How Many Different Compounds Share the Formula C4H10O?
Apply IUPAC rules to name organic compounds, distinguish chain, position and functional group isomerism and metamerism, and use the degree of unsaturation to work out which structures a molecular formula allows.
Why does every organic compound need a systematic name?
A vast number of organic compounds exist, and many share the same molecular formula. The formula alone fits seven different structures, some of them alcohols and some ethers. IUPAC names give each structure one unambiguous label, and the idea of isomerism explains how one formula can hide so many compounds.
This lesson covers IUPAC naming rules, the types of structural isomerism, and how the degree of unsaturation narrows down the possible structures.
This lesson covers IUPAC naming rules, the types of structural isomerism, and how the degree of unsaturation narrows down the possible structures.
How do you name organic compounds using IUPAC rules?
An IUPAC name is built from a word root for the parent carbon chain, a primary suffix for single, double or triple bonds, a secondary suffix for the principal functional group, and prefixes for substituents, all numbered to give the lowest locants.
Parts of a name:
- Word root — the number of carbons in the parent chain: meth (1), eth (2), prop (3), but (4), pent (5), hex (6)
- Primary suffix — -ane, -ene or -yne for single, double or triple bonds
- Secondary suffix — the principal functional group, such as -ol, -al, -one, -oic acid or -amine
- Prefixes — substituents such as methyl, chloro, nitro and methoxy
Rules, in order:
- Choose the longest chain that contains the principal functional group and as many multiple bonds as possible
- Number the chain so the principal functional group gets the lowest locant, then multiple bonds, then substituents
- Arrange prefixes alphabetically, ignoring di, tri and tetra
- Separate numbers with commas, and numbers from words with hyphens
Priority of functional groups: carboxylic acid > ester > acid chloride > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene and alkyne.
Worked examples:
- — a four-carbon chain with a methyl group on carbon 2: 2-methylbutane
- — numbering from the end nearer the OH group gives butan-2-ol, not butan-3-ol
- — locants 2 and 3 from the left beat 3 and 4 from the right, and chloro comes before methyl: 2-chloro-3-methylpentane
- — the acid outranks the ketone, which becomes a prefix: 3-oxobutanoic acid
An everyday example. Nail polish remover is mostly propan-2-one, better known as acetone, and kitchen vinegar contains ethanoic acid — IUPAC names that describe exactly what each molecule is.
The substance. The longest chain is not always the one drawn in a straight line — a chain can turn through a branch, so trace every path before choosing the parent.
Parts of a name:
- Word root — the number of carbons in the parent chain: meth (1), eth (2), prop (3), but (4), pent (5), hex (6)
- Primary suffix — -ane, -ene or -yne for single, double or triple bonds
- Secondary suffix — the principal functional group, such as -ol, -al, -one, -oic acid or -amine
- Prefixes — substituents such as methyl, chloro, nitro and methoxy
Rules, in order:
- Choose the longest chain that contains the principal functional group and as many multiple bonds as possible
- Number the chain so the principal functional group gets the lowest locant, then multiple bonds, then substituents
- Arrange prefixes alphabetically, ignoring di, tri and tetra
- Separate numbers with commas, and numbers from words with hyphens
Priority of functional groups: carboxylic acid > ester > acid chloride > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene and alkyne.
Worked examples:
- — a four-carbon chain with a methyl group on carbon 2: 2-methylbutane
- — numbering from the end nearer the OH group gives butan-2-ol, not butan-3-ol
- — locants 2 and 3 from the left beat 3 and 4 from the right, and chloro comes before methyl: 2-chloro-3-methylpentane
- — the acid outranks the ketone, which becomes a prefix: 3-oxobutanoic acid
An everyday example. Nail polish remover is mostly propan-2-one, better known as acetone, and kitchen vinegar contains ethanoic acid — IUPAC names that describe exactly what each molecule is.
The substance. The longest chain is not always the one drawn in a straight line — a chain can turn through a branch, so trace every path before choosing the parent.
What are chain, position and functional group isomerism and metamerism?
Structural isomers have the same molecular formula but different arrangements of atoms, and they are classed as chain, position or functional group isomers, or metamers, according to what differs between them.
Chain isomerism — different carbon skeletons:
- : butane, , and 2-methylpropane,
- : pentane, 2-methylbutane and 2,2-dimethylpropane
Position isomerism — the same skeleton and group, with the group or multiple bond in a different position:
- Propan-1-ol and propan-2-ol, both
- But-1-ene and but-2-ene, both
Functional group isomerism — different functional groups:
- Ethanol, , and methoxymethane, , both
- Propanal, , and propanone, , both
Metamerism — the same polyvalent functional group, with different alkyl groups on either side:
- Ethoxyethane, , and 1-methoxypropane,
- Pentan-2-one and pentan-3-one, both
**Worked example — the isomers of :
- Four alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol
- Three ethers: 1-methoxypropane, 2-methoxypropane and ethoxyethane
- In total, seven structural isomers, showing chain, position and functional group isomerism and metamerism within one formula
An everyday example. Many hand sanitisers rely on ethanol, a liquid, while its functional group isomer methoxymethane is a gas used as an aerosol propellant — the same formula with completely different behaviour.
The substance. Metamerism is not functional group isomerism** — metamers share the same functional group and differ only in the alkyl groups attached to it.
Chain isomerism — different carbon skeletons:
- : butane, , and 2-methylpropane,
- : pentane, 2-methylbutane and 2,2-dimethylpropane
Position isomerism — the same skeleton and group, with the group or multiple bond in a different position:
- Propan-1-ol and propan-2-ol, both
- But-1-ene and but-2-ene, both
Functional group isomerism — different functional groups:
- Ethanol, , and methoxymethane, , both
- Propanal, , and propanone, , both
Metamerism — the same polyvalent functional group, with different alkyl groups on either side:
- Ethoxyethane, , and 1-methoxypropane,
- Pentan-2-one and pentan-3-one, both
**Worked example — the isomers of :
- Four alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol
- Three ethers: 1-methoxypropane, 2-methoxypropane and ethoxyethane
- In total, seven structural isomers, showing chain, position and functional group isomerism and metamerism within one formula
An everyday example. Many hand sanitisers rely on ethanol, a liquid, while its functional group isomer methoxymethane is a gas used as an aerosol propellant — the same formula with completely different behaviour.
The substance. Metamerism is not functional group isomerism** — metamers share the same functional group and differ only in the alkyl groups attached to it.
Formula
How does the degree of unsaturation help you find possible isomers?
**The degree of unsaturation, , gives the total number of rings and pi bonds in a molecule, which tells you what kinds of structure a formula allows.**
where C, H, X and N are the numbers of carbon, hydrogen, halogen and nitrogen atoms; oxygen does not affect the count.
Worked examples:
- : , so there are no rings or double bonds — only alcohols and ethers are possible, never an aldehyde
- : , so one double bond or ring — propanal, propanone and prop-2-en-1-ol all fit
- : — propene or cyclopropane, which are ring-chain isomers
- : — three double bonds and one ring, as in benzene
Reading the value. A double bond or a ring counts as 1 and a triple bond as 2, and a value of 4 or more in a compound with at least six carbons often hints at a benzene ring.
An everyday example. A quality-control chemist at a perfume unit in Kannauj checking an unknown fragrance oil starts from its molecular formula, and the degree of unsaturation at once rules whole families of structures in or out.
The substance. The degree of unsaturation counts rings and pi bonds together — it cannot tell a ring from a double bond, so other evidence is needed to choose between them.
where C, H, X and N are the numbers of carbon, hydrogen, halogen and nitrogen atoms; oxygen does not affect the count.
Worked examples:
- : , so there are no rings or double bonds — only alcohols and ethers are possible, never an aldehyde
- : , so one double bond or ring — propanal, propanone and prop-2-en-1-ol all fit
- : — propene or cyclopropane, which are ring-chain isomers
- : — three double bonds and one ring, as in benzene
Reading the value. A double bond or a ring counts as 1 and a triple bond as 2, and a value of 4 or more in a compound with at least six carbons often hints at a benzene ring.
An everyday example. A quality-control chemist at a perfume unit in Kannauj checking an unknown fragrance oil starts from its molecular formula, and the degree of unsaturation at once rules whole families of structures in or out.
The substance. The degree of unsaturation counts rings and pi bonds together — it cannot tell a ring from a double bond, so other evidence is needed to choose between them.
Exam tip
What earns full marks on IUPAC names and isomers?
Draw the carbon chain and write locant numbers on it before writing the name — most lost marks come from numbering from the wrong end.
- Longest chain containing the principal functional group; lowest locant to that group first
- Prefixes in alphabetical order, ignoring di and tri
- Chain, position and functional group isomers, and metamers
-
The trap. Writing names such as 2-ethylbutane or 3-methylbutane. The first hides a five-carbon chain and should be 3-methylpentane; the second is numbered from the wrong end and should be 2-methylbutane.
- Longest chain containing the principal functional group; lowest locant to that group first
- Prefixes in alphabetical order, ignoring di and tri
- Chain, position and functional group isomers, and metamers
-
The trap. Writing names such as 2-ethylbutane or 3-methylbutane. The first hides a five-carbon chain and should be 3-methylpentane; the second is numbered from the wrong end and should be 2-methylbutane.
Did you know
How can glucose and fructose share a formula but not a taste?
Glucose and fructose are both , yet they are functional group isomers: in its open-chain form, glucose has an aldehyde group, while fructose has a ketone group.
That single difference changes how each molecule fits the sweet-taste receptors on the tongue, and fructose tastes noticeably sweeter than glucose.
Both sugars occur together in honey and in ripe mangoes, so every sweet bite holds a pair of isomers side by side.
That single difference changes how each molecule fits the sweet-taste receptors on the tongue, and fructose tastes noticeably sweeter than glucose.
Both sugars occur together in honey and in ripe mangoes, so every sweet bite holds a pair of isomers side by side.
Exam relevance
How do JEE Main and NEET test IUPAC naming and structural isomerism?
Organic Chemistry: Some Basic Principles and Techniques is a recurring chapter in both JEE Main and NEET, and naming and isomerism are needed in every organic chapter that follows.
What gets asked. The correct IUPAC name of a given structure, including compounds with more than one functional group, the type of isomerism between two compounds, and the number of structural isomers of a formula.
Question types. Single-correct questions on names and isomer types, and numerical-value questions in JEE Main asking for a number of isomers.
Why it matters later. Naming and isomerism return in Hydrocarbons, Haloalkanes and Haloarenes, Alcohols, Phenols and Ethers and Aldehydes, Ketones and Carboxylic Acids, and stereoisomerism builds on the same idea.
The trap that costs marks. Counting stereoisomers when a question asks for structural isomers, or the reverse — read which kind the question wants before counting.
What gets asked. The correct IUPAC name of a given structure, including compounds with more than one functional group, the type of isomerism between two compounds, and the number of structural isomers of a formula.
Question types. Single-correct questions on names and isomer types, and numerical-value questions in JEE Main asking for a number of isomers.
Why it matters later. Naming and isomerism return in Hydrocarbons, Haloalkanes and Haloarenes, Alcohols, Phenols and Ethers and Aldehydes, Ketones and Carboxylic Acids, and stereoisomerism builds on the same idea.
The trap that costs marks. Counting stereoisomers when a question asks for structural isomers, or the reverse — read which kind the question wants before counting.
Key takeaways
What must you be able to do from this lesson?
- IUPAC naming: word root, suffixes and prefixes, the longest chain with the principal group, lowest locants and alphabetical prefixes
- Structural isomerism: chain, position and functional group isomers, and metamers
- Degree of unsaturation: , counting rings and pi bonds together
How many structural isomers does have — and can you name each one?
- Structural isomerism: chain, position and functional group isomers, and metamers
- Degree of unsaturation: , counting rings and pi bonds together
How many structural isomers does have — and can you name each one?