Carbon and Its CompoundsClass 10 Chemistry Notes

Carbon and its Compounds

Carbon is a versatile element that is fundamental to all living organisms and many materials we use daily, such as food, clothes, medicines, and books. Although it is present in small amounts in the Earth's crust ($0.02\%$) and atmosphere ($0.03\%$), its importance is immense.

BONDING IN CARBON - THE COVALENT BOND

Unlike ionic compounds which have high melting and boiling points and conduct electricity in a molten state, most carbon compounds are poor conductors of electricity and have low melting and boiling points. This suggests that the forces of attraction between their molecules are not very strong, and their bonding does not create ions.

Carbon's Electronic Configuration Carbon has an atomic number of 6, with an electronic configuration of 2 electrons in the K shell and 4 electrons in the L (outermost) shell. To achieve a stable noble gas configuration, it would need to gain or lose 4 electrons.

• Gaining 4 electrons: It could form a $C^{4-}$ anion. However, it would be extremely difficult for a nucleus with only 6 protons to hold onto 10 electrons.
• Losing 4 electrons: It could form a $C^{4+}$ cation. This would require a huge amount of energy to remove four electrons from the atom.

Carbon overcomes this challenge by sharing its four valence electrons with other atoms. This sharing of electrons is known as covalent bonding.

What is a Covalent Bond? A covalent bond is a chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms. By sharing electrons, both atoms attain a stable, completely filled outermost shell (noble gas configuration).

• Intramolecular forces (bonds within the molecule) are very strong.
• Intermolecular forces (forces between molecules) are weak. This is why covalent compounds generally have low melting and boiling points.
• Since electrons are shared and no charged particles (ions) are formed, covalent compounds are typically poor conductors of electricity.

Examples of Covalent Bonding

• Single Bond (Hydrogen, $H_2$): Two hydrogen atoms, each with one electron, share their electrons to form a single shared pair. This allows each atom to have two electrons in its K shell, like the noble gas helium. A single bond is represented by a single line (H-H).

• Double Bond (Oxygen, $O_2$): An oxygen atom has six valence electrons and needs two more to complete its octet. Two oxygen atoms each share two electrons, forming two shared pairs. This is called a double bond and is represented by two lines (O=O).

• Triple Bond (Nitrogen, $N_2$): A nitrogen atom has five valence electrons and needs three more. Two nitrogen atoms each share three electrons, forming three shared pairs. This is a triple bond, represented by three lines (N≡N).

• Methane ($CH_4$): Carbon is tetravalent, meaning it has a valency of four. It shares its four valence electrons with four hydrogen atoms, forming four single covalent bonds. Methane is a major component of bio-gas and Compressed Natural Gas (CNG).

Allotropes of carbon

Allotropes are different structural forms of the same element in the same physical state. These forms have different physical properties but similar chemical properties. Carbon has several allotropes.

• Diamond: Each carbon atom is bonded to four other carbon atoms, forming a rigid, three-dimensional structure. This makes diamond the hardest known substance.
• Graphite: Each carbon atom is bonded to three other carbon atoms in the same plane, forming hexagonal arrays that are placed in layers. One of these bonds is a double bond, satisfying carbon's valency. Graphite is smooth, slippery, and a good conductor of electricity (unlike most non-metals).
• Fullerenes: This is another class of carbon allotropes. The first one identified was C-60, where carbon atoms are arranged in the shape of a football. It was named Buckminsterfullerene after the architect Buckminster Fuller, who designed geodesic domes that resembled its structure.

VERSATILE NATURE OF CARBON

Carbon forms an astonishingly large number of compounds, estimated to be in the millions. This is due to two unique properties:

1. Catenation: This is the unique ability of carbon to form strong covalent bonds with other carbon atoms, creating long chains, branched chains, and rings. The carbon-carbon bond is very strong and stable, allowing for the formation of large molecules. No other element exhibits catenation to the same extent.
2. Tetravalency: With a valency of four, a carbon atom can bond with four other atoms (which can be carbon or atoms of other elements like hydrogen, oxygen, nitrogen, sulfur, and halogens). The small size of the carbon atom allows its nucleus to hold onto the shared pairs of electrons strongly, making its bonds exceptionally stable.

Saturated and Unsaturated Carbon Compounds

Carbon compounds can be classified based on the types of bonds between their carbon atoms.

• Saturated Compounds: These are carbon compounds where the carbon atoms are linked only by single bonds. They are generally not very reactive. Alkanes are saturated hydrocarbons.
• Unsaturated Compounds: These are compounds containing at least one double or triple bond between carbon atoms. They are more reactive than saturated compounds.
  • Alkenes are unsaturated hydrocarbons with one or more double bonds.
  • Alkynes are unsaturated hydrocarbons with one or more triple bonds.

Chains, Branches and Rings

Carbon atoms can link together to form various structures.

• Straight Chains: Carbon atoms are linked one after another in a continuous chain (e.g., propane, butane).
• Branched Chains: A chain of carbon atoms has one or more smaller carbon chains branching off it.
• Rings (Cyclic Compounds): Carbon atoms are arranged in a ring (e.g., cyclohexane, benzene).

Structural Isomers Structural isomers are compounds that have the same molecular formula but different structural arrangements of atoms. For example, butane ($C_4H_{10}$) can exist as a straight chain or a branched chain, both having the same formula but different structures and properties.

Hydrocarbons Compounds containing only carbon and hydrogen are called hydrocarbons.

• Alkanes: Saturated hydrocarbons (single bonds). General formula: $C_nH_{2n+2}$.
• Alkenes: Unsaturated hydrocarbons (at least one double bond). General formula: $C_nH_{2n}$.
• Alkynes: Unsaturated hydrocarbons (at least one triple bond). General formula: $C_nH_{2n-2}$.

Will you be my Friend? (Functional Groups)

In a hydrocarbon chain, hydrogen atoms can be replaced by other atoms or groups of atoms. An atom replacing hydrogen is called a heteroatom (e.g., Cl, O, N, S).

A functional group is a heteroatom or a group of atoms that is attached to a carbon chain and is responsible for the characteristic chemical properties of the compound. The properties of the compound are largely determined by the functional group, regardless of the length of the carbon chain.

Table: Some functional groups in carbon compounds

Homologous Series

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

Characteristics of a Homologous Series:

• All members share the same general formula.
• Each successive member differs from the next by a $-CH_2-$ unit.
• The difference in molecular mass between successive members is 14 u.
• All members show similar chemical properties because they have the same functional group.
• There is a gradual change in physical properties (like melting point, boiling point, and solubility) as the molecular mass increases.

For example, the alcohols methanol ($CH_3OH$), ethanol ($C_2H_5OH$), and propanol ($C_3H_7OH$) form a homologous series.

Nomenclature of Carbon Compounds

The systematic naming of organic compounds follows a set of rules:

1. Identify the number of carbon atoms in the longest chain to determine the root name (e.g., 1 C = Meth-, 2 C = Eth-, 3 C = Prop-).
2. Identify the functional group and add the appropriate prefix or suffix.
   • If the suffix starts with a vowel (a, e, i, o, u), the final '-e' of the root alkane name is dropped (e.g., Propane - e + ol = Propanol).
3. Indicate saturation:
   • Single bonds: Suffix '-ane'
   • Double bonds: Suffix '-ene'
   • Triple bonds: Suffix '-yne'

Table: Nomenclature of organic compounds

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

Combustion

Combustion is the process of burning a substance in the presence of oxygen to release heat and light. Carbon and its compounds are excellent fuels because they release a large amount of energy upon combustion. $C + O_2 \rightarrow CO_2 + \text{heat and light}$ $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat and light}$ $CH_3CH_2OH + 3O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat and light}$

• Saturated hydrocarbons generally burn with a clean, blue flame due to complete combustion.
• Unsaturated hydrocarbons burn with a yellow, sooty flame due to incomplete combustion, which produces carbon (soot).
• Limited air supply leads to incomplete combustion even for saturated hydrocarbons, producing a sooty flame. This is why cooking vessels get blackened if the air holes of a gas stove are blocked.

Oxidation

Oxidation is a reaction that involves the addition of oxygen or removal of hydrogen. While combustion is a form of complete oxidation, other controlled oxidation reactions are also important. For example, alcohols can be oxidized to carboxylic acids using oxidising agents like alkaline potassium permanganate ($KMnO_4$) or acidified potassium dichromate ($K_2Cr_2O_7$). $CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 + \text{Heat}} CH_3COOH$ In this reaction, ethanol is oxidized to ethanoic acid.

Addition Reaction

Unsaturated hydrocarbons (alkenes and alkynes) undergo addition reactions, where atoms are added across the double or triple bond to form a saturated compound. This reaction typically occurs in the presence of a catalyst, a substance that changes the rate of a reaction without being consumed.

A common example is hydrogenation, the addition of hydrogen. $R_2C=CR_2 + H_2 \xrightarrow{\text{Ni catalyst}} R_2CH-CHR_2$ This reaction is used industrially to convert unsaturated vegetable oils (liquids) into saturated vegetable fats or margarine (solids).

Substitution Reaction

Saturated hydrocarbons are relatively unreactive, but they can undergo substitution reactions, where one or more hydrogen atoms are replaced by another atom or group. For example, in the presence of sunlight, methane reacts with chlorine, and the chlorine atoms replace the hydrogen atoms one by one. $CH_4 + Cl_2 \xrightarrow{\text{Sunlight}} CH_3Cl + HCl$

SOME IMPORTANT CARBON COMPOUNDS - ETHANOL AND ETHANOIC ACID

Properties of Ethanol ($C_2H_5OH$)

Ethanol, commonly known as alcohol, is a liquid at room temperature.

• Uses: It is the active ingredient in alcoholic beverages, a good solvent used in medicines (like tincture of iodine) and tonics, and is soluble in water in all proportions.
• Effects: Consumption of dilute ethanol causes drunkenness. Pure ethanol (absolute alcohol) is lethal. Long-term consumption leads to health problems.

Reactions of Ethanol

1. Reaction with Sodium: Ethanol reacts with sodium to produce sodium ethoxide and hydrogen gas. $2Na + 2CH_3CH_2OH \rightarrow 2CH_3CH_2O^-Na^+ + H_2$
2. Dehydration: When heated with excess concentrated sulphuric acid at 443 K, ethanol loses a molecule of water (dehydration) to form ethene, an unsaturated hydrocarbon. $CH_3CH_2OH \xrightarrow{\text{Hot Conc. } H_2SO_4} CH_2=CH_2 + H_2O$ Here, concentrated sulphuric acid acts as a dehydrating agent.

Properties of Ethanoic Acid ($CH_3COOH$)

Ethanoic acid, commonly known as acetic acid, belongs to the carboxylic acid group.

• A 5-8% solution of acetic acid in water is called vinegar, used as a preservative.
• The melting point of pure ethanoic acid is 290 K, so it often freezes in cold climates, earning it the name glacial acetic acid.
• Carboxylic acids are weak acids compared to mineral acids like HCl.

Reactions of Ethanoic Acid

1. Esterification Reaction: Ethanoic acid reacts with an alcohol (like ethanol) in the presence of an acid catalyst to form a sweet-smelling compound called an ester. $CH_3COOH + CH_3CH_2OH \xrightarrow{\text{Acid}} \underset{\text{(Ester)}}{CH_3COOCH_2CH_3} + H_2O$ Esters are used in perfumes and as flavouring agents.
2. Saponification: When an ester is treated with a base like sodium hydroxide ($NaOH$), it is converted back to the alcohol and the sodium salt of the carboxylic acid. This reaction is called saponification because it is used to make soap. $CH_3COOC_2H_5 + NaOH \rightarrow C_2H_5OH + CH_3COONa$
3. Reaction with a Base: Ethanoic acid reacts with a base like $NaOH$ to form a salt (sodium ethanoate or sodium acetate) and water. $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
4. Reaction with Carbonates and Hydrogencarbonates: Ethanoic acid reacts with carbonates and hydrogencarbonates to produce a salt, carbon dioxide, and water. $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$ $CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2$

Soaps and Detergents

Soaps and detergents are cleansing agents.

Structure of a Soap Molecule A soap molecule has two distinct parts:

1. A long hydrocarbon chain, which is hydrophobic (water-repelling) and dissolves in oil or grease.
2. An ionic end (e.g., $-COO^-Na^+$), which is hydrophilic (water-attracting) and dissolves in water.

Cleaning Action of Soap When soap is dissolved in water, the molecules form clusters called micelles. In a micelle, the hydrophobic tails point inwards, trapping an oil or dirt particle, while the hydrophilic heads face outwards into the water. This forms an emulsion, allowing the oily dirt to be suspended in the water and washed away.

Soap in Hard Water Hard water contains calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) salts. When soap is used in hard water, it reacts with these salts to form an insoluble, curdy precipitate called scum. This wastes soap and makes cleaning difficult.

Detergents Detergents are another class of cleansing agents, typically sodium salts of sulphonic acids or ammonium salts.

• Like soap, they have a hydrophobic tail and a hydrophilic head.
• The key difference is that the charged ends of detergent molecules do not form insoluble precipitates (scum) with the calcium and magnesium ions in hard water.
• Therefore, detergents remain effective cleansing agents even in hard water. They are widely used in shampoos and laundry products.