Each Step Up a Homologous Series Adds Exactly One CH2 Unit
Learn what makes a homologous series, how boiling point and physical state change steadily along the alkanes, alkenes and alkynes, how to spot nine functional groups in a structure, and how to write IUPAC and common names using the longest chain and lowest number rules.
How do chemists keep track of so many carbon compounds?
Part 1 showed that carbon can form an enormous number of compounds. Learning each one separately would be hopeless. Chemistry avoids that by sorting organic compounds into families, and within a family by giving every member a name that describes its structure.
The families are homologous series. Methane, ethane, propane and butane look different, but each is one unit longer than the one before, and they all react in the same way. Learn the reactions of one member and you know the reactions of all of them.
What decides the family is the functional group — the atom or group of atoms that gives the compound its characteristic reactions:
- **The group makes an alcohol, such as the ethanol in hand sanitiser
- The group makes a carboxylic acid, such as the acetic acid in vinegar
- The group makes an ester, such as the compounds that give many fruits their smell
The name then records everything. A name such as propan-2-ol** tells you three carbon atoms, only single bonds, and an group on the middle carbon. Once you can read names, you can draw the structure; once you can draw the structure, you can write the name.
This part covers:
- Homologous series and their characteristics
- Gradation in physical properties along a series, linked to molecular mass
- Nine functional groups to recognise in any structure
- IUPAC and common names, using the longest chain rule and the lowest number rule
An everyday way to see a homologous series. Cooking gas, petrol, kerosene, diesel and candle wax are all mixtures of alkanes — members of one homologous series with chains of increasing length. Their states run from gas to liquid to solid exactly as the gradation in properties predicts.
The link to Part 1. Every name in this part assumes you can draw the chain and check four bonds on every carbon. Naming is just drawing, written in words.
This page covers the second part of the ICSE Class 10 Chemistry chapter on organic chemistry: homologous series, gradation of properties, functional groups, and IUPAC nomenclature.
The families are homologous series. Methane, ethane, propane and butane look different, but each is one unit longer than the one before, and they all react in the same way. Learn the reactions of one member and you know the reactions of all of them.
What decides the family is the functional group — the atom or group of atoms that gives the compound its characteristic reactions:
- **The group makes an alcohol, such as the ethanol in hand sanitiser
- The group makes a carboxylic acid, such as the acetic acid in vinegar
- The group makes an ester, such as the compounds that give many fruits their smell
The name then records everything. A name such as propan-2-ol** tells you three carbon atoms, only single bonds, and an group on the middle carbon. Once you can read names, you can draw the structure; once you can draw the structure, you can write the name.
This part covers:
- Homologous series and their characteristics
- Gradation in physical properties along a series, linked to molecular mass
- Nine functional groups to recognise in any structure
- IUPAC and common names, using the longest chain rule and the lowest number rule
An everyday way to see a homologous series. Cooking gas, petrol, kerosene, diesel and candle wax are all mixtures of alkanes — members of one homologous series with chains of increasing length. Their states run from gas to liquid to solid exactly as the gradation in properties predicts.
The link to Part 1. Every name in this part assumes you can draw the chain and check four bonds on every carbon. Naming is just drawing, written in words.
This page covers the second part of the ICSE Class 10 Chemistry chapter on organic chemistry: homologous series, gradation of properties, functional groups, and IUPAC nomenclature.
What is a homologous series, and what are its characteristics?
A homologous series is a family of organic compounds with the same functional group and general formula, in which each member differs from the next by a CH2 group and has similar chemical properties.
Characteristics of a homologous series:
- All members have the same general formula
- **Successive members differ by one group**, which means a difference of in molecular mass
- All members have the same functional group, so they show similar chemical properties
- Physical properties change gradually as the molecular mass increases
- Members can be prepared by similar general methods
1. The alkane series — general formula , single bonds only.
- Methane , ethane , propane , butane , pentane
2. The alkene series — general formula , one double bond.
- Ethene , propene , butene , pentene
3. The alkyne series — general formula , one triple bond.
- Ethyne , propyne , butyne , pentyne
Worked example 1 — the next member. Write the formula and molecular mass of the alkane after pentane. C , H .
Check: pentane is , and — **exactly one more.
Worked example 2 — is it a member?** Could be an alkane? For , an alkane needs hydrogens. ** has only , so it is not a stable alkane — it is an alkyl group, the butyl group, which appears inside larger molecules.
Worked example 3 — which series?** Classify , and .
- ****: — alkyne
- ****: — alkane
- ****: — alkene
An everyday example. Cooking gas is mainly propane and butane; petrol and kerosene contain longer alkane chains; paraffin wax contains longer ones still. All belong to the same homologous series, and they burn in the same way because the members react alike.
The boundary case. A general formula alone does not always decide the series. ** fits the alkenes, but cyclobutane — a ring of four groups with only single bonds — has the same formula.** The structure, not just the formula, finally fixes the family.
Characteristics of a homologous series:
- All members have the same general formula
- **Successive members differ by one group**, which means a difference of in molecular mass
- All members have the same functional group, so they show similar chemical properties
- Physical properties change gradually as the molecular mass increases
- Members can be prepared by similar general methods
1. The alkane series — general formula , single bonds only.
- Methane , ethane , propane , butane , pentane
2. The alkene series — general formula , one double bond.
- Ethene , propene , butene , pentene
3. The alkyne series — general formula , one triple bond.
- Ethyne , propyne , butyne , pentyne
Worked example 1 — the next member. Write the formula and molecular mass of the alkane after pentane. C , H .
Check: pentane is , and — **exactly one more.
Worked example 2 — is it a member?** Could be an alkane? For , an alkane needs hydrogens. ** has only , so it is not a stable alkane — it is an alkyl group, the butyl group, which appears inside larger molecules.
Worked example 3 — which series?** Classify , and .
- ****: — alkyne
- ****: — alkane
- ****: — alkene
An everyday example. Cooking gas is mainly propane and butane; petrol and kerosene contain longer alkane chains; paraffin wax contains longer ones still. All belong to the same homologous series, and they burn in the same way because the members react alike.
The boundary case. A general formula alone does not always decide the series. ** fits the alkenes, but cyclobutane — a ring of four groups with only single bonds — has the same formula.** The structure, not just the formula, finally fixes the family.
How do physical properties change along a homologous series, and why?
As molecular mass increases along a series, boiling point, melting point and density rise and the physical state changes from gas to liquid to solid, while chemical properties stay similar because the functional group is the same.
Gradation in physical properties:
- Boiling and melting points increase as the chain gets longer
- Physical state changes: the first alkanes are gases, the middle ones liquids, and the long-chain ones solids
- Density generally increases with molecular mass
- Solubility in water is very low throughout for hydrocarbons
Why the properties change. Longer molecules have larger surfaces and greater molecular mass, so the attractive forces between molecules are stronger. More energy is needed to separate them, which raises the melting and boiling points.
Why chemical properties do not change. Chemical reactions happen at the functional group, and every member has the same one. **Adding a to the chain changes the size of the molecule, not the part that reacts.
Boiling points of the first five straight-chain alkanes, approximately:
- Methane**: — gas
- Ethane: — gas
- Propane: — gas
- Butane: about — gas at room temperature
- Pentane: — liquid at room temperature
Relating molecular mass to the general formula. For carbon atoms:
Worked example 1. Find the molecular mass of pentane from the formula.
Worked example 2. An alkene has molecular mass . Find its formula.
Worked example 3. An alkyne has molecular mass . Find its formula.
Worked example 4. An alkane has molecular mass . Is it a gas, liquid or solid at room temperature?
Hexane has a longer chain than pentane, which is already a liquid, so hexane is a liquid too.
An everyday example. Butane in a cooking gas cylinder is kept liquid under pressure, but because its boiling point is close to , it turns back into gas the moment the valve opens. Longer-chain alkanes in kerosene stay liquid at room temperature, and those in candle wax are solids — the same gradation seen in the kitchen.
The boundary case. The steady rise in boiling point applies to straight-chain members. A branched isomer boils lower than its straight-chain partner — 2-methylpropane lower than butane — because branched molecules cannot pack as closely, so the gradation must compare like with like.
Gradation in physical properties:
- Boiling and melting points increase as the chain gets longer
- Physical state changes: the first alkanes are gases, the middle ones liquids, and the long-chain ones solids
- Density generally increases with molecular mass
- Solubility in water is very low throughout for hydrocarbons
Why the properties change. Longer molecules have larger surfaces and greater molecular mass, so the attractive forces between molecules are stronger. More energy is needed to separate them, which raises the melting and boiling points.
Why chemical properties do not change. Chemical reactions happen at the functional group, and every member has the same one. **Adding a to the chain changes the size of the molecule, not the part that reacts.
Boiling points of the first five straight-chain alkanes, approximately:
- Methane**: — gas
- Ethane: — gas
- Propane: — gas
- Butane: about — gas at room temperature
- Pentane: — liquid at room temperature
Relating molecular mass to the general formula. For carbon atoms:
Worked example 1. Find the molecular mass of pentane from the formula.
Worked example 2. An alkene has molecular mass . Find its formula.
Worked example 3. An alkyne has molecular mass . Find its formula.
Worked example 4. An alkane has molecular mass . Is it a gas, liquid or solid at room temperature?
Hexane has a longer chain than pentane, which is already a liquid, so hexane is a liquid too.
An everyday example. Butane in a cooking gas cylinder is kept liquid under pressure, but because its boiling point is close to , it turns back into gas the moment the valve opens. Longer-chain alkanes in kerosene stay liquid at room temperature, and those in candle wax are solids — the same gradation seen in the kitchen.
The boundary case. The steady rise in boiling point applies to straight-chain members. A branched isomer boils lower than its straight-chain partner — 2-methylpropane lower than butane — because branched molecules cannot pack as closely, so the gradation must compare like with like.
How do you recognise halide, alcohol, aldehyde, ketone, carboxylic acid, ether and ester functional groups?
**Each functional group is a particular atom or arrangement of atoms — a halogen, a C=C or CC bond, –OH, –CHO, a C=O inside the chain, –COOH, –O– between two carbons, or –COO– between two carbons.
The functional groups, with examples:
- Halide (halo group)**, where X is Cl, Br or I: chloromethane , bromoethane
- Double bond, : ethene
- Triple bond, : ethyne
- Alcoholic group, : ethanol
- Aldehydic group, : methanal , ethanal
- Ketonic group, between two carbons, written : propanone
- Carboxylic group, : methanoic acid , ethanoic acid
- Ether group, between two carbons: methoxymethane
- Ester group, between two carbon groups: ethyl ethanoate
How to tell the similar-looking groups apart:
- Aldehyde versus ketone — both contain . In an aldehyde the carbonyl carbon carries a hydrogen and sits at the end of the chain; in a ketone it sits between two carbon atoms
- Alcohol versus ether — both contain oxygen with single bonds. An alcohol has O bonded to H; an ether has O bonded to two carbons
- Carboxylic acid versus ester — both contain . **An acid ends in ; in an ester the H is replaced by a carbon group
Worked example — identify the functional group.
- ** — at the end: aldehyde
- **** — between two carbons: ketone
- ** — O between two carbons: ether
- ** — ends in : carboxylic acid
- **** — between carbons: ester
- ** — bromine atom: halide
- ** — : alcohol
Worked check — molecular formulae of an alcohol and an ether. Ethanol, , and methoxymethane, , both contain 2 carbons, 6 hydrogens and 1 oxygen. Same formula, different functional groups — isomers that belong to different families.
Everyday examples of the groups:
- Vinegar contains ethanoic acid
- Nail polish remover is often propanone, a ketone
- Formalin, used to preserve biological specimens, is a solution of methanal, an aldehyde
- Hand sanitisers contain alcohols
- The fruity smells of many sweets and perfumes come from esters
The boundary case. A compound can contain more than one functional group, but this chapter deals with one functional group per molecule. Always look at the whole structure before naming the group — contains a and an , yet it is a carboxylic acid, not a ketone or an alcohol.
The functional groups, with examples:
- Halide (halo group)**, where X is Cl, Br or I: chloromethane , bromoethane
- Double bond, : ethene
- Triple bond, : ethyne
- Alcoholic group, : ethanol
- Aldehydic group, : methanal , ethanal
- Ketonic group, between two carbons, written : propanone
- Carboxylic group, : methanoic acid , ethanoic acid
- Ether group, between two carbons: methoxymethane
- Ester group, between two carbon groups: ethyl ethanoate
How to tell the similar-looking groups apart:
- Aldehyde versus ketone — both contain . In an aldehyde the carbonyl carbon carries a hydrogen and sits at the end of the chain; in a ketone it sits between two carbon atoms
- Alcohol versus ether — both contain oxygen with single bonds. An alcohol has O bonded to H; an ether has O bonded to two carbons
- Carboxylic acid versus ester — both contain . **An acid ends in ; in an ester the H is replaced by a carbon group
Worked example — identify the functional group.
- ** — at the end: aldehyde
- **** — between two carbons: ketone
- ** — O between two carbons: ether
- ** — ends in : carboxylic acid
- **** — between carbons: ester
- ** — bromine atom: halide
- ** — : alcohol
Worked check — molecular formulae of an alcohol and an ether. Ethanol, , and methoxymethane, , both contain 2 carbons, 6 hydrogens and 1 oxygen. Same formula, different functional groups — isomers that belong to different families.
Everyday examples of the groups:
- Vinegar contains ethanoic acid
- Nail polish remover is often propanone, a ketone
- Formalin, used to preserve biological specimens, is a solution of methanal, an aldehyde
- Hand sanitisers contain alcohols
- The fruity smells of many sweets and perfumes come from esters
The boundary case. A compound can contain more than one functional group, but this chapter deals with one functional group per molecule. Always look at the whole structure before naming the group — contains a and an , yet it is a carboxylic acid, not a ketone or an alcohol.
How do you write IUPAC and trivial names using the longest chain and lowest number rules?
An IUPAC name is built from a word root for the number of carbons in the longest chain, a suffix or prefix for the bond type or functional group, and numbers showing positions — with the chain numbered so the functional group gets the lowest possible number.
1. Word roots — number of carbons in the longest chain:
- Meth- , eth- , prop- , but- , pent- , hex-
2. Endings and prefixes for each family:
- Alkane: -ane — propane
- Alkene: -ene — propene
- Alkyne: -yne — propyne
- Halide: prefix chloro-, bromo-, iodo- — chloroethane
- Alcohol: -ol — ethanol
- Aldehyde: -al — ethanal
- Ketone: -one — propanone
- Carboxylic acid: -oic acid — ethanoic acid
- Ether: alkoxyalkane — methoxymethane
- Ester: alkyl alkanoate — ethyl ethanoate
3. The rules:
- Longest chain rule — choose the longest continuous carbon chain that contains the functional group or multiple bond
- Lowest number rule — number the chain from the end that gives the functional group or multiple bond the smallest number
- Write the position number before the part of the name it refers to
Common and IUPAC names side by side:
- — methane (marsh gas)
- — ethene (ethylene)
- — ethyne (acetylene)
- — trichloromethane (chloroform)
- — ethanol (ethyl alcohol)
- — methanal (formaldehyde)
- — ethanal (acetaldehyde)
- — propanone (acetone)
- — methanoic acid (formic acid)
- — ethanoic acid (acetic acid)
- — methoxymethane (dimethyl ether)
- — ethyl ethanoate (ethyl acetate)
Worked example 1. Name .
**Four carbons: but-. Double bond: -ene.** Numbering from the left puts the double bond at carbon ; from the right, at carbon . **Lowest number is : but-1-ene (also written 1-butene).
Worked example 2.** Name .
**Four carbons: butane.** From the left, chlorine is on carbon ; from the right, on carbon . Lowest number: 2-chlorobutane.
Worked example 3. Name .
**Four carbons including the carbon: butanal.** The aldehyde carbon is always carbon , so no number is needed.
Worked example 4. Name .
**Four carbons with on carbon from the left: butan-2-one, often just called butanone.
Worked example 5.** Name .
**Three carbons, on carbon from either end: propan-2-ol (isopropyl alcohol).
An everyday example. Many medicine and sanitiser labels list propan-2-ol or ethanol among their ingredients, and paint thinners list propanone. Reading those IUPAC names tells you the structure without any diagram.
Two boundary cases.
- The aldehyde and carboxylic acid carbons are always carbon , so their names never need a position number
- In an ester name, the alkyl group from the alcohol comes first as a separate word — ethyl** ethanoate — even though it appears last in the formula
1. Word roots — number of carbons in the longest chain:
- Meth- , eth- , prop- , but- , pent- , hex-
2. Endings and prefixes for each family:
- Alkane: -ane — propane
- Alkene: -ene — propene
- Alkyne: -yne — propyne
- Halide: prefix chloro-, bromo-, iodo- — chloroethane
- Alcohol: -ol — ethanol
- Aldehyde: -al — ethanal
- Ketone: -one — propanone
- Carboxylic acid: -oic acid — ethanoic acid
- Ether: alkoxyalkane — methoxymethane
- Ester: alkyl alkanoate — ethyl ethanoate
3. The rules:
- Longest chain rule — choose the longest continuous carbon chain that contains the functional group or multiple bond
- Lowest number rule — number the chain from the end that gives the functional group or multiple bond the smallest number
- Write the position number before the part of the name it refers to
Common and IUPAC names side by side:
- — methane (marsh gas)
- — ethene (ethylene)
- — ethyne (acetylene)
- — trichloromethane (chloroform)
- — ethanol (ethyl alcohol)
- — methanal (formaldehyde)
- — ethanal (acetaldehyde)
- — propanone (acetone)
- — methanoic acid (formic acid)
- — ethanoic acid (acetic acid)
- — methoxymethane (dimethyl ether)
- — ethyl ethanoate (ethyl acetate)
Worked example 1. Name .
**Four carbons: but-. Double bond: -ene.** Numbering from the left puts the double bond at carbon ; from the right, at carbon . **Lowest number is : but-1-ene (also written 1-butene).
Worked example 2.** Name .
**Four carbons: butane.** From the left, chlorine is on carbon ; from the right, on carbon . Lowest number: 2-chlorobutane.
Worked example 3. Name .
**Four carbons including the carbon: butanal.** The aldehyde carbon is always carbon , so no number is needed.
Worked example 4. Name .
**Four carbons with on carbon from the left: butan-2-one, often just called butanone.
Worked example 5.** Name .
**Three carbons, on carbon from either end: propan-2-ol (isopropyl alcohol).
An everyday example. Many medicine and sanitiser labels list propan-2-ol or ethanol among their ingredients, and paint thinners list propanone. Reading those IUPAC names tells you the structure without any diagram.
Two boundary cases.
- The aldehyde and carboxylic acid carbons are always carbon , so their names never need a position number
- In an ester name, the alkyl group from the alcohol comes first as a separate word — ethyl** ethanoate — even though it appears last in the formula
Exam tip
What earns full marks on homologous series and naming?
Quote the characteristics of a homologous series precisely, draw or read the full structure before naming, and number the chain from the end nearest the functional group.
- List at least four characteristics of a homologous series, including the and mass difference
- Use the general formulae to classify compounds and find the next member
- Explain gradation through increasing molecular mass and stronger forces between molecules
- Say chemical properties are similar because the functional group is the same
- Write functional groups exactly: , , , , ,
- Separate aldehyde from ketone by the position of the C=O
- Choose the longest chain containing the functional group
- Number from the end giving the lowest number to the functional group or multiple bond
- Give both names when asked for IUPAC and trivial names
- Check the name by drawing it back into a structure
The misconception to name. The longest chain is not always the one written in a straight line. A chain drawn with a bend or a branch may continue through a side group, giving a longer chain than the one written horizontally. Tracing every path through the carbons finds the true longest chain.
A second trap. Naming as ethanol. **The ending -al means aldehyde and -ol means alcohol** — one letter separates two different families, and examiners mark it strictly.
- List at least four characteristics of a homologous series, including the and mass difference
- Use the general formulae to classify compounds and find the next member
- Explain gradation through increasing molecular mass and stronger forces between molecules
- Say chemical properties are similar because the functional group is the same
- Write functional groups exactly: , , , , ,
- Separate aldehyde from ketone by the position of the C=O
- Choose the longest chain containing the functional group
- Number from the end giving the lowest number to the functional group or multiple bond
- Give both names when asked for IUPAC and trivial names
- Check the name by drawing it back into a structure
The misconception to name. The longest chain is not always the one written in a straight line. A chain drawn with a bend or a branch may continue through a side group, giving a longer chain than the one written horizontally. Tracing every path through the carbons finds the true longest chain.
A second trap. Naming as ethanol. **The ending -al means aldehyde and -ol means alcohol** — one letter separates two different families, and examiners mark it strictly.
Did you know
Why do bananas and pineapples smell so different when both get their scent from esters?
Peel a ripe banana or cut a pineapple and the room fills with a sweet, fruity smell. Much of that smell comes from esters — compounds containing the functional group from this lesson.
Esters are formed when a carboxylic acid combines with an alcohol, and fruits make them naturally as they ripen. Changing the acid or the alcohol changes the ester, and changing the ester changes the smell.
- The typical smell of banana is associated largely with an ester made from acetic acid and a five-carbon alcohol
- The smell of pineapple is associated with an ester made from a four-carbon acid and ethanol
- Other esters give notes of apple, pear, orange and many other fruits
So two molecules in the same homologous family, differing only in the length and shape of their carbon chains, smell completely different to us. Our noses are sensitive detectors of molecular shape.
This is also why sweets and soft drinks can taste of fruit that is not in them. Food flavourings often contain esters made in a factory — chemically identical to, or very like, those in the fruit — and their names follow exactly the IUPAC rule in this lesson: **the alcohol part first, then the acid part with the ending -oate.
Ethyl ethanoate, made from ethanol and ethanoic acid, is one of the simplest esters. It has a sweet smell and is used as a solvent, including in some nail polish removers, where you may notice its pleasant fruity odour.
The contrast with the acid is striking. Ethanoic acid smells sharp and sour, like vinegar; combine it with ethanol and the product smells sweet.** A single change of functional group — from to — turns a sour smell into a fruity one, which is a good reminder that the functional group, not the size of the molecule, decides how a compound behaves.
Esters are formed when a carboxylic acid combines with an alcohol, and fruits make them naturally as they ripen. Changing the acid or the alcohol changes the ester, and changing the ester changes the smell.
- The typical smell of banana is associated largely with an ester made from acetic acid and a five-carbon alcohol
- The smell of pineapple is associated with an ester made from a four-carbon acid and ethanol
- Other esters give notes of apple, pear, orange and many other fruits
So two molecules in the same homologous family, differing only in the length and shape of their carbon chains, smell completely different to us. Our noses are sensitive detectors of molecular shape.
This is also why sweets and soft drinks can taste of fruit that is not in them. Food flavourings often contain esters made in a factory — chemically identical to, or very like, those in the fruit — and their names follow exactly the IUPAC rule in this lesson: **the alcohol part first, then the acid part with the ending -oate.
Ethyl ethanoate, made from ethanol and ethanoic acid, is one of the simplest esters. It has a sweet smell and is used as a solvent, including in some nail polish removers, where you may notice its pleasant fruity odour.
The contrast with the acid is striking. Ethanoic acid smells sharp and sour, like vinegar; combine it with ethanol and the product smells sweet.** A single change of functional group — from to — turns a sour smell into a fruity one, which is a good reminder that the functional group, not the size of the molecule, decides how a compound behaves.
Exam relevance
How do homologous series and IUPAC naming prepare you for JEE and NEET?
This is foundation work for Class 11 Organic Chemistry – Some Basic Principles and Techniques, and for every Class 12 organic chapter — Haloalkanes and Haloarenes, Alcohols, Phenols and Ethers, and Aldehydes, Ketones and Carboxylic Acids — all examined in JEE Main and NEET Chemistry.
Where IUPAC naming leads. Class 11 extends the rules on this page to branched chains, substituents, multiple functional groups and priority order. Naming a given structure, or drawing the structure from a name, is a recurring question type in both exams, and the longest chain and lowest locant rules practised here are applied in every case.
Where functional groups lead. Each Class 12 organic chapter is organised by functional group — halides, alcohols and ethers, then aldehydes, ketones and acids. Recognising the group instantly is the first step in predicting any reaction, and questions often present a structure and ask for its class or its product.
Where homologous series lead. Class 11 uses the idea of a series to explain trends in boiling point, including the effect of branching. Arranging compounds in order of boiling point is a standard question, and the branched-versus-straight boundary case on this page is exactly what those questions test.
Where isomers across functional groups lead. The ethanol and methoxymethane pair — same formula, different functional group — is an example of functional group isomerism, introduced in Class 11. Identifying the type of isomerism between two structures appears in both exams.
Question types to expect. At this level: characteristics of a series, identifying functional groups, and writing IUPAC and common names. In competitive papers: IUPAC names of branched and polyfunctional compounds, boiling-point orders, types of isomerism, and structure-from-name questions.
The single trap that costs marks. Numbering the chain from the wrong end. The functional group or multiple bond must get the lowest possible number, and a candidate who numbers from the left out of habit writes a name that is marked wrong even though the structure was read correctly.
A second trap. Confusing the endings -ol, -al and -one. Alcohol, aldehyde and ketone differ by one or two letters in the name but completely in their chemistry, and options in objective questions are often built from exactly these near-misses.
Board versus competitive emphasis. The ICSE paper marks the characteristics, correct functional groups and names of simple compounds; a competitive paper marks names of complex structures and orders of physical properties. The transferable habit is always drawing the structure before naming it — the method that stays reliable as the molecules grow.
Where IUPAC naming leads. Class 11 extends the rules on this page to branched chains, substituents, multiple functional groups and priority order. Naming a given structure, or drawing the structure from a name, is a recurring question type in both exams, and the longest chain and lowest locant rules practised here are applied in every case.
Where functional groups lead. Each Class 12 organic chapter is organised by functional group — halides, alcohols and ethers, then aldehydes, ketones and acids. Recognising the group instantly is the first step in predicting any reaction, and questions often present a structure and ask for its class or its product.
Where homologous series lead. Class 11 uses the idea of a series to explain trends in boiling point, including the effect of branching. Arranging compounds in order of boiling point is a standard question, and the branched-versus-straight boundary case on this page is exactly what those questions test.
Where isomers across functional groups lead. The ethanol and methoxymethane pair — same formula, different functional group — is an example of functional group isomerism, introduced in Class 11. Identifying the type of isomerism between two structures appears in both exams.
Question types to expect. At this level: characteristics of a series, identifying functional groups, and writing IUPAC and common names. In competitive papers: IUPAC names of branched and polyfunctional compounds, boiling-point orders, types of isomerism, and structure-from-name questions.
The single trap that costs marks. Numbering the chain from the wrong end. The functional group or multiple bond must get the lowest possible number, and a candidate who numbers from the left out of habit writes a name that is marked wrong even though the structure was read correctly.
A second trap. Confusing the endings -ol, -al and -one. Alcohol, aldehyde and ketone differ by one or two letters in the name but completely in their chemistry, and options in objective questions are often built from exactly these near-misses.
Board versus competitive emphasis. The ICSE paper marks the characteristics, correct functional groups and names of simple compounds; a competitive paper marks names of complex structures and orders of physical properties. The transferable habit is always drawing the structure before naming it — the method that stays reliable as the molecules grow.
Key takeaways
What must you be able to do from this part?
One definition, three series, nine functional groups and a naming method.
- Homologous series: same general formula and functional group, successive members differ by ( in mass), similar chemical properties, gradual change in physical properties, similar methods of preparation
- Alkanes , alkenes , alkynes
- Molecular mass: , and ; mass alkene is , mass alkyne is
- Boiling point, melting point and density rise along a series; the state changes from gas to liquid to solid
- **Methane to pentane : the first four alkanes are gases, pentane is a liquid
- Chemical properties stay similar because the functional group is the same
- Branched isomers boil lower than straight-chain ones
- Functional groups**: halide , double bond, triple bond, alcohol , aldehyde , ketone , carboxylic acid , ether , ester
- Aldehyde C=O is at the chain end with an H; ketone C=O is between two carbons
- Word roots: meth, eth, prop, but, pent, hex
- Endings: -ane, -ene, -yne, -ol, -al, -one, -oic acid; prefixes chloro, bromo, iodo; alkoxyalkane; alkyl alkanoate
- Longest chain rule and lowest number rule
- Examples: but-1-ene, 2-chlorobutane, butanal, butan-2-one, propan-2-ol, ethyl ethanoate
- Trivial names: marsh gas, ethylene, acetylene, chloroform, ethyl alcohol, formaldehyde, acetaldehyde, acetone, formic acid, acetic acid, dimethyl ether, ethyl acetate
The sharpest self-test is a two-way drill. Write ten formulae from this page, name each without looking, then cover the names and draw each structure back from its name — any pair that does not match shows exactly which rule to revisit.
- Homologous series: same general formula and functional group, successive members differ by ( in mass), similar chemical properties, gradual change in physical properties, similar methods of preparation
- Alkanes , alkenes , alkynes
- Molecular mass: , and ; mass alkene is , mass alkyne is
- Boiling point, melting point and density rise along a series; the state changes from gas to liquid to solid
- **Methane to pentane : the first four alkanes are gases, pentane is a liquid
- Chemical properties stay similar because the functional group is the same
- Branched isomers boil lower than straight-chain ones
- Functional groups**: halide , double bond, triple bond, alcohol , aldehyde , ketone , carboxylic acid , ether , ester
- Aldehyde C=O is at the chain end with an H; ketone C=O is between two carbons
- Word roots: meth, eth, prop, but, pent, hex
- Endings: -ane, -ene, -yne, -ol, -al, -one, -oic acid; prefixes chloro, bromo, iodo; alkoxyalkane; alkyl alkanoate
- Longest chain rule and lowest number rule
- Examples: but-1-ene, 2-chlorobutane, butanal, butan-2-one, propan-2-ol, ethyl ethanoate
- Trivial names: marsh gas, ethylene, acetylene, chloroform, ethyl alcohol, formaldehyde, acetaldehyde, acetone, formic acid, acetic acid, dimethyl ether, ethyl acetate
The sharpest self-test is a two-way drill. Write ten formulae from this page, name each without looking, then cover the names and draw each structure back from its name — any pair that does not match shows exactly which rule to revisit.