Chemistry · General chemistry II · Worked example
Compare ethanol and dimethyl ether
Ethanol and dimethyl ether both have the formula C₂H₆O. Which representations tell them apart?
Compare the formulas
Both have 2 carbons, 6 hydrogens and 1 oxygen, so the formula C₂H₆O can’t tell them apart.
Compare the skeletal structures
In ethanol, CH₃CH₂OH, the oxygen is at the end of the chain and carries an H. In dimethyl ether, CH₃OCH₃, the oxygen sits between the two carbons and carries none.
Compare the Lewis structures
Both use 2(4) + 6(1) + 6 = 20 valence electrons, and each oxygen keeps two lone pairs. What differs is which atoms the oxygen is bonded to.
Compare the SMILES and names
Ethanol is CCO and dimethyl ether is COC. The names say the same: ethanol has an –OH on a two-carbon chain, and dimethyl ether has two methyl groups on an oxygen.
| Representation | Ethanol | Dimethyl ether |
|---|---|---|
| Formula | C₂H₆O | C₂H₆O |
| Condensed formula | CH₃CH₂OH | CH₃OCH₃ |
| SMILES | CCO | COC |
| Name | ethanol | dimethyl ether |
Result
Every representation except the molecular formula tells them apart. They are constitutional isomers: the same atoms, joined differently.
Your turn
Butane and 2-methylpropane share the formula C₄H₁₀. Write the SMILES of each.
Show the answer and explanation
Butane: CCCC. 2-Methylpropane: CC(C)C.
Butane is an unbranched chain of four carbons. In 2-methylpropane the middle carbon of a three-carbon chain carries the fourth carbon as a branch, written in parentheses.
Keep exploring
Open ethanol in Same Molecule, Different Representations and pick the oxygen: it lights up at the end of the skeletal chain, in the Lewis structure, in the formula, at the end of CCO and in the -ol of the name.
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