Chapter – 4

Carbon and its Compounds

In this post we have given the detailed notes of class 10 Science Chapter 4 (Carbon and its Compounds) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 4: Carbon and its Compounds

Carbon 🧪

• Carbon is a non-metal with the chemical symbol C and atomic number 6. It has three naturally occurring isotopes: ¹²C, ¹³C and ¹⁴C. Its electronic configuration is 2, 4, and its valency is 4 — this is why carbon is called tetravalent.
• Food, clothes, medicines, books and almost everything we can list is based on this versatile element, carbon. In other words, all living forms are built of carbon.
• Occurrence of carbon: Carbon forms an extremely large number of compounds in nature. In the earth’s crust, carbon is present only to the extent of 0.02% in the form of minerals (carbonates, hydrogen carbonates, coal and petroleum), and the atmosphere contains about 0.03% carbon dioxide. Carbon is an essential element found everywhere in the universe and it forms a very wide variety of compounds. Many living and non-living things around us — plants, animals, sugar, fuel, paper, food, clothes, thread, medicines, cosmetics — are all made of carbon. These are all organic compounds obtained either from plants or from living organisms. The chemistry of these carbon compounds is known as organic chemistry.

Allotropes of Carbon 💎

Allotropes

• Different physical forms of the same element, having different physical properties but similar chemical properties, are called allotropes of that element. Carbon has three well-known allotropes — graphite, diamond and buckminsterfullerene — all made up of carbon atoms only.

Graphite

• Each carbon atom is bonded to three other carbon atoms in the same plane, giving a hexagonal array. One of the bonds is a double bond, so the valency of carbon is satisfied. Graphite is a very good conductor of electricity, unlike other non-metals.

Diamond

• In diamond, each carbon atom is bonded to four other carbon atoms, forming a rigid three-dimensional structure. This makes diamond the hardest known natural substance and a very poor conductor of electricity.

Fullerenes

Fullerene is another class of carbon allotrope. The first one identified was C-60, in which carbon atoms are arranged in a football-like structure. Since it resembles the geodesic dome designed by American architect Buckminster Fuller, this molecule was named fullerene.

Bonding in Carbon

Carbon has four electrons in its outermost shell and needs to gain or lose four electrons to attain a noble gas configuration.

• It could gain four electrons to form a C^4- anion, but it would be difficult for the nucleus with six protons to hold on to ten electrons, i.e. four extra electrons.
• It could lose four electrons to form a C⁴⁺ cation, but this would require a huge amount of energy to remove four electrons, leaving behind a carbon cation with only two electrons around a nucleus of six protons.

Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements. Not just carbon, atoms of other elements also form compounds by sharing electrons in this manner.

Chemical Bonds

The bond formed between atoms of elements in a compound, due to the force acting between them, is called a chemical bond. Chemical bonds are of two types.

(i) Ionic bond: A bond formed by the complete transfer of electrons is called an ionic bond.

Example: Na⁺ + Cl^– → NaCl

(ii) Covalent bond: A bond formed between two atoms by the sharing of a pair of electrons is called a covalent bond. There are three types of covalent bonds:

• Single covalent bond: A covalent bond formed by the sharing of one electron pair between two atoms is called a single bond. It is represented by a single line (-) between two atoms. Example: H – H, Cl – Cl, Br – Br
• Double covalent bond: A covalent bond formed by the sharing of two electron pairs between two atoms is called a double bond. It is represented by two short lines (=) between two atoms. Example: O = O (double bond between two oxygen atoms).
• Triple covalent bond: A bond formed by the sharing of three electron pairs between two atoms is called a triple bond. It is represented by three short lines (≡) between two atoms. Example: N ≡ N (triple bond between two nitrogen atoms).

Properties of Compounds Having Covalent Bonds

• Compounds with covalent bonds generally have strong bonds within the molecule.
• The intermolecular forces of attraction in such compounds are weak.
• They generally have low melting and boiling points.
• These compounds are usually poor conductors of electricity.

Other Properties of Carbon

1. Catenation: Carbon has a unique ability to form bonds with other atoms of carbon itself, giving rise to large molecules. This property is called catenation. The covalent nature of the bonding gives carbon the property of forming a very large number of compounds.

2. Tetravalency: Since carbon has a valency of four, it can bond with four other atoms of carbon or atoms of some other monovalent elements. This property is called the tetravalency of carbon.

Some Properties of Carbon Bonds

• Bonds formed by carbon with most other elements are extremely strong, making the resultant compounds exceptionally stable.
• One reason for the strength of these bonds is the small size of the carbon atom.
• This allows the nucleus to hold on to the shared pairs of electrons strongly.
• Bonds formed by larger atoms are comparatively much weaker.

Why Carbon Forms a Very Large Number of Compounds

Because of the following properties of carbon, a very large number of organic compounds are found in nature:

• Formation of covalent bonds: Because of its ability to form covalent bonds, carbon forms a very large number of compounds.
• Catenation: Carbon-carbon bonds are very strong and stable. Because of this, carbon atoms can link with each other to give a large number of compounds.
• Tetravalency: Since carbon has a valency of four, it has the capacity to bond with four other atoms of carbon or some other monovalent elements, which results in a large number of compounds.

Hydrocarbons ⛽

• All those carbon compounds which are made up of only carbon and hydrogen are called hydrocarbons.

Difference Between Saturated and Unsaturated Carbon Compounds

Formulae of Organic Compounds

• General formula: A formula that represents the number of atoms (n) of each element in a molecule is called the general formula. Example: for alkanes, C_nH_2n+2
• Molecular formula: The molecular formula shows the actual number of atoms in a molecule. Example: for ethane, C₂H₆ — 2 carbon and 6 hydrogen atoms.
• Condensed formula: The condensed formula shows the groups of atoms attached to each carbon atom. Example: for ethane, CH₃CH₃
• Structural formula: This shows the exact arrangement of atoms in a molecule.
• Electron dot formula: This represents the sharing of electrons between the atoms of a molecule, also called the electron dot structure.

Saturated Carbon Compounds

Carbon compounds in which carbon atoms are linked with each other by only single bonds are called saturated carbon compounds. Example: all alkanes such as methane, ethane, propane and butane.

General formula of alkanes: C_nH_2n+2

Using this formula for methane (n = 1): C₁H₂₍₁₎₊₂ = CH₄

Similarly for ethane (n = 2): C₂H₂₍₂₎₊₂ = C₂H₆

In the same way, we can find the formulae for propane, butane, pentane and so on.

Alkane: A saturated hydrocarbon in which carbon atoms are linked only by single bonds is called an alkane.

Naming of Hydrocarbons

• 1 carbon atom — Meth-
• 2 carbon atoms — Eth-
• 3 carbon atoms — Prop-
• 4 carbon atoms — But-
• 5 carbon atoms — Pent-
• 6 carbon atoms — Hex-
• 7 carbon atoms — Hept-
• 8 carbon atoms — Oct-
• 9 carbon atoms — Non-
• 10 carbon atoms — Dec-

In alkanes, the name of the hydrocarbon is decided by the number of carbon atoms, with the suffix “-ane” added to the root.

Examples:

• CH₄ — Meth + ane = Methane
• C₂H₆ — Eth + ane = Ethane
• C₃H₈ — Prop + ane = Propane
• C₄H₁₀ — But + ane = Butane
• C₅H₁₂ — Pent + ane = Pentane
• C₆H₁₄ — Hex + ane = Hexane

Naming, Molecular Formula and Structure of Alkenes

The alkene series follows the general formula C_nH_2n and each member has one carbon-carbon double bond. The names of alkenes from C₂ to C₁₀ are: Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈), Pentene (C₅H₁₀), Hexene (C₆H₁₂), Heptene (C₇H₁₄), Octene (C₈H₁₆), Nonene (C₉H₁₈) and Decene (C₁₀H₂₀).

Structure of Alkynes

The general formula of alkynes is C_nH_2n-2. Ethyne is the simplest member of the alkyne series and has two carbon atoms.

Putting n = 2 in the formula: C₂H_2(2)-2 = C₂H₂, so ethyne = C₂H₂

Similarly for propyne, putting n = 3: C₃H_2(3)-2 = C₃H₄, so propyne = C₃H₄

Long chain formulae are written in a shorter way. For example, nonyne, CH≡CCH₂CH₂CH₂CH₂CH₂CH₂CH₃, is written as CH≡C (CH₂)₆CH₃.

Functional Groups

A functional group is an atom or group of atoms in a carbon compound joined in a particular way, which is largely responsible for the chemical reactions of that compound. Oxygen, chlorine, sulphur, nitrogen and other atoms may be present as functional groups in carbon compounds.

Heteroatom

• An element which replaces a hydrogen atom in a compound is called a heteroatom.
• Example: oxygen, chlorine, sulphur, nitrogen and other elements may be present in carbon compounds as part of a functional group — such elements are called heteroatoms.

Some Common Functional Groups

(i) Halogens: Halogens include non-metals like chlorine, fluorine, bromine and iodine, located in Group 17 of the modern periodic table.

(ii) Alcohol: Alcohol is another functional group that attaches to chains of hydrocarbons. A hydroxyl group (-OH) replaces a hydrogen atom from the hydrocarbon and forms an alcohol compound. Example: -OH attaches to alkanes to form alcohols like methanol, ethanol and propanol.

(iii) Aldehyde: A functional group in which a single oxygen atom is joined by a double bond to a carbon atom which is also attached to hydrogen.

(iv) Ketone: A functional group in which a carbon atom is joined by a double bond to a single oxygen atom, while attached to two other carbon groups.

(v) Carboxylic acid: A functional group in which a carbon atom is joined by a double bond to an oxygen atom, and also joined to a hydroxyl group.

Homologous Series

A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain, forming a series of molecules, is called a homologous series.

Example of a homologous series: As the molecular mass increases along a homologous series, physical properties such as melting point, boiling point and solubility in a given solvent show a regular pattern, but the chemical properties, which are determined by the functional group, remain similar throughout the series.

Nomenclature of carbon compounds: The systematic way of naming organic compounds is called nomenclature.

IUPAC name: A name given according to this system of naming is called the IUPAC name.

Chemical Properties of Carbon Compounds 🔥

1. Combustion

• Combustion is the process in which a compound burns in the presence of air/oxygen to give water and carbon dioxide.
• (i) Combustion of methane in air: CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light
• (ii) Combustion of ethanol in air gives CO₂, water, heat and light: CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + heat and light. This shows how carbon compounds release heat and light on combustion.
• Carbon compounds as fuels: Most carbon compounds also release a large amount of heat and light on burning.

2. Oxidation

• Oxidation is a reaction in which a carbon compound gains oxygen in the presence of an oxidising agent to form another carbon compound.
• Oxidising agents: Some substances have the ability to add oxygen to others; these are called oxidising agents. Example: alkaline potassium permanganate and acidified potassium dichromate are oxidising agents.
• When a few drops of alkaline potassium permanganate or acidified potassium dichromate are added to warm ethyl alcohol, it gets oxidised to acetic acid.

3. Addition Reaction

• Unsaturated compounds add atoms (usually hydrogen) in the presence of catalysts like nickel or palladium to form saturated compounds. This is called an addition reaction. This reaction is commonly used to hydrogenate vegetable oils.
• Catalyst: A catalyst is a substance that increases the rate of a reaction without itself being affected by the reaction.
• Hydrogenation reaction: Unsaturated hydrocarbons add hydrogen in the presence of catalysts like nickel or palladium to give saturated hydrocarbons. This is used industrially to hydrogenate vegetable oils (vanaspati ghee). Vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains.
• Which is better, and why: Unsaturated fatty acids (vegetable oils) are healthier. Fats from animals, like ghee, generally consist of saturated fatty acids which are harmful for health. Oils containing unsaturated fatty acids should be used for cooking since they are beneficial rather than harmful.

4. Substitution Reaction

• When an atom or group of atoms present in a saturated compound is replaced by another atom or group, it is called a substitution reaction. Chlorine is a heteroatom that replaces hydrogen in carbon compounds.
• When chlorine is added to a hydrocarbon in the presence of sunlight, it removes hydrogen atoms one by one. This is a very fast reaction.
• CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)

• Chemical properties of carbon compounds: Carbon compounds give carbon dioxide along with heat and light on combustion. Most carbon compounds also release a large amount of heat and light on burning.
• Carbon compounds can be easily oxidised on combustion.
• In the presence of catalysts such as palladium or nickel, unsaturated hydrocarbons add hydrogen to form saturated hydrocarbons.
• Saturated hydrocarbons are usually unreactive and are inert in the presence of most reagents.

Sooty Flame from Saturated Hydrocarbons

A limited supply of air causes incomplete combustion of the hydrocarbon, and this incomplete combustion produces a sooty flame even from saturated hydrocarbons. Domestic gas/kerosene stoves are designed with holes for air so that a sufficient supply of oxygen keeps the flame clean and blue.

If the bottom of cooking vessels turns black, it means:

• The air holes are blocked.
• The oxygen supply is not adequate.
• Your fuel is being wasted.

Harmful Effects of Burning Coal and Petroleum

• Their combustion produces oxides of sulphur and nitrogen, which are major pollutants in the environment.
• Incomplete combustion of coal and petroleum produces a sooty flame.
• Incomplete combustion of carbon and petroleum releases a dangerous pollutant called carbon monoxide.

Formation of Coal and Petroleum

Coal and petroleum are formed from biomass which has been subjected to various biological and geological processes. Coal is the remains of trees, ferns and other plants millions of years old that got buried under layers of rock, possibly due to earthquakes or volcanic eruptions, and gradually decayed to become coal. Oil and gas are remains of millions-of-years-old marine organisms whose bodies settled at the bottom of the sea and were covered with silt. Bacterial action on their remains, in the presence of high pressure, led to the formation of oil and gas.

Ethanol (CH₃CH₂OH)

• Ethanol is commonly known as alcohol.

Physical Properties of Ethanol

• Ethanol is a liquid at room temperature.
• It is a good solvent.
• Ethanol is soluble in water in all proportions.
• It is highly inflammable.

Chemical Properties of Ethanol

• Combustion: Ethanol burns in oxygen to give carbon dioxide and water.
• Dehydration: On heating with concentrated sulphuric acid, ethanol undergoes dehydration — water molecules are removed because concentrated sulphuric acid is a strong dehydrating agent.
• Oxidation: Carbon compounds can be oxidised using oxidising agents such as alkaline potassium permanganate or acidified potassium dichromate, since these substances add oxygen to carbon compounds.
• Esterification: The reaction of ethanol with a carboxylic acid forms an ester.

Uses of Ethanol

• It is an important component of all alcoholic drinks.
• It is also used industrially as a good solvent.
• It is used in medicines like tincture of iodine, cough syrup and various tonics.
• It is used to make methylated spirit for industrial use.
• On burning it gives carbon dioxide and water, so it can be used as a fuel.

Harmful Effects of Consuming Alcohol/Ethanol

• Consuming even small amounts of ethanol causes intoxication.
• Short-term intake of ethanol causes vomiting, headache, slurring of speech and drowsiness.
• Long-term consumption of ethanol leads to serious health problems such as alcohol toxicity, liver disease, nerve damage and permanent brain damage.
• It slows down metabolic processes and weakens the central nervous system. It reduces normal inhibition, leads to lack of coordination, mental confusion, drowsiness and numbness.

Denatured alcohol: To prevent the misuse of ethanol prepared for industrial use, a poisonous substance like methanol is added to it, making it unfit for drinking. A blue dye is also added to identify it. This is called denatured alcohol.

Reactions of Ethanol

(i) Reaction with sodium: Alcohol reacts with sodium to release hydrogen gas and form sodium ethoxide.

2Na + 2CH₃CH₂OH → 2CH₃CH₂O^–Na⁺ + H₂ (sodium ethoxide)

(ii) Reaction to obtain an unsaturated hydrocarbon: When ethanol is heated with excess concentrated sulphuric acid at 443K, it undergoes dehydration to form ethene.

Ethanoic Acid (CH₃COOH)

• Ethanoic acid is commonly called acetic acid and belongs to the carboxylic acid group.
• A 3-5% solution of acetic acid in water is called vinegar and is used as a preservative in pickles.
• Pure ethanoic acid has a melting point of 290K and so it freezes in cold climates in winter. This is why it is also called glacial acetic acid.

Properties of Ethanoic Acid

• It is acidic in nature.
• Ethanoic acid does not have a strong pungent smell (it is not odourless, but mild).
• The melting point of ethanoic acid is 290K.

Uses of Acetic Acid/Ethanoic Acid

• It is used as vinegar for the preservation of pickles.
• It is used as a laboratory reagent.
• It is used in the manufacture of white lead.
• It is used in the manufacture of rayon fibres.
• Acetic acid is used as a coagulant in the manufacture of rubber.
• It is also used as a solvent.

Reactions of Ethanoic Acid

(i) Esterification reaction: Esters are mainly formed by the reaction of an acid and an alcohol. Ethanoic acid reacts with pure ethanol in the presence of an acid catalyst to form an ester.

Ester: The compound formed by the reaction of ethanol and ethanoic acid is called an ester. Its molecular formula is CH₃COOCH₂CH₃.

Uses of esters: Esters have a sweet smell and are used:

• In making perfumes and as flavouring agents.
• In making soaps and detergents.
• Some esters are used in making polymers such as polyester.
• Esterification: The reaction by which an ester is formed is called esterification.

Saponification

The process by which an ester is broken down, in the presence of an acid or base, back into ethanol and ethanoic acid, is called saponification because it is used to make soap.

(ii) Reaction with a base: Like a mineral acid, ethanoic acid reacts with a base such as sodium hydroxide to form a salt (sodium ethanoate/sodium acetate) and water.

NaOH + CH₃COOH → CH₃COONa + H₂O

(iii) Reaction with carbonates and hydrogen carbonates: Ethanoic acid reacts with carbonates and hydrogen carbonates to form a salt, carbon dioxide and water.

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Soap and Detergent 🧼

Soap

• Soap molecules are sodium or potassium salts of long-chain carboxylic acids. The ionic part of a soap molecule dissolves in water while the carbon chain dissolves in oil. Soap cleans by forming a structure called a micelle.

Micelle

• When soap is at the surface of water, its molecules arrange themselves such that the ionic end is in water while the hydrocarbon tail is out of water and collects the oily dirt at its centre. This happens because a large group of molecules is formed. This structure is called a micelle.

The two ends of a soap molecule are important in forming a micelle:

• Hydrophilic end: The end of the soap molecule that is soluble in water is called hydrophilic.
• Hydrophobic end: The end of the soap molecule that is soluble in hydrocarbons, i.e. oily dirt, is called hydrophobic.

Difference Between Hydrophilic and Hydrophobic Ends

Hydrophilic end:

• It is soluble in water.
• It is the ionic end.
• In the micelle structure, it stays outside, dissolved in water.

Hydrophobic end:

• It is not soluble in water but is soluble in hydrocarbons (oil).
• It is not an ionic end.
• In the micelle structure, it stays inside, towards the oily part.

Cleansing action of soap: Soap cleans through micelle formation. The ionic end of soap molecules stays in water while the hydrocarbon tail dissolves in oily dirt, and they form a micelle structure. As a micelle, soap can clean because the oily dirt collects at the centre of the micelle, forming an emulsion in water. Micelles stay suspended as a colloid in the solution. The soap micelle helps dissolve dirt in water, and as the micelles float, the dirt is easily removed, leaving our clothes clean.

Properties of Micelles

• As micelles, soap is able to clean effectively.
• Micelles remain as a colloid in solution.
• They do not precipitate because of ion-ion repulsion.
• Soap micelles can scatter light.
• The soap micelle helps dissolve dirt in water.

Why Soap Does Not Lather with Hard Water

• When we wash with soap in hard water, we find that lather forms with difficulty, and even after washing, some insoluble substance (scum) remains. This is because soap reacts with the calcium and magnesium salts present in hard water, and so more soap needs to be used.

Detergents Are Effective Even in Hard Water

• Detergents are usually ammonium or sulphonate salts of long carboxylic acid chains. The charged ends of these compounds do not form insoluble substances with the calcium and magnesium ions in hard water. This is why they remain effective even in hard water.

Difference Between Soap and Detergent

Soap

• Soap molecules are sodium and potassium salts of long-chain carboxylic acids.
• It is not effective in hard water and does not lather well.
• Its cleaning action involves the formation of a micelle.
• It increases the hardness of water.

Detergent

• Detergents are ammonium or sulphonate salts of long carboxylic acid chains.
• It is effective in hard water and lathers well.
• Its cleaning action does not require micelle formation in the same way.
• It reduces the hardness of water.

Key Points to Remember

• Carbon is tetravalent with valency 4 and forms strong covalent bonds due to catenation and small atomic size. 🧪
• The three allotropes of carbon are diamond, graphite and fullerene, each with different physical properties. 💎
• Hydrocarbons made of only single bonds are saturated (alkanes); those with double or triple bonds are unsaturated (alkenes and alkynes).
• Functional groups such as -OH (alcohol), -CHO (aldehyde), -COOH (carboxylic acid) and halogens determine the chemical behaviour of carbon compounds.
• A homologous series is a family of compounds with the same functional group and similar chemical properties, differing by a -CH₂– unit.
• Ethanol is a common alcohol used as a solvent and in medicines, but is harmful when consumed in excess.
• Ethanoic acid (acetic acid) is used as vinegar and undergoes esterification and saponification.
• Soap cleans through micelle formation but does not work well in hard water, unlike detergents.

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