Class 10 Science Carbon and Its Compound Exam Notes

This Class 10 Science exam notes page covers the chapter Carbon and its Compounds. It explains carbon and its covalent bonding, the forms and allotropes of carbon (diamond, graphite and fullerenes), the homologous series of hydrocarbons, IUPAC naming rules, functional groups, the chemical properties of ethanol and ethanoic acid, esterification, soaps and detergents, and the important chemical reactions of carbon compounds.

Topics Covered

1. Introduction and important terms
2. Covalent bonding in carbon compounds
3. Properties of covalent compounds and allotropy
4. Allotropes of carbon and uses
5. Unique nature of carbon and hydrocarbons
6. Homologous series, functional groups and IUPAC naming
7. Alcohols, aldehydes, ketones and carboxylic acids
8. Chemical properties of carbon compounds
9. Ethanol and ethanoic acid
10. Esters, soaps and detergents
11. Important chemical reactions and points to remember

INTRODUCTION

Carbon is the most important element because it forms the largest number of compounds, which are useful in our daily life. In this chapter, we shall study carbon and its compounds.

Important Terms and Concepts

1. Carbon: Its atomic number is 6. Its mass number is 12. Its atomic mass is 12.011. Its melting point is 3550 ºC and its boiling point is 4830 ºC. It occurs in the free state as well as in the combined state. About 70% of our body is made up of carbon. It forms the largest number of compounds. The earth's crust contains only 0.02% of carbon.
2. Organic compounds: Those compounds which consist of carbon essentially and hydrogen mostly, along with other elements like oxygen, sulphur, nitrogen, halogens, etc., are called organic compounds.
3. Coal: It is a naturally occurring solid fuel which exists in the form of varying depths below the earth's surface. It is formed by the decay of vegetation that grew 40 to 300 million years ago, followed by chemical processes of condensation and polymerisation under the influence of temperature, pressure and time.
4. Petroleum: It is an oil found in rocks. It is a mixture of solid, liquid and gaseous hydrocarbons. It is a source of petrol, diesel, kerosene, petroleum ether, petroleum coke, petroleum wax, etc.
5. Carbonates: They are compounds of carbonic acid. They are found in the earth's crust, e.g., CaCO₃, MgCO₃, Na₂CO₃, ZnCO₃. They are thermally stable.
6. Hydrogen carbonates: They contain HCO₃^- ions. They are formed by replacing one H⁺ of carbonic acid, e.g., NaHCO₃, Ca(HCO₃)₂, Mg(HCO₃)₂. They are soluble in water. They are thermally unstable, i.e., they decompose on heating to form carbonates, CO₂ and H₂O.
7. Chemical bond: It is a force of attraction which holds two atoms together.
8. Covalent bond: It is the bond formed by the equal sharing of electrons. For example, hydrogen has one valence electron. It can share one valence electron with another hydrogen atom to form the H₂ molecule, so as to acquire the nearest noble gas configuration. The bond between two hydrogen atoms by sharing one electron each is called a covalent bond.

Covalency of Carbon

Carbon has four valence electrons. It cannot lose four electrons, since a very high amount of energy would be required to lose four electrons to form the C⁴⁺ ion. There is a strong force of attraction between the nucleus and the valence electrons.
Carbon cannot gain four electrons to form the C^4- ion, because six protons cannot hold 10 electrons easily and there would be strong inter-electronic repulsion.
Carbon can share four electrons easily with other atoms of carbon and with other elements to acquire a stable electronic configuration.

Covalent Bonds in Simple Molecules

1. Hydrogen molecule: When two atoms of hydrogen share one electron each, a single covalent bond is formed, as shown below.
H-H (single covalent bond between two hydrogen atoms)
2. Chlorine molecule: Chlorine has 7 valence electrons. It can share one electron with another chlorine atom to form Cl₂.
Cl-Cl (single covalent bond between two chlorine atoms)
3. Single covalent bond: It is a bond formed by the sharing of one electron by each of the atoms. It is represented by a line between two atoms.
4. Hydrogen fluoride (HF): One hydrogen atom shares one electron with one electron of fluorine. Hydrogen acquires two electrons, whereas fluorine acquires 8 electrons and becomes stable. They form a single covalent bond.
H-F (single covalent bond between hydrogen and fluorine)
5. Water (H₂O): Each hydrogen atom shares one electron with the oxygen atom, so that oxygen completes its octet and hydrogen acquires the nearest noble gas configuration.
H-O-H
6. Ammonia (NH₃): Nitrogen has five valence electrons. It shares one electron with each of the three hydrogen atoms to form NH₃.
H-N-H (with H below N)
7. Methane (CH₄): Carbon has four valence electrons. It needs four more electrons to complete its octet. It shares four electrons with four hydrogen atoms and forms four single covalent bonds.
H-C-H (with H above and below C)
8. Double covalent bond: When two atoms share two electrons each to acquire a stable electronic configuration, a double covalent bond is formed. It is denoted by = (two lines).
9. Oxygen molecule (O₂): When two oxygen atoms share two electrons each to complete their octet, a double covalent bond is formed. O=O
10. Ethene (C₂H₄): Two carbon atoms share two electrons with each other, and each carbon shares two electrons with two hydrogen atoms. They complete their octet and form a double covalent bond between the two carbon atoms.
H₂C=CH₂, i.e., H-C=C-H (each carbon bonded to two hydrogens)
11. Triple covalent bond: When an atom shares three valence electrons with another atom, a triple covalent bond is formed. It is denoted by ≡ (three lines).
12. Nitrogen molecule (N₂): Nitrogen has five valence electrons. It needs three more electrons to complete its octet. It shares three electrons with another atom of nitrogen to form a triple covalent bond. N≡N
13. Ethyne (C₂H₂): Two carbon atoms share three electrons with each other, and each carbon shares one electron with a hydrogen atom. They complete their octet and form a triple covalent bond with each other. H-C≡C-H
14. Ethane (C₂H₆): In ethane, two carbon atoms share one electron each, forming a single covalent bond with each other. Each carbon shares one electron with each of three hydrogen atoms to complete its octet. CH₃-CH₃
15. Carbon dioxide (CO₂): Carbon has four valence electrons. It shares two electrons with one oxygen atom and two electrons with the other oxygen atom, forming a double covalent bond with each. O=C=O
16. Methyl chloride (CH₃Cl): Carbon shares one electron with the chlorine atom and one electron with each of three hydrogen atoms, forming four single bonds. CH₃-Cl
17. Carbon tetrachloride (CCl₄): Carbon shares one electron with each of four chlorine atoms, forming four single covalent bonds. CCl₄

Electron dot structures of the above molecules (each shared pair of electrons is shown between the atoms): H:H; Cl:Cl (each Cl with three lone pairs); H:F (F with three lone pairs); H:O:H (O with two lone pairs); H:N:H with a third H below N (N with one lone pair); methane with C at the centre and four H around it, each sharing one pair; O::O (each O with two lone pairs); ethene H₂C::CH₂; N:::N (each N with one lone pair); ethyne H:C:::C:H; ethane H₃C:CH₃; O::C::O (each O with two lone pairs); CH₃Cl and CCl₄ with every shared pair between C and the attached atom (Cl atoms have three lone pairs each).

Properties of Covalent Compounds

(i) Physical state: Covalent compounds can exist in solid, liquid as well as gaseous states, e.g., CH₄ is a gas, CHCl₃ is a liquid, and glucose is a solid.
(ii) Solubility: (a) They are generally insoluble in water and in polar solvents because they cannot form ions in aqueous solution. (b) They are soluble in non-polar organic solvents like ether, benzene, CCl₄, CS₂, CHCl₃, acetone, etc.
(iii) Electrical conductivity: Covalent compounds are poor conductors of electricity because they do not contain ions or free electrons for conduction. For example, CCl₄, benzene and toluene do not conduct electricity.
(iv) Melting and boiling points: The melting and boiling points of covalent compounds are low due to weak forces of attraction between molecules. Less energy is required to overcome these forces.

Melting and boiling points of some covalent compounds:

Allotropy and Isotopes of Carbon

Allotropy: It is a property due to which an element can exist in more than one form. These forms differ in physical properties but have similar chemical properties. For example, carbon, sulphur, phosphorus and oxygen show allotropy.

Isotopes of carbon: Naturally occurring carbon has two stable isotopes, ¹²₆C (98.9%) and ¹³₆C (1.1%), in addition to traces of the radioactive ¹⁴₆C isotope. The radioactive ¹⁴₆C isotope is used to determine the age of archaeological specimens of organic origin. The isotope ¹²₆C is the international standard for atomic mass measurement and is assigned a mass of 12.00000 units.

Allotropes of Carbon

Carbon exists in both crystalline and amorphous forms. The two well-known allotropes of carbon are diamond and graphite.

Fullerenes: A third form of carbon, known as fullerenes, was discovered by H.W. Kroto, R.F. Curl and R.E. Smalley. Fullerenes consist of a hollow cage of carbon atoms. They are large spheroidal molecules of composition C_2n. Two important members of this family are C₆₀ and C₇₀. The 1996 Nobel Prize was awarded to these scientists for the discovery of fullerenes.
[Figure: Structure of C₆₀, Buckminsterfullerene. The molecule has the shape of a soccer ball (football).]

Differences between diamond and graphite:

[Figure: Structure of diamond, showing carbon-carbon bond length of 154 pm.]
[Figure: Structure of graphite, showing flat hexagonal layers. The carbon-carbon bond length within a layer is 141.5 pm and the distance between two layers is 340 pm.]

Other forms of carbon: Other forms of elemental carbon are carbon black, coke and charcoal. They are impure forms of graphite or fullerenes. Carbon black is obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke are obtained by heating wood or coal, respectively, at high temperatures in the absence of air.

Uses of carbon:

Unique Nature of Carbon

Carbon has a small size and therefore can form strong covalent bonds with other atoms. It forms the maximum number of compounds. Our bodies are made up of carbon compounds like proteins, fats and nucleic acids.

Catenation: It is the property due to which carbon can form bonds with other atoms of carbon. Carbon shows catenation to the maximum extent because it is small in size and can form strong covalent bonds.

Tetravalency of carbon: Carbon has four valence electrons. It can share four electrons with other atoms of carbon, as well as with oxygen, hydrogen, nitrogen, sulphur and halogens.

Large number of organic compounds: They are due to the tetravalency of carbon and the property of catenation.

Vital force theory: It was proposed that a 'vital force' is necessary for the formation of organic compounds. According to this theory, they can only be obtained from living organisms.

Preparation of the first organic compound in the laboratory: In 1828, Wohler prepared the first organic compound, urea, by heating ammonium cyanate. This is an isomerisation reaction.
NH₄CNO → (heat) NH₂CONH₂
Ammonium cyanate → Urea

Hydrocarbons

Hydrocarbons are compounds which contain carbon and hydrogen only, e.g., CH₄ (methane), C₂H₆ (ethane), C₂H₄ (ethene), C₂H₂ (ethyne), etc.

Saturated hydrocarbons: Hydrocarbons which contain single bonds only are called saturated hydrocarbons, e.g., CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane), etc.
Methane: CH₄
Ethane: CH₃-CH₃
Propane: CH₃-CH₂-CH₃
Butane: CH₃-CH₂-CH₂-CH₃

Unsaturated hydrocarbons: Hydrocarbons in which the valency of carbon is satisfied by a double or triple bond are called unsaturated hydrocarbons, e.g., C₂H₄, C₃H₆, C₂H₂.
Ethene: CH₂=CH₂
Propene: CH₂=CH-CH₃
Ethyne: CH≡CH

Straight chain compounds: Compounds which contain straight carbon chains are called straight chain compounds, e.g., n-butane CH₃-CH₂-CH₂-CH₃, n-pentane CH₃-CH₂-CH₂-CH₂-CH₃, n-hexane CH₃-CH₂-CH₂-CH₂-CH₂-CH₃.

Branched chain compounds: Compounds which are branched are called branched chain compounds, e.g., 2-methylpropane (CH₃-CH(CH₃)-CH₃), 2-methylbutane (CH₃-CH(CH₃)-CH₂-CH₃) and 2,2-dimethylpropane (CH₃-C(CH₃)₂-CH₃).

Closed chain or ring compounds: Cyclic compounds are called closed chain or ring compounds, e.g., cyclopropane (C₃H₆), cyclobutane (C₄H₈), cyclopentane (C₅H₁₀) and cyclohexane (C₆H₁₂).

Aromatic compounds: Benzene (C₆H₆) and its derivatives, which contain a benzene ring, are called aromatic compounds. Toluene is methylbenzene (C₆H₅CH₃).

Homologous Series and Functional Groups

Functional group: It is an atom or a group of atoms, or the reactive part of a compound, which largely determines the chemical properties of the compound. For example, -OH (alcohol), -CHO (aldehyde), >C=O (ketone), -COOH (carboxylic acid) and -X (halogens, where X is Cl, Br, F or I).

Homologous series: It is a series of compounds which are derived from the same general formula, have the same functional group, show similar chemical properties and show gradation in physical properties. Each member differs from the successive member by -CH₂-. The difference in molecular weight between two successive members is 12 u.

Characteristics of a homologous series:
(i) They have the same general formula.
(ii) They have the same functional group.
(iii) They have general methods of preparation.
(iv) They have similar chemical properties.
(v) They show gradation in physical properties, like melting and boiling points, which increase with increase in molecular weight. For example, the boiling point of alcohols goes on increasing with increase in molecular weight.
(vi) Solubility in a particular solvent shows gradation with increase in molecular weight. For example, the solubility of alcohols in water decreases with increase in molecular weight.

Alkanes

All compounds in which carbon and hydrogen are attached with single bonds are called alkanes. The general formula is C_nH_2n+2, where n is the number of carbon atoms.

IUPAC Naming of Organic Compounds

IUPAC stands for International Union of Pure and Applied Chemistry. IUPAC names are used for international communication. The rules are as follows.
(i) Select the longest possible chain containing the functional group. For example, CH₃-CH₂-CH(CH₃)-CH₂-CH₃ has a longest chain of 5 carbon atoms.
(ii) The number of carbon atoms in the parent compound is denoted by a prefix: meth (one), eth (two), prop (three), but (four), pent (five), hex (six), hept (seven), oct (eight), non (nine). For example, CH₃-CH₂-CH₂-CH₂-CH₂-CH₃ is hexane. The suffix -ane is used for alkanes, which have single bonds only.
(iii) Groups attached to the parent chain are indicated by their names, prefixed by the number of the carbon atom to which they are attached. The alkyl group has the general formula C_nH_2n+1. CH₃- is called methyl, C₂H₅- is called ethyl, and CH₃CH₂CH₂- is called n-propyl. For example, CH₃-CH(CH₃)-CH₃ is 2-methylpropane, because the methyl group is attached to the second carbon atom.
(iv) The carbon chain is numbered so that the carbon attached to the alkyl or functional group gets the minimum number. For example, CH₃-CH₂-CH(CH₃)-CH₃ is 2-methylbutane and not 3-methylbutane.
(v) If more than one identical group is attached to the same or different carbon atoms, the numbers of the carbon atoms to which they are attached are prefixed. The number of such groups is indicated by di (two), tri (three), tetra (four), and so on. For example, CH₃-C(CH₃)₂-CH₃ is 2,2-dimethylpropane, and CH₃-CH(CH₃)-CH(CH₃)-CH₃ is 2,3-dimethylbutane.
(vi) For a double bond in alkenes, the suffix -ene is used, and for a triple bond in alkynes, the suffix -yne is used. The number of the carbon atom after which the double or triple bond starts is also prefixed. For example, CH₃-CH=CH-CH₃ is but-2-ene, because the double bond is after the second carbon atom.

Another example: CH₃-CH₂-C(=CH₂)-CH₂-CH₃ (numbered 4, 3, 2, 1 along CH₃-CH₂-C=CH₂, with a CH₂-CH₃ branch on carbon 2). The longest chain containing the double bond has 4 carbon atoms.

Electronic Formula and Shape of Methane

In methane, the carbon atom shares four electrons, one each with four hydrogen atoms, forming four covalent bonds. The four hydrogen atoms in methane are arranged at the corners of a regular tetrahedron, with the carbon atom at the centre of the tetrahedron.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are those which contain at least one double or triple bond between two carbon atoms. A double bond is formed by sharing of two pairs of electrons, e.g., ethene (H₂C=CH₂). A triple bond is formed by sharing of three pairs of electrons between two carbon atoms, e.g., ethyne (HC≡CH).

Alkenes: They have the general formula C_nH_2n, where n is the number of carbon atoms.

Alkynes: The general formula is C_nH_2n-2.

Alcohols

Alcohols are carbon compounds containing the -OH group attached to a carbon atom. The general formula of alcohol is R-OH, where R is an alkyl group and -OH is the functional group. The name of an alcohol is derived by replacing the -e of the name of the alkane with the suffix -ol. For example, methanol (CH₃OH) is derived by substituting one H of methane with -OH.

Alkyl Halides

The general formula of an alkyl halide is C_nH_2n+1X, where X is Cl, Br, I or F.

Aldehydes and Ketones

Aldehydes and ketones are compounds containing the carbonyl group (C=O). In aldehydes, the carbon of the C=O group is attached to an alkyl group and a hydrogen atom, i.e., RCHO. In ketones, the carbon of the carbonyl group is attached to two alkyl groups, which may be the same or different, i.e., RCOR'. Aldehydes are named by replacing the -e of the alkane with the suffix -al, and ketones are named by replacing the -e of the alkane with the suffix -one.

Aldehydes (general formula C_nH_2n+1CHO)

Ketones (general formula C_nH_2n+1COC_nH_2n+1)

Carboxylic Acids

Compounds containing the carboxyl group (-COOH) are known as carboxylic acids. They are named by substituting the -e of the corresponding alkane with -oic acid. Their general formula is C_nH_2n+1COOH.

Chemical Properties of Carbon Compounds

Combustion of carbon: Carbon, in all allotropic forms, burns in the presence of oxygen to form carbon dioxide, with the evolution of heat and light energy. Diamond, graphite and fullerene burn completely to form CO₂, because they are the purest forms of carbon.
C + O₂ → CO₂ + heat + light

Most carbon compounds are combustible and burn in the presence of oxygen to form CO₂ and H₂O. For example:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + heat + light
2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l) + heat + light
2CH₃OH(g) + 3O₂(g) → 2CO₂(g) + 4H₂O(l) + heat + light
CH₃CH₂OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) + heat
CH₃COOH(l) + 2O₂(g) → 2CO₂(g) + 2H₂O(l) + heat

Combustion of hydrocarbons: If hydrocarbons are burnt in a limited supply of oxygen, a smoky flame is produced due to incomplete combustion. In an excess of oxygen, complete combustion takes place, and a non-luminous, bluish flame with high temperature is produced.

Oxidising agent: Substances which can add oxygen to the starting material are called oxidising agents, e.g., alkaline KMnO₄ and acidified potassium dichromate.

Addition reactions: Reactions in which unsaturated compounds react with a molecule like H₂, Cl₂, etc., to form saturated compounds are called addition reactions.

Hydrogenation: It is a process in which an unsaturated compound reacts with hydrogen in the presence of nickel as a catalyst to form a saturated compound.
Vegetable oil + H₂ → (Ni) Vegetable ghee (saturated)

Catalyst: It is a substance which increases the rate of a reaction without itself undergoing any permanent chemical change, e.g., Ni, Pt and V₂O₅ are used as catalysts.

Substitution reactions: Reactions in which an atom or group of atoms of a compound is replaced by another atom or group of atoms are called substitution reactions.
Saturated hydrocarbons are less reactive and do not react with most reagents. They react with halogens in the presence of sunlight and undergo substitution reactions. The reaction is very fast. It is a photochemical reaction because it takes place in the presence of sunlight.
CH₄(g) + Cl₂(g) → (sunlight) CH₃Cl(g) + HCl(g) (chloromethane)
CH₃Cl(g) + Cl₂(g) → (sunlight) CH₂Cl₂(g) + HCl(g) (dichloromethane)
CH₂Cl₂(g) + Cl₂(g) → (sunlight) CHCl₃(g) + HCl(g) (trichloromethane)
CHCl₃(l) + Cl₂(g) → (sunlight) CCl₄(l) + HCl(g) (tetrachloromethane, carbon tetrachloride)

Test for unsaturation: Add a few drops of bromine water to a test tube containing ethyne. Shake and observe. The reddish-brown colour of bromine water is decolourised.
HC≡CH + 2Br₂(aq) → CHBr₂-CHBr₂ (1,1,2,2-tetrabromoethane)

Addition of hydrogen: Ethyne reacts with hydrogen in the presence of a catalyst to give ethane. Two molecules of hydrogen are added across the carbon-carbon triple bond.
HC≡CH + 2H₂ → (Ni, 573 K) CH₃-CH₃ (ethane)

Addition of chlorine: Two molecules of chlorine react with ethyne to form 1,1,2,2-tetrachloroethane.
HC≡CH + 2Cl₂ → CHCl₂-CHCl₂ (1,1,2,2-tetrachloroethane)

Addition of HCl: Ethyne reacts with HCl in the presence of mercuric chloride (HgCl₂) to form vinyl chloride, which is the monomer of polyvinyl chloride (PVC), a plastic.
HC≡CH + HCl → (HgCl₂) CH₂=CHCl (vinyl chloride, chloroethene)

Combustion of acetylene: Acetylene burns in the presence of oxygen to form CO₂ and H₂O.
2C₂H₂(g) + 5O₂(g) → 4CO₂(g) + 2H₂O(l) + heat

Uses of ethyne:
(i) Oxy-acetylene flame is used for welding purposes.
(ii) It is used for lighting purposes.
(iii) It is used to prepare benzene (C₆H₆).
(iv) It is used for making vinyl chloride, which is used for making PVC (plastic).

Ethanol (C₂H₅OH)

Physical properties of ethanol:
(i) Pure ethanol is a colourless liquid.
(ii) It has a specific smell and a burning taste.
(iii) Its boiling point is 351 K, which is higher than that of the corresponding alkanes.
(iv) It is soluble in water, i.e., it is miscible with water in all proportions.

Chemical properties of ethanol:

(i) Dehydration: Ethanol, when heated with concentrated H₂SO₄ at 443 K, or with Al₂O₃ at 623 K, undergoes dehydration, i.e., it loses a molecule of water to form an alkene.
CH₃CH₂OH → (conc. H₂SO₄, 443 K or Al₂O₃, 623 K) CH₂=CH₂ + H₂O

(ii) Reaction with sodium: Alcohols are very weakly acidic. Ethanol reacts with sodium metal to form sodium ethoxide and hydrogen gas.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

(iii) Oxidation with chromic anhydride (CrO₃): Ethanol is oxidised to ethanal, which on further oxidation gives ethanoic acid.
CH₃CH₂OH → (CrO₃) CH₃CHO → CH₃COOH

(iv) Oxidation with alkaline KMnO₄: Ethanol is oxidised to ethanoic acid.
CH₃CH₂OH + [O] → (alkaline KMnO₄) CH₃COOH + H₂O

(v) Oxidation with acidified potassium dichromate: Ethanol is oxidised to ethanoic acid with the help of acidified K₂Cr₂O₇.
CH₃CH₂OH + 2[O] → (K₂Cr₂O₇/H₂SO₄, conc.) CH₃COOH + H₂O
During this reaction, the orange colour of K₂Cr₂O₇ changes to green. Therefore, this reaction can be used for the identification of alcohols.

(vi) Esterification: Ethanol reacts with ethanoic acid in the presence of concentrated H₂SO₄ to form ethyl ethanoate and water. The compound formed by the reaction of an alcohol with a carboxylic acid is known as an ester, and the reaction is called esterification. Esters are sweet, fruity-smelling compounds because they occur in fruits. They are used in ice creams, cold drinks and perfumes.
CH₃COOH + C₂H₅OH → (conc. H₂SO₄) CH₃COOC₂H₅ + H₂O
Conc. H₂SO₄ acts as a dehydrating agent, i.e., it removes the water formed, otherwise the ester formed would get hydrolysed.

(vii) Ethanol is a highly inflammable liquid, i.e., it catches fire very easily. It burns with a blue flame in the presence of oxygen to form carbon dioxide and water.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O(l)

Uses of ethanol:
(i) Ethanol is present in alcoholic beverages such as beer, wine and whisky.
(ii) Ethanol is used as an antiseptic for sterilising wounds.
(iii) Ethanol is used in cough syrups, digestive syrups and tonics.
(iv) Ethanol is mixed with petrol and used as a motor fuel. This mixture is called power alcohol.
(v) A mixture of ethanol and water has a lower freezing point than water. This mixture is known as antifreeze and is used in the radiators of vehicles in cold countries and at hill stations.
(vi) Ethanol is used for the preparation of chloroform, iodoform, ethanoic acid, ethanal, ethyl ethanoate, etc.
(vii) Ethyl alcohol is used as a hypnotic (induces sleep).

Harmful effects of drinking alcohol:
(i) If ethanol is mixed with CH₃OH (methanol) and consumed, it may cause serious poisoning and loss of eyesight.
(ii) It causes addiction (habit forming) and mixes with blood. It damages the liver if taken regularly in large amounts.
(iii) The person loses the sense of discrimination under its influence.
(iv) A higher amount of consumption leads to loss of body control and consciousness. It may even cause death.
Therefore, alcohol should not be consumed under any circumstances, because it leads to wastage of time and wealth and spoils health.

Alcohol as a fuel: Alcohol is added to petrol up to 20%. The mixture is called 'gasohol'. It is a cleaner fuel because it creates less pollution. Alcohol, on combustion, gives only CO₂ and H₂O.

Fermentation: It is a process in which controlled microbial action takes place to give useful products. For example, ethanol can be prepared by the fermentation of molasses.
C₁₂H₂₂O₁₁ + H₂O → (invertase) C₆H₁₂O₆ + C₆H₁₂O₆ (glucose and fructose)
C₆H₁₂O₆ → (zymase) 2C₂H₅OH + 2CO₂

Ethanoic Acid (Acetic Acid), CH₃COOH

Ethanoic acid is most commonly known as acetic acid. Its dilute solution in water (5 to 8%) is known as vinegar, which is used for preserving food items such as sausages and pickles.

Physical properties:
(i) Ethanoic acid is a liquid with the smell of vinegar. The lower carboxylic acids are liquids, whereas the higher ones are solids.
(ii) Ethanoic acid is sour in taste. Other lower carboxylic acids are also sour in taste.
(iii) Ethanoic acid has a boiling point of 391 K. Carboxylic acids have higher boiling points than the corresponding alcohols, aldehydes and ketones.
(iv) Acetic acid is soluble in water, i.e., it is miscible with water in all proportions. The lower carboxylic acids are soluble in water, but solubility in water decreases with increase in molecular weight.
(v) Acetic acid freezes at 290 K. Thus, in cold weather, crystallisation of acetic acid may take place. That is why pure acetic acid is called glacial acetic acid.

Chemical properties:

(i) Ethanoic acid is a weak acid, but it turns blue litmus red.

(ii) Reaction with metals: Ethanoic acid reacts with metals like Na, K and Zn to form metal ethanoates and hydrogen gas.
2CH₃COOH + 2Na → 2CH₃COONa + H₂ (sodium ethanoate)
2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂ (zinc ethanoate)

(iii) Reaction with carbonates and bicarbonates: Ethanoic acid reacts with carbonates and bicarbonates and produces brisk effervescence due to the formation of carbon dioxide.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O (sodium carbonate)
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ (sodium bicarbonate, sodium hydrogen carbonate)

(iv) Reaction with a base: Ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water.
CH₃COOH + NaOH → CH₃COONa + H₂O

(v) Decarboxylation (removal of CO₂): When the sodium salt of ethanoic acid, i.e., sodium ethanoate, is heated with soda lime (3 parts of NaOH and 1 part of CaO), methane gas is formed.
CH₃COONa + NaOH → (CaO, heat) CH₄ + Na₂CO₃
This reaction is known as decarboxylation, because a molecule of CO₂ is removed from a molecule of the acid.

(vi) Reaction with alcohols: Ethanoic acid reacts with ethanol in the presence of concentrated sulphuric acid to form esters, which are pleasant, fruity-smelling compounds.
CH₃COOH + C₂H₅OH → (conc. H₂SO₄) CH₃COOC₂H₅ + H₂O (ethyl ethanoate)

(vii) Reduction: Acetic acid, on reduction with lithium aluminium hydride (LiAlH₄), gives ethanal, which on further reduction gives ethanol.
CH₃COOH → (LiAlH₄) CH₃CHO → (LiAlH₄) CH₃CH₂OH

Uses of ethanoic acid:
(i) It is used for making vinegar.
(ii) It is used as a laboratory reagent.
(iii) It is used for the preparation of white lead [2PbCO₃.Pb(OH)₂], which is used in white paints.
(iv) It is used for the coagulation of rubber from latex and of casein (a protein) from milk.
(v) It is used in the preparation of acetone, ethyl acetate, acetic anhydride and aspirin, which is used in medicines.
(vi) It is used in the preparation of cellulose acetate, which is used for making photographic film.
(vii) Its esters are used in artificial flavours and in perfumes.
(viii) Its 5% solution is bactericidal (destroys bacteria).
(ix) Its compound basic copper acetate (verdigris) is used as a green pigment.
(x) Aluminium acetate and chromium acetate are used as mordants in dyeing and in the waterproofing of fabrics.

Esters

Esters are pleasant, fruity-smelling compounds formed by the reaction of carboxylic acids and alcohols. They are used in making ice creams, cold drinks, perfumes and as flavouring agents.

Acidic hydrolysis of esters: Esters, on hydrolysis in the presence of H⁺, give a carboxylic acid and an alcohol.
CH₃COOC₂H₅ + H₂O → (H⁺) CH₃COOH + C₂H₅OH

Saponification: It is a process in which an ester reacts with sodium hydroxide to form the sodium salt of the acid and an alcohol.
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
Saponification is also used for the preparation of soap.

Soaps and Synthetic Detergents

Soaps: Soaps are sodium or potassium salts of higher fatty acids. Fatty acids are carboxylic acids containing 12 or more carbon atoms.

Glycerides: They are esters of glycerol, an alcohol containing three hydroxyl groups, and fatty acids. Glycerides are present in fats or oils of animal and vegetable origin.

Saponification (of fats and oils): The process in which an oil or fat (glyceride) is hydrolysed with sodium hydroxide to get soap and glycerol is called saponification.
Glyceryl stearate + 3NaOH → Glycerol + 3C₁₇H₃₅COONa (sodium stearate, soap)
Other examples of soaps are sodium palmitate (C₁₅H₃₁COONa), sodium oleate (C₁₇H₃₃COONa) and sodium linoleate (C₁₇H₃₁COONa).

Advantages of soap:
(i) Soap is cheaper and readily available.
(ii) It works well for cleaning clothes with soft water (water which does not contain Ca²⁺ and Mg²⁺).
(iii) Soaps are 100% biodegradable, i.e., they are decomposed by micro-organisms present in sewage, and therefore they do not create water pollution.

Disadvantages of soap:
(i) It does not work well with hard water containing Ca²⁺ or Mg²⁺. It reacts with these ions to form a white precipitate called scum, and the soap is wasted.
(C₁₇H₃₅COO)₂Ca + 2Na⁺ is formed from 2C₁₇H₃₅COONa + Ca²⁺ (calcium stearate)
(C₁₇H₃₅COO)₂Mg is formed from 2C₁₇H₃₅COONa + Mg²⁺ (magnesium stearate)
Thus, soap solution forms less lather with hard water.
(ii) Soap is not suitable for washing woollen garments, because it is basic in nature and woollen garments have acidic dyes.
(iii) Soaps are less effective in saline water and acidic water.

Detergents: Detergents are sodium or potassium salts of sulphonic acids of hydrocarbons of the alkene type. They have the -SO₃H group, i.e., the sulphonic acid group.
Examples:
(i) Sodium lauryl sulphate: CH₃(CH₂)₁₀CH₂OSO₃^-Na⁺
(ii) Sodium dodecylbenzenesulphonate: C₁₂H₂₅-C₆H₄-SO₃^-Na⁺

Advantages of detergents over soaps:
(i) Detergents work well even with hard water, but soaps do not.
(ii) Detergents may be used in saline or acidic water.
(iii) Detergents are more easily soluble in water than soaps.
(iv) Detergents can be used for washing woollen garments, whereas soaps cannot be used.
(v) Detergents having a linear hydrocarbon chain are biodegradable.

Disadvantages of detergents over soaps:
(i) Synthetic detergents having a branched hydrocarbon chain are not fully biodegradable, i.e., they are not decomposed by micro-organisms in sewage and create water pollution.
(ii) They are more expensive than soaps.

Difference between soaps and detergents:

Cleansing Action of Soaps and Detergents

Soaps and detergents consist of a large hydrocarbon tail with a negatively charged head. The hydrocarbon tail is hydrophobic (water-hating or water-repelling), and the negatively charged head is hydrophilic (water-loving).

In aqueous solution, water molecules, being polar in nature, surround the ions and not the hydrocarbon part of the molecule. When a soap or detergent is dissolved in water, the molecules associate together as clusters called micelles. In a micelle, the tails stick inwards and the heads point outwards.

[Figure: (a) Hydrophilic (water-loving) and hydrophobic (water-repellent) ends of a soap molecule, showing the long hydrocarbon chain as the hydrophobic end and the COO^-Na⁺ group as the polar hydrophilic end. (b) Hydrophilic and hydrophobic ends of a detergent molecule, showing the hydrophobic hydrocarbon part and the SO₃^-Na⁺ hydrophilic end. (c) Micelle formed by detergent molecules in water.]

In cleansing, the hydrocarbon tail attaches itself to the oily dirt. When water is agitated (shaken vigorously), the oily dirt tends to lift off from the dirty surface and breaks up into fragments. This gives an opportunity to other tails to stick to the oil. The solution now contains small globules of oil surrounded by detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and forming aggregates. Thus, the oily dirt is removed.

In the past, detergents caused pollution in rivers and water bodies. The long carbon chains present in the detergents used earlier contained a lot of branching. These branched-chain detergent molecules were degraded very slowly by micro-organisms present in sewage, discharge septic tanks and water bodies. Thus, the detergents persisted in water for a long time and made water unfit for aquatic life. Nowadays, detergents are made of molecules in which branching is kept at a minimum. These are degraded more easily than branched-chain detergents.

Important Chemical Reactions

Decarboxylation
1. CH₃COONa + NaOH → (CaO, heat) CH₄ + Na₂CO₃ (sodium acetate, soda lime, methane)
2. CH₂=CH₂ + H₂O → (H₃PO₄ or H₂SO₄) CH₃-CH₂OH (ethene, ethanol)

Fermentation
3. C₁₂H₂₂O₁₁ + H₂O → (invertase, yeast) C₆H₁₂O₆ + C₆H₁₂O₆ (sucrose; glucose, fructose)
4. C₆H₁₂O₆ → (zymase, yeast) 2C₂H₅OH + 2CO₂ (glucose, ethanol)
5. (C₆H₁₀O₅)_n + nH₂O → (diastase) nC₁₂H₂₂O₁₁ (starch, maltose)
6. C₁₂H₂₂O₁₁ + H₂O → (maltase, yeast) 2C₆H₁₂O₆ (maltose, glucose)
7. C₆H₁₂O₆ → (zymase, yeast) 2C₂H₅OH + 2CO₂ (glucose, ethanol)

Reactions with sodium
8. 2CH₃OH + 2Na → 2CH₃ONa + H₂ (methanol, sodium methoxide)
9. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ (ethanol, sodium ethoxide)
23. 2CH₃COOH + 2Na → 2CH₃COONa + H₂ (ethanoic acid, sodium ethanoate)

Combustion
10. 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O (methanol)
11. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (ethanol)
15. CH₃CH₂OH + O₂ → CH₃COOH (ethyl alcohol, acetic acid)

Esterification
12. CH₃COOH + C₂H₅OH → (conc. H₂SO₄) CH₃COOC₂H₅ + H₂O (ethanoic acid, ethanol, ethyl ethanoate)
13. CH₃COOH + CH₃OH → (conc. H₂SO₄) CH₃COOCH₃ + H₂O (ethanoic acid, methanol, methyl ethanoate)

Oxidation
14. CH₃CH₂OH → (K₂Cr₂O₇/H₂SO₄, [O]) CH₃COOH (ethanol, ethanoic acid)
17. CH₃CH₂OH → (CrO₃) CH₃CHO + H₂O (ethanol, ethanal)
18. CH₃CH₂OH + 2[O] → (alkaline KMnO₄) CH₃COOH + H₂O (ethanol, ethanoic acid)
20. 2CH₃OH + O₂ → (873-923 K, MoO₃) 2HCHO + 2H₂O (methanol, methanal)
30. CH₃OH + 2[O] → (CrO₃) HCOOH + H₂O (methanol, methanoic acid)

Neutralisation and other reactions of ethanoic acid
16. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ (ethanoic acid, sodium bicarbonate, sodium acetate)
24. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Reduction
28. CH₃COOH → (LiAlH₄ or NaBH₄) CH₃CHO → (LiAlH₄ or NaBH₄) CH₃CH₂OH (ethanoic acid, ethanal, ethanol)

Saponification
19. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (ethyl ethanoate, sodium ethanoate, ethanol)
26. Glyceryl stearate + 3NaOH → Glycerol + 3C₁₇H₃₅COONa (fat or oil, sodium stearate, soap)

Dehydration and addition reactions
27. C₂H₅OH → (conc. H₂SO₄, 443 K or Al₂O₃, 623 K) CH₂=CH₂ + H₂O (ethanol, ethene)
29. CH₂=CH₂ + Br₂(aq) → BrCH₂-CH₂Br (ethene, 1,2-dibromoethane)
31. CH₃OH + Cu or Ag (573 K) → HCHO + H₂ (methanol, methanal)
21. (CH₃)₂C=O + HCN → (CH₃)₂C(OH)CN (propanone, propanone cyanohydrin)
22. CH₃OH + CO → (I₂/Rh) CH₃COOH (methanol, ethanoic acid)

POINTS TO REMEMBER

Carbon always forms covalent bonds.
Carbon is present in all substances of animal and vegetable origin.
The ability of carbon to unite with an indefinite number of carbon atoms in straight, branched or cyclic chains is known as catenation.
Carbon and hydrogen combine together in different proportions to form a large number of compounds called hydrocarbons.
There are two types of hydrocarbons: saturated and unsaturated.
Alkanes are represented by the general formula C_nH_2n+2.
Alkenes are represented by the general formula C_nH_2n.
Alkynes are represented by the general formula C_nH_2n-2.
Organic compounds having the same functional group and common properties, but differing in molecular formula from the next member by one CH₂ group, form a homologous series, and such compounds are called homologues.
Compounds with the same molecular formula but different structural formulae are known as isomers.
The decomposition of alkanes on heating in the absence of oxygen is known as cracking.
Methane is prepared by heating a mixture of sodium acetate and soda lime.
When ethanol is heated with an excess of concentrated sulphuric acid at 160 °C, ethene gas is produced.
Natural gas is a mixture of gaseous hydrocarbons, mainly methane, ethane, propane and butane.
Compressed natural gas (CNG) is used as an alternative to petrol as an automobile fuel.
Natural gas is a rich source of hydrogen gas, which is required for the manufacture of fertilisers.
Liquefied petroleum gas (LPG) is used as a domestic fuel.
Petrol is a complex mixture of hydrocarbons such as hexane, heptane and octane.
Petrol is used as a motor fuel.
Alcohols are organic compounds which contain the hydroxyl group (-OH) bonded to a carbon atom.
Alcohols are neutral to litmus.
Alcohols are poor conductors of electricity.
Alcohol reacts with sodium to liberate hydrogen gas.
Ethanol is a constituent of beverages like wine and beer.
Ethanol is used as a hypnotic and is highly addictive.
Organic compounds containing the carboxyl group (-COOH) are called carboxylic acids.
Ethanoic acid reacts with sodium carbonate to produce carbon dioxide gas.
A dilute aqueous solution of 4 to 6% ethanoic acid is called vinegar.
Pure acetic acid (about 99%) is called glacial acetic acid.
A soap is a sodium or potassium salt of a long-chain carboxylic acid. Sodium palmitate and sodium stearate are examples of soaps.
The process of splitting fats or oils using alkalis is called saponification.
Soaps do not work well with hard water, but synthetic detergents do.
Soaps are biodegradable, but synthetic detergents are not always biodegradable.

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