Use a systematic approach to drawing isomers
When drawing all of the possible constitutional (structural) isomers that fit a particular description, use a systematic approach.
Draw the carbon skeleton with bonded carbon atoms separated by dashes.
Place your mouse over the chain above to see the dashes converted to H's.
If the carbon skeleton is NOT specified, start with a linear one containing all of the carbon atoms. Proceed as below.
Then repeat using a chain that is one carbon shorter and has a one-carbon branch, and so on with shorter chains and more branches until all of the possibilities have been considered.
Generate the isomers by positioning the functional group at different carbon atoms in the chain.
Monovalent groups (like OH and Cl) replace one H at each non-equivalent carbon atoms.
Divalent groups (like =O) replace two H at each non-equivalent carbon atom bearing two H.
A carbon-carbon double bond replaces two H on different pairs of adjacent carbons.
Be aware that some carbon atoms are equivalent (NOT different). Be aware that carbons may not have sufficient hydrogens to be replaced by the functional group.
Carbons bonded to four other carbons cannot have a functional group or be involved in a carbon-carbon double bond.
There is only one alcohol or halide isomer and no alkene isomers for the carbon skeleton shown.
Draw the carbon skeleton with bonded carbon atoms separated by dashes.
Place your mouse over the chain above to see the dashes converted to H's.
| C | | ||
| C — | C — | C |
If the carbon skeleton is NOT specified, start with a linear one containing all of the carbon atoms. Proceed as below.
Then repeat using a chain that is one carbon shorter and has a one-carbon branch, and so on with shorter chains and more branches until all of the possibilities have been considered.
Generate the isomers by positioning the functional group at different carbon atoms in the chain.
Monovalent groups (like OH and Cl) replace one H at each non-equivalent carbon atoms.
Divalent groups (like =O) replace two H at each non-equivalent carbon atom bearing two H.
A carbon-carbon double bond replaces two H on different pairs of adjacent carbons.
For a linear chain, the end carbon atoms are equivalent.
Thus, as described above, there are only two C3 alcohol isomers putting an OH at either end would give propan-1-ol.
For longer chains, some central carbon atoms may be equivalent.
For example, the two central carbon atoms of a C-4 chain are equivalent
Thus, as described above, there are only two C3 alcohol isomers putting an OH at either end would give propan-1-ol.
| OH | | OH | | |||||
| C — | C — | C | C — | C — | C |
For example, the two central carbon atoms of a C-4 chain are equivalent
| C | – | C | - |  | - | C | – | C |
Branched compounds have equivalent carbons if branches attached to the same carbon have the same composition.
Thus two red carbons are equivalent in the example shown.
There are four alcohol or halide isomers possible.
There are three alkene isomers,
The double bond can be
between red and blue carbons or
between black and blue carbons or
between the pair of black carbons.
| C | | ||||
| C — | C — | C | — | C |
There are four alcohol or halide isomers possible.
There are three alkene isomers,
The double bond can be
between red and blue carbons or
between black and blue carbons or
between the pair of black carbons.
| C | | ||
| C — | C — | C |
| | C |
There is only one alcohol or halide isomer and no alkene isomers for the carbon skeleton shown.