Wednesday, September 1, 2021

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

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 WΓΆhler (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, C5H10 and so on.


General formula for alkenes is CnH2n [n = 2, 3, 4].

General formula for alkanes is CnH2n+2.

General formula for alkynes is CnH2n-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:


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