Introduction to Organic Chemistry
Senior 3 · Chemistry
Introduction to Organic Chemistry
Lesson 3 of 3
25 min
The small alcohol wipes used before an injection are soaked in "propan-2-ol". To a chemist anywhere in the world, that name is a complete description: prop means three carbon atoms, an means single bonds only, ol means an alcohol, and 2 means the group is on the middle carbon. The rules that build such names are set by the International Union of Pure and Applied Chemistry, IUPAC, and at this level they need only three steps.
Find the longest continuous chain of carbon atoms (it must contain the functional group). Its length gives the stem.
Table 3.1: Stems for chains of 1 to 10 carbon atoms
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| stem | meth | eth | prop | but | pent | hex | hept | oct | non | dec |
Table 3.2: Endings (suffixes)
| Series | Ending | Example |
|---|---|---|
| alkane | -ane | propane, |
| alkene | -ene | propene, |
| alkyne | -yne | propyne, |
| alcohol | -anol | propanol, |
| carboxylic acid | -anoic acid | propanoic acid, |
From four carbons for alkenes and alkynes, and from three for alcohols, the functional group can sit in different places. Number the chain from the end nearer the functional group, and put the lowest possible number just before the ending (Figure 3.1). In a carboxylic acid the carbon is always carbon 1, so no number is needed.
But-1-ene and but-2-ene have the same molecular formula, , but different structures: they are isomers. So are propan-1-ol and propan-2-ol.
Definition
IUPAC name: the systematic name of a compound, built from a stem (number of carbons), an ending (the homologous series) and position numbers. Example: pent-2-yne is a five-carbon chain with a triple bond between carbons 2 and 3.
Isomers: compounds with the same molecular formula but different structures. Example: but-1-ene and but-2-ene, both .
Worked example
Problem. Name these compounds: (a) ; (b) ; (c) ; (d) ; (e) ; (f) .
Solution. (a) four carbons, single bonds: butane. (b) four carbons, C=C between carbons 2 and 3: but-2-ene. (c) three carbons, OH on an end carbon: propan-1-ol. (d) OH on the middle carbon: propan-2-ol. (e) three carbons including the COOH carbon: propanoic acid. (f) four carbons, triple bond starting at carbon 1: but-1-yne.
Worked example
Problem. Draw (as condensed formulas) and give the molecular formula of (a) pent-1-ene; (b) hexan-1-ol; (c) methanoic acid.
Solution. (a) , . (b) , . (c) HCOOH: one carbon, which is the carboxyl carbon, .
Table 3.3: Organic compounds in daily life
| Area | Examples |
|---|---|
| fuels | methane (natural gas, biogas), propane and butane (cooking gas, LPG), petrol, diesel and kerosene (mixtures of alkanes), ethanol |
| food | carbohydrates, fats and oils, proteins and vitamins; ethanoic acid in vinegar |
| medicines and health | paracetamol, aspirin and antibiotics; ethanol and propan-2-ol as antiseptics |
| materials | plastics such as poly(ethene) and PVC, synthetic fibres, rubber, paints and dyes |
| cleaning | soaps and detergents |
| agriculture | urea fertiliser, pesticides and herbicides |
Organic chemistry also brings problems to solve. Plastics made from alkenes can last for hundreds of years as litter, which is why Rwanda's Law N° 17 of 2019 prohibits plastic carry bags and single-use plastic items, and burning fuels releases carbon dioxide (Unit 1).
Warning
Common mistakes. (1) Number the chain from the end that gives the functional group the lowest number: is but-1-ene, not but-3-ene. (2) The longest chain must be continuous; a chain drawn with a bend is still one chain. (3) Count the carbon of the group in the stem: is ethanoic acid (two carbons), not methanoic acid.
Tip
"Monkeys Eat Peeled Bananas": meth, eth, prop, but for 1 to 4 carbons; from 5 on, the stems are the number prefixes pent, hex, hept, oct, non, dec.
Practice
Further reading
Sources and credits
