Answers to Organic Basics questions

i) Name this molecule:3methylpentane

– the longest carbon chain is 5 long, so it’s a pentane

– there is a methyl group on the 3rd carbon, so it 3-methylpentane


ii) Why can’t we have 5-methylheptane?

– because if we number from the other end of the chain we’d get a smaller number in the name. Its correctly named 2-methylheptane

or 1-methylhexane?

– because a methyl group on the first carbon of a chain just means the chain is one carbon longer than you thought ! It would be heptane.


iii) What’s wrong with calling a molecule 2-ethylpentane?

– an ethyl group on the second carbon means you haven’t found the longest chain. In fact the ethyl group is part of the main chain and you have a methyl group on the second carbon, meaning the molecule is actually 2-methylhexane.


iv) What’s wrong with calling a molecule trimethylhexane?

– you don’t know where the three methyl groups go. There are lots of possible molecules all of which could be called trimethylhexane, such as 2,2,3-trimethylhexane, or 2,3,4-trimethylhexane etc.


v) How many different alkanes with C6H14 molecular formula can you make? Draw their skeletal formulae and name them.


isomers of hexane


vi) What are the isomers of dibromopropane ?