Why Carbon Can Build Chains, Branches and Rings Out of Its Own Atoms
Understand the tetravalency and catenation that let carbon form so many compounds, represent single, double and triple bonds, tell straight chains from branched chains and rings including benzene, draw alkanes, alkenes and alkynes up to five carbons, and find chain and position isomers.
Why does one element have a whole branch of chemistry to itself?
The cooking gas in a kitchen cylinder, the kerosene in a lamp, the wax of a candle, the sugar in tea, the proteins in dal and the plastic of a water bottle all have one thing in common: their molecules are built around carbon. Carbon forms far more compounds than any other element, so many that their study — organic chemistry — is treated as a subject of its own.
Two properties of the carbon atom make this possible.
- Tetravalency — every carbon atom forms four covalent bonds
- Catenation — carbon atoms bond to each other, building long chains, branched chains and rings
Put those together with carbon's ability to form double and triple bonds, and to bond with hydrogen, oxygen, nitrogen and the halogens, and the number of possible molecules becomes enormous.
One more idea multiplies the count further. The same set of atoms can often be joined in more than one arrangement, giving different compounds with the same molecular formula. These are isomers, and they are the last topic of this part.
This part builds the language of organic chemistry:
- Why carbon is unique — tetravalency and catenation
- How bonds and chain types are represented, including the ring of benzene
- How to draw alkanes, alkenes and alkynes with up to five carbon atoms
- What isomerism is, with chain and position isomers
The link to the bonding chapter. Everything here rests on the covalent bond — the shared pair of electrons. A line in an organic structure is simply a shared pair, a double line two shared pairs, and a triple line three. Drawing organic molecules is drawing dot structures in shorthand.
The skill that matters most. Every carbon atom in every structure you draw must have exactly four bonds. Checking that one rule catches almost every error in this chapter.
This page covers the first part of the ICSE Class 10 Chemistry chapter on organic chemistry: the unique nature of carbon, types of bonds and chains, structural formulae of hydrocarbons, and isomerism.
Two properties of the carbon atom make this possible.
- Tetravalency — every carbon atom forms four covalent bonds
- Catenation — carbon atoms bond to each other, building long chains, branched chains and rings
Put those together with carbon's ability to form double and triple bonds, and to bond with hydrogen, oxygen, nitrogen and the halogens, and the number of possible molecules becomes enormous.
One more idea multiplies the count further. The same set of atoms can often be joined in more than one arrangement, giving different compounds with the same molecular formula. These are isomers, and they are the last topic of this part.
This part builds the language of organic chemistry:
- Why carbon is unique — tetravalency and catenation
- How bonds and chain types are represented, including the ring of benzene
- How to draw alkanes, alkenes and alkynes with up to five carbon atoms
- What isomerism is, with chain and position isomers
The link to the bonding chapter. Everything here rests on the covalent bond — the shared pair of electrons. A line in an organic structure is simply a shared pair, a double line two shared pairs, and a triple line three. Drawing organic molecules is drawing dot structures in shorthand.
The skill that matters most. Every carbon atom in every structure you draw must have exactly four bonds. Checking that one rule catches almost every error in this chapter.
This page covers the first part of the ICSE Class 10 Chemistry chapter on organic chemistry: the unique nature of carbon, types of bonds and chains, structural formulae of hydrocarbons, and isomerism.
What makes the carbon atom unique — tetravalency and catenation?
Carbon has four outer electrons and completes its octet by sharing all four, forming four covalent bonds; its small size and strong carbon–carbon bonds let it link to itself in long chains and rings, which accounts for the huge number of organic compounds.
1. Tetravalency. Carbon, , has four valence electrons.
- Gaining four electrons to form would be very difficult — six protons cannot hold ten electrons easily
- Losing four electrons to form would need a great deal of energy
- So carbon shares its four electrons, forming four covalent bonds
In methane, one carbon shares one electron pair with each of four hydrogen atoms:
The four bonds point to the corners of a tetrahedron, as far apart as possible.
2. Catenation. Catenation is the property of an element to form bonds with atoms of the same element, building chains or rings.
- Carbon atoms are small, so the shared electrons are held close to both nuclei
- Carbon–carbon bonds are strong, so long chains are stable
- Chains can be straight, branched or closed into rings, and can contain double and triple bonds
Why these two properties produce so many compounds:
- Chains of any length — one, two, three or thousands of carbon atoms
- Branches at any carbon atom
- Rings of different sizes
- Single, double and triple bonds between carbon atoms
- Bonds to other elements — hydrogen, oxygen, nitrogen, sulphur and the halogens
- Isomers — different arrangements of the same atoms
Worked example — electrons in ethane. Ethane, , has one carbon–carbon bond and six carbon–hydrogen bonds. How many shared pairs are there, and does each carbon have an octet?
**Each carbon has three C–H bonds and one C–C bond: bonds, so electrons — an octet.
An everyday example. The cooking gas in an LPG cylinder is mainly propane and butane, chains of three and four carbon atoms; kerosene has longer chains, and paraffin wax longer still. The same two properties give gas, liquid and solid fuels simply by lengthening the chain.
Two boundary cases.
- Silicon, just below carbon in group 14, is also tetravalent, but its bonds to other silicon atoms are much weaker, so it shows only limited catenation — which is why there is no comparable chemistry of silicon chains
- Not every carbon compound is classed as organic.** Carbon dioxide, carbon monoxide, carbonates and cyanides are usually studied in inorganic chemistry
1. Tetravalency. Carbon, , has four valence electrons.
- Gaining four electrons to form would be very difficult — six protons cannot hold ten electrons easily
- Losing four electrons to form would need a great deal of energy
- So carbon shares its four electrons, forming four covalent bonds
In methane, one carbon shares one electron pair with each of four hydrogen atoms:
The four bonds point to the corners of a tetrahedron, as far apart as possible.
2. Catenation. Catenation is the property of an element to form bonds with atoms of the same element, building chains or rings.
- Carbon atoms are small, so the shared electrons are held close to both nuclei
- Carbon–carbon bonds are strong, so long chains are stable
- Chains can be straight, branched or closed into rings, and can contain double and triple bonds
Why these two properties produce so many compounds:
- Chains of any length — one, two, three or thousands of carbon atoms
- Branches at any carbon atom
- Rings of different sizes
- Single, double and triple bonds between carbon atoms
- Bonds to other elements — hydrogen, oxygen, nitrogen, sulphur and the halogens
- Isomers — different arrangements of the same atoms
Worked example — electrons in ethane. Ethane, , has one carbon–carbon bond and six carbon–hydrogen bonds. How many shared pairs are there, and does each carbon have an octet?
**Each carbon has three C–H bonds and one C–C bond: bonds, so electrons — an octet.
An everyday example. The cooking gas in an LPG cylinder is mainly propane and butane, chains of three and four carbon atoms; kerosene has longer chains, and paraffin wax longer still. The same two properties give gas, liquid and solid fuels simply by lengthening the chain.
Two boundary cases.
- Silicon, just below carbon in group 14, is also tetravalent, but its bonds to other silicon atoms are much weaker, so it shows only limited catenation — which is why there is no comparable chemistry of silicon chains
- Not every carbon compound is classed as organic.** Carbon dioxide, carbon monoxide, carbonates and cyanides are usually studied in inorganic chemistry
How are single, double and triple bonds, chains and rings, and benzene represented?
A single line stands for one shared pair, a double line for two and a triple line for three; carbon chains can be straight, branched or closed into rings, and benzene is a six-carbon ring with alternating single and double bonds.
1. Bonds between carbon atoms.
- Single bond — one shared pair: ethane,
- Double bond — two shared pairs: ethene,
- Triple bond — three shared pairs: ethyne,
Saturated and unsaturated.
- Saturated compounds contain only single bonds between carbon atoms — alkanes
- Unsaturated compounds contain at least one double or triple bond — alkenes and alkynes
2. Types of carbon chain.
- Straight chain — carbon atoms joined one after another, with no branches:
- Branched chain — at least one carbon atom joined to three or four other carbons:
- Cyclic or closed chain — the chain joins back on itself to form a ring: cyclohexane, , is a ring of six groups
3. The structure of benzene, .
- Six carbon atoms form a flat hexagonal ring
- Each carbon is bonded to one hydrogen atom
- The ring is drawn with alternating single and double bonds between the carbon atoms
- Because the electrons of the double bonds are in fact spread evenly round the ring, benzene is also drawn as a hexagon with a circle inside
Worked check — does every carbon in benzene have four bonds? Each carbon has:
Worked check — does every carbon in 2-methylpropane have four bonds? The central carbon is bonded to three carbons and one hydrogen: . Each of the three outer carbons is bonded to one carbon and three hydrogens: . Molecular formula: .
An everyday example. Naphthalene balls, used to keep insects away from stored clothes, are made of a compound whose molecules contain two benzene-type rings fused together. Their strong smell and their slow disappearance from a cupboard come from those ring molecules escaping into the air.
The boundary case. A structure written in a bent or zig-zag line is still a straight chain if no carbon is joined to more than two others. Branching is about how carbon atoms are connected, not about how the chain is drawn on paper — a distinction that becomes vital when counting isomers.
1. Bonds between carbon atoms.
- Single bond — one shared pair: ethane,
- Double bond — two shared pairs: ethene,
- Triple bond — three shared pairs: ethyne,
Saturated and unsaturated.
- Saturated compounds contain only single bonds between carbon atoms — alkanes
- Unsaturated compounds contain at least one double or triple bond — alkenes and alkynes
2. Types of carbon chain.
- Straight chain — carbon atoms joined one after another, with no branches:
- Branched chain — at least one carbon atom joined to three or four other carbons:
- Cyclic or closed chain — the chain joins back on itself to form a ring: cyclohexane, , is a ring of six groups
3. The structure of benzene, .
- Six carbon atoms form a flat hexagonal ring
- Each carbon is bonded to one hydrogen atom
- The ring is drawn with alternating single and double bonds between the carbon atoms
- Because the electrons of the double bonds are in fact spread evenly round the ring, benzene is also drawn as a hexagon with a circle inside
Worked check — does every carbon in benzene have four bonds? Each carbon has:
Worked check — does every carbon in 2-methylpropane have four bonds? The central carbon is bonded to three carbons and one hydrogen: . Each of the three outer carbons is bonded to one carbon and three hydrogens: . Molecular formula: .
An everyday example. Naphthalene balls, used to keep insects away from stored clothes, are made of a compound whose molecules contain two benzene-type rings fused together. Their strong smell and their slow disappearance from a cupboard come from those ring molecules escaping into the air.
The boundary case. A structure written in a bent or zig-zag line is still a straight chain if no carbon is joined to more than two others. Branching is about how carbon atoms are connected, not about how the chain is drawn on paper — a distinction that becomes vital when counting isomers.
How do you draw the structural formulae of alkanes, alkenes and alkynes up to five carbons?
Draw the carbon chain, place the double or triple bond, then add hydrogen atoms until every carbon has four bonds — and check the formula against the general formula of the series.
General formulae:
- Alkanes:
- Alkenes:
- Alkynes:
1. Alkanes — single bonds only.
- Methane,
- Ethane, :
- Propane, :
- Butane, :
- Pentane, :
2. Alkenes — one double bond.
- Ethene, :
- Propene, :
- But-1-ene, :
- Pent-1-ene, :
3. Alkynes — one triple bond.
- Ethyne, :
- Propyne, :
- But-1-yne, :
- Pent-1-yne, :
Worked check 1 — hydrogens in pentane.
Worked check 2 — bonds in propene, carbon by carbon:
- First carbon: hydrogens (double bond)
- Second carbon: hydrogen (double bond) (single bond)
- Third carbon: hydrogens (single bond)
Worked check 3 — hydrogens in pent-1-yne.
The carbon of the triple bond that is bonded to another carbon has no hydrogen, because its three bonds to one carbon and one bond to the next already make four.
Why each series loses two hydrogens. Turning a single bond into a double bond uses two more bonding positions — one on each carbon — so two hydrogen atoms must go. A triple bond removes two more. **That is the difference of between the general formulae.
An everyday example. Ethyne, better known as acetylene, burns with oxygen in the very hot flame of a gas welding torch used in metal workshops across the country. Its triple bond stores a great deal of energy, released when the molecule burns.
The boundary case. There is no alkene or alkyne with one carbon atom, because a double or triple bond needs two carbon atoms to join. Each of these series begins at two carbons**, while the alkanes begin at one.
General formulae:
- Alkanes:
- Alkenes:
- Alkynes:
1. Alkanes — single bonds only.
- Methane,
- Ethane, :
- Propane, :
- Butane, :
- Pentane, :
2. Alkenes — one double bond.
- Ethene, :
- Propene, :
- But-1-ene, :
- Pent-1-ene, :
3. Alkynes — one triple bond.
- Ethyne, :
- Propyne, :
- But-1-yne, :
- Pent-1-yne, :
Worked check 1 — hydrogens in pentane.
Worked check 2 — bonds in propene, carbon by carbon:
- First carbon: hydrogens (double bond)
- Second carbon: hydrogen (double bond) (single bond)
- Third carbon: hydrogens (single bond)
Worked check 3 — hydrogens in pent-1-yne.
The carbon of the triple bond that is bonded to another carbon has no hydrogen, because its three bonds to one carbon and one bond to the next already make four.
Why each series loses two hydrogens. Turning a single bond into a double bond uses two more bonding positions — one on each carbon — so two hydrogen atoms must go. A triple bond removes two more. **That is the difference of between the general formulae.
An everyday example. Ethyne, better known as acetylene, burns with oxygen in the very hot flame of a gas welding torch used in metal workshops across the country. Its triple bond stores a great deal of energy, released when the molecule burns.
The boundary case. There is no alkene or alkyne with one carbon atom, because a double or triple bond needs two carbon atoms to join. Each of these series begins at two carbons**, while the alkanes begin at one.
What is isomerism, and how are chain and position isomers drawn?
Isomers are compounds with the same molecular formula but different structural formulae; chain isomers differ in the arrangement of the carbon chain, and position isomers differ in the position of a double bond, triple bond or functional group.
Definition. Isomerism is the existence of two or more compounds with the same molecular formula but different structural formulae, and therefore different properties. Isomers of this kind are structural isomers.
1. Chain isomerism — the carbon skeleton is arranged differently.
**Butane, , has two chain isomers:**
**Pentane, , has three chain isomers:**
2. Position isomerism — the chain is the same, but a multiple bond or functional group sits in a different position.
**Butene, :**
**Butyne, :**
**Chloropropane, :**
Worked check — are but-1-ene and but-2-ene really isomers? Count the atoms in each:
- But-1-ene: carbons ; hydrogens
- But-2-ene: carbons ; hydrogens
**Both are , with different structures — so they are isomers.
Worked check — 2,2-dimethylpropane. The central carbon is bonded to four** methyl groups: bonds. Hydrogens ; carbons . ** — the same formula as pentane.
Where isomerism begins. Methane, ethane and propane have no chain isomers, because there is only one way to join one, two or three carbon atoms in a chain. Butane is the first alkane with an isomer.
Isomers have different properties. Branched chains pack together less closely, so 2-methylpropane boils at a lower temperature than butane, even though they have the same formula.
An everyday example. The gas in some LPG cylinders and aerosol cans contains both butane and its isomer 2-methylpropane**, sometimes called isobutane — two different compounds that happen to share the formula .
The boundary case — a common false isomer. drawn with the last carbon bent downwards is not a new isomer; it is butane drawn differently. Numbering the longest continuous chain tells you whether two drawings are the same compound or genuine isomers.
Definition. Isomerism is the existence of two or more compounds with the same molecular formula but different structural formulae, and therefore different properties. Isomers of this kind are structural isomers.
1. Chain isomerism — the carbon skeleton is arranged differently.
**Butane, , has two chain isomers:**
**Pentane, , has three chain isomers:**
2. Position isomerism — the chain is the same, but a multiple bond or functional group sits in a different position.
**Butene, :**
**Butyne, :**
**Chloropropane, :**
Worked check — are but-1-ene and but-2-ene really isomers? Count the atoms in each:
- But-1-ene: carbons ; hydrogens
- But-2-ene: carbons ; hydrogens
**Both are , with different structures — so they are isomers.
Worked check — 2,2-dimethylpropane. The central carbon is bonded to four** methyl groups: bonds. Hydrogens ; carbons . ** — the same formula as pentane.
Where isomerism begins. Methane, ethane and propane have no chain isomers, because there is only one way to join one, two or three carbon atoms in a chain. Butane is the first alkane with an isomer.
Isomers have different properties. Branched chains pack together less closely, so 2-methylpropane boils at a lower temperature than butane, even though they have the same formula.
An everyday example. The gas in some LPG cylinders and aerosol cans contains both butane and its isomer 2-methylpropane**, sometimes called isobutane — two different compounds that happen to share the formula .
The boundary case — a common false isomer. drawn with the last carbon bent downwards is not a new isomer; it is butane drawn differently. Numbering the longest continuous chain tells you whether two drawings are the same compound or genuine isomers.
Exam tip
What earns full marks when drawing organic structures and isomers?
Give every carbon exactly four bonds, check each structure against its general formula, and name each isomer so the examiner can see it is genuinely different.
- Define tetravalency and catenation in one line each, with the reason for catenation — small size and strong C–C bonds
- Draw every bond when a question asks for full structural formulae, including C–H bonds
- Check four bonds on every carbon, counting a double bond as two and a triple bond as three
- Check the hydrogen count against , or
- Describe benzene as a flat six-carbon ring with one hydrogen on each carbon and alternating single and double bonds
- Label straight chain, branched chain and cyclic structures correctly
- Name each isomer — butane and 2-methylpropane; but-1-ene and but-2-ene
- State the type of isomerism — chain or position
- Do not redraw the same molecule bent differently and call it an isomer
- Say that methane, ethane and propane have no isomers
The misconception to name. A carbon with five bonds is never correct. **Drawing gives the first carbon five bonds — three hydrogens plus a double bond — and is one of the most common errors in the chapter. Counting bonds on every carbon prevents it.
A second trap. Treating but-1-ene and but-3-ene as two different compounds. But-3-ene is but-1-ene numbered from the wrong end**, and the lowest number is always used.
- Define tetravalency and catenation in one line each, with the reason for catenation — small size and strong C–C bonds
- Draw every bond when a question asks for full structural formulae, including C–H bonds
- Check four bonds on every carbon, counting a double bond as two and a triple bond as three
- Check the hydrogen count against , or
- Describe benzene as a flat six-carbon ring with one hydrogen on each carbon and alternating single and double bonds
- Label straight chain, branched chain and cyclic structures correctly
- Name each isomer — butane and 2-methylpropane; but-1-ene and but-2-ene
- State the type of isomerism — chain or position
- Do not redraw the same molecule bent differently and call it an isomer
- Say that methane, ethane and propane have no isomers
The misconception to name. A carbon with five bonds is never correct. **Drawing gives the first carbon five bonds — three hydrogens plus a double bond — and is one of the most common errors in the chapter. Counting bonds on every carbon prevents it.
A second trap. Treating but-1-ene and but-3-ene as two different compounds. But-3-ene is but-1-ene numbered from the wrong end**, and the lowest number is always used.
Did you know
Why is diamond so hard and graphite so soft when both are pure carbon?
Diamond is the hardest natural substance, used to cut glass and drill through rock. Graphite is soft enough to leave a grey mark on paper — it is the so-called lead in a pencil. Both are nothing but carbon atoms. The difference is entirely in how catenation joins those atoms together.
In diamond, every carbon atom uses all four of its bonds to link to four other carbon atoms, arranged in a tetrahedron. The result is one continuous three-dimensional network of strong covalent bonds running through the whole crystal.
- To scratch or break diamond, you must break carbon–carbon bonds in every direction
- Every electron is locked in a bond, so diamond does not conduct electricity
In graphite, each carbon atom bonds to only three others, forming flat sheets of hexagons like a honeycomb, rather like endless sheets of joined benzene rings without the hydrogens.
- Within a sheet, the bonds are strong
- Between sheets, only weak forces hold them together, so the layers slide over each other easily — which is why graphite feels slippery and marks paper
- The fourth electron of each carbon is free to move along the sheets, so graphite conducts electricity, which is why it is used for electrodes
So one element, one kind of bond, and two completely different materials — decided only by whether catenation builds a three-dimensional network or stacked flat layers.
Carbon can do still more. Chemists have made hollow cages of carbon atoms shaped like footballs, and single sheets of graphite just one atom thick, each with properties unlike either diamond or graphite. All of them follow from the two properties of this lesson: four bonds per atom, and a willingness to bond with itself.
And the link to organic chemistry. In diamond and graphite, carbon bonds only to carbon. Let some of those bonds go to hydrogen, oxygen and nitrogen instead, and the same catenation builds the chains and rings of fuels, sugars, proteins and plastics.
In diamond, every carbon atom uses all four of its bonds to link to four other carbon atoms, arranged in a tetrahedron. The result is one continuous three-dimensional network of strong covalent bonds running through the whole crystal.
- To scratch or break diamond, you must break carbon–carbon bonds in every direction
- Every electron is locked in a bond, so diamond does not conduct electricity
In graphite, each carbon atom bonds to only three others, forming flat sheets of hexagons like a honeycomb, rather like endless sheets of joined benzene rings without the hydrogens.
- Within a sheet, the bonds are strong
- Between sheets, only weak forces hold them together, so the layers slide over each other easily — which is why graphite feels slippery and marks paper
- The fourth electron of each carbon is free to move along the sheets, so graphite conducts electricity, which is why it is used for electrodes
So one element, one kind of bond, and two completely different materials — decided only by whether catenation builds a three-dimensional network or stacked flat layers.
Carbon can do still more. Chemists have made hollow cages of carbon atoms shaped like footballs, and single sheets of graphite just one atom thick, each with properties unlike either diamond or graphite. All of them follow from the two properties of this lesson: four bonds per atom, and a willingness to bond with itself.
And the link to organic chemistry. In diamond and graphite, carbon bonds only to carbon. Let some of those bonds go to hydrogen, oxygen and nitrogen instead, and the same catenation builds the chains and rings of fuels, sugars, proteins and plastics.
Exam relevance
How does the structure of carbon compounds prepare you for JEE and NEET?
This is foundation work for Class 11 Organic Chemistry – Some Basic Principles and Techniques and Class 11 Hydrocarbons, both examined in JEE Main and NEET Chemistry, and the base of every organic chapter in Class 12.
Where tetravalency leads. Class 11 explains the shapes of carbon compounds through hybridisation: carbon with four single bonds is sp³ and tetrahedral, with a double bond sp² and planar, with a triple bond sp and linear. Identifying the hybridisation of each carbon in a structure is a recurring question in both exams, and it starts with counting the bonds you draw here.
Where structural formulae lead. Class 11 uses several ways of writing structures — complete, condensed and bond-line formulae. Converting between them without losing or adding a hydrogen depends entirely on the four-bond check practised in this lesson.
Where isomerism leads. Class 11 extends structural isomerism to functional group isomerism and metamerism, and introduces stereoisomerism, including geometrical isomerism of alkenes such as but-2-ene. Counting structural isomers of a given formula is a standard objective question — pentane has three and hexane five, and candidates who miss a branched structure lose the mark.
Where benzene leads. The aromatic nature of benzene, its resonance structures and its substitution reactions are studied in Class 11 Hydrocarbons. The picture of electrons spread evenly round the ring is the idea of resonance in its simplest form.
Where saturated and unsaturated lead. The addition reactions of alkenes and alkynes and the substitution reactions of alkanes follow directly from whether multiple bonds are present — the topic of Part 3 of this chapter and of Class 11 Hydrocarbons.
Question types to expect. At this level: explaining catenation and tetravalency, drawing structures and isomers, and identifying chain types. In competitive papers: hybridisation, number of structural isomers, bond-line formulae, geometrical isomerism and aromaticity.
The single trap that costs marks. Counting the same molecule twice when listing isomers. Two drawings are the same compound if their longest chain and branch positions match, and both exams use isomer-counting questions precisely to catch duplicates and omissions.
A second trap. Giving a carbon atom five bonds when converting a structure to a condensed or bond-line form. The four-bond check on every carbon is the protection, at every level.
Board versus competitive emphasis. The ICSE paper marks correct full structures, the two kinds of isomerism and named examples; a competitive paper marks an isomer count, a hybridisation or a stereochemical relationship. The transferable habit is drawing the carbon skeleton first and adding hydrogens last — the method that works for every structure you will ever need to draw.
Where tetravalency leads. Class 11 explains the shapes of carbon compounds through hybridisation: carbon with four single bonds is sp³ and tetrahedral, with a double bond sp² and planar, with a triple bond sp and linear. Identifying the hybridisation of each carbon in a structure is a recurring question in both exams, and it starts with counting the bonds you draw here.
Where structural formulae lead. Class 11 uses several ways of writing structures — complete, condensed and bond-line formulae. Converting between them without losing or adding a hydrogen depends entirely on the four-bond check practised in this lesson.
Where isomerism leads. Class 11 extends structural isomerism to functional group isomerism and metamerism, and introduces stereoisomerism, including geometrical isomerism of alkenes such as but-2-ene. Counting structural isomers of a given formula is a standard objective question — pentane has three and hexane five, and candidates who miss a branched structure lose the mark.
Where benzene leads. The aromatic nature of benzene, its resonance structures and its substitution reactions are studied in Class 11 Hydrocarbons. The picture of electrons spread evenly round the ring is the idea of resonance in its simplest form.
Where saturated and unsaturated lead. The addition reactions of alkenes and alkynes and the substitution reactions of alkanes follow directly from whether multiple bonds are present — the topic of Part 3 of this chapter and of Class 11 Hydrocarbons.
Question types to expect. At this level: explaining catenation and tetravalency, drawing structures and isomers, and identifying chain types. In competitive papers: hybridisation, number of structural isomers, bond-line formulae, geometrical isomerism and aromaticity.
The single trap that costs marks. Counting the same molecule twice when listing isomers. Two drawings are the same compound if their longest chain and branch positions match, and both exams use isomer-counting questions precisely to catch duplicates and omissions.
A second trap. Giving a carbon atom five bonds when converting a structure to a condensed or bond-line form. The four-bond check on every carbon is the protection, at every level.
Board versus competitive emphasis. The ICSE paper marks correct full structures, the two kinds of isomerism and named examples; a competitive paper marks an isomer count, a hybridisation or a stereochemical relationship. The transferable habit is drawing the carbon skeleton first and adding hydrogens last — the method that works for every structure you will ever need to draw.
Key takeaways
What must you be able to do from this part?
Two properties of carbon, three kinds of bond and chain, fifteen structures and two kinds of isomer.
- Tetravalency: carbon, , shares four electrons to form four covalent bonds
- Catenation: carbon bonds to itself in chains and rings, because it is small and forms strong C–C bonds
- These, with multiple bonds, bonds to other elements and isomerism, explain the huge number of organic compounds
- Silicon shows only limited catenation; CO2, CO, carbonates and cyanides are usually treated as inorganic
- Single, double, triple bonds: , ,
- Saturated compounds have single bonds only; unsaturated have double or triple bonds
- Straight chain: butane; branched chain: 2-methylpropane; cyclic: cyclohexane and benzene
- Benzene, : flat hexagonal ring, one H per carbon, alternating single and double bonds
- Alkanes : methane to pentane
- Alkenes : ethene, propene, but-1-ene, pent-1-ene
- Alkynes : ethyne, propyne, but-1-yne, pent-1-yne
- Every carbon has four bonds; check hydrogens against the general formula
- Isomers: same molecular formula, different structural formula
- Chain isomers: butane and 2-methylpropane; pentane, 2-methylbutane and 2,2-dimethylpropane
- Position isomers: but-1-ene and but-2-ene; but-1-yne and but-2-yne; 1-chloropropane and 2-chloropropane
- Methane, ethane and propane have no isomers
The sharpest self-test is one formula and a blank page. Draw every structural isomer of and every position isomer of with a straight chain, check four bonds on every carbon — then cross out any drawing that turns out to be a repeat.
- Tetravalency: carbon, , shares four electrons to form four covalent bonds
- Catenation: carbon bonds to itself in chains and rings, because it is small and forms strong C–C bonds
- These, with multiple bonds, bonds to other elements and isomerism, explain the huge number of organic compounds
- Silicon shows only limited catenation; CO2, CO, carbonates and cyanides are usually treated as inorganic
- Single, double, triple bonds: , ,
- Saturated compounds have single bonds only; unsaturated have double or triple bonds
- Straight chain: butane; branched chain: 2-methylpropane; cyclic: cyclohexane and benzene
- Benzene, : flat hexagonal ring, one H per carbon, alternating single and double bonds
- Alkanes : methane to pentane
- Alkenes : ethene, propene, but-1-ene, pent-1-ene
- Alkynes : ethyne, propyne, but-1-yne, pent-1-yne
- Every carbon has four bonds; check hydrogens against the general formula
- Isomers: same molecular formula, different structural formula
- Chain isomers: butane and 2-methylpropane; pentane, 2-methylbutane and 2,2-dimethylpropane
- Position isomers: but-1-ene and but-2-ene; but-1-yne and but-2-yne; 1-chloropropane and 2-chloropropane
- Methane, ethane and propane have no isomers
The sharpest self-test is one formula and a blank page. Draw every structural isomer of and every position isomer of with a straight chain, check four bonds on every carbon — then cross out any drawing that turns out to be a repeat.