11.1 Formulae, functional groups and terminology
- Syllabus
- 0620–2026–2027
- Topic
- 11.1
- Level
- —
Learning objectives
11.1.1And interpret the displayed formula of• Draw and interpret the displayed formula of a molecule to show all the atoms and all the bonds11.1.2Write and interpret general formulae• Write and interpret general formulae of compounds in the same homologous series, (a) alkanes, C nH2n+2 (b) alkenes, CnH2n (c) alcohols, C nH2n+1OH (d) carboxylic acids, CnH2n+1COOH11.1.3Functional group as an atom or group• Identify a functional group as an atom or group of atoms that determine the chemical properties of a homologous series11.1.7Structural formula is an unambiguous• State: a structural formula is an unambiguous description of the way the atoms in a molecule are arranged, including CH 2=CH2, CH3CH2OH, CH3COOCH311.1.8Structural isomers as compounds with• Define structural isomers as compounds with the same molecular formula, but different structural formulae, including C4H10 as CH3CH2CH2CH3 and CH3CH(CH3)CH3 and C4H8 as CH3CH2CH=CH2 and CH3CH=CHCH311.1.4Homologous series is a family of• State: a homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group11.1.5Saturated compound has molecules in• State: a saturated compound has molecules in which all carbon–carbon bonds are single bonds11.1.6An unsaturated compound has molecules• State: an unsaturated compound has molecules in which one or more carbon– carbon bonds are not single bonds11.1.9Homologous series features: same• Describe homologous series features: same functional group and general formula, adjacent members differ by -CH2-, physical properties show trends, chemical properties are similar
Draw and interpret displayed formulae
A displayed formula shows every atom and every covalent bond in a molecule. Each line represents one shared pair of electrons in a covalent bond.
Displayed formula of methane:
H
|
H—C—H
|
H
Displayed formula of ethene:
H H
| |
C=C
| |
H H
The double line shows the carbon–carbon double bond.
First place the atoms in the required arrangement, then add bonds until carbon has four bonds, hydrogen one bond, oxygen two bonds and halogens one bond. Finally count both atoms and bonds against the molecular formula.
A structural formula such as CH₃CH₂OH does not show every individual C–H bond, so it is not a displayed formula.
Use general formulae for four homologous series
| Homologous series | General formula | Example when n = 3 |
|---|---|---|
| alkanes | CₙH₂ₙ₊₂ | C₃H₈ |
| alkenes | CₙH₂ₙ | C₃H₆ |
| alcohols | CₙH₂ₙ₊₁OH | C₃H₇OH |
| carboxylic acids | CₙH₂ₙ₊₁COOH | C₃H₇COOH |
A general formula represents every member of one homologous series. Substitute the stated value of n, then simplify the number of atoms without changing the functional group.
In CₙH₂ₙ₊₁COOH, n counts the carbon atoms in the alkyl part before COOH. The COOH group contributes one additional carbon atom to the complete molecule.
Do not use CₙH₂ₙ for every molecule containing a double bond: here it is the general formula for the alkene homologous series. Keep CₙH₂ₙ₊₁OH and CₙH₂ₙ₊₁COOH intact when substituting n.
Identify the functional group
A functional group is an atom or group of atoms that determines the characteristic chemical properties of a homologous series.
| Homologous series | Functional group | Structural clue |
|---|---|---|
| alkenes | carbon–carbon double bond | C=C |
| alcohols | hydroxyl group | –OH |
| carboxylic acids | carboxyl group | –COOH |
Scan the structure for the characteristic group before using the rest of the carbon chain. Molecules with the same functional group normally undergo similar types of chemical reaction.
The functional group is not the whole molecule and is not chosen merely because an element is present. For example, the –OH arrangement identifies an alcohol functional group.
Read and write structural formulae
A structural formula is an unambiguous description of how the atoms in a molecule are arranged. It groups atoms to show connectivity without drawing every bond.
| Molecule type | Structural formula | Arrangement shown |
|---|---|---|
| alkene | CH₂=CH₂ | two carbon atoms joined by C=C |
| alcohol | CH₃CH₂OH | two-carbon chain ending in –OH |
| ester | CH₃COOCH₃ | CH₃COO– joined to CH₃ |
Read from left to right and use brackets for branches when needed. The order of grouped atoms must preserve which atoms are bonded to which.
A molecular formula gives only the number of each type of atom. A structural formula adds the arrangement, but unlike a displayed formula it need not draw every C–H bond.
Recognise and construct structural isomers
Structural isomers are compounds with the same molecular formula but different structural formulae.
| Molecular formula | Structural isomer 1 | Structural isomer 2 | Difference |
|---|---|---|---|
| C₄H₁₀ | CH₃CH₂CH₂CH₃ | CH₃CH(CH₃)CH₃ | straight and branched carbon skeletons |
| C₄H₈ | CH₃CH₂CH=CH₂ | CH₃CH=CHCH₃ | different position of C=C |
To test a proposed pair, count every atom to confirm the molecular formulae are identical, then compare connectivity to confirm the structural formulae differ.
To generate another isomer, change the carbon skeleton, the position of a functional group or double bond, or—where allowed—the functional group, while preserving the exact atom count and valid valencies.
Different drawings of the same connectivity are not different structural isomers. Compounds with only the same general formula, but different molecular formulae, are homologues rather than isomers.
Define a homologous series
A homologous series is a family of similar compounds with similar chemical properties because they contain the same functional group.
The shared functional group controls the characteristic reactions. The remaining carbon chain can change in length without changing the family identity.
| Evidence | Same homologous series? |
|---|---|
| same functional group and fits the same general formula | yes |
| same number of carbon atoms only | not enough |
| similar physical state only | not enough |
Members do not need identical physical properties or the same molecular formula. The defining chemical similarity comes from the same functional group.
Recognise saturated compounds
A saturated compound has molecules in which all carbon–carbon bonds are single bonds.
Inspect only the bonds between carbon atoms. If every C–C bond is a single bond, the molecule is saturated under this definition.
Ethane, CH₃CH₃, is saturated because its two carbon atoms are joined by a single bond.
Saturated does not mean that every bond in the molecule is a C–C bond; it means there is no carbon–carbon double or triple bond.
Recognise unsaturated compounds
An unsaturated compound has one or more carbon–carbon bonds that are not single bonds.
Look for a carbon–carbon double bond, C=C, or carbon–carbon triple bond, C≡C. Finding either makes the compound unsaturated.
Ethene, CH₂=CH₂, is unsaturated because it contains a C=C bond.
A double bond such as C=O does not by itself meet this carbon–carbon definition. The non-single bond must be between two carbon atoms.
Describe all features of a homologous series
| Feature | What it means |
|---|---|
| same functional group | characteristic reactions are similar |
| same general formula | one expression represents every member |
| neighbouring members differ by –CH₂– | molecular mass increases by 14 each step |
| trend in physical properties | properties such as boiling point change gradually |
| similar chemical properties | members undergo the same characteristic types of reaction |
When comparing adjacent members, add one carbon and two hydrogens. Predict a gradual physical-property change, but keep the functional group and characteristic chemistry unchanged.
Physical properties show a trend rather than being identical. Chemical properties are similar because the functional group is shared.
Successive members differ by CH₂, not CH₃. They share a general formula, not one identical molecular formula.