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Introduction to Organic Chemistry: Question 1

Syllabus 13.1

Multiple choice AS 1 mark

An organic chemist draws the structural formula of a branched, brominated alkane:

CH3CH(CH3)CH2CHBrCH3\text{CH}_3\text{CH(CH}_3\text{)CH}_2\text{CHBrCH}_3

What is the correct IUPAC name for this compound?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the longest carbon chain

Writing out the five carbons in order, left to right:

CH3⏟a−CH(CH3)⏟b−CH2⏟c−CHBr⏟d−CH3⏟e\underbrace{\text{CH}_3}_{a}-\underbrace{\text{CH(CH}_3\text{)}}_{b}-\underbrace{\text{CH}_2}_{c}-\underbrace{\text{CHBr}}_{d}-\underbrace{\text{CH}_3}_{e}

The longest continuous chain running through carbons aa-ee has five carbons, so the parent name is pentane. There is a methyl branch on carbon bb and a bromine atom on carbon dd.

Step 2: Number the chain in both directions

  • Numbering from the aa end: a=1, b=2, c=3, d=4, e=5a=1,\ b=2,\ c=3,\ d=4,\ e=5. Methyl is at locant 22, bromo is at locant 44. Locant set ={2,4}=\{2,4\}.
  • Numbering from the ee end: e=1, d=2, c=3, b=4, a=5e=1,\ d=2,\ c=3,\ b=4,\ a=5. Bromo is at locant 22, methyl is at locant 44. Locant set ={2,4}=\{2,4\}.

Both directions give the same locant set, {2,4}\{2,4\}, so there is a tie. Comparing locant sets alone does not decide the numbering direction.

Step 3: Break the tie using the alphabetical rule

When two numbering directions give an identical locant set, the lower locant is assigned to whichever substituent is cited first in alphabetical order. Comparing “bromo” and “methyl”, b comes before m, so bromo must get the lower locant.

This means the chain must be numbered from the ee end, giving bromo the locant 22 and methyl the locant 44.

Step 4: Assemble the name

Substituents are listed alphabetically in the name (bromo before methyl), each with its correct locant:

2-bromo-4-methylpentane\textbf{2-bromo-4-methylpentane}

Why the other options are wrong

  • B, 4-bromo-2-methylpentane, numbers the chain the other way round, giving the lower locant (2) to methyl and the higher locant (4) to bromo. This ignores the alphabetical tie-break rule, which requires bromo (cited first alphabetically) to get the lower locant.
  • C, 4-methyl-2-bromopentane, actually uses the correct locants (bromo at 2, methyl at 4) but cites the substituents in the wrong order in the name; IUPAC names always list substituent prefixes alphabetically, so “bromo” must be written before “methyl” regardless of which locant each one carries.
  • D, 2-bromo-4-methylhexane, incorrectly treats the parent chain as six carbons (hexane) rather than five (pentane).

Final answer

A, 2-bromo-4-methylpentane.