From a condensed formula to a skeletal structure
How to draw a skeletal structure from a condensed formula such as CH3CH2CH(CH3)CH2OH: branches, (CH2)n, CHO, COOH, COO and CN, with worked examples.
A condensed formula writes every carbon with the hydrogens attached to it, one group after another: CH3CH2OH, or with bonds drawn between the groups, CH3–CH2–OH. A skeletal structure shows the same molecule as lines, with a carbon at every corner and line end and the hydrogens on carbon left out. This guide turns the first into the second; the guide to reading skeletal structures explains the drawing conventions themselves.
The method in four steps
- Split the formula into groups: carbon groups (CH3, CH2, CH, C) and functional groups (OH, CHO, CO, COOH, COO, CN, NH2, a halogen).
- Find the main chain. Groups written one after the other are bonded in a row. A group in parentheses is a branch on the atom just before it, and (CH2)3 stands for three CH2 groups in a row.
- Draw the main chain as a zigzag, one corner or line end for every carbon of the chain.
- Add the branches and the functional groups, writing out every atom other than carbon with the hydrogens on it. Then check that every carbon has four bonds: its hydrogens in the condensed formula plus its bonds to other atoms.
What the groups mean
| In the condensed formula | In the skeletal structure |
|---|---|
| CH3– | the end of a line (a carbon with one bond) |
| –CH2– | a corner of the chain |
| –CH(CH3)– | a corner with a one-carbon branch |
| –C(CH3)2– | a carbon with two one-carbon branches and no hydrogen |
| –CHO | an aldehyde at the end of the chain: a carbon with =O (its H is not drawn) |
| –CO– | a ketone: a chain carbon with =O |
| –COOH | a carboxylic acid: a carbon with =O and –OH |
| –COO– | an ester: a carbon with =O, bonded through the other O to the next group |
| –CN or –C≡N | a nitrile: one more carbon, triple-bonded to N, drawn in a straight line |
| –O– | an ether oxygen inside the chain |
Worked examples
(CH3)2CHCH2CH(CH3)2
(CH3)2CH– is a carbon with two methyl groups, and so is –CH(CH3)2 at the other end. Between them is one CH2. Draw a five-carbon zigzag through one methyl group at each end and add the other two methyl groups to C2 and C4.
(CH3)3CCH2OH
(CH3)3C– is one carbon carrying three methyl groups and no hydrogen. It is bonded to CH2, which carries the OH. In the drawing the central carbon has four lines and the OH is written out.
CH3COCH2CH2CH3
CO between two carbon groups is a ketone: the second carbon of the chain has a double bond to oxygen. There are five carbons in all, so the chain is a five-carbon zigzag with =O on C2.
CH3CH2COOCH2CH3
COO joins two carbon groups: it is an ester. Its carbon has a double bond to one oxygen and a single bond to the other, and that second oxygen is bonded to CH2CH3. Draw CH3CH2C(=O)– as a zigzag, then –O–, then the two-carbon group.
CH3(CH2)3CN
(CH2)3 is three CH2 groups in a row, and CN is a nitrile carbon triple-bonded to nitrogen. That makes five carbons: CH3, three CH2 and the nitrile carbon, which is easy to forget because its letter C sits next to the N.
CH2=CHCH2Cl
A double bond written in the condensed formula stays a double bond: two lines between the first two carbons. The chlorine is on the third carbon and is written out.
The other way: skeletal to condensed
Go along the main chain and write each carbon with its hydrogens: a carbon at a line end with one bond is CH3, a corner with two bonds is CH2, a carbon with three bonds is CH, and one with four bonds has no hydrogen. Put each branch in parentheses after the carbon it is on. A C=O counts as two bonds, so a chain-end carbon with =O is CHO and a chain carbon with =O between two carbons is CO.
Common mistakes
- Forgetting the carbon in CN, CHO or COOH. Each of them contains a carbon, which is part of the chain.
- Putting a branch on the wrong carbon. In CH3CH(CH3)CH2CH3 the methyl group in parentheses is on the CH just before it, the second carbon.
- Reading an ester backward. CH3COOCH2CH3 has its C=O in the left part, on the two-carbon side, and CH3CH2COOCH3 has it on the three-carbon side. They are different esters with the same formula.
- Drawing the hydrogens on carbon. They are implied in a skeletal structure; only hydrogens on O, N and other atoms are written.
Practice
The worked answer of every exercise without a ring or stereochemistry shows the condensed formula, so you can compare it with your drawing. Start on the practice page, or draw a structure on the free drawing page and compare the condensed formula it shows with the one you wrote.
Want to check a structure of your own? Draw it in free drawing and see its name, formula and functional groups.