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The alkanes
Example 1: Write the structural formula for 2-methylpentane.
Start decoding the name from the bit that counts the number of carbon atoms
in the longest chain - pent counts 5 carbons.
Are there any carbon-carbon double bonds? No - an tells you there aren't any.
Now draw this carbon skeleton:
Put a methyl group on the number 2 carbon atom:
Does it matter which end you start counting from? No - if you counted from the
other end, you would draw the next structure. That's exactly the same as the
first one, except that it has been flipped over.
Finally, all you have to do is to put in the correct number of hydrogen atoms on
each carbon so that each carbon is forming four bonds.
If you had to name this yourself:
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Count the longest chain of carbons that you can find. Don't assume that
you have necessarily drawn that chain horizontally. 5 carbons means
pent.
Are there any carbon-carbon double bonds? No - therefore pentane.
There's a methyl group on the number 2 carbon - therefore 2methylpentane. Why the number 2 as opposed to the number 4 carbon?
In other words, why do we choose to number from this particluar end?
The convention is that you number from the end which produces the
lowest numbers in the name - hence 2- rather than 4-.
Example 2: Write the structural formula for 2,3-dimethylbutane.
Start with the carbon backbone. There are 4 carbons in the longest chain (but)
with no carbon-carbon double bonds (an).
This time there are two methyl groups (di) on the number 2 and number 3
carbon atoms.
Completing the formula by filling in the hydrogen atoms gives:
Example 3: Write the structural formula for 2,2-dimethylbutane.
This is exactly like the last example, except that both methyl groups are on the
same carbon atom. Notice that the name shows this by using 2,2- as well as di.
The structure is worked out as before:
Example 4: Write the structural formula for 3-ethyl-2-methylhexane.
hexan shows a 6 carbon chain with no carbon-carbon double bonds.
This time there are two different alkyl groups attached - an ethyl group on the
number 3 carbon atom and a methyl group on number 2.
Filling in the hydrogen atoms gives:
If you had to name this yourself:
How do you know what order to write the different alkyl groups at the beginning
of the name? The convention is that you write them in alphabetical order.
The cycloalkanes
In a cycloalkane the carbon atoms are joined up in a ring - hence cyclo.
Example: Write the structural formula for cyclohexane.
hexan shows 6 carbons with no carbon-carbon double bonds. cyclo shows that they are in a ring.
Drawing the ring and putting in the correct number of hydrogens to satisfy the bonding requirements
of the carbons gives:
The alkenes
Example 1: Write the structural formula for propene.
prop counts 3 carbon atoms in the longest chain. en tells you that there is a
carbon-carbon double bond. That means that the carbon skeleton looks like
this:
Putting in the hydrogens gives you:
Example 2: Write the structural formula for 1 - butene.
but counts 4 carbon atoms in the longest chain and en tells you that there is a
carbon-carbon double bond. The number in the name tells you where the
double bond starts.
No number was necessary in the propene example above because the double
bond has to start on one of the end carbon atoms. In the case of butene,
though, the double bond could either be at the end of the chain or in the middle
- and so the name has to code for the its position.
The carbon skeleton is:
And the full structure is:
Incidentally, you might equally well have decided that the right-hand carbon
was the number 1 carbon, and drawn the structure as:
Compounds containing halogens
Example 1: Write the structural formula for 1,1,1-trichloroethane.
This is a two carbon chain (eth) with no double bonds (an). There are three
chlorine atoms all on the first carbon atom.
Example 2: Write the structural formula for 2-bromo-2-methylpropane.
First sort out the carbon skeleton. It's a three carbon chain with no double
bonds and a methyl group on the second carbon atom.
Draw the bromine atom which is also on the second carbon.
And finally put the hydrogen atoms in.
If you had to name this yourself:
Notice that the whole of the hydrocarbon part of the name is written together as methylpropane - before you start adding anything else on to the name
Alcohols
All alcohols contain an -OH group. This is shown in a name by the ending ol.
Example 1: Write the structural formula for methanol.
The ol ending shows it's an alcohol and so contains an -OH group.
Example 2: Write the structural formula for 2-methylpropan-1-ol.
The carbon skeleton is a 3 carbon chain, with a methyl group on the number 2
carbon.
The -OH group is attached to the number 1 carbon.
The structure is therefore:
Aldehydes
All aldehydes contain the group:
If you are going to write this in a condensed form, you write it as -CHO - never
as -COH, because that looks like an alcohol.
The names of aldehydes end in al.
Example 1: Write the structural formula for propanal.
This is a 3 carbon chain with no carbon-carbon double bonds. The al ending
shows the presence of the -CHO group. The carbon in that group counts as
one of the chain.
Example 2: Write the structural formula for 2-methylpentanal.
This time there are 5 carbons in the longest chain, including the one in the CHO group. There aren't any carbon-carbon double bonds. A methyl group is
attached to the number 2 carbon. Notice that in aldehydes, the carbon in the CHO group is always counted as the number 1 carbon.
Ketones
Ketones contain a carbon-oxygen double bond just like aldehydes, but this time
it's in the middle of a carbon chain. There isn't a hydrogen atom attached to the
group as there is in aldehydes.
Ketones are shown by the ending one.
Example 1: Write the structural formula for propanone.
This is a 3 carbon chain with no carbon-carbon double bond. The carbonoxygen double bond has to be in the middle of the chain and so must be on the
number 2 carbon.
Ketones are often written in this way to emphasize the carbon-oxygen double
bond.
Example 2: Write the structural formula for 3 - pentanone.
This time the position of the carbon-oxygen double bond has to be stated
because there is more than one possibility. It's on the third carbon of a 5
carbon chain with no carbon-carbon double bonds. If it was on the second
carbon, it would be pentan-2-one.
This could equally well be written:
Carboxylic acids
Carboxylic acids contain the -COOH group, which is better written out in full as:
Carboxylic acids are shown by the ending oic acid. When you count the carbon
chain, you have to remember to include the carbon in the -COOH group. That
carbon is always thought of as number 1 in the chain.
Example 1: Write the structural formula for 3-methylbutanoic acid.
This is a four carbon acid with no carbon-carbon double bonds. There is a
methyl group on the third carbon (counting the -COOH carbon as number 1).
Esters
Esters are one of a number of compounds known collectively as acid
derivatives. In these the acid group is modified in some way. In an ester, the
hydrogen in the -COOH group is replaced by an alkyl group (or possibly some
more complex hydrocarbon group).
Example 1: Write the structural formula for methyl propanoate.
An ester name has two parts - the part that comes from the acid (propanoate)
and the part that shows the alkyl group (methyl).
Start by thinking about propanoic acid - a 3 carbon acid with no carbon-carbon
double bonds.
The hydrogen in the -COOH group is replaced by an alkyl group - in this case, a
methyl group.
Ester names are confusing because the name is written backwards from the
way the structure is drawn. There's no way round this - you just have to get
used to it!
In the shortened version, this formula would be written CH3CH2COOCH3.
Example 2: Write the structural formula for ethyl ethanoate.
This is probably the most commonly used example of an ester. It is based on
ethanoic acid ( hence, ethanoate) - a 2 carbon acid. The hydrogen in the COOH group is replaced by an ethyl group.
Make sure that you draw the ethyl group the right way round. A fairly common
mistake is to try to join the CH3 group to the oxygen. If you count the bonds if
you do that, you will find that both the CH3 carbon and the CH2 carbon have the
wrong number of bonds.
Primary amines
A primary amine contains the group -NH2 attached to a hydrocarbon chain or
ring. You can think of amines in general as being derived from ammonia, NH3.
In a primary amine, one of the hydrogens has been replaced by a hydrocarbon
group.
Example 1: Write the structural formula for ethylamine.
In this case, an ethyl group is attached to the -NH2 group.
This name (ethylamine) is fine as long as you've only got a short chain where
there isn't any ambiguity about where the -NH2 group is found. But suppose you
had a 3 carbon chain - in this case, the -NH2 group could be on an end carbon
or on the middle carbon. How you get around that problem is illustrated in the
next example.
Example 2: Write the structural formula for 2-aminopropane.
The name shows a 3 carbon chain with an amino group attached to the second
carbon. amino shows the -NH2 group.
Ethylamine (example 1 above) could equally well have been called
aminoethane.
Secondary and tertiary amines
Example 1: Write the structural formula for dimethylamine.
In this case, two of the hydrogens in ammonia have been replaced by methyl
groups.
Rules for Naming Amides
Amide is a group of organic compounds containing the carbonyl group that is
bonded to a nitrogen atom, CONH2 and, usually, a hydrocarbon. Simply
change the carboxylic acid reactant -oic acid ending and replace it with -amide.
Example:
CH3CONH2
Ethanamide