So far so good. But if this was the end of the story we still would not be here. Life as we know it would still be unsupportable.
There is no law that limits a carbon atom from bonding to more than two others. Bonds to three and even four other carbon atoms are well-known. We refer to such arrangements as branches and naming the products can be quite involved. For our purposes we will restrict ourselves to simple cases. You don’t really want to have to figure out the structure of methyl β-2,3,4,6-tetra-O-methyl-D-glucoside, do you?
First rules for naming carbon compounds.
1) Find the longest chain. It does not have to be the straightest chain, but it does have to be the longest. In the molecules below the longest chain is five carbons. (The two molecules are in fact identical.)
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Two views of 3-methylpentane,
condensed structural formula.
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2) If the compound contains a double or triple bond, the longest chain must include this bond, regardless of the rule 1, and the name must end with the corresponding suffix. (If a compound contains both a double and triple bond, or more than two of either all on separate branches, it violates my promise of simple cases and may assumed to be a typographical error.) In the structure below left, the double bond is already part of the longest chain. The name will end in pent-1-ene. In the structure below right, however, the double bond occurs on a four carbon chain. In this case the name would end in but-1-ene.
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3-methylpent-1-ene
(left) and 2-ethylbut-1-ene.
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3) Locate the branches, and start counting off carbons on the longest chain (as determined by rules 1 and 2) to provide for the lowest numbers for the branching carbons.
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CH3-CH2-CH-CH2-CH3 |
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CH3
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3-methylpentane.
In this example it makes no difference which end you start from. The branch comes from the third carbon from either end (C3, red).
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CH2=CH-CH-CH2-CH3 |
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CH3
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3-methylpent-1-ene.
Branch is still from C3, but double bond connects lowest possible numbers (C1 and C2), giving 3-methylpent-1-ene rather than 3-methylpent-4-ene.
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CH2=C-CH2-CH3 |
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CH2
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CH3
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2-ethylbut-1-ene.
Double bond to lowest numbers (C1 and C2 again). Branch is off C2.
4) List the branches in alphabetical order, that
is, ethyl before methyl,
but include the number of the carbon from which they branch.
5) If a length occurs more than once, give the
prefix indicating the number of occurences after listing the branching
carbons. The prefixes do not effect the alphabetisation in Rule 4. You
still list ethyl before dimethyl.
6) There is no need to include redundancy. The 1’s in eth-1-ene and prop-1-ene are redundant. So is the ‘3,3’ in 3,3-diethylpentane. The names ethene, propene and diethylpentane carry just as much information. If this is not clear, try drawing a propene that isn’t prop-1-ene or a diethylpentane that where the ethyl branches are not on C3. If you think you have done this, check to make sure you have found the longest chain.
Before going on let’s go back and work out the names of the example structures below.
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Top, left is a hept-1-ene with methyl branches on C4 and C5; 4,5-dimethylhept-1-ene.
Top, right is a heptane with methyl branches on C3 and C4; 3,4-dimethylheptane.
For the bottom, would you accept 2-ethyl-3-methylhex-1-ene?
Practice: These compounds are all dimethylpentanes.
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Find the longest chain in each. It is five carbons. Locate the two methyl groups. Structure (a) is 2,2-dimethylpentane.
Problem 4. Name the other three structures systematically.
Problem 5. Why isn’t 3,4-dimethylpentane included?
Related Topics: Alternative Naming Exercises
Next page: Family Traits