i) Name this molecule:
– 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
– 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.
vi) What are the isomers of dibromopropane ?