CARBON AND COMPOUNDS

Introduction

Carbon is a non-metallic element that forms the basis of a large number of compounds. It is present in food, fuels, medicines, plastics, and living organisms.

• Symbol: C
• Atomic number: 6
• Electronic configuration: 2, 4
• Valency: 4

Carbon forms a large number of compounds because of its two important properties: tetravalency and catenation.

Covalent Bonding in Carbon

Carbon has four electrons in its outermost shell. It shares electrons with other atoms to complete its octet and form covalent bonds.

Covalent bond: A chemical bond formed by the mutual sharing of electron pairs between atoms.

Types of Covalent Bonds

1. Single bond: One pair of electrons is shared. Example: Ethane (C₂H₆).
2. Double bond: Two pairs of electrons are shared. Example: Ethene (C₂H₄).
3. Triple bond: Three pairs of electrons are shared. Example: Ethyne (C₂H₂).

Properties of Covalent Compounds

• They generally have low melting and boiling points.
• They are generally poor conductors of electricity.
• Many are insoluble in water but soluble in organic solvents.
• Many exist as gases or liquids at room temperature.

These are general properties; some covalent substances, such as diamond, have very high melting points.

Versatile Nature of Carbon

A. Tetravalency

Carbon has a valency of four. It can form four covalent bonds with atoms of carbon and other elements.

B. Catenation

Catenation is the ability of an element to form bonds with atoms of the same element, producing chains, branched structures, or rings.

Carbon exhibits extensive catenation because carbon–carbon bonds are strong and stable.

C. Multiple Bond Formation

Carbon can form single, double, and triple bonds with other carbon atoms.

Examples:

• Ethane: CH₃–CH₃
• Ethene: CH₂=CH₂
• Ethyne: HC≡CH

Allotropes of Carbon

Allotropy is the property of an element to exist in different forms in the same physical state.

Allotrope	Properties and uses
Diamond	Extremely hard; used in jewellery and cutting tools
Graphite	Soft and slippery; conducts electricity
Fullerene	Carbon atoms arranged in cage-like structures

Diamond and graphite have different properties because their carbon atoms are arranged differently.

Hydrocarbons

Hydrocarbons are compounds containing only carbon and hydrogen atoms.

Saturated Hydrocarbons

Hydrocarbons containing only single bonds between carbon atoms are called saturated hydrocarbons. They are known as alkanes.

Examples: Methane (CH₄), ethane (C₂H₆), propane (C₃H₈).

General formula: CₙH₂ₙ₊₂

Name	n	Molecular Formula	Condensed Form
Methane	1	CH₄	CH₄
Ethane	2	C₂H₆	CH₃–CH₃
Propane	3	C₃H₈	CH₃–CH₂–CH₃
Butane	4	C₄H₁₀	CH₃–(CH₂)₂–CH₃
Petane	5	C₅H₁₂	CH₃–(CH₂)₃–CH₃
Hexan	6	C₆H₁₄	CH₃–(CH₂)₄–CH₃

Unsaturated Hydrocarbons

Hydrocarbons containing at least one carbon–carbon double or triple bond are called unsaturated hydrocarbons.

Alkenes: Contain at least one double bond.

Example: Ethene (C₂H₄).

General formula for open-chain alkenes with one double bond: CₙH₂ₙ

Name	n	Molecular Formula	Condensed Form
Ethene	2	C₂H₄	CH₂=CH₂
Propene	3	C₃H₆	CH₂=CH–CH₃
Butene	4	C₄H₈	CH₂=CH–CH₂–CH₃
Pentene	5	C₅H₁₀	CH₂=CH–CH₂–CH₂–CH₃
Hexene	6	C₆H₁₂	CH₂=CH–CH₂–CH₂–CH₂–CH₃

Alkynes: Contain at least one triple bond.

Example: Ethyne (C₂H₂).

General formula for open-chain alkynes with one triple bond: CₙH₂ₙ₋₂

Name	n	Molecular Formula	Condensed Form
Ethyne	2	C₂H₂	HC≡CH
Propyne	3	C₃H₄	HC≡C–CH₃
Butyne	4	C₄H₆	HC≡C–CH₂–CH₃
Pentyne	5	C₅H₈	HC≡C–CH₂–CH₂–CH₃
Hexyne	6	C₆H₁₀	HC≡C–CH₂–CH₂–CH₂–CH₃

Saturated and Unsaturated Hydrocarbons

Saturated hydrocarbons	Unsaturated hydrocarbons
Contain only single carbon–carbon bonds	Contain double or triple carbon–carbon bonds
Called alkanes	Called alkenes or alkynes
Generally less reactive towards addition reactions	Commonly undergo addition reactions
Example: Ethane	Examples: Ethene and ethyne

Nomenclature of Carbon Compounds

Nomenclature is the systematic method of naming chemical compounds.

Prefixes Based on the Number of Carbon Atoms

Carbon atoms	Prefix	Example
1	Meth-	Methane
2	Eth-	Ethane
3	Prop-	Propane
4	But-	Butane
5	Pent-	Pentane
6	Hex-	Hexane

Suffixes Based on the Type of Bond

Compound	Bond	Suffix	Example
Alkane	Single	-ane	Ethane
Alkene	Double	-ene	Ethene
Alkyne	Triple	-yne	Ethyne

Functional Groups

A functional group is an atom or group of atoms that gives an organic compound its characteristic chemical properties.

Functional group	Representation	Example
Halo group	–Cl, –Br	Chloromethane
Alcohol	–OH	Ethanol
Aldehyde	–CHO	Ethanal
Ketone	>C=O	Propanone
Carboxylic acid	–COOH	Ethanoic acid

Important compounds:

• Methane: CH₄
• Ethane: C₂H₆
• Ethene: C₂H₄
• Ethyne: C₂H₂
• Ethanol: C₂H₅OH
• Ethanoic acid: CH₃COOH

HALO GROUP

Functional group: –Cl
General representation: R–Cl

Molecular Formula	Condensed Structural Formula	IUPAC Name	Common Name
CH₃Cl	CH₃–Cl	Chloromethane	Methyl chloride
C₂H₅Cl	CH₃–CH₂–Cl	Chloroethane	Ethyl chloride
C₃H₇Cl	CH₃–CH₂–CH₂–Cl	1-Chloropropane	n-Propyl chloride
C₄H₉Cl	CH₃–CH₂–CH₂–CH₂–Cl	1-Chlorobutane	n-Butyl chloride
C₅H₁₁Cl	CH₃–CH₂–CH₂–CH₂–CH₂–Cl	1-Chloropentane	n-Amyl chloride
C₆H₁₃Cl	CH₃–CH₂–CH₂–CH₂–CH₂–CH₂–Cl	1-Chlorohexane	n-Hexyl chloride

Important points:

• The halo group containing chlorine is represented by –Cl.
• In IUPAC nomenclature, chlorine is written as the prefix chloro-.
• In common nomenclature, the name is generally written as alkyl group + chloride.
• The table shows straight-chain compounds with chlorine attached to the first carbon atom.

ALCOHOL GROUP

Functional group: –OH
General representation: R–OH

Molecular Formula	Condensed Structural Formula	IUPAC Name	Common Name
CH₄O	CH₃–OH	Methanol	Methyl alcohol
C₂H₆O	CH₃–CH₂–OH	Ethanol	Ethyl alcohol
C₃H₈O	CH₃–CH₂–CH₂–OH	Propan-1-ol	n-Propyl alcohol
C₄H₁₀O	CH₃–CH₂–CH₂–CH₂–OH	Butan-1-ol	n-Butyl alcohol
C₅H₁₂O	CH₃–CH₂–CH₂–CH₂–CH₂–OH	Pentan-1-ol	n-Amyl alcohol
C₆H₁₄O	CH₃–CH₂–CH₂–CH₂–CH₂–CH₂–OH	Hexan-1-ol	n-Hexyl alcohol

Important points:

• The functional group of alcohols is the hydroxyl group (–OH).
• In IUPAC nomenclature, the suffix -ol is used.
• In common nomenclature, the name is generally written as alkyl group + alcohol.
• The table shows straight-chain primary alcohols with the –OH group attached to the first carbon atom.

KETONE GROUP

Functional group: >C=O

Molecular Formula	Condensed Formula	IUPAC Name	Common Name
C₃H₆O	CH₃COCH₃	Propanone	Acetone
C₄H₈O	CH₃COCH₂CH₃	Butan-2-one	Methyl ethyl ketone
C₅H₁₀O	CH₃COCH₂CH₂CH₃	Pentan-2-one	Methyl propyl ketone
C₆H₁₂O	CH₃COCH₂CH₂CH₂CH₃	Hexan-2-one	Methyl butyl ketone

Important points:

• The functional group of ketones is the carbonyl group (>C=O) within the carbon chain.
• In IUPAC nomenclature, the suffix “-one” is used.
• The carbonyl group in a ketone is attached to two carbon atoms.
• The general formula for saturated, open-chain ketones is CₙH₂ₙO.
• The simplest ketone is propanone (acetone). Ketones cannot have only one or two carbon atoms.

ALDEHYDE GROUP

Functional group: –CHO

Molecular Formula	Condensed Formula	IUPAC Name	Common Name
CH₂O	HCHO	Methanal	Formaldehyde
C₂H₄O	CH₃CHO	Ethanal	Acetaldehyde
C₃H₆O	C₂H₅CHO	Propanal	Propionaldehyde
C₄H₈O	C₃H₇CHO	Butanal	Butyraldehyde
C₅H₁₀O	C₄H₉CHO	Pentanal	Valeraldehyde
C₆H₁₂O	C₅H₁₁CHO	Hexanal	Caproaldehyde

Important points:

• The functional group of aldehydes is –CHO.
• In IUPAC nomenclature, the suffix -al is used.
• The aldehyde group is present at the end of the carbon chain.
• The general formula for saturated, open-chain aldehydes is CₙH₂ₙO.

CARBOXYLIC ACID GROUP

Functional group: –COOH

Molecular Formula	Condensed Formula	IUPAC Name	Common Name
CH₂O₂	HCOOH	Methanoic acid	Formic acid
C₂H₄O₂	CH₃COOH	Ethanoic acid	Acetic acid
C₃H₆O₂	C₂H₅COOH	Propanoic acid	Propionic acid
C₄H₈O₂	C₃H₇COOH	Butanoic acid	Butyric acid
C₅H₁₀O₂	C₄H₉COOH	Pentanoic acid	Valeric acid
C₆H₁₂O₂	C₅H₁₁COOH	Hexanoic acid	Caproic acid

Important points:

• The functional group of carboxylic acids is –COOH.
• In IUPAC nomenclature, the suffix “-oic acid” is used.
• The –COOH group is always at the end of the carbon chain.
• The general formula for saturated, open-chain monocarboxylic acids is CₙH₂ₙO₂.

Homologous Series

A homologous series is a group of organic compounds having the same functional group and similar chemical properties, in which consecutive members differ by a –CH₂– unit.

Characteristics

1. All members have the same functional group.
2. They have the same general formula.
3. Successive members differ by –CH₂–.
4. The difference in molecular mass between successive members is 14 u.
5. They have similar chemical properties.
6. Their physical properties show a gradual change with increasing molecular mass.

Example: Homologous series of alkanes

Name	Formula	Molecular mass
Methane	CH₄	16 u
Ethane	C₂H₆	30 u
Propane	C₃H₈	44 u
Butane	C₄H₁₀	58 u

ISOMERISM

Isomerism is the phenomenon in which two or more compounds have the same molecular formula but different structural arrangements of atoms. Such compounds are called isomers.

Isomerism occurs because carbon atoms can form straight chains, branched chains, and rings. The same number of carbon and hydrogen atoms can therefore be arranged in different ways.

Isomers of Butane (C₄H₁₀)

Name of Isomer	Molecular Formula	Structure
Butane (straight chain)	C₄H₁₀	CH₃–CH₂–CH₂–CH₃
2-Methylpropane (branched chain)	C₄H₁₀	CH₃–CH(CH₃)–CH₃

Note: Butane has two structural isomers with the same molecular formula (C₄H₁₀) but different arrangements of carbon atoms.

Isomers of Pentane (C₅H₁₂)

Name of Isomer	Molecular Formula	Structure
Pentane (straight chain)	C₅H₁₂	CH₃–CH₂–CH₂–CH₂–CH₃
2-Methylbutane (branched chain)	C₅H₁₂	CH₃–CH(CH₃)–CH₂–CH₃
2,2-Dimethylpropane (branched chain)	C₅H₁₂	C(CH₃)₄

Important Points to Remember

Note: Pentane has three structural isomers. All have the same molecular formula (C₅H₁₂) but different arrangements of carbon atoms.

• Isomers have the same molecular formula.
• Isomers have different structural formulae.
• Isomerism is common in carbon compounds because carbon exhibits catenation.
• Butane (C₄H₁₀) is the first alkane in the homologous series to show chain isomerism.

Cyclic Compounds

Cycli compounds are carbon compounds in which carbon atoms are joined together to form a closed chain or ring.

Benzene (C₆H₆)

Benzene is a cyclic, unsaturated hydrocarbon consisting of six carbon atoms arranged in a ring, with alternating double bonds in its conventional structural representation.

Chemical Properties of Carbon Compounds

Combustion

Combustion is a chemical reaction in which a substance reacts with oxygen, releasing energy as heat and often light.

Example: Combustion of methane

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Carbon compounds are commonly used as fuels because many release considerable energy during combustion.

Incomplete combustion may produce carbon monoxide or soot when oxygen is insufficient.

Oxidation

Oxidation is a reaction in which a substance gains oxygen or loses hydrogen.

Ethanol is oxidised to ethanoic acid using suitable oxidising agents such as alkaline potassium permanganate or acidified potassium dichromate.

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Addition Reaction

An addition reaction occurs when atoms are added to an unsaturated compound.

Example: Hydrogenation of ethene

CH₂=CH₂ + H₂ → CH₃–CH₃

Condition: Nickel or another suitable catalyst.

Ethene is converted into ethane.

Hydrogenation is also used to make certain solid or semi-solid fats from unsaturated vegetable oils.

Substitution Reaction

A substitution reaction occurs when one atom or group of atoms is replaced by another.

Example: Reaction of methane with chlorine

CH₄ + Cl₂ → CH₃Cl + HCl

CH₄ + Cl₂ → CH₂Cl₂ + HCl

CH₄ + Cl₂ → CHCl₃ + HCl

CH₄ + Cl₂ → CCl₄ + HCl

Condition: Sunlight.

One hydrogen atom of methane is replaced by a chlorine atom.

Ethanol

Ethanol is an alcohol with the molecular formula C₂H₅OH.

Properties

• Colourless liquid with a characteristic smell.
• Soluble in water.
• Neutral to litmus under ordinary conditions.
• Combustible and releases energy on burning.
• Used as a solvent in various products.

Important Reactions

A. Reaction with sodium

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

Products: Sodium ethoxide and hydrogen gas.

B. Dehydration of ethanol

When ethanol is heated with concentrated sulphuric acid at 443 K, it forms ethene and water.

C₂H₅OH → CH₂=CH₂ + H₂O

C. Combustion of Ethanol

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat

Uses of Ethanol

• Used as a solvent in medicines and perfumes.
• Used in certain fuel blends.
• Used in the manufacture of chemicals.
• Used in suitable disinfectant formulations.

Ethanol is flammable. Alcohol intended for industrial or laboratory use must never be consumed.

Denaturation of Alcohol

The process of making ethanol unfit for drinking by adding poisonous substances such as methanol is called denaturation of alcohol.

Methanol is highly poisonous and can cause blindness or even death if consumed.

Real-life Example: Denatured alcohol is used in spirit lamps in laboratories and as a cleaning solvent. The alcohol is made unfit for drinking to prevent its misuse.

Uses of Denatured Alcohol:

1. Used as an industrial solvent.
2. Used as a fuel in spirit lamps.
3. Used in various industrial processes.

Note: Denatured alcohol is poisonous and must never be consumed.

Ethanoic Acid

Ethanoic acid, also called acetic acid, is a carboxylic acid with the formula CH₃COOH.

A dilute solution containing approximately 5–8% ethanoic acid in water is commonly known as vinegar.

Properties

• Colourless liquid with a characteristic pungent smell.
• Soluble in water.
• Turns blue litmus red.
• It is a weak acid.
• Pure ethanoic acid freezes at approximately 16.6°C and is called glacial acetic acid.

Important Reactions

A. Reaction with sodium hydroxide

CH₃COOH + NaOH → CH₃COONa + H₂O

Products: Sodium ethanoate and water.

B. Reaction with sodium hydrogen carbonate

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Carbon dioxide gas is released, causing effervescence.

C. Reaction with sodium carbonate

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Esterification

Esterification is the reaction between a carboxylic acid and an alcohol to form an ester and water, usually in the presence of concentrated sulphuric acid.

Example: Ethanoic acid reacts with ethanol

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Condition: Concentrated H₂SO₄ and warming.

Products: Ethyl ethanoate and water.

Esters generally have pleasant, fruity smells and are used in perfumes and flavouring agents.

Soaps and Detergents

Soap

Soap is generally the sodium or potassium salt of a long-chain fatty acid.

Soap is prepared by heating fats or oils with sodium hydroxide or potassium hydroxide. This process is called saponification.

Detergents

Detergents are cleansing agents, commonly synthetic, that are generally effective in both soft and hard water.

Cleansing Action of Soap

A soap molecule has two parts:

1. Hydrophilic head: Attracted to water.
2. Hydrophobic tail: Attracted to oil and grease.

The hydrophobic tails attach to oily dirt while the hydrophilic heads remain in contact with water. Soap molecules form structures called micelles, which help suspend dirt in water so that it can be washed away.

Soaps and Detergents

Soaps	Detergents
Usually salts of long-chain fatty acids	Commonly synthetic cleansing agents
Form scum with calcium and magnesium ions	Many work effectively in hard water
Generally less effective in hard water	Generally more effective in hard water
Some soaps are readily biodegradable	Biodegradability depends on their formulation

Important Chemical Equations for Revision

1. Combustion of methaneCH₄ + 2O₂ → CO₂ + 2H₂O
2. Oxidation of ethanolCH₃CH₂OH + 2[O] → CH₃COOH + H₂O
3. Addition of hydrogen to etheneCH₂=CH₂ + H₂ → CH₃–CH₃
4. Substitution reaction of methaneCH₄ + Cl₂ → CH₃Cl + HCl
5. Ethanol with sodium2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
6. Ethanoic acid with sodium hydrogen carbonateCH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
7. EsterificationCH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Important Questions for Examination

Very Short-Answer Questions

1. What is catenation?
2. Define a covalent bond.
3. Why does carbon form four covalent bonds?
4. What are hydrocarbons?
5. Define a homologous series.
6. Write the molecular formula of ethene and ethyne.
7. Name the functional group present in ethanol.
8. What is vinegar?
9. What is esterification?
10. What is saponification?

Short-Answer Questions

1. Explain tetravalency and catenation.
2. Differentiate between saturated and unsaturated hydrocarbons.
3. State three characteristics of a homologous series.
4. Explain the difference between ethanol and ethanoic acid.
5. Why are soaps ineffective in hard water?
6. Explain the cleansing action of soap.
7. Write balanced chemical equations for the combustion of methane and the reaction between ethanoic acid and sodium hydrogen carbonate.
8. Explain addition and substitution reactions with examples.

Long-Answer Questions

1. Explain the chemical properties of carbon compounds with suitable equations.
2. Describe the properties and chemical reactions of ethanol and ethanoic acid.
3. Explain the formation of esters and mention their uses.
4. Compare soaps and detergents and explain their cleansing action.