IUPAC naming of alkanes
Find the longest carbon chain for the parent name (meth, eth, prop, but, pent, hex...). Number from the end that gives the lowest numbers to branches. Name each branch (methyl, ethyl) with its number and list them alphabetically.
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Analogy
Like a postal address: the number says exactly which house on the street.
Example
Longest chain: 4 carbons (butane)One CH₃ branch on carbon 2Name: 2-methylbutaneFormula: C₅H₁₂
In practice
A clear name lets a chemist anywhere draw the exact molecule from words alone.
Practice
Flashcards
- What is the first step in naming an alkane?Find the longest carbon chain.
Interview questions
- Draw and name the isomers of C₅H₁₂ and explain how branching affects boiling point.
- Three isomers: pentane, 2-methylbutane, 2,2-dimethylpropane
- More branching, more compact shape
- Less surface contact, weaker London forces
- Boiling points fall as branching increases
Questions people ask
Why do we number to give the lowest numbers to branches?
The rules need one agreed name for each molecule. Numbering from the end that gives the lowest first point of difference removes the choice. For a methyl on the second carbon from one end and the fourth from the other, we call it 2-methyl, not 4-methyl.
What if there are two of the same branch?
Use di, tri or tetra before the branch name and give each position. Two methyl groups on carbons 2 and 3 of butane make 2,3-dimethylbutane. The prefix di is ignored when ordering alphabetically, but the letters in ethyl or methyl count.