Introduction
Carbon is
one of the most important elements present in nature. Although the amount of
carbon present in the Earth’s crust and atmosphere is very small, its
importance is enormous because all living organisms and most everyday
substances are carbon-based.
Food,
clothes, medicines, fuels, plastics, paper, and even our body contain compounds
of carbon. Carbon is therefore called the backbone of organic chemistry.
Carbon
exists in:
- Coal
- Petroleum
- Carbon
dioxide
- Carbonates
- Living
organisms
The unique
nature of carbon allows it to form millions of compounds.
4.1 BONDING
IN CARBON – THE COVALENT BOND
Most carbon
compounds:
- Have
low melting and boiling points
- Are
poor conductors of electricity
This
indicates that carbon compounds are not ionic compounds. Instead, carbon forms covalent
bonds by sharing electrons.
Electronic
Configuration of Carbon
Atomic
number of carbon = 6
Electronic
configuration:
K shell = 2
electrons
L shell = 4 electrons
Thus, carbon
has 4 valence electrons.
Carbon
cannot:
- Gain 4
electrons easily
- Lose 4
electrons easily
Therefore,
carbon completes its octet by sharing electrons.
Covalent
Bond
A chemical
bond formed by sharing of electrons between atoms is called a covalent bond.
Formation of
Hydrogen Molecule (H₂)
Each
hydrogen atom contributes one electron.
A single
shared pair of electrons forms a single covalent bond.
Formation of
Oxygen Molecule (O₂)
Oxygen has 6
valence electrons and needs 2 more electrons to complete its octet.
Each oxygen
atom shares two electrons.
This forms a
double covalent bond.
Formation of
Nitrogen Molecule (N₂)
Nitrogen has
5 valence electrons and requires 3 more electrons.
Each
nitrogen atom shares three electrons.
This forms a
triple covalent bond.
Formation of
Methane (CH₄)
Methane is
the simplest carbon compound.
Carbon
shares its four valence electrons with four hydrogen atoms.
Important
Features of Covalent Compounds
- Low
melting and boiling points
- Generally
poor conductors of electricity
- Molecules
are electrically neutral
Allotropes
of Carbon
Different
physical forms of the same element are called allotropes.
Major
allotropes of carbon:
- Diamond
- Graphite
- Fullerenes
Diamond
In diamond:
- Each
carbon atom is bonded to four other carbon atoms.
- Strong
three-dimensional structure formed.
Properties
- Hardest
natural substance
- High
melting point
- Poor
conductor of electricity
Graphite
In graphite:
- Each
carbon atom is bonded to three other carbon atoms.
- Layered
hexagonal structure formed.
Properties
- Soft
and slippery
- Conducts
electricity
- Used in
electrodes and lubricants
Fullerene
(C₆₀)
Carbon atoms
are arranged like a football.
Also called:
Buckminsterfullerene
4.2
VERSATILE NATURE OF CARBON
Carbon forms
an extremely large number of compounds due to two special properties:
- Catenation
- Tetravalency
1.
Catenation
The ability
of carbon to form bonds with other carbon atoms is called catenation.
Carbon atoms
can form:
- Long
chains
- Branched
chains
- Ring
structures
Example:
- Methane
- Ethane
- Propane
- Benzene
Carbon-carbon
bonds are very strong, making carbon compounds stable.
2.
Tetravalency
Carbon has
valency 4.
Thus, one
carbon atom can bond with:
- Four
hydrogen atoms
- Other
carbon atoms
- Oxygen
- Nitrogen
- Sulphur
- Chlorine
This leads
to the formation of a huge number of compounds.
Organic
Compounds
Compounds
containing carbon are generally called organic compounds.
Exceptions:
- Carbonates
- Bicarbonates
- Oxides
of carbon
- Carbides
Saturated
and Unsaturated Carbon Compounds
Saturated
Compounds
Carbon
compounds containing only single bonds are called saturated compounds.
These are
called:
Alkanes
Examples
|
Compound |
Formula |
|
Methane |
CH₄ |
|
Ethane |
C₂H₆ |
|
Propane |
C₃H₈ |
|
Butane |
C₄H₁₀ |
Unsaturated
Compounds
Carbon
compounds containing:
- Double
bonds OR
- Triple
bonds
are called
unsaturated compounds.
Alkenes
Contain
double bonds.
Example
(Ethene)
Alkynes
Contain
triple bonds.
Example
(Ethyne)
Unsaturated
compounds are generally more reactive.
Chains,
Branches and Rings
Carbon atoms
can form:
1. Straight
Chains
Example:
- Pentane
- Hexane
2. Branched
Chains
Example:
- Isobutane
3. Cyclic
Compounds
Example:
- Cyclohexane
- Benzene
Structural
Isomers
Compounds
having:
- Same
molecular formula
- Different
structural arrangements
are called
structural isomers.
Example:
Butane
(C₄H₁₀)
Hydrocarbons
Compounds
containing only carbon and hydrogen are called hydrocarbons.
|
Type |
Bond Present |
|
Alkane |
Single
bond |
|
Alkene |
Double
bond |
|
Alkyne |
Triple
bond |
Functional
Groups
Atoms or
groups of atoms that determine the chemical properties of compounds are called
functional groups.
Important
Functional Groups
|
Functional Group |
Formula |
Example |
|
Alcohol |
–OH |
Ethanol |
|
Aldehyde |
–CHO |
Ethanal |
|
Ketone |
–CO |
Propanone |
|
Carboxylic
acid |
–COOH |
Ethanoic
acid |
|
Halo group |
–Cl, –Br |
Chloropropane |
Homologous
Series
A series of
compounds:
- Having
same functional group
- Similar
chemical properties
- Consecutive
members differ by –CH₂
is called a
homologous series.
Characteristics
of Homologous Series
- Same
functional group
- Similar
chemical properties
- Gradation
in physical properties
- Consecutive
members differ by 14u mass
General
Formulae
|
Series |
General Formula |
|
Alkane |
CₙH₂ₙ₊₂ |
|
Alkene |
CₙH₂ₙ |
|
Alkyne |
CₙH₂ₙ₋₂ |
Nomenclature
of Carbon Compounds
Naming
depends on:
- Number
of carbon atoms
- Functional
group
- Type of
bonding
Prefixes
Based on Carbon Atoms
|
Carbon Atoms |
Prefix |
|
1 |
Meth |
|
2 |
Eth |
|
3 |
Prop |
|
4 |
But |
|
5 |
Pent |
|
6 |
Hex |
Suffixes
|
Functional Group |
Suffix |
|
Alcohol |
–ol |
|
Aldehyde |
–al |
|
Ketone |
–one |
|
Carboxylic
acid |
–oic acid |
|
Double
bond |
–ene |
|
Triple
bond |
–yne |
4.3 CHEMICAL
PROPERTIES OF CARBON COMPOUNDS
1.
Combustion
Carbon
compounds burn in air to produce:
- Carbon
dioxide
- Water
- Heat
and light
Carbon
Burning
Methane
Combustion
Ethanol
Combustion
Types of
Flame
|
Compound Type |
Flame |
|
Saturated
compounds |
Clean blue
flame |
|
Unsaturated
compounds |
Yellow
sooty flame |
Incomplete
combustion produces black soot.
Fossil Fuels
Coal and
petroleum are called fossil fuels because they are formed from dead plants and
animals over millions of years.
2. Oxidation
Reaction
Addition of
oxygen or removal of hydrogen is called oxidation.
Oxidation of
Ethanol
Oxidising
Agents
- Alkaline
KMnO₄
- Acidified
K₂Cr₂O₇
These
convert alcohol into carboxylic acid.
3. Addition
Reaction
Unsaturated
hydrocarbons add hydrogen in the presence of catalyst.
Hydrogenation
of Ethene
Catalyst
Used
- Nickel
- Palladium
Industrial
Use
Conversion
of vegetable oils into vanaspati ghee.
4.
Substitution Reaction
In saturated
hydrocarbons, one atom replaces another atom.
Methane with
Chlorine
4.4
IMPORTANT CARBON COMPOUNDS
ETHANOL
(C₂H₅OH)
Commonly
called:
Alcohol
Properties:
- Colourless
liquid
- Soluble
in water
- Used in
medicines and perfumes
- Used as
solvent
Excess
consumption affects nervous system.
Reactions of
Ethanol
1. Reaction
with Sodium
Hydrogen gas
is evolved.
2.
Dehydration Reaction
Heating
ethanol with concentrated sulphuric acid at 443 K produces ethene.
Concentrated
sulphuric acid acts as dehydrating agent.
Denatured
Alcohol
Industrial
alcohol made unfit for drinking by adding poisonous substances like methanol.
ETHANOIC
ACID (CH₃COOH)
Commonly
called:
Acetic Acid
5–8%
solution in water is called vinegar.
Properties
- Sour
smell
- Weak
acid
- Turns
blue litmus red
Pure
ethanoic acid freezes in winter and is called:
Glacial
acetic acid
Reactions of
Ethanoic Acid
1.
Esterification Reaction
Acid reacts
with alcohol to form ester.
Product
Ethyl
ethanoate (pleasant fruity smell)
Used in:
- Perfumes
- Flavouring
agents
Saponification
Conversion
of ester into alcohol and sodium salt using NaOH.
Soap
manufacturing is based on this reaction.
2. Reaction
with Base
Produces:
- Sodium
acetate
- Water
3. Reaction
with Carbonates
4. Reaction
with Hydrogencarbonates
Carbon
dioxide gas turns lime water milky.
4.5 SOAPS
AND DETERGENTS
Soap
Molecules
Soap
molecules contain:
- Hydrophilic
end → attracts water
- Hydrophobic
end → attracts oil
Micelles
Soap
molecules arrange themselves in water forming structures called micelles.
Working
- Oil
gets trapped inside micelle.
- Dirt is
removed with water.
This is the
cleaning action of soap.
Hard Water
and Soap
Soap reacts
with calcium and magnesium salts present in hard water to form:
Scum
This reduces
cleaning efficiency.
Detergents
Detergents
do not form scum with hard water.
Therefore:
- Work
effectively in hard water
- Used in
shampoos and washing powders
