Organic Chemistry- ICSE-Class 10|Biswajit Das

1. Introduction to Organic Chemistry

• Definition: Organic chemistry is the branch of chemistry that deals with the study of hydrocarbons and their derivatives. Hydrocarbons are compounds made up of carbon and hydrogen.
• Vital Force Theory: This theory, proposed by Berzelius, stated that organic compounds could only be produced by living organisms and a “vital force” was essential for their synthesis. This theory was disproved by Friedrich Wöhler in 1828 when he synthesized urea (an organic compound) from ammonium cyanate (an inorganic compound).
• Special Properties of Carbon: The uniqueness of carbon is due to its valency of four and its ability to form strong covalent bonds with other carbon atoms.
  • Catenation: The ability of carbon atoms to link with one another to form long chains (straight, branched), rings, or complex three-dimensional structures.
  • Isomerism: The existence of two or more compounds with the same molecular formula but different structural formulas and properties.
  • Tetravalency: Carbon has a valency of four, allowing it to form single, double, or triple covalent bonds with other atoms.

2. Classification of Hydrocarbons

Hydrocarbons are the parent organic compounds and are classified based on the types of bonds present between the carbon atoms.

• Saturated Hydrocarbons (Alkanes): These contain only single covalent bonds between carbon atoms. Their general formula is Cn​H2n+2​. They are relatively unreactive and are known as paraffins.
  • Examples: Methane (CH4​), Ethane (C2​H6​).
• Unsaturated Hydrocarbons: These contain at least one double or triple bond between carbon atoms.
  • Alkenes: Contain at least one carbon-carbon double bond. Their general formula is Cn​H2n​. They are more reactive than alkanes.
    • Example: Ethene (C2​H4​).
  • Alkynes: Contain at least one carbon-carbon triple bond. Their general formula is Cn​H2n−2​. They are the most reactive of the three.
    • Example: Ethyne (C2​H2​).

3. Functional Groups

A functional group is an atom or group of atoms that determines the characteristic chemical properties of an organic compound. The hydrocarbon part of the molecule (alkyl group) remains largely unreactive.

4. Homologous Series

A homologous series is a series of organic compounds that have the same functional group and similar chemical properties. Each successive member differs from the next by a CH2​ group.

• Characteristics:
  • Same general formula.
  • Same functional group.
  • Members can be prepared using general methods.
  • Physical properties (like boiling point) show a gradual change.

5. Isomerism

Isomerism is the phenomenon where two or more compounds have the same molecular formula but different structural arrangements, leading to different physical and chemical properties. This chapter focuses on structural isomerism, where the atoms are connected in different sequences.

• Chain Isomerism: Occurs due to the difference in the arrangement of the carbon skeleton (straight chain vs. branched chain).
  • Example: Butane (C4​H10​) has two isomers: n-butane and isobutane.
• Position Isomerism: The same carbon skeleton exists, but the position of a functional group or a multiple bond is different.
  • Example: 1-Butene and 2-Butene (C4​H8​).
• Functional Isomerism: The compounds have the same molecular formula but different functional groups.
  • Example: Ethanol (C2​H5​OH) and Dimethyl ether (CH3​OCH3​) both have the formula C2​H6​O.

Methane (CH4​)

• Preparation: Methane is prepared in the lab by heating a mixture of sodium acetate (CH3​COONa) and soda lime (a mixture of NaOH and CaO). This reaction is called decarboxylation.CH3​COONa+NaOHCaO,Δ​CH4​+Na2​CO3​
• Reactions: Being a saturated hydrocarbon, methane primarily undergoes substitution reactions.
  • Combustion: It burns in sufficient oxygen to produce carbon dioxide and water.CH4​+2O2​→CO2​+2H2​O
  • Halogenation: In the presence of sunlight, it reacts with halogens like chlorine, replacing hydrogen atoms one by one.CH4​+Cl2​Sunlight​CH3​Cl+HCl

Ethane (C2​H6​)

• Preparation: Ethane can be prepared by heating sodium propionate (C2​H5​COONa) with soda lime. This is another example of decarboxylation.C2​H5​COONa+NaOHCaO,Δ​C2​H6​+Na2​CO3​
• Reactions: Like methane, ethane is a saturated hydrocarbon and undergoes substitution reactions.
  • Combustion: It burns in air to produce carbon dioxide and water.2C2​H6​+7O2​→4CO2​+6H2​O
  • Halogenation: Ethane reacts with halogens in the presence of light to form haloalkanes.C2​H6​+Cl2​Sunlight​C2​H5​Cl+HCl

Ethene (C2​H4​)

• Preparation: Ethene is prepared by the dehydration of ethanol (C2​H5​OH). This is done by heating ethanol with excess concentrated sulfuric acid at 170°C.CH3​CH2​OHConc.H2​SO4​,170°C​CH2​=CH2​+H2​O
• Reactions: Ethene is an unsaturated hydrocarbon and undergoes addition reactions where the double bond breaks to add other atoms.
  • Hydrogenation: Addition of hydrogen in the presence of a catalyst (like Nickel) converts ethene to ethane.CH2​=CH2​+H2​Ni,Δ​CH3​−CH3​
  • Halogenation: Ethene decolorizes bromine water, forming 1,2-dibromoethane. This is a key test for unsaturation.CH2​=CH2​+Br2​→CH2​Br−CH2​Br
  • Combustion: It burns with a smoky, luminous flame due to its high carbon content.C2​H4​+3O2​→2CO2​+2H2​O

Ethyne (C2​H2​)

• Preparation: Ethyne (acetylene) is prepared in the lab by the reaction of calcium carbide (CaC2​) with water.CaC2​+2H2​O→Ca(OH)2​+C2​H2​
• Reactions: Being an alkyne, it has a triple bond and is even more reactive than ethene, undergoing addition reactions.
  • Hydrogenation: It can be hydrogenated in two steps, first forming ethene and then ethane.C2​H2​+H2​Ni​C2​H4​C2​H4​+H2​Ni​C2​H6​
  • Combustion: It burns with a very sooty flame. In a pure oxygen supply (oxy-acetylene torch), it burns with an intense white flame used for welding and cutting metals.2C2​H2​+5O2​→4CO2​+2H2​O

Ethanol (C2​H5​OH)

• Preparation: Ethanol is prepared industrially by the fermentation of sugars found in molasses, grains, or fruits. The fermentation process uses yeast, which contains the enzyme zymase.C12​H22​O11​+H2​OInvertase​C6​H12​O6​(glucose)+C6​H12​O6​(fructose)C6​H12​O6​Zymase​2C2​H5​OH+2CO2​
• Reactions:
  • Oxidation: Ethanol can be oxidized to ethanoic acid using an oxidizing agent like acidified potassium dichromate (K2​Cr2​O7​) or potassium permanganate (KMnO4​).CH3​CH2​OHK2​Cr2​O7​,H+​CH3​COOH
  • Esterification: It reacts with a carboxylic acid (like acetic acid) in the presence of a strong acid catalyst to form a sweet-smelling ester.CH3​COOH+C2​H5​OHH+​CH3​COOC2​H5​+H2​O
  • Dehydration: Dehydration with concentrated sulfuric acid at different temperatures can yield ethene (at 170°C) or diethyl ether (at 140°C).

Acetic Acid (CH3​COOH)

• Preparation: Acetic acid is prepared commercially by the oxidation of ethanol in the presence of a catalyst. This can also occur via microbial fermentation, which produces vinegar.CH3​CH2​OH+O2​Bacteria​CH3​COOH+H2​O
• Reactions: Acetic acid is a typical weak acid and undergoes the following reactions:
  • Reaction with Metals: It reacts with active metals to produce hydrogen gas.2CH3​COOH+2Na→2CH3​COONa+H2​
  • Reaction with Carbonates/Bicarbonates: It reacts with metal carbonates and bicarbonates to produce a salt, water, and carbon dioxide gas, which is a characteristic test for carboxylic acids.CH3​COOH+NaHCO3​→CH3​COONa+H2​O+CO2​
  • Esterification: It reacts with alcohols to form esters.CH3​COOH+C2​H5​OHH+​CH3​COOC2​H5​+H2​O