4. CARBON AND ITS COMPOUNDS

VERSATILE NATURE OF CARBON

Carbon has two unique properties called Catenation & Tetravalency. So it can form millions of compounds.

This outnumbers the compounds formed by all the other elements put together.

1. Catenation

It is the ability of carbon to form bonds with other atoms of carbon, giving rise to large molecules.

They may be long chains, branched chains or ring forms.

No other element exhibits catenation like carbon. Silicon forms compounds with hydrogen which have chains of up to 7 or 8 atoms, but these are very reactive. Carbon-carbon bond is very strong & stable. This gives large number of compounds.

2. Tetravalency

Carbon can bond with four other atoms of carbon or some other monovalent elements.

Carbon compounds are formed with oxygen, hydrogen, nitrogen, sulphur, chlorine etc. giving specific properties.

Carbon atom is small-sized. So the nucleus can hold the shared pairs of electrons strongly. So carbon can make very stable compounds with other elements. The bonds formed by elements having bigger atoms are weaker.

It was thought that organic or carbon compounds could only be formed with the help of a vital force (i.e., a living system is needed).

Friedrich Wöhler (1828) disproved this by preparing urea from ammonium cyanate.

But carbon compounds, except for carbides, oxides of carbon, carbonate and hydrogencarbonate salts are studied under organic chemistry.

Saturated and Unsaturated Carbon Compounds

Saturated compounds: They are linked by only single bonds between the carbon atoms. These are not very reactive. E.g. Ethane (C₂H₆).

Structure of ethane:

Structure of propane (C₃H₈):

Unsaturated compounds: They have double or triple bonds between carbon atoms. They are more reactive.

E.g. Ethene (C₂H₄): It needs double bond to satisfy the valency.

Ethyne (C₂H₂): It has triple bond between carbon atoms to satisfy the valency (H – C ≡ C – H).

Electron dot structure for ethyne

Chains, Branches and Rings

Chains of carbon atoms contain more carbon atoms. E.g.

No. of C atoms	Name	Formula	Structure
1	Methane	CH₄
2	Ethane	C₂H₆
3	Propane	C₃H₈
4	Butane	C₄H₁₀
5	Pentane	C₅H₁₂
6	Hexane	C₆H₁₄

Carbon ‘skeleton’ of 4 carbon atoms has two forms:

Complete molecules for two structures with formula C₄H₁₀

These structures have same formula C₄H₁₀. Such compounds with identical molecular formula but different structures are called structural isomers.

Some compounds have carbon atoms arranged in the form of a ring. E.g., cyclohexane (C₆H₁₂).

Straight chain, branched-chain & cyclic compounds may be saturated or unsaturated. E.g. benzene (C₆H₆).

All carbon compounds that contain only carbon and hydrogen are called hydrocarbons.

Saturated hydrocarbons are called alkanes.

The unsaturated hydrocarbons which contain one or more double bonds are called alkenes. Those containing one or more triple bonds are called alkynes.

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Carbon also bonds with other elements such as halogens, oxygen, nitrogen & sulphur.

In a hydrocarbon chain, one or more hydrogens are replaced by these elements.

The element replacing hydrogen is called a heteroatom.

Heteroatoms & the group containing these give specific properties to the compound, regardless of the length and nature of the chain. So they are called functional groups.

Some functional groups in carbon compounds

Free valency or valencies of the group are shown by the single line. The functional group is attached to the carbon chain through this valency.

Homologous Series

A functional group such as alcohol decides the properties of the carbon compound. E.g. chemical properties of CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH are very similar.

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

Homologous series for alkanes: Succeeding members differ by a –CH₂- unit. E.g.

CH₄ and C₂H₆ – differ by a –CH₂- unit

C₂H₆ and C₃H₈– differ by a –CH₂- unit

C₃H₈ and C₄H₁₀– differ by a –CH₂- unit

They show a difference of 14 U in molecular masses b/w the pairs (atomic mass of carbon = 12 u, hydrogen = 1 u).

Homologous series for alkenes: They also differ by a –CH₂– unit. First member is ethene (C₂H₄). Succeeding members are C₃H₆, C₄H₈, C₅H₁₀and so on.

General formula for alkenes is C_nH_2n [n = 2, 3, 4].

General formula for alkanes is C_nH_2n+2.

General formula for alkynes is C_nH_2n-2.

As the molecular mass increases, physical properties such as melting & boiling points, solubility in solvent etc. also increase. But chemical properties remain similar.

Homologous series of Alcohols:

Compounds	Difference in formula	Difference in molecular mass
CH₃OH & C₂H₅OH	–CH₂-	14 U
C₂H₅OH & C₃H₇OH	–CH₂-	14 U
C₃H₇OH & C₄H₉OH	–CH₂-	14 U
C₄H₉OH & C₅H₁₁OH	–CH₂-	14 U

Nomenclature of Carbon Compounds

Method of naming a carbon compound:

1. Identify the number of carbon atoms. E.g. three-carbon compound is named propane.

2. Presence of functional group is indicated by a prefix or a suffix.

3. If the suffix of the functional group begins with a vowel, the final letter ‘e’ is deleted from the name of the carbon chain. E.g., Propane with a ketone group is named as

Propane – ‘e’ = propan + ‘one’ = propanone.

4. For unsaturated carbon chain, the final ‘ane’ is substituted by ‘ene’ or ‘yne’. E.g., propene (double bond), propyne (triple bond) etc.

Nomenclature of organic compounds:

Select your Next Topic 👇

👉 Part 1: Bonding in Carbon- the Covalent Bond
👉 Part 2: Versatile Nature of Carbon
👉 Part 3: Chemical properties of Carbon Compounds
👉 Part4: Some important Carbon Compounds
👉 Part 5: Soaps and Detergents

4. Carbon and its Compounds | Class 10 CBSE | Web Notes - Part 2