Class 10 Science Chapter 4 Carbon and its Compounds NCERT Notes

Chapter 4 Carbon and its Compounds Class 10 Science CBSE NCERT Notes

Bonding in Carbon – The Covalent Bond

Two or more elements combine to form compound. There are two types of compounds- Organic Compound and Inorganic Compounds. Organic compounds are the one which are made up of carbon and hydrogen.

Covalent Bond

The bond formed by sharing a pair of electrons between two atoms are known as Covalent Bond. Carbon forms covalent bond. Carbon exists in two forms as free state and as combined state.

Free form of carbon is found in graphite, diamond and fullerene. In combined state, carbon exists as Carbon-dioxide, Glucose, Sugar etc.

The atomic number of carbon is 6. Its electronic configuration is 2, 4. It requires, 4 electrons to achieve the inert gas electronic configuration. But carbon cannot form an ionic bond It could gain four electrons forming C⁴⁺ cation. But it would be difficult for the nucleus with six protons to hold on to ten electrons. It could lose four electrons forming C⁴⁺ cations. But it requires a large amount of energy to remove four electrons. Thus, carbon overcomes this problem by sharing of its valence electrons with other carbon atoms or with atoms of other elements. The bond formed by mutual sharing of electron pairs between two atoms in a molecule is known as Covalent Bond.

Types of Covalent Bond

Single Covalent Bond: When a single pair of electrons are shared between two atoms in a molecule. For example; F₂, Cl₂, H₂ etc.

Double Covalent Bond: When two pairs of electrons are shared between two atoms in a molecule. For example; O₂, CO₂ etc.

Triple Covalent Bond: When three pairs of electrons are shared between two atoms in a molecule. For example; N₂ etc.

Electron Dot Structure

The electron dot structures provides a picture of bonding in molecules in terms of the shared pairs of electrons and octet rule.

The atomic number of carbon is 6 and its electronic configuration is 2, 4. To attain a noble gas configuration it can:
Gain 4 electrons but it would be difficult for nucleus to hold 4 extra electrons.
Lose 4 electrons but it would require a large amount of energy to remove 4 electrons.

It is difficult thus for an atom of carbon to either gain or lose electrons. Carbon attains the noble gas configuration by sharing its valence electrons with other atoms. Atoms of other elements like hydrogen, oxygen, nitrogen, chlorine also show sharing of valence electrons.

Formation of Hydrogen Molecule

Single bond between two hydrogen atoms

Double bond between two oxygen atoms

Triple bond between two nitrogen atoms

It is evident that the number of shared pair of electrons can be one, two or three. Try making the structures of H₂O and CH₄. Bond formed by the sharing of an electron pair between two atoms is called covalent bond.

Covalently bonded molecules have low melting and boiling points because of comparatively weaker intermolecular forces, unlike ionic compounds. These molecules are generally poor conductor of electricity since no charged particles are formed.

Allotropes of Carbon

Different forms of an element that has same chemical properties but different physical properties are known as Allotropes. There are three allotropes of carbon- diamond, graphite and fullerene.

Diamond

Diamond exits as three-dimensional network with strong carbon-carbon covalent bonds. Diamond is hard in nature with high melting point. It shines in presence of light and it is a bad conductor of electricity. The most common use of diamond is in making jewellery. It is also used in cutting and drilling tools.

Graphite

Graphite is made from weak van der wall forces. Each carbon atom is bonded with other three carbon atoms in order to form hexagonal rings. It serves as good conductor of heat and electricity. It is used as dry lubricant for machine parts as well as it is used in lead pencils.

Fullerene

It is a hollow cage which exits in the form of sphere. Its structure is similar to fullerene. But along with hexagonal rings, sometimes pentagonal or heptagonal rings are also present.

Versatile Nature of Carbon Atoms

Two important properties of carbon atom enable carbon to form enormously large number of compounds.

Catenation is a property of carbon atom to form bond with other atoms of carbon is called catenation. Like carbon, silicon forms compounds with hydrogen upto seven or eight atoms of silicon.

Tetravalency is having a valency of 4, carbon atom is capable of bonding with atoms of oxygen, hydrogen, nitrogen, sulphur, chlorine and other elements. The smaller size of carbon atom enables nucleus to hold the shared pair of electrons strongly, thus carbon compounds are very stable in general.

Saturated and Unsaturated Carbon Compounds

Compounds which are made up of carbon and hydrogen they are known as Hydrocarbons. There are two types of hydrocarbons found are Saturated Hydrocarbons and Unsaturated Hydrocarbons.

Saturated Hydrocarbons consist of single bonds between the carbon atoms. For Example, Alkanes. Alkanes are saturated hydrocarbons represented by a formula, C_nH_2n+2.

Unsaturated Hydrocarbons are the one with double or triple bonds between the carbon atoms. For Example, Alkenes and Alkynes. Alkenes are represented as C_nH_2n whereas alkynes are represented as C_nH_2n-2.

Electron dot structure of ethane

Structure of ethene

Some functional groups in carbon compounds

Homologous Series

Series of organic compounds having the same functional group and chemical properties and successive members differ by a CH₂ unit or 14 mass units are known as Homologous series.

Characteristic of Homologous Series

The successive members in homologous series differ by CH2 unit or 14 mass unit.
Members of given homologous series have the same functional group.
All the members of homologous series shows similar chemical properties.

Nomenclature of Carbon Compounds

In IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature, the names are correlated with the structures such that the learner can deduce the structure from the name.

Before the IUPAC system of nomenclature, organic compounds were assigned trivial or common names based on their origin or certain properties.

A series of organic compounds containing a particular characteristic group is called a homologous group.

While naming hydrocarbons, the first part of the name, called the root name, represents the number of carbon atoms and the last three letters represent the homologous series to which the alkane belongs.

Alkanes: General formula C_nH2_n+2, Suffix-ane
Alkenes: General formula C_nH_2n, Suffix -ene
Alkynes: General formula C_nH_2n-2, Suffix -yne
Alkyl halides: General formula C_nH_2n+1X, Prefix – halo
Alcohols: General formula C_nH_2n+1OH, Suffix -ol
Aldehydes: General formula C_nH2_n+1CHO, Suffix -al
Carboxylic acid: General formula C_nH_2n+1COOH, Suffix -oic acid

A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon and the functional group(s) attached to it.

Functional groups are structural units within organic compounds that are defined by specific bonding arrangements between specific atoms. To name a compound:

Step I: Select the longest carbon chain.
Step II: Assign lowest number to the side chain.
Step III: Arrangement of prefixes.
Step IV: Lowest number for functional group.

Using the IUPAC of an organic compound, it’s structure can be determined. The following rules help in accomplishing the task:

Step I: Identify the root word. It forms the carbon skeleton in the structure.
Step II: Write the number of carbon atoms as per the root word and number them from any end.
Step III: As per the suffix in the name, ascertain the type of bond present in the
compound. If any multiple bond is present, place it between the carbon atoms as stated in the IUPAC name.
Step IV: Place the substituents at the carbon atoms mentioned in the IUPAC name.
Step V: Place the functional group at the designated carbon atom.

Chemical Properties of Carbon Compounds

Combustion

Carbon along with its compound is used as a fuel as it burns in presence of oxygen to release energy. Saturated hydrocarbons produce blue and non-sooty flame whereas unsaturated hydrocarbons produce yellow sooty flame.

CH₄ + 2O₂ → CO₂ + 2H₂O

Oxidation

Alcohol can be oxidized to aldehydes whereas aldehydes in turn can be oxidized to carboxylic acid. Oxidizing agent such as potassium permanganate can be used for oxidation.

Addition Reaction

Hydrogenation of vegetable oil is an example of addition reaction. Addition of hydrogen in presence of catalyst such as nickel or palladium. This converts oil into ghee.

Substitution Reaction

When one atom in hydrocarbon is replaced by chlorine, bromine, etc. this is known as Substitution Reaction.

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

Some Important Compounds – Ethanol and Ethanoic Acid

Ethanol

Ethanol is a volatile liquid with low melting point. Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks.

Reaction with sodium: Alcohols react with sodium leading to the evolution of hydrogen. With ethanol, the other product is sodium ethoxide.
2Na + 2CH₃CH₂OH → 2CH₃CH₂O^–Na⁺ (Sodium ethoxide) + H₂

Reaction to give unsaturated hydrocarbon: Heating ethanol at 443 K with excess concentrated sulphuric acid results in the dehydration of ethanol to give ethene.

The concentrated sulphuric acid can be regarded as a dehydrating agent which removes water from ethanol.

Ethanoic Acid

Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.

Esterification reaction: Esters are most commonly formed by reaction of an acid and an alcohol. Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester. Generally, esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents.

On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.

Reaction with a base: Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:
NaOH + CH₃COOH → CH₃COONa + H₂O

Reaction with carbonates and hydrogencarbonates: Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water. The salt produced is commonly called sodium acetate.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO2
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Soaps and Detergents

Sodium or potassium salt of carboxylic acid is known as Soap. They work most effectively in soap water. Detergents are sulphonate or ammonium salt of long chain of carboxylic acid. They can work effectively on soft as well as hard water.

Cleansing Action of Soaps and Detergents

Cleansing action of soaps and detergents is due to ability to minimize the surface tension of water, to emulsify oil or grease and to hold them in a suspension of water. When soap dissolves in water, it forms soap anions and soap cations. The hydrophobic part of soaps and detergents are soluble in grease and hydrophilic part is soluble in water.

Soap and Micelle Formation

When dirt and grease are mixed with soap water, soap molecules arrange them in tiny clusters known as Micelle. The hydrophilic part sticks to the water and form outer surface of the micelle and hydrophobic part binds to oil and grease.