Chapter 4: Carbon and its Compounds | CBSE Class 10 | NCERT Notes

CARBON AND ITS COMPOUNDS

Most of the things are made up of compounds of carbon.

When a carbon compound is burnt, CO2 & water are produced. The presence of CO2 can be confirmed by passing it through lime water which turns milky.

All living structures and many non-living structures such as food, clothes, medicines, books etc. are carbon-based.

Earth’s crust has only 0.02% carbon (as minerals like carbonates, hydrogen carbonates, petroleum, coal etc.). The atmosphere has 0.03% CO2. But carbon has immense importance.

BONDING IN CARBON – THE COVALENT BOND

Carbon compounds are poor conductors of electricity. So the bonding does not form ions. They have low melting & boiling points as compared to ionic compounds.

 

Carbon Compounds	Melting point (K)	Boiling point (K)
Acetic acid (CH3COOH)	290	391
Chloroform (CHCl3)	209	334
Ethanol (CH3CH2OH)	156	351
Methane (CH4)	90	111

Atomic number (Z) of Carbon= 6.

Electronic configuration= 2, 4 (1s² 2s² 2p²).

Carbon has 4 electrons in the outermost shell. Gaining or losing 4 electrons is not possible to attain noble gas configuration because:

• Gaining 4 electrons (C^4–  anion) makes it difficult to hold 6 protons and 10 electrons.
• Losing 4 electrons (C⁴⁺ cation) needs high energy to leave 6 protons and two electrons.

This problem is overcome by sharing valence electrons with other atoms of carbon or other elements. Thus both atoms attain noble gas configuration.

The simplest molecule formed by the sharing of valence electrons is that of hydrogen (Z= 1). It has one electron in K shell and needs one more electron to fill the K shell. So two hydrogen atoms share their electrons to form a hydrogen molecule (H2) and attain the nearest noble gas (helium- 2 electrons in K shell) configuration.

To represent valence electrons, dots or crosses are used.

The shared pair of electrons constitute a covalent bond between 2 hydrogen atoms. It is represented by a line.

Electron dot structure of Chlorine:

Atomic number = 17.

Electronic configuration= 2, 8, 7 (7 electrons in valence shell). It forms a diatomic molecule (Cl2).

Electron dot structure of Oxygen:

Atomic number = 8. It has 6 electrons in L shell.

It requires two more electrons to complete its octet.

So the oxygen atom shares 2 electrons with another oxygen atom forming a double bond.

Electron dot structure for water (H2O):

H – O – H

Electron dot structure of Nitrogen:

Atomic number = 7. Electronic configuration= 2, 5.

It forms a diatomic molecule (N2).

To attain an octet, each nitrogen atom in a nitrogen molecule contributes 3 electrons forming triple bond.

Electron dot structure for methane (CH4):

Methane is one of the simplest compounds of carbon.

It is used as a fuel and is a major component of biogas and Compressed Natural Gas (CNG).

Valency of Hydrogen= 1.

Valency of Carbon= 4.

Carbon shares these electrons with 4 hydrogen atoms to get a noble gas configuration.

The bonds formed by sharing of an electron pair between two atoms are called covalent bonds.

Covalently bonded molecules have strong bonds within the molecule, but intermolecular forces are weak. This gives rise to the low melting & boiling points.

Since the electrons are shared between atoms and no charged particles are formed, covalent compounds are generally poor conductors of electricity.

ALLOTROPES (DIFFERENT FORMS) OF CARBON

E.g. Diamond, Graphite & Fullerenes.

In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure.

In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array. One bond is double-bond to satisfy valency. Hexagonal arrays are placed in layers one above the other.

Diamond & Graphite have different physical properties but same chemical properties.

Diamond is the hardest substance. Graphite is smooth and slippery and a very good conductor of electricity.

Synthetic diamonds can be produced by subjecting pure carbon to very high pressure and temperature. These are small but indistinguishable from natural diamonds. 

Fullerenes: The first identified one was C-60 which has carbon atoms arranged as a football. This looked like the geodesic dome designed by US architect Buckminster Fuller. So it was named fullerene.

CARBON AND ITS COMPOUNDS

VERSATILE NATURE OF CARBON

Carbon has two unique properties called Catenation & Tetravalency. So it can form millions of compounds.

This outnumbers the compounds formed by all the other elements put together.

1. CATENATION

It is the ability of carbon to form bonds with other atoms of carbon, giving rise to large molecules.

They may be long chains, branched chains or ring forms.

No other element exhibits catenation like carbon. Silicon forms compounds with hydrogen which have chains of up to 7 or 8 atoms, but these are very reactive. Carbon-carbon bond is very strong & stable. This gives large number of compounds.

2. TETRAVALENCY

Carbon can bond with four other atoms of carbon or some other monovalent elements.

Carbon compounds are formed with oxygen, hydrogen, nitrogen, sulphur, chlorine etc. giving specific properties.

Carbon atom is small-sized. So the nucleus can hold the shared pairs of electrons strongly. So carbon can make very stable compounds with other elements. The bonds formed by elements having bigger atoms are weaker.

It was thought that organic or carbon compounds could only be formed with the help of a vital force (i.e., a living system is needed).

Friedrich Wöhler (1828) disproved this by preparing urea from ammonium cyanate.

But carbon compounds, except for carbides, oxides of carbon, carbonate and hydrogencarbonate salts are studied under organic chemistry.

Saturated and Unsaturated Carbon Compounds

Saturated compounds: They are linked by only single bonds between the carbon atoms. These are not very reactive. E.g. Ethane (C₂H₆).

Structure of ethane:

Structure of propane (C₃H₈):

Unsaturated compounds: They have double or triple bonds between carbon atoms. They are more reactive. 

E.g. Ethene (C₂H₄): It needs double bond to satisfy the valency. 

Ethyne (C₂H₂): It has triple bond between carbon atoms to satisfy the valency (H – C ≡ C – H).

Electron dot structure for ethyne

Chains, Branches and Rings

Chains of carbon atoms contain more carbon atoms. E.g.

No. of C atoms	Name	Formula	Structure
1	Methane	CH4
2	Ethane	C2H6
3	Propane	C3H8
4	Butane	C4H10
5	Pentane	C5H12
6	Hexane	C6H14

Carbon ‘skeleton’ of 4 carbon atoms has two forms:

Complete molecules for two structures with formula C₄H₁₀

These structures have same formula C₄H₁₀. Such compounds with identical molecular formula but different structures are called structural isomers. 

Some compounds have carbon atoms arranged in the form of a ring. E.g., cyclohexane (C₆H₁₂).

Straight chain, branched-chain & cyclic compounds may be saturated or unsaturated. E.g. benzene (C₆H₆).

All carbon compounds that contain only carbon and hydrogen are called hydrocarbons.

Saturated hydrocarbons are called alkanes.

The unsaturated hydrocarbons which contain one or more double bonds are called alkenes. Those containing one or more triple bonds are called alkynes.

Will you be my Friend?

Carbon also bonds with other elements such as halogens, oxygen, nitrogen & sulphur. 

In a hydrocarbon chain, one or more hydrogens are replaced by these elements. 

The element replacing hydrogen is called a heteroatom.

Heteroatoms & the group containing these give specific properties to the compound, regardless of the length and nature of the chain. So they are called functional groups.

Some functional groups in carbon compounds

Free valency or valencies of the group are shown by the single line. The functional group is attached to the carbon chain through this valency.

Homologous Series

A functional group such as alcohol decides the properties of the carbon compound. E.g. chemical properties of CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH are very similar.

Such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.

Homologous series for alkanes: Succeeding members differ by a –CH₂- unit. E.g.

CH₄ and C₂H₆ – differ by a –CH₂- unit

C₂H₆ and C₃H₈ – differ by a –CH₂- unit

C₃H₈ and C₄H₁₀ – differ by a –CH₂- unit

They show a difference of 14 U in molecular masses b/w the pairs (atomic mass of carbon = 12 u, hydrogen = 1 u).

Homologous series for alkenes: They also differ by a –CH₂– unit. First member is ethene (C₂H₄). Succeeding members are C₃H₆, C₄H₈, C₅H₁₀ and so on.

General formula for alkenes is C_nH_2n [n = 2, 3, 4].

General formula for alkanes is C_nH_2n+2.

General formula for alkynes is C_nH_2n-2.

As the molecular mass increases, physical properties such as melting & boiling points, solubility in solvent etc. also increase. But chemical properties remain similar.

Homologous series of Alcohols:

Compounds	Difference in formula	Difference in molecular mass
CH3OH & C2H5OH	–CH2-	14 U
C2H5OH & C3H7OH	–CH2-	14 U
C3H7OH & C4H9OH	–CH2-	14 U
C4H9OH & C5H11OH	–CH2-	14 U

Nomenclature of Carbon Compounds

Method of naming a carbon compound:

1.  Identify the number of carbon atoms. E.g. three-carbon compound is named propane.

2.  Presence of functional group is indicated by a prefix or a suffix.

3.  If the suffix of the functional group begins with a vowel, the final letter ‘e’ is deleted from the name of the carbon chain. E.g., Propane with a ketone group is named as

Propane – ‘e’ = propan + ‘one’ = propanone.

4.  For unsaturated carbon chain, the final ‘ane’ is substituted by ‘ene’ or ‘yne’. E.g., propene (double bond), propyne (triple bond) etc.

Nomenclature of organic compounds:

CARBON AND ITS COMPOUNDS

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

Combustion

Carbon (all allotropic forms) & most carbon compounds burn in oxygen to give CO₂ releasing heat and light. These are oxidation reactions.

C + O₂ → CO₂ + heat & light

CH₄ + 2O₂ → CO₂ + 2H₂O + heat & light

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + heat & light

Saturated hydrocarbons generally give a clean flame.

Unsaturated carbon compounds give a yellow flame with black smoke or sooty deposit (carbon). E.g. Camphor & Naphthalene are unsaturated hydrocarbons. So they burn with yellow flame and leave residues.

Alcohol is saturated and burns with clean blue flame.

Light a Bunsen burner and adjust the air hole at the base to get different types of flames/presence of smoke.

If there is no sufficient supply of air, it results in incomplete combustion of even saturated hydrocarbons giving a yellow, sooty flame. In presence of sufficient supply of air with oxygen, it gives blue flame.

The gas/kerosene stove has inlets for sufficient supply of air, the fuel is burnt to give a clean blue flame.

Blackening of the bottom of cooking vessel indicates that the air holes are blocked and fuel is getting wasted.

Coal and petroleum have some nitrogen & sulphur. Their combustion forms oxides of sulphur & nitrogen. They are major air pollutants.

Why do substances burn with or without a flame?

A flame is produced only when gaseous substances burn. So a candle or LPG burns with a flame.

Wood, coal or charcoal burn with a flame at first due to the volatile substances in them. After that they just glow red and gives out heat.

Atoms of gas substance are heated and glow to produce flame. Each element produces characteristic colour. E.g. heating a copper wire in flame gives bluish green flame.

Yellow colour of a candle flame is due to the incomplete combustion of carbon particles. When light falls on them, they scatter yellow colour.

Formation of coal and petroleum (fossil fuels)

Fossil fuels were formed from biomass by biological and geological processes.

Millions of years ago, trees, ferns and other plants were crushed into the earth due to earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal.

Dead marine tiny plants and animals sank to the sea bed and were covered by silt. Due to bacterial action, they turned into oil & gas under high pressure. The silt was compressed into rock. The oil & gas seeped into porous rock parts, and got trapped like water in a sponge.

Oxidation

Carbon compounds are easily oxidised on combustion.

Alcohols can be oxidised to carboxylic acids. Here, oxidising agents like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇) are used. (oxidising agents: The substances that can add oxygen to others).

E.g. Take 3 mL ethanol in a test tube and warm gently in a water bath. Add a 5% solution of alkaline KMnO₄ drop by drop. Purple colour of KMnO₄ disappears initially.

When more KMnO₄ is added, the colour persists because all the alcohol gets consumed and the reaction stops.

Addition Reaction

Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. (Catalysts: The substances that influence the rate of a reaction without changing itself).

This reaction is commonly used in the hydrogenation of vegetable oils using a nickel catalyst.

Vegetable oils generally have long unsaturated carbon chains (fatty acids). So they are healthy.

Animal fats generally contain saturated fatty acids which are harmful to health.

Substitution Reaction

Saturated hydrocarbons are unreactive and inert in the presence of most reagents.

However, in the presence of sunlight, hydrocarbons undergo a substitution reaction very fast. E.g.

CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)

Here, chlorine replaces the hydrogen atoms one by one.

Higher homologues of alkanes can form many products. 

CARBON AND ITS COMPOUNDS

SOME IMPORTANT CARBON COMPOUNDS: ETHANOL & ETHANOIC ACID

Properties of Ethanol

Ethanol is liquid at room temperature.

It is commonly called alcohol and is the active ingredient of all alcoholic drinks.

It is a good solvent. So it is used in medicines such as tincture iodine, cough syrups, and tonics.

Ethanol is soluble in water in all proportions.

Consumption of small quantities of dilute ethanol causes drunkenness. Intake of even a small quantity of pure ethanol (absolute alcohol) is lethal. Long-term intake leads to many health problems.

Ethanol for industrial use is made unfit for drinking by adding methanol. It is called denatured alcohol. Blue dyes are added to alcohol to identify easily.

Some countries use alcohol as an additive in petrol since it is a cleaner fuel. It releases only CO₂ & water.

Reactions of Ethanol

a. Reaction with sodium:

Alcohols react with sodium evolving hydrogen. E.g. Drop a small piece of sodium into pure ethanol. It produces sodium ethoxide (2CH₃CH₂O^–Na⁺) & H₂.

2Na + 2CH₃CH₂OH → 2CH₃CH₂O^–Na⁺ + H₂

b. Reaction to give unsaturated hydrocarbon:

Heating ethanol at 443 K with excess conc. H₂SO₄ results in dehydration of ethanol to give ethene. Conc. H₂SO₄ is a dehydrating agent (removes water from ethanol).

Properties of Ethanoic acid (Acetic acid)

It belongs to carboxylic acids (weak acids).

5-8% solution of acetic acid in water is called vinegar. It is used as a preservative in pickles.

The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter. So it is known as glacial acetic acid.

Acetic acid is weak acid and HCl is a strong acid.

Reactions of ethanoic acid:

a. Esterification reaction: 

It is the formation of esters by the reaction of an acid and an alcohol.

E.g. Take 1 mL absolute ethanol + 1 mL glacial acetic acid + few drops of conc. H₂SO₄ in a test tube.

Warm in a water bath for 5 minutes.

Pour into a beaker containing 20-50 mL of water. The resulting mixture is an ester.

Here, ethanoic acid reacts with ethanol in presence of an acid catalyst to give an ester. 

Esters have a sweet smell.

Uses of esters: To make perfumes & as flavouring agents.

On treating with NaOH (an alkali), the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is called saponification because it is used in the preparation of soap.

b. Reaction with a base:

Ethanoic acid reacts with a base like NaOH to give a salt (sodium ethanoate or sodium acetate) and water.

NaOH + CH₃COOH → CH₃COONa + H₂O

c. Reaction with carbonates & hydrogen carbonates:

Take a spatula full of sodium carbonate in a test tube and add 2 mL dilute ethanoic acid. Following reaction occurs:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

                         (Sodium acetate)

Pass the gas produced through lime-water. Lime-water turns milky. It means that the gas is CO₂.

  Reaction with sodium hydrogen carbonate:

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

CARBON AND ITS COMPOUNDS

SOAPS AND DETERGENTS

Take 10 mL of water each in two test tubes A & B.

Add a drop of cooking oil to both the test tubes.

To the test tube B, add few drops of soap solution.

Shake test tubes vigorously to get unclear mixtures.

Leave the test tubes undisturbed for some time. Oil layer separates out in both test tubes. But this happens first in test tube A.

This activity demonstrates the effect of soap in cleaning.

Most dirt is oily and does not dissolve in water.

Soap molecules are sodium / potassium salts of long-chain carboxylic acids.

Ionic-end (hydrophilic) of soap interacts with water while carbon chain (hydrophobic tail) interacts with oil. The soap molecules, thus form structures called micelles. In this, one end of the molecules is towards the oil droplet and the ionic-end faces outside. It forms an emulsion in water. The soap micelle thus helps in pulling out the dirt in water and clothes become clean.

Effect of soap in cleaning

At the surface of water, soap aligns such that its ionic end is in water and hydrocarbon tail protrude out of water.

Inside water, these molecules form clusters in which the hydrophobic tails are oriented towards interior and the ionic ends towards exterior. This cluster is called a micelle.

The oily dirt is collected in the centre of micelle. The micelles stay as a colloid and will not come together to precipitate because of ion-ion repulsion. So, the dirt in micelles is easily rinsed away.

The soap micelles are large enough to scatter light. Hence a soap solution appears cloudy.

The water containing sulphates/ chlorides/ hydrogen carbonates of calcium or magnesium is called hard water. E.g. Water from tube well or hand-pump. 

It is difficult to produce foam by soaps in hard water. So bathing and washing become difficult. After washing, an insoluble substance (scum) remains in hard water. It can be demonstrated by the following experiment:

• Take 10 mL distilled water (or rain water) and 10 mL hard water in separate test tubes (hard water can be prepared by dissolving salts of Ca or Mg in water).
• Add few drops of soap solution to both and shake well for same period.
• Test tube with distilled water gets more foam.
• Test tube with hard water gets white curdy precipitate.

This scum or precipitate is caused by the reaction of soap with the calcium and magnesium salts.

Detergents

These are sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc.

Both have long hydrocarbon chain. Charged ends of these compounds do not form insoluble precipitates with Ca & Mg ions. Thus, they are also effective in hard water.

It can be demonstrated by the following experiment:

• Take two test tubes with 10 mL hard water in each.
• Add five drops of soap solution to one and five drops of detergent solution to the other.
• Shake both test tubes for the same period.
• The test tube with detergent gets more foam.
• In test tube with soap, curdy precipitate is formed.

Detergents are used to make shampoos and products to clean clothes.