group position.

Methyl acetate is an ester with the molecular formula C3H6O2. Its structural formula is CH3COOCH3, which reveals the ester functional group (–COO–) linking two alkyl groups.

In the formula, a methyl group (CH3) is bonded to a carbonyl group (C=O), which is connected via an oxygen atom to another methyl group (OCH3). This arrangement explains the molecule's properties, such as its characteristic fruity smell and its reactivity in hydrolysis reactions.

The structural formula distinguishes methyl acetate from other isomers like propanol (C3H7OH).

• Shows ester functional group (–COO–).
• Includes carbonyl (C=O) and oxygen linkage.
• Clarifies connectivity of alkyl groups.

Structural formulas are crucial for predicting and explaining chemical behaviour, as molecules with the same molecular formula can have different structures and properties (isomers).

They allow chemists to identify functional groups, understand reaction mechanisms, and communicate molecular information clearly.

In examinations, correctly drawing or interpreting structural formulas can demonstrate understanding of molecular structure and functional groups, which is essential for achieving high marks.

• Distinguishes isomers with same molecular formula.
• Identifies functional groups and bonding.
• Essential for understanding chemical properties and reactions.

Structural isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms, meaning their structural formulas are different. This difference in structure leads to variations in physical and chemical properties despite having identical numbers and types of atoms.

For example, two compounds with the molecular formula C4H10 can have different arrangements of carbon and hydrogen atoms, resulting in distinct compounds called structural isomers.

• Same molecular formula (e.g., C4H10)
• Different structural formula (different atom connectivity)
• Different physical and chemical properties

Butane (C4H10) exists as two structural isomers: n-butane and isobutane (methylpropane).

n-Butane has a straight chain of four carbon atoms: CH3–CH2–CH2–CH3.

Isobutane has a branched chain: (CH3)3CH, where three methyl groups are attached to a central carbon atom.

• Both have molecular formula C4H10
• Different structural formulas: straight chain vs branched chain
• Different boiling points and physical properties

Compounds with the molecular formula C4H8 can also show structural isomerism. Examples include:

Butene isomers (alkenes with a double bond in different positions): 1-butene (CH2=CH–CH2–CH3) and 2-butene (CH3–CH=CH–CH3).

Cyclobutane, a cyclic compound with four carbon atoms forming a ring (C4H8).

• Same molecular formula C4H8
• Different structures: alkenes with double bonds in different positions, and cyclic compounds
• Different chemical reactivity and physical properties

Structural isomers differ in their physical properties such as boiling point, melting point, and density due to differences in shape and intermolecular forces.

Chemical properties can also vary, especially when functional groups or bond types differ in position or connectivity.

For example, n-butane and isobutane have different boiling points: n-butane boils at approximately −0.5 °C, while isobutane boils at about −11.7 °C, reflecting the effect of branching on intermolecular forces.

• Physical properties affected by molecular shape and intermolecular forces
• Chemical properties influenced by atom connectivity and functional group position
• Branching generally lowers boiling points due to decreased surface area

Structural formulas clearly show the arrangement of atoms and bonds in a molecule, distinguishing isomers.

Displayed formulas or skeletal formulas can be used to illustrate differences between isomers.

For example, the displayed formula of n-butane shows a continuous chain of carbon atoms, whereas isobutane shows a central carbon bonded to three methyl groups.

• Use displayed or skeletal formulas to distinguish isomers
• Structural formulas reveal atom connectivity
• Essential for understanding differences in properties

A homologous series is a family of organic compounds that share the same functional group. The functional group is a specific atom or group of atoms responsible for the characteristic chemical reactions of the series. For example, the alcohols all contain the hydroxyl group (-OH) as their functional group, which defines their chemical behaviour.

Because all members have the same functional group, they exhibit similar chemical properties. This means they generally undergo the same types of chemical reactions, although the rate or conditions may vary slightly with chain length.

• Functional group defines chemical properties.
• Same functional group means similar chemical reactions.

Each homologous series has a general formula that applies to all its members. This formula expresses the ratio of atoms in the compounds. For example, alkanes have the general formula CnH2n+2, where n is the number of carbon atoms.

Members of a homologous series differ from each other by a -CH2- unit. This means that moving from one member to the next involves adding one carbon atom and two hydrogen atoms. For example, methane (CH4) to ethane (C2H6) differs by one -CH2- group.

This incremental change leads to a gradual increase in molecular size and mass along the series.

• General formula applies to all members.
• Difference between members is one -CH2- unit.
• Example: methane (CH4) to ethane (C2H6) differs by -CH2-.

Physical properties such as boiling point, melting point, and solubility show a regular trend within a homologous series. As the number of carbon atoms increases, the molecular mass and size increase, affecting these properties.

For example, boiling points increase with increasing chain length due to stronger van der Waals forces between larger molecules. This trend is predictable and helps identify unknown compounds.

Solubility in water generally decreases as the hydrocarbon chain length increases because the non-polar hydrocarbon part dominates over the polar functional group.

• Boiling point increases with chain length.
• Melting point shows a trend with molecular size.
• Solubility in water decreases as chain length increases.

Because all members have the same functional group, they undergo similar chemical reactions. For example, all alcohols can undergo oxidation reactions to form aldehydes or carboxylic acids under suitable conditions.

The chemical properties are largely determined by the functional group rather than the hydrocarbon chain. This similarity allows chemists to predict reactions of unknown members based on known ones.

Example: The combustion of alkanes follows the general equation: CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O (balanced for complete combustion).

• Chemical reactions depend on the functional group.
• Similar reactions occur across the series.
• Example: combustion of alkanes produces CO2 and H2O.

Consider the alkanes, which have the general formula CnH2n+2 and contain only single bonds between carbon atoms.

Methane (CH4) is the first member (n=1). Ethane (C2H6) is the next member, differing by one -CH2- unit.

Boiling points increase from methane (-161 °C) to ethane (-89 °C) and continue rising with longer chains due to increased van der Waals forces between molecules.

• Alkanes general formula: CnH2n+2.
• Members differ by -CH2- unit.
• Boiling point increases with chain length.