Basics and Hydrocarbons – NDA Chemistry Notes

Organic chemistry is the study of carbon compounds. Carbon forms more compounds than any other element in the periodic table. 

This chapter builds the foundation for organic chemistry. It covers why carbon is special, how organic compounds are classified, and what hydrocarbons are.

The chapter is entirely conceptual. No complex calculations are needed. What is needed is a clear understanding of terms, classifications, and the properties of key hydrocarbons.

1. What is Organic Chemistry?

Think of sugar, cooking oil, petrol, candle wax, medicines, plastics, and cotton. All of these are organic compounds. Organic compounds are compounds that contain carbon.

The term “organic” originally meant compounds found only in living organisms. Scientists once believed that living organisms had a special “vital force” needed to produce these compounds. This idea was disproved in 1828 when Friedrich Wohler synthesised urea (an organic compound) from ammonium cyanate (an inorganic compound) in a laboratory. This proved that organic compounds can be made artificially.

Today, organic chemistry means the chemistry of carbon compounds.

2. Why is Carbon Special?

Carbon has four properties that make it unique and allow it to form millions of compounds.

Property 1: Tetravalency. Carbon has atomic number 6. Its electronic configuration is 2, 4. Carbon has four electrons in its outermost shell. It needs four more electrons to complete its octet. So carbon always forms four covalent bonds. This is called tetravalency.

The maximum covalency of carbon is 4. [NDA 2006-I]

Because carbon forms exactly four bonds, it can connect to four other atoms at the same time. This gives enormous variety in how carbon atoms can arrange themselves.

Property 2: Catenation. Carbon atoms can form bonds with other carbon atoms. Long chains of carbon atoms can be built. This property is called catenation.

No other element shows catenation to the same extent as carbon. Silicon can form chains too, but much shorter ones. Catenation allows carbon to form straight chains, branched chains, and rings.

Property 3: Multiple Bonds. Carbon can form single bonds (C-C), double bonds (C=C), and triple bonds (C≡C) with other carbon atoms. This gives more variety in organic structures.

Property 4: Small Size. Carbon atoms are small. Small size means carbon can form strong, stable bonds with other atoms including hydrogen, oxygen, nitrogen, sulphur, and halogens.

These four properties together explain why carbon forms millions of compounds.

3. Organic vs Inorganic Compounds

Feature	Organic Compounds	Inorganic Compounds
Contains	Carbon (always)	Any element except usually carbon
Source	Living organisms or lab synthesis	Minerals, rocks, industrial chemicals
Solubility	Usually soluble in organic solvents	Usually soluble in water
Melting/boiling point	Usually low	Usually high
Electrical conductivity	Usually non-conductors	Many conduct electricity
Bond type	Mostly covalent	Ionic or covalent
Examples	Sugar, petrol, alcohol, protein	NaCl, H₂SO₄, CaCO₃

Exceptions exist: CO, CO₂, and carbonates are carbon compounds but they are treated as inorganic.

4. Classification of Organic Compounds

Organic compounds are first divided into two broad groups based on their structure.

Group 1: Acyclic (Open Chain) Compounds. These compounds have carbon atoms arranged in open chains. The chain can be straight or branched. They are also called aliphatic compounds. Examples: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).

Group 2: Cyclic (Closed Chain) Compounds. These compounds have carbon atoms arranged in a ring. There are two types of cyclic compounds.

Type 1: Alicyclic compounds. These are ring compounds that behave like aliphatic compounds. Example: cyclohexane.

Type 2: Aromatic compounds. These contain a special type of ring called a benzene ring. Benzene (C₆H₆) is the simplest aromatic compound. Aromatic compounds have a distinctive pleasant smell.

5. Functional Groups

A functional group is an atom or group of atoms in an organic molecule that determines its chemical properties.

Different functional groups give different chemical behaviour. The same functional group behaves similarly regardless of the carbon chain attached to it.

Functional Group	Name	Example
-OH	Hydroxyl (alcohol)	Ethanol (C₂H₅OH)
-COOH	Carboxyl (carboxylic acid)	Acetic acid (CH₃COOH)
-CHO	Aldehyde	Formaldehyde (HCHO)
-CO-	Ketone	Acetone (CH₃COCH₃)
-NH₂	Amino	Glycine (amino acid)
-X (F, Cl, Br, I)	Halide	Chloroform (CHCl₃)
-NO₂	Nitro	Nitrobenzene

6. Homologous Series

A homologous series is a group of organic compounds that have the same general formula, the same functional group, and similar chemical properties. Each member in the series differs from the next by a CH₂ unit.

Properties of a homologous series:

• Same general formula for all members
• Same functional group
• Each consecutive member differs by CH₂ (14 in molecular mass)
• Similar chemical properties
• Gradually changing physical properties (melting point, boiling point, density increase as chain length increases)

Example of a homologous series: alkanes (methane, ethane, propane, butane…).

7. Isomers

Isomers are compounds that have the same molecular formula but different structural arrangements. They have the same number and type of atoms but different properties.

Chain Isomerism: Same molecular formula, different arrangement of the carbon chain (straight vs branched). Example: Butane (C₄H₁₀) can be straight chain (n-butane) or branched (isobutane/2-methylpropane).

Position Isomerism: Same molecular formula and carbon chain, but the functional group is at a different position on the chain. Example: 1-propanol and 2-propanol are both C₃H₇OH but -OH is at different positions.

Functional Group Isomerism: Same molecular formula but different functional groups. Example: Ethanol (C₂H₅OH, an alcohol) and dimethyl ether (CH₃OCH₃, an ether) both have formula C₂H₆O.

Alkyl halides cannot show functional group isomerism because they have only one possible functional group type.

8. Hydrocarbons

Hydrocarbons are organic compounds that contain only carbon and hydrogen. They are the simplest organic compounds. Hydrocarbons are divided into two main groups.

Group 1: Saturated Hydrocarbons. In saturated hydrocarbons, all carbon-carbon bonds are single bonds (C-C). No double or triple bonds between carbon atoms. They are called saturated because they cannot absorb any more hydrogen atoms. The alkanes form the most important family of saturated hydrocarbons.

Group 2: Unsaturated Hydrocarbons. In unsaturated hydrocarbons, there is at least one double bond (C=C) or triple bond (C≡C) between carbon atoms. They are called unsaturated because they can absorb more hydrogen. Alkenes (one double bond) and alkynes (one triple bond) are unsaturated hydrocarbons. Aromatic hydrocarbons (benzene and its derivatives) are also considered unsaturated.

Alkanes (CₙH₂ₙ₊₂)

Alkanes are saturated hydrocarbons. They have only single bonds. Their general formula is CₙH₂ₙ₊₂.

Name	Formula	Number of C atoms	Common use
Methane	CH₄	1	Natural gas, biogas, CNG
Ethane	C₂H₆	2	Component of LPG
Propane	C₃H₈	3	LPG (cooking gas)
Butane	C₄H₁₀	4	LPG (cooking gas)
Pentane	C₅H₁₂	5	Petrol/gasoline fraction
Hexane	C₆H₁₄	6	Solvent

Properties of Alkanes: Alkanes are relatively unreactive. They do not react with acids, bases, or most chemicals at room temperature.

Alkanes undergo combustion. Complete combustion gives CO₂ and H₂O with a clean blue flame. Incomplete combustion gives CO and soot with a yellow sooty flame. Alkanes burn with a relatively clean, blue flame because they have a high proportion of hydrogen relative to carbon.

Methane is the main constituent of natural gas and biogas. Methane is also the major constituent of gobar gas (biogas produced from cow dung).

Alkenes (CₙH₂ₙ)

Alkenes are unsaturated hydrocarbons with one carbon-carbon double bond (C=C). Their general formula is CₙH₂ₙ.

Name	Formula	Double bond
Ethene (Ethylene)	C₂H₄	One C=C
Propene	C₃H₆	One C=C
Butene	C₄H₈	One C=C

Properties of Alkenes: Alkenes are more reactive than alkanes because of the double bond. The double bond can be broken to add other atoms. This type of reaction is called an addition reaction.

Ethylene (ethene) is used in gas welding. Ethylene is used to ripen fruits artificially. Polyethylene (polythene) is made from ethylene by addition polymerisation.

Alkynes (CₙH₂ₙ₋₂)

Alkynes are unsaturated hydrocarbons with one carbon-carbon triple bond (C≡C). Their general formula is CₙH₂ₙ₋₂.

Name	Formula	Triple bond
Ethyne (Acetylene)	C₂H₂	One C≡C
Propyne	C₃H₄	One C≡C
Butyne	C₄H₆	One C≡C

Properties of Alkynes: Alkynes are the most reactive of the three alkane/alkene/alkyne groups.

Acetylene (ethyne) burns with an extremely hot flame when burnt in oxygen. This is used in gas welding and metal cutting.  The flame in oxyacetylene welding reaches about 3500°C. This is hot enough to melt any metal.

Property	Alkanes	Alkenes	Alkynes
Bond type	Only single bonds (C-C)	One double bond (C=C)	One triple bond (C≡C)
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ	CₙH₂ₙ₋₂
Saturated or unsaturated	Saturated	Unsaturated	Unsaturated
Reactivity	Low	Medium	High
Typical reaction	Substitution	Addition	Addition
Flame	Clean blue	Yellow, slightly sooty	Very hot, luminous
Example	Methane (CH₄)	Ethene (C₂H₄)	Ethyne (C₂H₂)

Aromatic Hydrocarbons (Benzene)

Benzene (C₆H₆) is the simplest and most important aromatic compound. It contains a ring of 6 carbon atoms. Each carbon is bonded to one hydrogen. The ring has alternating double bonds.

Benzene is a colourless liquid with a sweet smell. It is highly toxic. It is a known carcinogen (causes cancer).

Benzene burns with a very sooty yellow flame because it has a high carbon-to-hydrogen ratio. [NDA 2016-I]

To fully saturate one mole of benzene (convert all double bonds to single bonds), three moles of H₂ are needed. [NDA 2016-I] C₆H₆ + 3H₂ → C₆H₁₂ (cyclohexane).

Benzene is obtained from coal tar. Coal tar is a by-product of the destructive distillation of coal.

Compounds with sooty flames (high carbon content): Benzene, Naphthalene, Anthracene. [NDA 2016-I]

Compound that does NOT give a sooty flame: Hexane. [NDA 2016-I] Hexane is an alkane (C₆H₁₄) with a high hydrogen content relative to carbon. It burns more cleanly than aromatic compounds.

9. Freon

Freon is the trade name for chlorofluorocarbons (CFCs). These are compounds where hydrogen atoms in hydrocarbons are replaced by fluorine and chlorine atoms.

Freon is used as a refrigerant in air conditioners and refrigerators.

Freon is chemically known as a chlorofluoro hydrocarbon (CFC).

CFCs damage the ozone layer when they reach the stratosphere. This is why CFCs are now banned under the Montreal Protocol.

10. Petroleum and Natural Gas

Petroleum (crude oil) is a naturally occurring mixture of hydrocarbons found underground. It is formed over millions of years from the remains of marine organisms.

Petroleum is refined by fractional distillation. Different fractions are separated based on their different boiling points.

Fraction	Approximate Boiling Range	Use
Petroleum gas (LPG)	Below 30°C	Cooking fuel (propane, butane)
Gasoline (Petrol)	30-200°C	Motor fuel (lowest boiling point fraction)
Kerosene	150-300°C	Fuel for lamps, jet fuel
Diesel	250-350°C	Diesel engines
Lubricating oil	300-400°C	Machine lubrication
Paraffin wax	Solid at room temp	Candles, wax polish
Bitumen/Tar	Residue	Road surfacing

Gasoline has the lowest boiling point among liquid petroleum fractions.

Cracking is the process of breaking large hydrocarbon molecules into smaller, more useful molecules by heating them at high temperature with a catalyst. This converts heavy fractions into petrol and other lighter products.

11. Coal Tar

Coal tar is a black, viscous liquid obtained as a by-product when coal is heated strongly without air (destructive distillation of coal). Coal tar is an important source of aromatic compounds.

Benzene, toluene, naphthalene, and anthracene are all obtained from coal tar.

Naphthalene and anthracene are aromatic hydrocarbons. Both give sooty flames when burned.

12. Reactions of Hydrocarbons

Combustion: All hydrocarbons burn in oxygen to produce CO₂ and H₂O.

Complete combustion (excess oxygen): CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O (blue flame, clean). Incomplete combustion (limited oxygen): produces CO and soot (yellow, sooty flame).

Unsaturated hydrocarbons (alkenes, alkynes, aromatics) have higher carbon content. They tend to give sootier flames than saturated hydrocarbons of similar size.

Addition Reaction (characteristic of alkenes and alkynes): A molecule adds across the double or triple bond. No atoms are lost. Example: C₂H₄ + H₂ → C₂H₆ (hydrogenation of ethene). Example: C₂H₄ + Br₂ → C₂H₄Br₂ (addition of bromine).

Bromine water test: Alkenes and alkynes decolourise brown bromine water. Alkanes do NOT decolourise bromine water. This is a simple test to distinguish saturated from unsaturated hydrocarbons.

Substitution Reaction (characteristic of alkanes): One or more hydrogen atoms are replaced by another atom or group, usually a halogen. Example: CH₄ + Cl₂ → CH₃Cl + HCl (chlorination of methane in sunlight).