Contents: 8 sections
Cambridge IGCSE Chemistry 0620 · Core and Extended
Syllabus points
- Name and draw the structures of methane, ethane, ethene, ethanol and ethanoic acid.
- State the type of compound present, given a name ending in -ane, -ene, -ol or -oic acid, or given a structure.
- Extended: name and draw the structures of unbranched alkanes, alkenes, alcohols and carboxylic acids containing up to four carbon atoms.
- Extended: name and draw the structures of the unbranched esters made from unbranched alcohols and carboxylic acids, each containing up to four carbon atoms.
A name is built in two pieces
Stem plus suffix. The stem counts the carbon atoms and the suffix names the functional group.
| Carbon atoms | Stem |
|---|---|
| 1 | meth |
| 2 | eth |
| 3 | prop |
| 4 | but |
| Functional group | Suffix |
|---|---|
| Alkane, only single bonds | -ane |
| Alkene, C=C | -ene |
| Alcohol, -OH | -ol |
| Carboxylic acid, -COOH | -oic acid |
| Ester, -COO- | -yl ...-oate |
So "propanol" splits as prop plus ol: three carbon atoms, alcohol. "Butanoic acid" splits as but plus oic acid: four carbon atoms in total, carboxylic acid. Reading a name in those two pieces answers the Core requirement of stating the type of compound from the name alone, and it works for a name you have never met.
The five compounds every candidate must know
| Name | Structural formula | Functional group |
|---|---|---|
| Methane | CH₄ | none |
| Ethane | CH₃CH₃ | none |
| Ethene | CH₂=CH₂ | C=C |
| Ethanol | CH₃CH₂OH | -OH |
| Ethanoic acid | CH₃COOH | -COOH |
Drawn out in full, ethene has four C-H bonds and one C=C, giving each carbon its four bonds. Ethanoic acid has a CH₃ group joined to a carbon that carries one double bonded oxygen and one O-H group. Writing the acid as CH₃COH or CH₃CHO names a different compound entirely, so both oxygens must appear.
Naming up to four carbons (Extended)
| n | Alkane CₙH₂ₙ₊₂ | Alkene CₙH₂ₙ | Alcohol CₙH₂ₙ₊₁OH | Acid CₙH₂ₙ₊₁COOH |
|---|---|---|---|---|
| 1 | Methane | (none) | Methanol | Methanoic acid |
| 2 | Ethane | Ethene | Ethanol | Ethanoic acid |
| 3 | Propane | Propene | Propan-1-ol | Propanoic acid |
| 4 | Butane | Butene | Butan-1-ol | Butanoic acid |
There is no alkene with one carbon, because a C=C bond needs two carbon atoms.
For a carboxylic acid the stem counts every carbon, including the one in the COOH group. Propanoic acid is CH₃CH₂COOH, with three carbons in total, not four.
Position numbers
From three carbons upwards the functional group can sit in more than one place, and the name has to say where. Number the carbon atoms along the chain and put the number in front of the suffix, choosing the end that gives the lowest number.
Alcohols.
- CH₃CH₂CH₂OH is propan-1-ol. The OH is on an end carbon.
- CH₃CH(OH)CH₃ is propan-2-ol. The OH is on the middle carbon.
Numbering propan-1-ol from the other end would call it propan-3-ol, which is wrong because 1 is lower than 3.
Alkenes.
- CH₂=CHCH₂CH₃ is but-1-ene. The double bond starts at carbon 1.
- CH₃CH=CHCH₃ is but-2-ene. The double bond starts at carbon 2.
The number given is the position of the first of the two carbons in the double bond.
Methanol, ethanol, ethene and propene need no number, because there is only one possible position. Carboxylic acids never need one either: the COOH group is always at the end of the chain, so it is carbon 1 by definition.
Naming esters (Extended)
An ester is made from an alcohol and a carboxylic acid, and its name is built from both, in this order:
alkyl group from the alcohol, then -oate from the acid.
Worked example. Ethanol plus ethanoic acid.
The alcohol is ethanol, which contributes an ethyl group. The acid is ethanoic acid, which contributes ethanoate. The ester is ethyl ethanoate, CH₃COOCH₂CH₃.
Read the formula from the middle outwards. The COO linkage sits between the two parts. The carbon on the left of the COO came from the acid; the group on the right of the second O came from the alcohol.
Three more, worth working through:
| Alcohol | Acid | Ester | Formula |
|---|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate | CH₃COOCH₃ |
| Ethanol | Methanoic acid | Ethyl methanoate | HCOOCH₂CH₃ |
| Propan-1-ol | Butanoic acid | Propyl butanoate | CH₃CH₂CH₂COOCH₂CH₂CH₃ |
Methyl ethanoate and ethyl methanoate are different compounds with the same molecular formula, C₃H₆O₂. Swapping the two halves of the name is the single most common error in this section, and it is examined precisely because it looks careless rather than difficult.
The check that catches it: the part of the ester name ending in -oate always contains the C=O group, and that carbon came from the acid.
Reading a structure backwards
Given a structure, work through three questions in order.
- Which functional group is present? A C=C gives -ene. An -OH gives -ol. A -COOH gives -oic acid. A -COO- in the middle of the chain gives an ester.
- How many carbons? Count every carbon in the chain, including any inside the functional group, and take the stem from the table.
- Does it need a position number? Only if the group could sit in more than one place.
Worked example. CH₃CH₂CH(OH)CH₃. There is an OH group, so it is an alcohol and the name ends in -ol. There are four carbons, so the stem is but. Numbering from the right-hand end puts the OH on carbon 2; from the left-hand end it would be carbon 3. The lower number wins, so the name is butan-2-ol.
Common mistakes
- Naming CH₃CH₂COOH as butanoic acid by forgetting that the COOH carbon counts.
- Writing ethanoic acid as CH₃CHO, which is an aldehyde, or as CH₃COH.
- Numbering from the wrong end and producing propan-3-ol or but-3-ene.
- Putting a position number on ethanol, ethene or any carboxylic acid.
- Reversing an ester name, so that ethanol and methanoic acid are said to give methyl ethanoate.
- Assuming "butene" is one compound. Without a number it could be but-1-ene or but-2-ene.