13.1 Formulas, functional groups and nomenclature
- Syllabus
- 9701–2028–2029
- Topic
- 13.1
- Level
- AS
Learning objectives
13.1.1Terms: hydrocarbon as a compound made up of C• Define: hydrocarbon as a compound made up of C and H atoms only13.1.2Alkanes are simple hydrocarbons with no• Understand: alkanes are simple hydrocarbons with no functional group13.1.3Compounds in the table on pages 29 and 30• Understand: the compounds in the table on pages 29 and 30 contain a functional group which dictates their physical/chemical properties13.1.4Interpret/use the general, structural,• Interpret/use: the general, structural, displayed and skeletal formulas of the classes of compound stated in the table on pages 29 and 3013.1.5Systematic nomenclature of simple aliphatic• Understand and use systematic nomenclature of simple aliphatic organic molecules with functional groups detailed in the table on pages 29 and 30, up to six carbon atoms (six plus six for esters, straight chains only for esters and nitriles)13.1.6The molecular and/or empirical formula• Deduce the molecular and/or empirical formula of a compound, given its structural, displayed or skeletal formula
A hydrocarbon contains carbon and hydrogen only
A hydrocarbon is a compound made up of carbon and hydrogen atoms only. This is a composition test: the molecule may contain single or multiple carbon–carbon bonds, but it must contain no other element.
Check every element symbol in the formula or structure. CH₄ and CH₂=CH₂ are hydrocarbons; CH₃CH₂OH, CH₃Cl and CH₃CN are not because each contains an element other than C or H.
Organic compound and hydrocarbon are not synonyms. Hydrocarbons are one subset of organic compounds.
Alkanes are simple hydrocarbons with no functional group
An alkane is a simple hydrocarbon with only C–C and C–H single bonds and no functional group. In an acyclic alkane, every carbon has four single bonds and the general formula is CₙH₂ₙ₊₂.
Ethane, CH₃CH₃, is an alkane. Ethene contains a C=C functional group, and ethanol contains an –OH functional group, so neither is an alkane even though each has a carbon chain.
“No functional group” does not mean “no bonds” or “incapable of reacting”. It distinguishes the alkane family from the functional-group families in the syllabus table; their reactions are taught later.
A functional group dictates characteristic properties
A functional group is the atom, bond or connected group of atoms that dictates the characteristic physical and chemical properties of an organic family. Molecules with the same functional group therefore show a shared pattern of reactions.
The group changes electron distribution and intermolecular attractions. For example, an alcohol –OH group can form hydrogen bonds, so an alcohol and a hydrocarbon with similar size need not have similar boiling points or water solubility.
The group also identifies the part of the molecule where characteristic chemistry occurs: C=C marks an alkene, –CHO an aldehyde and –COOH a carboxylic acid. Inspect connectivity, not just which elements are present.
The carbon skeleton can still influence a numerical property such as boiling point, and one molecule may contain more than one functional group. “Dictates” does not mean that the rest of the structure disappears.
Translate among general, structural, displayed and skeletal formulas
| Formula type | What it communicates | Example / reading rule |
|---|---|---|
| general | the atom-number pattern shared by a homologous series | acyclic alkanes: CₙH₂ₙ₊₂; alkenes with one C=C: CₙH₂ₙ |
| structural | connectivity in a compact line | CH₃CH(OH)CH₃ fixes the OH on carbon 2 |
| displayed | every atom and every covalent bond | use it to check the complete bonding around each atom |
| skeletal | the carbon framework without writing C atoms or H atoms bonded to C | every unlabelled line end and vertex is C; add enough H to each C for four bonds |
When converting, preserve the same atom connectivity, functional-group atoms and bond orders. In a skeletal formula, write heteroatoms and hydrogens attached to them, count each line end and vertex once, then infer only the hydrogens on carbon that are needed to complete valency four.
A molecular formula records atom counts but not connectivity, so it cannot by itself replace a structural, displayed or skeletal formula. Different structures can share one molecular formula.
Build systematic names from parent, position and functional group
Name simple aliphatic molecules in the syllabus families with up to six carbon atoms. Esters may contain up to six carbons on each side of –COO–; only straight-chain esters and nitriles are required.
- Identify the principal functional group. 2. Choose the longest parent chain that contains it and the required multiple bond. 3. Number from the end that gives the principal suffix the lowest locant; the carbon of –CHO, –COOH or –C≡N is carbon 1. 4. Add double-bond position and substituent prefixes with their locants. 5. Use commas between numbers and hyphens between numbers and words.
| Syllabus family | Name signal | Example |
|---|---|---|
| alkene | -ene | but-1-ene |
| halogenoalkane | fluoro-, chloro-, bromo-, iodo- | 1-bromopropane |
| alcohol | -ol | propan-2-ol |
| aldehyde | -al | propanal |
| ketone | -one | butan-2-one |
| carboxylic acid | -oic acid | propanoic acid |
| ester | O-side alkyl + acid-side alkanoate | methyl propanoate |
| primary amine | -amine | propan-1-amine |
| nitrile | -nitrile | butanenitrile |
For an ester, name the alkyl group attached directly to the single-bonded O first; then name the chain containing the carbonyl carbon as an alkanoate. CH₃CH₂COOCH₃ is methyl propanoate, not propyl methanoate.
Do not apply the straight-chain restriction to every family: the syllabus states it only for esters and nitriles. A locant describes a position; it does not change which atoms belong to the parent chain.
Count atoms first, then reduce only for an empirical formula
A molecular formula gives the actual number of each type of atom in one molecule. An empirical formula gives the simplest whole-number ratio of those atom counts.
From a structural or displayed formula, count every written atom. From a skeletal formula, count each unlabelled line end and vertex as a carbon, keep all labelled atoms, and add the hydrogens needed to give each carbon four bonds. Multiple bonds use two or three of those bonds. Finally, divide every molecular-formula subscript by their greatest common divisor only if an empirical formula is requested.
| Step for CH₃COOCH₂CH₃ | Result |
|---|---|
| count carbon atoms | 4 |
| count hydrogen atoms | 3 + 0 + 2 + 3 = 8 |
| count oxygen atoms | 2 |
| molecular formula | C₄H₈O₂ |
| divide 4:8:2 by 2 | empirical formula C₂H₄O |
Check that each carbon has total bond order four and that no vertex or line end was counted twice. Do not simplify C₄H₈O₂ when the question asks for the molecular formula.