In methane, the central carbon atom shares 4 pairs of electrons with 4 hydrogen atoms:

Each pair of dots ( : ) represents one shared covalent pair (single bond) satisfying hydrogen's duplet and carbon's octet.

In ethene, two carbon atoms share two electron pairs (double bond) between them, and each carbon shares one pair with two hydrogens:

Carbon shares 3 pairs with hydrogen and 1 pair with oxygen; oxygen shares 1 pair with hydrogen and retains 2 lone pairs:

(The symbol ¨ represents a non-bonding lone pair of valence electrons on the oxygen atom).

The oxygen atom shares 2 pairs with two hydrogen atoms and has 2 unshared lone pairs:

Propane has only one continuous open-chain structural formula (no structural isomers exist):

IUPAC Name: Propane

C₄H₁₀ has two structural isomers:

(i) Straight chain: n-Butane

IUPAC Name: Butane

(ii) Branched chain: Isobutane (2-Methylpropane)

IUPAC Name: 2-Methylpropane

C₃H₄ has two open-chain structural isomers:

(i) Propyne (contains one triple bond):

IUPAC Name: Propyne

(ii) Propadiene / Allene (contains two consecutive double bonds):

IUPAC Name: Propa-1,2-diene (Allene)

The phenomenon where two or more compounds have the same molecular formula but different structural arrangements of atoms in the molecule is called structural isomerism.

Example: Butane (C₄H₁₀) exists as two distinct structural isomers: straight-chain n-butane (boiling point -0.5°C) and branched-chain 2-methylpropane (isobutane) (boiling point -11.7°C).

A chemical bond formed by the mutual sharing of valence electron pairs between two atoms to achieve a stable electronic configuration (duplet or octet) is called a covalent bond.

Example: In the hydrogen molecule (H₂), two hydrogen atoms each sharing 1 electron form a single covalent bond (H–H). In oxygen (O₂), two oxygen atoms share 2 pairs forming a double covalent bond (O=O).

An atom of an element other than carbon and hydrogen (such as halogens, oxygen, nitrogen, sulfur) that replaces one or more hydrogen atoms in a hydrocarbon chain is termed a hetero atom.

Example: In chloromethane (CH₃Cl), chlorine (Cl) is the hetero atom replacing one hydrogen. In ethanol (CH₃CH₂OH), oxygen (O) is the hetero atom.

An atom or a specific bonded group of atoms that imparts characteristic chemical properties to an organic compound, regardless of the size or structure of the remaining carbon skeleton, is known as a functional group.

Example: The hydroxyl group (–OH) is the functional group of alcohols. Both methanol (CH₃OH) and ethanol (C₂H₅OH) share similar chemical behaviors due to the –OH group.

Alkanes are saturated hydrocarbons in which carbon atoms are bonded only by single covalent bonds (C–C). Their general formula is C_nH_2n+2.

Example: Methane (CH₄, n=1), Ethane (C₂H₆, n=2), Propane (C₃H₈, n=3).

Hydrocarbons containing at least one carbon-carbon multiple bond (either a double bond or a triple bond) are called unsaturated hydrocarbons. They readily undergo addition reactions.

Examples:

• Alkenes (Double bond C=C): Ethene (CH₂=CH₂), general formula C_nH_2n.
• Alkynes (Triple bond C≡C): Ethyne (CH≡CH), general formula C_nH_2n-2.

A polymer made by repeatedly joining only a single type of monomer unit is called a homopolymer.

Example: Polyethylene [–(CH₂–CH₂)–_n], formed only from repeating ethylene (CH₂=CH₂) monomer units; Polyvinyl chloride (PVC), formed from vinyl chloride.

A small simple molecule that joins together repeatedly to form a large polymer molecule is called a monomer.

Example: Ethylene (ethene, C₂H₄) is the monomer for polyethylene; Glucose is the monomer for cellulose and starch.

In organic chemistry, a chemical reaction involving the gain of hydrogen atoms, loss of oxygen atoms, or addition of electrons to a compound is defined as reduction.

Example: Catalytic hydrogenation of ethene to ethane using nickel catalyst: CH₂=CH₂ + H₂ ➔ CH₃–CH₃ (Addition of hydrogen is reduction).

An oxidant or oxidizing agent is a chemical substance capable of releasing nascent oxygen or removing electrons/hydrogen from another reactant in a redox reaction.

Examples: Alkaline potassium permanganate (KMnO₄), acidified potassium dichromate (K₂Cr₂O₇), and hydrogen peroxide (H₂O₂). For instance, alkaline KMnO₄ oxidizes ethanol to ethanoic acid.

Longest continuous carbon chain: 4 carbon atoms. Parent alkane: Butane. No functional group or substituent.

IUPAC Name: Butane

Longest carbon chain: 3 carbon atoms (Propane). Functional group: –OH (alcohol, suffix -ol) located at carbon-2.

IUPAC Name: Propan-2-ol

Longest chain: 3 carbon atoms (Propane). Functional group: –COOH (carboxylic acid, suffix -oic acid).

IUPAC Name: Propanoic acid

Longest chain: 2 carbon atoms (Ethane). Functional group: –NH₂ (amine, suffix -amine).

IUPAC Name: Ethanamine

Longest chain: 2 carbon atoms (Ethane). Functional group: –CHO (aldehyde, suffix -al).

IUPAC Name: Ethanal

Longest chain: 4 carbon atoms (Butane). Functional group: –CO– (ketone, suffix -one). Numbering from the left gives the carbonyl carbon the lowest position (C2).

IUPAC Name: Butan-2-one

Millions of carbon compounds exist in nature due to four extraordinary characteristics of carbon:

1. Catenation Power: Carbon has the unique ability to form strong, stable covalent bonds with other carbon atoms, creating extensive straight chains, branched chains, or ring structures of varying sizes.
2. Tetravalency: Having 4 valence electrons, a carbon atom forms 4 covalent bonds with carbon atoms or atoms of other elements (H, O, N, halogens, S, P), generating an immense variety of chemical structures.
3. Formation of Multiple Bonds: Carbon atoms can share one, two, or three electron pairs to form single (C–C), double (C=C), or triple (C≡C) bonds, yielding alkanes, alkenes, and alkynes.
4. Isomerism: A single molecular formula can produce multiple distinct structural arrangements (isomers) with different physical and chemical characteristics.

Saturated hydrocarbons contain only single covalent bonds between carbon atoms and are classified based on their carbon skeleton into:

1. Straight-chain alkanes: Carbon atoms form an unbranched, continuous linear chain.
   Example: Propane (CH₃–CH₂–CH₃) or Butane (CH₃–CH₂–CH₂–CH₃).
2. Branched-chain alkanes: One or more carbon branches (alkyl groups) are attached to the main parent carbon chain.
   Example: Isobutane / 2-Methylpropane [CH₃–CH(CH₃)–CH₃].
3. Ring or Cyclic alkanes (Cycloalkanes): Carbon atoms link to form a closed ring structure.
   Example: Cyclohexane (C₆H₁₂) or Cyclopropane (C₃H₆).

1. Vinegar:
   • Meaning: A 5% to 8% aqueous solution of acetic acid (ethanoic acid, CH₃COOH) in water.
   • Uses: Widely used as a preservative in pickles and food sauces, as a cooking condiment for tenderizing meat, and as a flavoring agent in salads.
2. Gasohol:
   • Meaning: An alternative automobile fuel prepared by blending 10% anhydrous ethanol with 90% petrol (gasoline).
   • Uses: Used as a cleaner-burning motor fuel that improves engine efficiency and cuts down toxic vehicular carbon emissions.

A catalyst is a substance that accelerates the rate of a chemical reaction without itself undergoing any permanent chemical change or being consumed in the reaction.

Example reaction (Hydrogenation of oils):

Unsaturated vegetable oils react with hydrogen gas in the presence of a finely divided nickel (Ni) catalyst at 60°C to form saturated vegetable ghee (Vanaspati):