Comprehensive Notes on Carbon and Its Compounds

Carbon and Its Compounds

Introduction

• In the Earth's crust, carbon is present at 0.02% in the form of minerals like carbonates, hydrogen-carbonates, coal, and petroleum.
• The atmosphere contains 0.03% carbon dioxide.
• Carbon is a versatile element due to its ability to form bonds, both in elemental and combined forms.

Bonding in Carbon - The Covalent Bond

• Most carbon compounds are poor conductors of electricity.
• Carbon compounds have low melting and boiling points, indicating weak forces of attraction between molecules.
• Bonding in carbon compounds doesn't give rise to ions.
• Atomic number of carbon is 6, with an electronic configuration of 2,4.
• Carbon has 4 valence electrons.

Achieving Noble Gas Configuration

• Carbon needs to gain or lose four electrons to attain noble gas configuration.
• Gaining 4 electrons to form $C^{4-}${}$ anion is difficult because the nucleus with six protons would struggle to hold onto ten electrons.
• Losing 4 electrons to form $C^{4+}${}$ cation requires a large amount of energy.
• Carbon overcomes this by sharing its valence electrons with other atoms, forming covalent bonds.

Covalent Bond Formation

• The shared electrons 'belong' to the outermost shells of both atoms, leading to both atoms attaining noble gas configuration.
• Hydrogen ($H_2$) molecule: Each hydrogen atom shares one electron to attain the electronic configuration of helium.
• A shared pair of electrons constitutes a single covalent bond, represented by a line between the atoms.
• Chlorine ($Cl_2$) forms a diatomic molecule with a single covalent bond between the two chlorine atoms.
• Oxygen ($O_2$) forms a double bond, where each oxygen atom shares two electrons.
• Nitrogen ($N_2$) forms a triple bond, where each nitrogen atom shares three electrons.
• Ammonia ($NH_3$) has single covalent bonds.

Methane ($CH_4$)

• Methane is a simple carbon compound widely used as a fuel.
• Carbon shares its four valence electrons with four hydrogen atoms.
• Covalent bonds are strong within the molecule, but intermolecular forces are weak, resulting in low melting and boiling points.
• Covalent compounds are generally poor conductors of electricity because electrons are shared, and no charged particles are formed.

Allotropes of Carbon

• Carbon exists in different forms with varying physical properties, such as diamond, graphite, and fullerenes.
• Diamond: Each carbon atom is bonded to four other carbon atoms, forming a rigid three-dimensional structure.
• Graphite: Each carbon atom is bonded to three other carbon atoms in the same plane, forming a hexagonal array. The hexagonal arrays are placed in layers one above the other.
• Diamond is the hardest substance, while graphite is smooth and slippery.
• Graphite is a good conductor of electricity.
• Diamonds can be synthesized by subjecting pure carbon to high pressure and temperature.
• Fullerenes: Carbon atoms arranged in the shape of a football (e.g., C-60), also known as Buckminsterfullerene named after architect Buckminster Fuller.

Versatile Nature of Carbon

• Carbon forms a large number of compounds; the number of carbon compounds known to chemists is in the millions.
• Two factors contribute to this:
  • Catenation: The unique ability of carbon to form bonds with other carbon atoms, leading to large molecules with long chains, branched chains, or rings.
    • Carbon atoms can be linked by single, double, or triple bonds.
    • Compounds with only single bonds are called saturated compounds.
    • Compounds with double or triple bonds are called unsaturated compounds.
    • The carbon-carbon bond is very strong and stable.
  • Tetravalency: Carbon has a valency of four, enabling it to bond with four other atoms.

Bonding with Other Elements

• Carbon forms compounds with oxygen, hydrogen, nitrogen, sulfur, chlorine, and other elements, giving rise to compounds with specific properties.
• Carbon forms strong bonds with most other elements due to its small size, enabling the nucleus to hold onto shared pairs of electrons strongly.

Organic Compounds

• Organic compounds were initially extracted from natural substances and thought to be formed only within a living system (vital force theory).
• Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate.
• Carbon compounds (except carbides, oxides of carbon, carbonate, and hydrogencarbonate salts) are studied under organic chemistry.

Saturated and Unsaturated Carbon Compounds

• Ethane ($C<em>2H</em>6$): Carbon atoms linked with a single bond, with the remaining valencies satisfied by hydrogen atoms.
• Propane ($C<em>3H</em>8$): Valencies of all atoms are satisfied by single bonds; such compounds are called saturated compounds and are generally not very reactive.
• Ethene ($C<em>2H</em>4$): Contains a double bond between the two carbons and is an unsaturated compound.
• Ethyne ($C<em>2H</em>2$): Contains a triple bond between the two carbon atoms and is an unsaturated carbon compound, being more reactive than saturated compounds.

Chains, Branches, and Rings

• Chains of carbon atoms can contain many more carbon atoms (e.g., methane, ethane, propane, butane, pentane, hexane).
• Structural Isomers: Compounds with identical molecular formula but different structures (e.g., butane).
• Cyclic Compounds: Some compounds have carbon atoms arranged in the form of a ring (e.g., cyclohexane, $C<em>6H</em>{12}$).
• Benzene ($C<em>6H</em>6$) also has a ring structure.

Hydrocarbons

• Carbon compounds containing only carbon and hydrogen are called hydrocarbons.
• Saturated hydrocarbons are called alkanes.
• Unsaturated hydrocarbons containing one or more double bonds are called alkenes.
• Unsaturated hydrocarbons containing one or more triple bonds are called alkynes.

Heteroatoms and Functional Groups

• In a hydrocarbon chain, one or more hydrogens can be replaced by elements like halogens, oxygen, nitrogen, and sulfur, referred to as heteroatoms.
• Heteroatoms and groups containing them confer specific properties and are called functional groups.

Homologous Series

• A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain.
• The presence of a functional group (like alcohol) determines the properties of the carbon compound.
• Successive compounds differ by a $-CH_2-$ unit.
• As molecular mass increases, a gradation in physical properties is seen (melting and boiling points increase).
• Chemical properties remain similar within a homologous series.
• The general formula for alkenes: $C<em>nH</em>{2n}$, where $n = 2, 3, 4$.

Nomenclature of Carbon Compounds

• Names of compounds are based on the name of the basic carbon chain modified by a prefix or suffix indicating the functional group.
• Naming method:
  1. Identify the number of carbon atoms.
  2. Indicate the functional group with a prefix or suffix.
  3. If the suffix starts with a vowel, delete the final ‘e’ from the carbon chain name.
  4. If the carbon chain is unsaturated, substitute ‘ane’ with ‘ene’ or ‘yne’.

Chemical Properties of Carbon Compounds

• Most fuels are carbon or its compounds, so combustion is important.

Combustion

• Carbon burns in oxygen to give carbon dioxide, heat, and light.
• Most carbon compounds release a large amount of heat and light on burning (oxidation reactions).
  • $C + O<em>2 \rightarrow CO</em>2 + \text{heat and light}$
  • $CH<em>4 + O</em>2 \rightarrow CO<em>2 + H</em>2O + \text{heat and light}$
  • $CH<em>3CH</em>2OH + O<em>2 \rightarrow CO</em>2 + H_2O + \text{heat and light}$
• Saturated hydrocarbons generally give a clean flame, while unsaturated carbon compounds give a yellow flame with black smoke.
• Limiting air supply results in incomplete combustion, giving a sooty flame.
• Oxides of sulfur and nitrogen, formed from combustion of fuels like coal and petroleum, are major pollutants.

Oxidation

• Carbon compounds can be easily oxidized on combustion.
• Alcohols can be converted to carboxylic acids.
• Oxidizing agents (e.g., alkaline potassium permanganate or acidified potassium dichromate) add oxygen to the starting material.

Addition Reaction

• Unsaturated hydrocarbons add hydrogen in the presence of catalysts (palladium or nickel) to give saturated hydrocarbons.
• This reaction is used in the hydrogenation of vegetable oils.
• Vegetable oils have long unsaturated carbon chains, while animal fats have saturated carbon chains.

Substitution Reaction

• Saturated hydrocarbons are fairly unreactive.
• In the presence of sunlight, chlorine can be added to hydrocarbons, replacing hydrogen atoms one by one.
• $CH<em>4 + Cl</em>2 \rightarrow CH_3Cl + HCl$ (in the presence of sunlight)

Some Important Carbon Compounds – Ethanol and Ethanoic Acid

Properties of Ethanol

• Ethanol is a liquid at room temperature, commonly called alcohol.
• It is the active ingredient in alcoholic drinks and a good solvent used in medicines.
• Ethanol is soluble in water in all proportions.
• Intake of even a small quantity of pure ethanol (absolute alcohol) can be lethal; long-term consumption leads to health problems.

Reactions of Ethanol

• Reaction with sodium: Alcohols react with sodium, leading to the evolution of hydrogen.
  • $2Na + 2CH<em>3CH</em>2OH \rightarrow 2CH<em>3CH</em>2O^-Na^+ + H_2$ (Sodium ethoxide)
• Reaction to give unsaturated hydrocarbon:
  • Heating ethanol at 443 K with excess concentrated sulfuric acid results in dehydration to give ethene.
    • $CH<em>3CH</em>2OH \xrightarrow[Hot Conc. H<em>2SO</em>4]{} CH<em>2=CH</em>2 + H_2O$

How Alcohols Affect Living Beings

• Large quantities of ethanol slow metabolic processes and depress the central nervous system.
• Methanol, when oxidized in the liver, forms methanal, which reacts rapidly with cell components and can cause blindness.
• Denatured alcohol: Ethanol produced for industrial use is made unfit for drinking by adding poisonous substances like methanol and dyes.

Properties of Ethanoic Acid

• Ethanoic acid is commonly called acetic acid and belongs to carboxylic acids.
• A 5-8% solution of acetic acid in water is called vinegar, used as a preservative.
• Melting point of pure ethanoic acid is 290 K, often freezing during winter (glacial acetic acid).
• Carboxylic acids are weak acids, unlike mineral acids like HCl.

Reactions of Ethanoic Acid

• Esterification reaction: Ethanoic acid reacts with absolute ethanol to give an ester.
  • $CH<em>3COOH + CH</em>3CH<em>2OH \xrightarrow[Acid]{} CH</em>3COOCH<em>2CH</em>3 + H_2O$
• Esters are sweet-smelling substances used in perfumes and flavoring agents.
• Saponification: Treating esters with sodium hydroxide converts them back to alcohol and sodium salt of carboxylic acid (used in soap preparation).
  • $CH<em>3COOC</em>2H<em>5 + NaOH \rightarrow C</em>2H<em>5OH + CH</em>3COONa$
• Reaction with a base: Ethanoic acid reacts with a base (e.g., sodium hydroxide) to give a salt and water.
  • $NaOH + CH<em>3COOH \rightarrow CH</em>3COONa + H_2O$
• Reaction with carbonates and hydrogencarbonates: Ethanoic acid reacts to give a salt, carbon dioxide, and water.
  • $2CH<em>3COOH + Na</em>2CO<em>3 \rightarrow 2CH</em>3COONa + H<em>2O + CO</em>2$
  • $CH<em>3COOH + NaHCO</em>3 \rightarrow CH<em>3COONa + H</em>2O + CO_2$

Soaps and Detergents

• Soaps are sodium or potassium salts of long-chain carboxylic acids.
• Soap molecules form micelles in water, with ionic ends interacting with water and carbon chains interacting with oil.
• Micelles help in pulling out dirt in water, enabling washing.
• Detergents are generally sodium salts of sulfonic acids or ammonium salts.
• Detergents don't form insoluble precipitates with calcium and magnesium ions in hard water, making them effective in hard water.