Stem + suffix builds every name
The stem gives the carbon count: meth- = 1, eth- = 2, prop- = 3, but- = 4. The suffix gives the compound type: -ane = alkane (all carbon-carbon bonds single), -ene = alkene (at least one C=C double bond), -ol = alcohol (an hydroxyl group). Read both halves: propene = 3 carbons plus a C=C; butane = 4 carbons, all single. Core requires methane, ethane, ethene and ethanol by name.
(Extended) Structural vs displayed formula
A structural formula groups atoms along the chain in shorthand -- butane as -- showing the arrangement but not every individual bond. A displayed formula (Core) draws every atom separately and every bond as an explicit line, a double bond as two parallel lines. Both convey the arrangement of atoms; a bare molecular formula such as gives only atom counts.
(Extended) Lowest locant names but-1-ene vs but-2-ene
A four-carbon alkene, , has two unbranched structures, so the name must state where the C=C sits. Number the chain from whichever end gives the double bond the lowest possible number: but-1-ene has the C=C between carbons 1 and 2, but-2-ene between carbons 2 and 3. Same molecular formula and same (56), different structures -- they are structural isomers. 0654 does not require the cis/trans distinction.
Drawn from real examiner reports.
-ane means single, -ene means double
-ane names an alkane: every carbon-carbon bond is single (saturated). -ene names an alkene: at least one C=C double bond (unsaturated). Reversing them -- calling propane unsaturated, or propene all-single -- is a documented examiner complaint, and saturated is also confused with water content. The check is one letter: -ane = all single; -ene = a double bond.
Flagged Jun 2022 P41 Q2e; Nov 2023 P42 Q2c
Ethanol is not a hydrocarbon
A hydrocarbon contains only carbon and hydrogen, and it is a compound -- not an element, atom or mixture. Ethanol, , carries oxygen in its group, so it is an alcohol, not a hydrocarbon, despite sharing the eth- stem with ethane. Classifying from a molecular formula, scan for an O first: if one is there, hydrocarbon is ruled out.
Flagged Nov 2022 P32 Q8c; Nov 2023 P41 Q2a
(Extended) Writing "butene" with no position number
A four-carbon alkene has two unbranched structures, so "butene" alone does not identify a compound. It earns partial credit at best: the alkene family is right, but the locant that separates but-1-ene from but-2-ene is missing. Statement 3(b) requires the number as part of the name -- match the amount of detail you give to the marks on offer.
Miscounting carbons gives the wrong stem
The stem is fixed by the number of carbon atoms, so a miscount renames the compound entirely. read as one carbon becomes "methane" instead of ethane; read as four carbons becomes "butane" instead of propane. Count the carbon subscript in the formula, or the carbons in a displayed chain, before choosing the stem.
A double bond drawn as one line
In a displayed formula a C=C double bond must be drawn as two parallel lines. A single line turns ethene into an ethane skeleton, or but-1-ene into butane, and the mark is lost. Examiners specifically report candidates failing to show the double bond when asked to draw ethene, so check the bond lines against the suffix before moving on.
Flagged Jun 2023 P32 Q8b
Ethanol O-H merged into the chain
The displayed formula of ethanol must show the oxygen as its own atom, bonded to a carbon on one side and to a hydrogen on the other, as . The usual errors are absorbing the oxygen into the carbon chain, or omitting the O-H bond line so the hydroxyl hydrogen is left floating unattached to anything.
(Extended) Different compounds, identical Mr
Because but-1-ene and but-2-ene are different compounds, candidates assume their relative formula masses must differ too. depends only on the atom counts in the molecular formula, and both compounds are : for each. Only the position of the double bond differs, not the mass.
Stem, then suffix, then position
Fixed order: 1) Count the carbons for the stem: 1 = meth-, 2 = eth-, 3 = prop-, 4 = but-. 2) Check bonding for the suffix: all single = -ane; a C=C = -ene; an = -ol. 3) For a four-carbon alkene, number the chain to give the double bond its lowest position.
Check for oxygen before "hydrocarbon"
Given a molecular formula, look for an O before classifying. An O with a plausible group means alcohol. Only C and H means hydrocarbon -- then compare hydrogens per carbon to judge saturation, since an alkane carries more hydrogens than an alkene of the same length.
Displayed means every atom, every bond
If the question says displayed, use no shorthand: draw each atom separately, never a grouped . Draw every bond as a line, including the C-H bonds on the end carbons, which are most often left out. Double bonds get two lines, in the position the name specifies.
Answer the command word, to the tariff
"Name" means the full name, position number included; "state the type of compound" means alkane, alkene or alcohol, not a formula. Give as many points as marks, and keep every point on the question actually asked -- scientifically true but off-question chemistry scores nothing.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.2 "Naming organic compounds". Core: name and draw the displayed formulas of methane, ethane, ethene and ethanol; state the type of compound present given a chemical name ending in -ane, -ene or -ol, or from a molecular/displayed formula. Supplement (Extended): name and draw the structural formulas and displayed formulas of unbranched alkanes and alkenes -- including but-1-ene and but-2-ene (not cis/trans) -- containing up to four carbon atoms per molecule.
| Term | Mark-scheme-precise meaning |
|---|---|
| Structural formula (Ext) | Shows how the atoms in a molecule are grouped along the chain, using shorthand groups (e.g. ), without drawing every individual bond |
| Displayed formula | Shows all the atoms in a molecule and all the bonds between them, drawn out individually (a double bond = two parallel lines) |
| Alkane (-ane suffix) | A saturated hydrocarbon: all carbon-carbon bonds are single bonds |
| Alkene (-ene suffix) | An unsaturated hydrocarbon: contains at least one carbon-carbon double bond (C=C) |
| Alcohol (-ol suffix) | Contains a hydroxyl () group; NOT a hydrocarbon, because it contains oxygen as well as carbon and hydrogen |
Naming stems (number of carbon atoms):
| Stem | Carbon atoms |
|---|---|
| meth- | 1 |
| eth- | 2 |
| prop- | 3 |
| but- | 4 |
Named compounds required by C11.2:
| Name | Molecular formula | Compound type | Tier |
|---|---|---|---|
| Methane | Alkane | Core | |
| Ethane | Alkane | Core | |
| Ethene | Alkene | Core | |
| Ethanol | Alcohol | Core | |
| Propane | Alkane | Extended | |
| Butane | Alkane | Extended | |
| Propene | Alkene | Extended | |
| But-1-ene | Alkene (double bond at C1-C2) | Extended | |
| But-2-ene | Alkene (double bond at C2-C3) | Extended |
Classifying a compound type from its formula (statement 2): check for oxygen first -- if the formula contains an O, and there is a plausible group, the compound is an alcohol (e.g. ethanol, ), NOT a hydrocarbon. If the formula contains only C and H, it is a hydrocarbon: count the hydrogens relative to the carbons to judge saturation (an alkane has more hydrogens per carbon than an alkene of the same chain length -- specific general formulas do not need to be recalled, but comparing H counts for a given formula is a useful check).
(Extended) Define "structural formula".
A compound has the molecular formula , and its displayed formula shows only single bonds throughout.
(a) Name this compound. (1 mark)
(b) Calculate its relative formula mass, . (2 marks)
