Chapter 4: Carbon and its Compounds

Delving into the world of organic chemistry

1. Introduction to Carbon

Carbon is an element of immense significance in both its elemental and combined forms.

Many everyday items like food, clothes, medicines, and books are carbon-based, and all living structures are carbon-based.

Despite its importance, the amount of carbon in the Earth's crust (0.02% in minerals like carbonates, hydrogen-carbonates, coal, and petroleum) and atmosphere (0.03% as carbon dioxide) is quite meagre.

2. Bonding in Carbon – The Covalent Bond

Most carbon compounds are poor conductors of electricity and have low melting and boiling points compared to ionic compounds.

This indicates that the forces of attraction between molecules are not very strong and that bonding does not give rise to ions.

Carbon's atomic number is 6, meaning it has four electrons in its outermost shell.

To achieve a stable noble gas configuration, carbon needs to gain or lose four electrons.

  • Gaining four electrons to form C⁴⁻ anion is difficult because the nucleus (6 protons) would struggle to hold 10 electrons.
  • Losing four electrons to form C⁴⁺ cation requires a large amount of energy.

Carbon overcomes this by sharing its valence electrons with other carbon atoms or atoms of other elements.

Covalent bonds are formed by the sharing of an electron pair between two atoms, allowing both atoms to attain a noble gas configuration.

Examples of covalent bonding:

  • Hydrogen (H₂): Each hydrogen atom (atomic number 1, 1 electron in K shell) shares one electron with another hydrogen atom to achieve helium's configuration (2 electrons in K shell), forming a single covalent bond.
  • Oxygen (O₂): Each oxygen atom (atomic number 8, 6 electrons in L shell) shares two electrons with another oxygen atom to complete its octet, forming a double bond.
  • Nitrogen (N₂): Each nitrogen atom (atomic number 7) contributes three electrons to share with another nitrogen atom to attain an octet, forming a triple bond.
  • Methane (CH₄): Carbon (tetravalent) shares its four valence electrons with four hydrogen atoms (valency 1), forming four single covalent bonds.

Covalently bonded molecules have strong bonds within the molecule, but weak inter-molecular forces, leading to low melting and boiling points.

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

3. Allotropes of Carbon

Carbon exists in different forms with widely varying physical properties, known as allotropes. The difference lies in how carbon atoms are bonded.

  • Diamond: Each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. It is the hardest substance known and can be synthesized under high pressure and temperature.
  • Graphite: Each carbon atom is bonded to three other carbon atoms in the same plane forming a hexagonal array. One of these bonds is a double bond, satisfying carbon's valency. Graphite structures are layered. It is smooth and slippery and a very good conductor of electricity.
  • Fullerenes: Another class of carbon allotropes, with the first identified being C-60, which has carbon atoms arranged in the shape of a football.

4. Versatile Nature of Carbon

Carbon forms a huge number of compounds (millions are known) due to two key properties:

  • (i) Catenation: The unique ability of carbon to form bonds with other carbon atoms, leading to large molecules. These compounds can have long chains, branched chains, or ring structures. Carbon atoms can be linked by single, double, or triple bonds. The carbon-carbon bond is very strong and stable. No other element exhibits catenation to this extent.
  • (ii) Tetravalency: Carbon has a valency of four, enabling it to bond with four other atoms (carbon or other monovalent elements). Carbon forms strong bonds with oxygen, hydrogen, nitrogen, sulphur, chlorine, and many other elements, leading to compounds with specific properties. The small size of carbon allows its nucleus to hold shared electron pairs strongly, contributing to strong bonds.

Organic Compounds: Carbon compounds (except carbides, oxides of carbon, carbonate, and hydrogencarbonate salts) are studied under organic chemistry. Initially, it was thought they could only be formed within living systems, but Friedrich Wöhler disproved this in 1828 by synthesizing urea from ammonium cyanate.

5. Saturated and Unsaturated Carbon Compounds (Hydrocarbons)

Hydrocarbons:

Hydrocarbons are carbon compounds that contain only carbon and hydrogen.

Saturated Compounds (Alkanes):

  • Have only single bonds between carbon atoms.
  • Are generally not very reactive.
  • Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀), Pentane (C₅H₁₂), Hexane (C₆H₁₄).

Unsaturated Compounds (Alkenes and Alkynes):

  • Have double or triple bonds between carbon atoms.
  • Are more reactive than saturated carbon compounds.
  • Alkenes: Contain one or more double bonds (e.g., Ethene, C₂H₄).
  • Alkynes: Contain one or more triple bonds (e.g., Ethyne, C₂H₂).

Chains, Branches, and Rings:

Carbon compounds can exist as straight chains, branched chains, or cyclic (ring) structures.

Structural Isomers: Compounds with identical molecular formulas but different structures (e.g., Butane, C₄H₁₀, can have a straight chain or a branched chain).

Examples of cyclic structures include Cyclohexane (C₆H₁₂) and Benzene (C₆H₆).

6. Heteroatoms and Functional Groups

Heteroatoms: In a hydrocarbon chain, one or more hydrogen atoms can be replaced by other elements (like halogens, oxygen, nitrogen, sulphur).

Functional Groups: These heteroatoms or groups containing them confer specific properties to the compound, regardless of the carbon chain's length. They are attached to the carbon chain by replacing one or more hydrogen atoms.

Examples of Functional Groups:

  • Haloalkane (e.g., -Cl, -Br)
  • Alcohol (-OH)
  • Aldehyde (-CHO)
  • Ketone (C=O within a chain)
  • Carboxylic acid (-COOH)

7. Homologous Series

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

Characteristics of a Homologous Series:

  • Successive members differ by a -CH₂- unit (e.g., CH₄ and C₂H₆, C₂H₆ and C₃H₈).
  • There is a gradation in physical properties as molecular mass increases (e.g., melting and boiling points increase with increasing molecular mass).
  • Chemical properties remain similar, as they are determined solely by the functional group.

General Formulas: For alkenes, the general formula is CnH₂n (where n = 2, 3, 4...).

8. Nomenclature of Carbon Compounds

The naming of compounds in a homologous series is based on the basic carbon chain, modified by a prefix or suffix indicating the functional group.

Steps for Naming:

  1. Identify the number of carbon atoms (e.g., 3 carbons = propane).
  2. Indicate functional group with a prefix or suffix (from Table 4.4).
  3. If the functional group suffix begins with a vowel (a, e, i, o, u), delete the final 'e' from the carbon chain name before adding the suffix (e.g., Propane - 'e' + 'one' = Propanone).
  4. If the carbon chain is unsaturated, substitute 'ane' with 'ene' for double bonds or 'yne' for triple bonds (e.g., Propene, Propyne).

9. Chemical Properties of Carbon Compounds

Combustion:

Carbon (in all allotropic forms) burns in oxygen to produce carbon dioxide, heat, and light. Most carbon compounds also release heat and light on burning.

  • Saturated hydrocarbons generally give a clean flame.
  • Unsaturated carbon compounds generally give a yellow flame with lots of black smoke (sooty deposit), indicating incomplete combustion. Limiting air supply also results in sooty flame for saturated hydrocarbons.

Fuels like coal and petroleum contain nitrogen and sulphur, producing oxides of sulphur and nitrogen upon combustion, which are major pollutants.

A flame is produced when gaseous substances burn; a luminous flame occurs when gaseous atoms are heated and glow.

Oxidation:

Carbon compounds are easily oxidized on combustion.

Alcohols can be converted to carboxylic acids.

Oxidising agents are substances that add oxygen to others (e.g., alkaline potassium permanganate or acidified potassium dichromate).

Addition Reaction:

Unsaturated hydrocarbons add hydrogen in the presence of catalysts (like palladium or nickel) to form saturated hydrocarbons.

Catalysts are substances that change the rate of a reaction without being affected themselves.

This reaction is used in the hydrogenation of vegetable oils (converting unsaturated fatty acids to saturated fatty acids, often considered less healthy than unsaturated oils).

Substitution Reaction:

Saturated hydrocarbons are generally unreactive but react rapidly with halogens (e.g., chlorine) in the presence of sunlight.

Chlorine replaces hydrogen atoms one by one. It's called a substitution reaction because one atom/group replaces another.

10. Some Important Carbon Compounds

Ethanol (CH₃CH₂OH):

Properties:

  • Liquid at room temperature, commonly called alcohol, active ingredient in alcoholic drinks, good solvent (used in medicines like tincture iodine, cough syrups), soluble in water.

Effects of Consumption:

  • Small quantities cause drunkenness; pure ethanol is lethal; long-term consumption leads to health problems. Methanol is highly toxic, causing blindness or death. Denatured alcohol (ethanol mixed with poisonous substances like methanol and dyes) is used for industrial purposes to prevent misuse.

Reactions:

  • With Sodium: Alcohols react with sodium to evolve hydrogen gas and form sodium alkoxides (e.g., sodium ethoxide).
  • Dehydration: Heating ethanol at 443 K with excess concentrated sulphuric acid (a dehydrating agent) results in the formation of ethene (an unsaturated hydrocarbon) and water.

As a Fuel:

Sugarcane juice can be fermented to produce ethanol, which is used as an additive in petrol because it's a cleaner fuel, producing only carbon dioxide and water on burning.

Ethanoic Acid (CH₃COOH):

Properties:

  • Commonly called acetic acid, belongs to carboxylic acids. A 5-8% solution in water is vinegar (used as a preservative). Pure ethanoic acid has a melting point of 290 K and freezes in cold climates, giving rise to its name glacial acetic acid. Carboxylic acids are weak acids, unlike strong mineral acids (e.g., HCl).

Reactions:

  • Esterification: Reacts with absolute ethanol in the presence of an acid catalyst to form an ester (sweet-smelling substances used in perfumes and flavouring agents).
  • Saponification: Esters can be converted back to alcohol and a sodium salt of carboxylic acid by treating them with sodium hydroxide (an alkali); this reaction is used in soap preparation.
  • With a Base: Reacts with bases like sodium hydroxide to give a salt (e.g., sodium ethanoate) and water, similar to mineral acids.
  • With Carbonates and Hydrogencarbonates: Reacts with carbonates (e.g., sodium carbonate) and hydrogencarbonates (e.g., sodium hydrogencarbonate) to produce a salt, carbon dioxide, and water.

11. Soaps and Detergents

Soap:

Composition:

  • Sodium or potassium salts of long-chain carboxylic acids.

Structure:

  • Soap molecules have two distinct ends: a hydrophilic (ionic) end that interacts with water, and a hydrophobic (carbon chain) end that interacts with oil/hydrocarbons.

Cleaning Action (Micelle Formation):

  • Most dirt is oily and doesn't dissolve in water.
  • When soap is added to water, the hydrophobic tails of soap molecules gather in the interior, while the hydrophilic ionic ends face outwards, forming micelles.
  • The oily dirt is collected in the center of the micelle.
  • Micelles form an emulsion in water. They stay in solution as a colloid due to ion-ion repulsion and do not precipitate.
  • This allows the soap micelle to pull out dirt into the water, which can then be rinsed away. Soap solution appears cloudy because micelles are large enough to scatter light.

Hard Water Problem with Soap:

  • Hard water contains calcium and magnesium salts.
  • Soap reacts with these salts to form an insoluble substance called scum.
  • This reduces foam formation and requires a larger amount of soap to achieve cleaning.

Detergents:

Composition:

  • Generally sodium salts of sulphonic acids or ammonium salts with chloride/bromide ions, both having long hydrocarbon chains.

Advantage in Hard Water:

  • The charged ends of detergents do not form insoluble precipitates with calcium and magnesium ions in hard water.
  • Therefore, detergents remain effective in hard water.

Used in shampoos and products for cleaning clothes.

Agitation: Necessary to get clean clothes, as it helps in the formation of micelles and the dispersion of dirt.