Here we are providing the short notes on Chemistry which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.
Organic chemistry is defined as the study of hydrocarbons and their derivatives. Most atoms are only capable of forming small molecules. However one or two can form larger molecules.
Urea was the first organic compound prepared in laboratory. It was prepared by Wohler (1828) from inorganic compound i.e. ammonium cyanate.
Acetic acid was the first organic compound synthesized from the elements by Kolbe.
Functional group is responsible for the chemical properties of the molecules ex. OH is alcoholic group
Isomers Compounds having the same molecular formula but different structures, e.g. C2H6O can have the structure, i.e. CH3OCH3 (dimethyl ether) and C2H5OH (ethanol).
These are the compounds of only carbon and hydrogen.
Saturated hydrocarbons They contain only single bonds. These are also called alkanes or paraffins and have general formula . Methane is the first member of this group.
Unsaturated hydrocarbons They have general formula for alkene and for alkynes. These have atleast one double (=) or triple (≡) bond and are called alkenes and alkynes respectively.
Aromatic hydrocarbons They have ring structure with alternate double bonds and (4n + 2) (Huckel’s rule) e.g. benzene.
Important Hydrocarbons and their Uses
Methane (CH4) It is also known as marsh gas or damp fire. Natural gas contains mainly 90% methane along with ethane, propane, butane etc. Rice agriculture is a big source of atmospheric methane.
- It is the cause of occurrence of the explosions in mines.
- It is used as a fuel gas in making carbon black.
Produced during decay of biomass in the absence of oxygen. Methane (75%) is the main constituent of biogas).
Natural gas contains approx. 10% ethane. Its hexachloro derivative is used as an artificial camphor.
It is the main constituent of LPG (liquefied petroleum gas).
In World war | (1914-18), it was used for the manufacturing of mustard gas (poisonous gas). It is used as an anesthetic for the preservation and artificial ripening of green fruits.
Acetylene (CH ≡ CH)
It is the simplest aromatic hydrocarbon. It was discovered by Faraday in 1825. It is also used as a motor fuel under the name benzol.
It is used as a commercial solvent in the manufacturing of explosive (TNT), drugs (chloramines-T) and dyestuffs. Used in the manufacturing of saccharin and printing inks. toluene is used as antifreeze.
It is used for preventing moths in clothes, as an insecticide.
Halogen Derivatives of Hydrocarbons
- It was discovered by Sir James Young Simpson.
- It is stored in closed dark coloured bottles completely filled because it is oxidized by air in the presence of sunlight to an extremely poisonous gas phosgene .
- It reacts with conc. and form chloropicrin . Chloropicrin is an insecticide and also used as poisonous gas at the time of war.
- The major use of chloroform today is in the production of the Freon refrigerant, R-22.
It is used as an antiseptic due to liberation of free iodine.
used as a fire extinguishers under the name pyrene.
Dichloro diphenyl trichloro ethane (DDT)
It was the first chlorinated organic insecticides and originally prepared in 1873.
- It is also known as wood spirit or wood naphtha.
- Methyl alcohol is poisonous in nature and when taken internally it can cause blindness and even death.
- It is used for denaturing alcohol (methylated spirit is denatured ethyl alcohol).
It is simply known as alcohol, spirit of wine or grain alcohol.
- It is an important trihydric alcohol known as glycerine.
- It is sweet in taste and very hygroscopic in nature. It is used in the manufacturing of cosmetics and transparent soaps.
It is a monohydric benzene derivative. It is commonly known as carbolic acid or benzenol.
Leakage of this gas is responsible for Bhopal gas tragedy.
- It is believed that is was formed by (carbonization). Different varieties of coal are anthracite (90% carbon), bituminous (70% carbon), lignite (40% carbon) and peat (10-15% carbon).
- On heating at 1270-1675 K in the absence of air, coal decomposes and gives the following products.
- Coke is the solid residue left after the distillation.
- Coal tar It is a mixture of about 700 substances.
- Now-a-days bitumen, a petroleum product, is used in place of coal tar for metalling the roads.
- The most significant characteristics of Indian coal are its high ash content, entrained gasifires and low sulphur content.
- The process of separation of various constituents/ fractions of petroleum is known as refining.
- Knocking – In a petrol engine, vapours of petrol and air are first compressed to a small volume and then ignited by a spark. If the quality of petrol is not good, it leads to the pre-ignition of fuel in the cylinder. This gives rise to a metallic sound known as knocking. Tetraethyl lead (TEL) and Benzene – Toluene – Xylene (BTX) are common antiknock compounds.
- Octane number – The antiknocking property of petrol is measured in terms of octane number. Higher the octane number, better is the quality of fuel. Gasoline used in automobiles has an octane number 80 or higher while in aeroplane, it has an octane number 100 or over higher.
- Producer gas is a mixture of carbon monoxide and nitrogen. Water gas in mixture of carbon monoxide and hydrogen.
- Coal gas is a mixture of hydrogen, methane, carbon monoxide, ethane, acetylene, carbon dioxide, nitrogen and oxygen.