Chemical Properties of Carbon Compounds
Carbon compounds undergo different types of chemical reactions.
- Thermal cracking.
All carbon compounds react with oxygen to produce heat and light along with carbon dioxide and water. This reaction of carbon with oxygen is called combustion.
Carbon Compound + Oxygen → Carbon dioxide + water + heat and light
CH4 + 2O2 → CO2 + 2H2O + Heat and light.
- Aliphatic compounds on combustion produce a non-sooty flame.
- Aromatic compounds on combustion produce sooty flame.
Alcohols undergo oxidation in presence of oxidising agents like alkaline potassium permanganate
or acidified potassium dichromate to form carboxylic acids.
Ethyl alcohol on oxidation with alkaline potassium permanganate or acidified potassium dichromate gives acetic acid.
CH3–CH2–OH Alkaline KMnO4 or Acidified K 2 Cr 2 O 7 → CH3–COOH
A chemical reaction is said to be an addition reaction if two substances combine and form a third substance. In general unsaturated hydrocarbons like alkenes and alkynes prefers to undergo addition reactions.
In addition reactions molecules add across double bond or triple bond.
Hydrogenation reaction involves the addition of hydrogen to unsaturated hydrocarbons in presence of catalyst like nickel or platinum to form saturated hydrocarbons.
Addition of hydrogen to ethene
Addition of hydrogen ethyne.
CH ≡ CH + 2H2 Ni or Pt → CH3–CH3
Addition of halogens to alkenes.
CH2 = CH2 + X2 → CH2X – CH2X
Similar to alkenes, addition reactions are also characteristic of alkynes.
Ethyne reacts readily with hydrogen in the presence of a suitable catalyst to form ethene, an intermediate and then adds another hydrogen molecule to give ethane.
CHΞCH + H2 + (Catalyst) → CH2=CH2 + H2 → CH3–CH3
Halogens, especially chlorine, react readily with alkynes to produce tetra-halogen derivatives.
The addition of one molecule of chlorine to ethyne produces 1, 2-dichloroethene. The product obtained contains a carbon-carbon double bond and can further add one molecule of chlorine thus yielding a tetra halogen derivative, 1, 1, 2, 2-tetrachloroethane.
CHΞCH + Cl2 → CH(Cl)=CH(Cl) + Cl2 → CH(Cl2)–CH(Cl2)
Addition of unsymmetrical reagents:
When unsymmetrical reagents like HCl, HBr or H2O across the double bond in such a way that one part of the molecule attaches itself to one carbon of the double bond, while the other part of the molecule attaches itself to the other carbon of the double bond.
Ethene on reaction with HCl, produces chloroethane.
CH2=CH2 + HCl → CH3-CH2Cl
The addition product of ethene and HBr is bromoethane.
CH2=CH2 + HBr → CH3–CH2Br
A reaction in which an atom or group of atoms replaces another atom or group of atoms is called substitution reaction. Alkanes undergo substitution reactions.
Chlorination of methane in presence of sunlight gives a mixture of products like methyl chloride,
methylene chloride, chloroform and carbon tetrachloride.
CH4 + Cl2 Sunlight → CH3Cl + HCl
CH3Cl+Cl2 Sunlight → CH2Cl2 + HCl
CH2Cl2+Cl2 Sunlight → CHCl3 +HCl
CHCl3+Cl2 Sunlight → CCl4+HCl
Alkenes and alkynes at higher temperatures under polymerization to form bigger molecules called as polymers.
Ethene at 400 °C undergoes polymerization to form polyehene.
nCH2 = CH2 → [–CH2 –CH2 – CH2 – CH2–]n
The polymer is usually named by adding the word “poly” to the name of the monomer. Thus, the polymer of ethene is named polyethene or polythene.
A variety of industrially important polymers are obtained by using substituted ethenes in place of ethene.
Propene gives polypropene on polymerisation.
Chloroethene, commonly known as vinyl chloride, yields poly vinyl chloride or PVC, on polymerisation.
(Teflon) Tetra fluoroethene, on polymerisation, yields poly tetra fluoroethene, commonly known as Teflon.
Applications of polymers:
Polythene, polypropene and PVC are common plastics widely used to make plastic bags, bottles, electrical insulation, pipes and many more things.
Teflon is used in the manufacture of non-stick cookware.
When hydrocarbons of high molecular masses are heated to high temperatures under high pressures, they decompose, forming hydrocarbons of lower molecular masses. This breaking up of large hydrocarbon molecules into smaller at high temperatures is known as thermal cracking. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons.
The products of thermal cracking depend upon the nature of the hydrocarbon, temperature, pressure, and the catalyst used. Thermal cracking of decane gives hexane and butene.
C10H22 Cracking at 600 – 700 ℃ → C6H14 + C4H8