Structural isomers explained: chain, position, functional and metamers

Guide · Updated 2026-11-10

How one molecular formula can give several compounds, how to sort isomers into chain, position, functional and metamer types, and a worked example with C₅H₁₀O.

Open the Isomer explorer Free, in your browser, nothing uploaded.

The isomer explorer lists every open-chain structural isomer of a small molecular formula, from butane up to heptane and on through alkenes, alcohols, ethers, aldehydes, ketones, acids and esters. Each one comes with its IUPAC name, a condensed formula and its family, and when you tap any two the page says what kind of isomers they are and why. It is built for Class 11 students checking they have not missed or double-counted a structure. To name a chain you have drawn yourself, use the IUPAC namer; to work out how many rings or double bonds a formula allows, use the unsaturation; and to see how a single isomer twists about its bonds, try the newman projection spinner.

What it is

A molecular formula such as C₅H₁₀O only counts atoms. It says nothing about how they are joined. Isomers are different compounds that share a molecular formula. When the atoms are connected in a different order, they are structural isomers. Structural isomers come in four kinds:

• Chain isomers have different carbon skeletons: one straight chain, another branched.
• Position isomers share a skeleton and a family, but the functional group (or a double bond) sits on a different carbon.
• Functional isomers belong to different families altogether, such as an aldehyde and a ketone, or an alcohol and an ether.
• Metamers are ethers or esters whose carbon groups are split differently on the two sides of the oxygen.

Two further marks appear on some cards. Cis and trans means the compound has a C=C double bond with two different groups on each end, so it exists as two geometric isomers that cannot turn into each other without breaking the bond. Chiral carbon means one carbon holds four different groups, so the molecule has a mirror-image twin it cannot be laid on top of: a pair of optical isomers.

The tool finds every carbon tree, then every place for the double bond or oxygen, and drops repeats. Rings are left out.

How to use it

1. Choose a Molecular formula from the list. Each option names the families it covers, for example “aldehydes and ketones”.
2. Read the headline count and the note below it. The note splits the total by family, gives the count if cis and trans forms are counted separately, says how many have a chiral carbon, and flags ring compounds that are not shown.
3. Scan the cards. Each shows the isomer number, the name, the condensed formula, the family and any cis–trans or chiral-carbon tag.
4. Tap two cards. The box above the list names their relationship and gives the reason. Tap a picked card again to drop it; tapping a third replaces the oldest pick.
5. In Check yourself, answer two counting questions and two “what kind of isomers?” questions, then press Check. New questions gives a fresh set.

A worked example

Pick C₅H₁₀O (aldehydes and ketones). The page finds 7 open-chain isomers.

Aldehydes (–CHO at the end of a chain):

1. pentanal, CH₃–CH₂–CH₂–CH₂–CHO
2. 2-methylbutanal, CH₃–CH₂–CH(CH₃)–CHO
3. 3-methylbutanal, CH₃–CH(CH₃)–CH₂–CHO
4. 2,2-dimethylpropanal, CH₃–C(CH₃)₂–CHO

Ketones (C=O inside the chain):

5. pentan-2-one, CH₃–CH₂–CH₂–C(=O)–CH₃
6. pentan-3-one, CH₃–CH₂–C(=O)–CH₂–CH₃
7. 3-methylbutan-2-one, CH₃–CH(CH₃)–C(=O)–CH₃

That is 4 aldehydes + 3 ketones = 7. Now compare pairs:

• pentan-2-one and pentan-3-one: same five-carbon chain, C=O on carbon 2 versus carbon 3, so position isomers.
• 2-methylbutanal and 3-methylbutanal: same branched skeleton, the –CHO attached at a different end, so the tool also calls these position isomers.
• pentanal and 2,2-dimethylpropanal: straight chain versus a heavily branched one, so chain isomers.
• pentanal and pentan-2-one: an aldehyde and a ketone, so functional isomers.

Only one card carries a chiral carbon tag: 2-methylbutanal. Its carbon 2 holds H, CH₃, C₂H₅ and CHO, four different groups. None of the seven has a C=C, so none is marked cis and trans.

Where it is used

• Lab identification. Isomers have the same mass, so a chemist confirming a product must rule out the others with boiling points or spectra. A full list shows what to rule out.
• Medicines. Many drugs have a chiral carbon, and the two mirror forms can act very differently in the body, which is why manufacturers test each form separately.
• Fuels. Branched alkane isomers burn more smoothly in engines than straight ones; octane rating is built on comparing such isomers.

Common mistakes

• Counting the same molecule twice. Bending or flipping a drawing does not make a new compound. If two drawings get the same IUPAC name, they are one isomer.
• Forgetting the other family. C₄H₁₀O is not only alcohols; ethers fit too. Ask which families a general formula allows before you start.
• Calling every same-family pair “position”. If the skeletons differ, the pair are chain isomers, even if the group also moved.
• Mixing up metamers and position isomers in ethers. For ethers, the question is how the carbons split around the oxygen (2 + 2 versus 3 + 1). That is metamerism.
• Missing a chiral carbon. Check each carbon for four different groups; CH₃ and C₂H₅ count as different.

Questions people ask

Why does the count grow so fast with more carbons? Each extra carbon can attach in several new places, and every new skeleton gives new positions for the functional group, so the numbers multiply rather than add.

Is a cis–trans pair one isomer or two? Two compounds, but one structural isomer. The note under the count tells you the total when you count them separately.

Do isomers have the same properties? Usually not. Pentanal boils at a different temperature from pentan-3-one, and branched isomers tend to boil lower than straight ones because their molecules touch less.