ICSE Class 10 Chemistry Chapter 12 Organic Chemistry

12 Organic Chemistry

Syllabus

(i) Introduction to Organic compounds. Unique nature of Carbon atom - tetravalency, catenation, formation of single, double and triple bonds, straight chain, branched chain and cyclic compounds.

(ii) Structure and Isomerism. Structure of compounds with single, double and triple bonds; Isomerism - structural (chain, position)

(iii) Homologous series - characteristics with examples. Alkane, alkene, alkyne series and their gradation in properties and the relationship with the molecular mass or molecular formula.

(iv) Simple nomenclature. Simple nomenclature - of the hydrocarbons with simple functional groups - (double bond, triple bond, alcoholic, ether, aldehydic, keto, carboxylic group) longest chain rule and smallest number for functional groups rule - trivial and IUPAC names.

(v) Hydrocarbons : alkanes, alkenes, alkynes. Alkanes - general formula; methane (green house gas) and ethane - methods of preparation from sodium ethanoate (sodium acetate, sodium propanoate (sodium propionate)), from iodomethane (methyl iodide) and bromoethane (ethyl bromide). Oxidation of methane and ethane in presence of oxygen under suitable conditions reaction of methane and ethane with chlorine through substitution. Alkenes - (unsaturated hydrocarbons with a double bond); ethene as an example. Methods of preparation of ethene by dehydration reaction and dehydration reactions. Alkynes - (unsaturated hydrocarbons with a triple bond); ethyne as an example of alkyne; Methods of preparation from calcium carbide and 1, 2 dibromoethane ethylene dibromide). Only main properties, particularly addition products with hydrogen and halogen namely Cl, Br and I; structural formulae must be given for : alkanes (up to butane); alkene (C-H-2); alkynes (C-H-2-2). Uses of methane, ethane, ethene, acetylene.

(vi) Alcohols : ethanol - preparation, properties and uses. Preparation of ethanol : hydration of ethene; by hydrolysis of alkyl halide; Properties - Physical : Nature, Solubility, Density, Boiling Points, Chemical : Combustion, Oxidation with acidified Potassium dichromate, action with sodium, ester formation with acetic acid, dehydration with conc. Sulphuric acid with reference to Ethanol; hydration of ethene; by hydrolysis of alkyl halide; Important uses of Ethanol

(vii) Carboxylic acids (aliphatic - mono carboxylic acid) : Acetic acid - preparation, properties and uses of acetic acid. Preparation of acetic acid from Ethyl alcohol. Properties of Acetic Acid : Physical properties - odour (vinegar), glacial acetic acid (effect of sufficient cooling to produce ice like crystals). Chemical properties - action with litmus, alkalis and alcohol (idea of esterification) Uses of acetic acid.

12a. Organic Compounds

12.1 Introduction

The word 'organic' means pertaining to life. When comparatively little chemistry was known, people thought that substances like sugar, starch, protein and acetic acid could be obtained only from living sources like plants and animals. So they called such substances organic compounds (the compounds derived from living organisms) and the chemistry dealing with them was named organic chemistry. As against these compounds, substances like common salt, blue vitriol, nitrate, etc., which could be produced from minerals and non-living sources, were called inorganic compounds, and the chemistry dealing with them was called inorganic chemistry.

Since these organic compounds were obtained compounds now supplies to the compounds of carbon and organic chemistry is defined as the study of carbon compounds. This however excludes oxides of carbon, metallic carbonates and related compounds like metal cyanides, metal carbides, etc.

Sources of organic compounds

1. Plants : Compounds like sugar, starch and cellulose, as well as several drugs, are obtained from plants.

2. Animals : Urea, proteins, fats, etc., are obtained from animals.

3. Coal : Destructive distillation of coal produces benzene, toluene, naphthalene, dyes, drugs, perfumes, etc.

4. Petroleum : A large number of organic compounds, like gasoline, fuel gases, petrol, naphtha, etc., are obtained from petroleum.

5. Fermentation : Compounds like ethyl alcohol and acetic acid are obtained by fermentation.

6. Wood : Methyl alcohol, acetone, etc., are obtained by destructive distillation of wood.

7. Synthetic Methods : Most organic compounds are synthesized in the laboratory.

12.2 Organic Compounds

Based on the fact that all organic compounds essentially contain carbon, the term organic compounds now supplies to the compounds of carbon and organic chemistry is defined as the study of carbon compounds. This however excludes oxides of carbon, metallic carbonates and related compounds like metal cyanides, metal carbides, etc.

12.2.1 Comparison between organic and inorganic compounds

Application of Organic Chemistry

Organic compounds are extremely useful to us in our daily life. The soaps and shampoos we use while taking bath, the powders, perfumes, etc., we apply on the body, the clothes we wear, food we eat i.e., carbohydrates, proteins, fats, vitamins, etc., fuels we use, natural gas, petroleum products, medicines, explosives, dyes, insecticides, etc., are all organic compounds. There is hardly any walk of life where we do not need organic compounds.

12.3 Unique Nature of Carbon Atoms

Carbon shows some unique properties like tetravalency and catenation, which are discussed below.

(i) Tetravalency of the carbon atom : Carbon has four valence electrons (At. no. of C = 6; Elec. Config. = 2, 4). Since it can neither lose nor gain electrons to attain octet, it forms covalent bonds by sharing its four electrons with other atoms. This characteristic of the carbon atom, by virtue of which it forms four covalent bonds, is called the tetravalency of carbon.

(ii) Catenation : Carbon atom possesses an unique property to link together (self linking) to form very long chains. This property is referred to as catenation.

The property of self-linking of atoms of an element through covalent bonds in order to form straight chains, branched chains and cyclic chains of different sizes is known as catenation.

The property of catenation is shown by other elements also but carbon exhibits this property to maximum extent. This is due to greater strength of carbon-carbon bond and due to tetra-covalency of carbon. In this process of catenation carbon atoms form straight, branch and cyclic chains of atoms, and can involve single (-), double (=) or triple (-) covalent bonds.

(a) Formation of straight, branched and cyclic chains of carbon atoms : The combination of carbon atoms with one another gives rise to straight or branched or cyclic chains.

The carbon-carbon chain can be very long. Organic molecules containing as many as seventy carbon atoms joined together, one after the other, are known to exist.

(b) Formation of single, double and triple covalent bonds : The valency of carbon is four, i.e. it is tetravalent. In order to satisfy its valency, it forms single, double and triple covalent bonds by sharing one, two or three pairs of electrons respectively between two carbon atoms as well as with other atoms like oxygen, nitrogen etc.

This unique nature of carbon atom (catenation and tetravalency) gives rise to the formation of a large number of compounds. More than 5 million organic compounds are known today and thousands of new compounds are added every year to the existing lot.

All this demands a separate branch of chemistry, i.e., organic chemistry.

Organic chemistry may be defined as the chemistry of hydrocarbons and their derivatives.

Features of covalent bonding involving carbon

Carbon atoms can join to each other to form long chains. Atoms of other elements can then attach to the chain.

The carbon atoms in a chain can be linked by single, double or triple covalent bonds.

Carbon atoms can also arranged themselves in rings.

12.4 Types of Organic Compounds

The organic compounds have been classified as hydrocarbons whereas their derivatives are classified as alcohol, aldehydes, carboxylic acids, ethers, halides, etc. These compounds may contain any number of other elements, including hydrogen, nitrogen, oxygen, the halogens as well as phosphorus, silicon and sulphur.

Table 12.1 : Organic Compounds

12.5 Hydrocarbons

Hydrocarbons are compounds that are made up only of carbon and hydrogen atoms.

Classification of Hydrocarbons

Hydrocarbons are sub-divided into two main groups, the aliphatic (open) and cyclic (closed) chain compounds.

The open chain compounds are further sub-divided into saturated compounds and unsaturated compounds.

Saturated Compounds

The simplest open chain hydrocarbon is alkane. It is represented by the formula CnH2n + 2, where n represents natural number.

It is a saturated hydrocarbon, as all the four valencies of carbon are satisfied by a single covalent bond. (For further details refer section 12B).

Unsaturated Compounds

Open chain compounds where all the four valencies are not satisfied by single covalent bonds, double or triple bonds are required to satisfy the valencies. These compounds are known as unsaturated compounds e.g., Alkenes are the hydrocarbons with double bond and Alkynes are the hydrocarbons with triple bond between two carbon atoms. (For further details refer sections 12C and 12D).

Comparison of Saturated and Unsaturated Hydrocarbons

* Substitution reaction : A reaction in which one atom of a molecule is replaced by another atom (or group of atoms) is called a substitution reaction. e.g. Cl2 + CH4 - CH3Cl + HCl.

** Addition reaction : A reaction involving addition of atom(s) or molecule(s) to the double or the triple bond of an unsaturated compound so as to yield a saturated product is known as an addition reaction, e.g. C2H4 + Br2 - C2H4Br2.

Teacher's Note

Organic chemistry is everywhere around us - from the fuel in our cars to the clothes we wear. Understanding how carbon atoms bond helps explain why plastic bags don't dissolve in water but sugar does.

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