Carbon and its Compounds Class 10 Notes

A covalent bond is a bond formed by the sharing of one or more pairs of electrons between two atoms. Carbon has the configuration 2,4, so it has four electrons in its outermost shell and needs four more to attain the stable octet. It therefore forms four bonds, and each of these bonds is a shared pair, which is why every compound of carbon is a covalent compound.A single shared pair is a single bond, two shared pairs form a double bond and three shared pairs form a triple bond. Carbon is the only element that readily forms double and triple bonds with itself and with other elements, and that single fact explains most of the size of organic chemistry.

• The need: carbon has four valence electrons, and sharing four more completes its octet, so carbon is tetravalent.
• Single bond: one shared pair, as in methane, CH₄, in which all four bonds are to hydrogen.
• Double bond: two shared pairs, as in the carbon to carbon bond of ethene, C₂H₄.
• Triple bond: three shared pairs, as in ethyne, C₂H₂, the molecule used in an oxy-acetylene flame.
• Carbon forms only covalent compounds, because there is no metal in the picture, and there are an unlimited number of carbon to carbon arrangements.

Versatility is the standard term for the fact that carbon forms a very large number of compounds, and the syllabus asks for the reasons. There are four, and they are best written as four numbered points because a four-mark question maps exactly onto four reasons.Catenation and isomerism are the two that students forget, while tetravalency and double and triple bonds are the two that come easily. A complete answer names all four.

• Catenation: carbon atoms link with one another in long chains and in rings, so chains of any length are possible, which is the largest single source of the number of compounds.
• Tetravalency: each carbon atom forms four bonds, and each bond may be to hydrogen or to another carbon, so straight chains, branched chains and rings are all possible.
• Formation of double and triple bonds: carbon can share two or three pairs of electrons, which shortens a chain and gives entirely new series of compounds such as alkenes and alkynes.
• Isomerism: the same molecular formula can be arranged in different structures, so butane and isobutane both have the formula C₄H₁₀ but different properties.

A homologous series is a family of organic compounds with the same general formula, the same functional group, and therefore similar chemical properties. Consecutive members differ from one another by a single CH₂ group, which is 14 units of relative mass, and this single fact generates the pattern that the series shows.Because the properties are almost identical within a series, a knowledge of one member allows the properties of the rest to be predicted. The alkanes are the standard example, running from methane, CH₄, through ethane, C₂H₆, propane, C₃H₈, butane, C₄H₁₀, to pentane, C₅H₁₂, with the general formula CnH2n+2.

• Same general formula: every member of the alkane series can be written as CnH2n+2.
• Successive difference of CH₂: methane to ethane adds a CH₂ unit, and so does ethane to propane, so consecutive members differ by 14 in relative mass.
• Similar chemical properties: all members burn in air, all of them are almost insoluble in water, and all of them show a common set of reactions because the functional group is the same.
• A gradual change in physical properties: boiling point and density rise steadily along the series, so the later members are solids while the early ones are gases.
• Functional group: a particular atom or group of atoms that gives a compound its characteristic properties, such as the alcohol group in ethanol and the carboxyl group in ethanoic acid.

Nomenclature is the system of naming organic compounds, and the Class 10 system works in two halves. The first half is the root, which gives the number of carbon atoms in the longest chain: meth for one, eth for two, prop for three, but for four and pent for five. The second half is the suffix, which gives the functional group present in the compound.The suffixes that must be known are the alcohol suffix -ol, the aldehyde suffix -al, the ketone suffix -one and the carboxylic acid suffix -oic acid. A halogen is shown as a prefix rather than a suffix, so a chlorine attached to ethane is named chloroethane.

• Root names: meth for 1 carbon, eth for 2, prop for 3, but for 4, pent for 5, as in methanol, ethanol, propanone and ethanoic acid.
• Alcohol, the -ol suffix: the -OH group is present, as in ethanol, C₂H₅OH, whose common name is ethyl alcohol.
• Aldehyde, the -al suffix: the -CHO group is at the end of the chain, as in ethanal, CH₃CHO.
• Ketone, the -one suffix: the C=O group is inside the chain and never at an end, as in propanone, CH₃COCH₃, the substance in nail polish remover.
• Carboxylic acid, the -oic acid suffix: the -COOH group is at the end of the chain, as in ethanoic acid, CH₃COOH. A halogen is a prefix: chloroethane, bromoethane, fluoroethane, iodoethane.

Hydrocarbons containing only single bonds are saturated, because every carbon is bonded to the maximum number of hydrogen atoms that it can hold. Hydrocarbons with at least one double or triple bond are unsaturated, because they carry fewer hydrogen atoms than the saturated compound with the same number of carbons.The board separates the two families by a laboratory test rather than by definition alone. Alkenes and alkynes decolourise bromine water, turning it from reddish brown to colourless, and they also decolourise acidified potassium permanganate. Alkanes do neither, and this test is the practical file question that the chapter is built around.

• Alkanes, general formula CnH2n+2, contain only carbon to carbon single bonds and are saturated, so they are relatively less reactive.
• Alkenes, general formula CnH2n, contain at least one carbon to carbon double bond and are unsaturated.
• Alkynes, general formula CnH2n-2, contain at least one carbon to carbon triple bond and are also unsaturated.
• The test: alkenes and alkynes decolourise bromine water, the reddish brown colour disappearing, and they also decolourise acidified KMnO₄, the purple colour disappearing.
• Addition reaction: a substance is added across a double or triple bond, and ethene adds bromine to give 1,2-dibromoethane, CH₂Br-CH₂Br. Alkanes undergo substitution instead.

Four types of reaction cover the whole of organic chemistry at this level: combustion, oxidation, addition and substitution. Combustion is a redox reaction, oxidation converts an alcohol first to an aldehyde and then to a carboxylic acid, addition is possible only across a double or triple bond, and substitution is the characteristic reaction of the saturated alkanes.The practical file asks for four of these reactions to be performed, and the observations are the marks. Burning a candle, oxidising ethanol to ethanoic acid, decolourising bromine water, and the reaction of ethane with chlorine in sunlight are the four that are asked.

• Combustion: a carbon compound burns in air to give carbon dioxide and water, and the observation is a blue flame, which confirms that the gas produced is carbon dioxide.
• Incomplete combustion: with a limited supply of air, the flame is yellow and sooty and carbon monoxide is produced, which is why an incomplete combustion is dangerous.
• Oxidation: ethanol is first oxidised to ethanal by heating with copper(II) oxide, and then to ethanoic acid, so C₂H₅OH + 2[O] → CH₃COOH + H₂O.
• Addition: an unsaturated hydrocarbon adds a substance across the double or triple bond, as CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br, which is why bromine water is decolourised.
• Substitution: an alkane reacts with chlorine in bright sunlight, and one hydrogen atom is replaced by one chlorine atom, with hydrogen chloride released.

Ethanol, C₂H₅OH, and ethanoic acid, CH₃COOH, are the two compounds the syllabus names individually, and the instruction is to give their properties and uses. Both are colourless liquids at room temperature and both are members of a homologous series of their own type, so a knowledge of the series predicts the behaviour of each.Ethanoic acid is the acid of vinegar. Commercial vinegar is a dilute solution of ethanoic acid in water, usually about five to eight per cent, and it is this solution rather than the pure acid that is used in cooking and as a preservative.

• Ethanol, C₂H₅OH, also called ethyl alcohol, is a colourless liquid with a pleasant smell, and it is miscible with water in all proportions.
• Ethanol has a lower boiling point than water, about 78 °C against 100 °C, which is why it evaporates faster and produces a cooling effect when it is used on the skin or as a jet fuel.
• Ethanol burns with a clean blue flame, and its uses are as a fuel in alcohol stoves, as a disinfectant, as a solvent, and as a coolant for quick freezing.
• Ethanoic acid, CH₃COOH, a colourless liquid with a pungent smell and a sour taste, turns blue litmus red, and a solution of it in water is vinegar.
• Ethanoic acid reacts with ethanol in the presence of an acid catalyst to form an ester with a fruity smell, and its uses are as a preservative in vinegar, in pickling, and in the manufacture of esters and perfumes.

A soap is a sodium or potassium salt of a long chain carboxylic acid, obtained from oils and fats. A detergent has a similar structure, but the salts are made from sulphonic acids or from alkyl sulphates instead. Both clean by the same mechanism, in which a molecule has a water-loving head and a grease-loving tail.The cleansing action works because the tails gather in the oil or grease and the heads point out towards the water, and a ball of grease surrounded by such molecules is called a micelle. The grease is held in the middle of the micelle and is carried away when the micelle is rinsed off. The difference between a soap and a detergent appears in hard water, where a soap forms an insoluble scum and a detergent does not.

• Soap: a sodium or potassium salt of a long chain carboxylic acid, made from oils and fats, and it forms scum in hard water because it reacts with calcium and magnesium ions.
• Detergent: a sodium or potassium salt of a sulphonic acid or an alkyl sulphate, and it does not form scum because its calcium and magnesium salts remain soluble.
• The two parts of a soap molecule: the long hydrocarbon tail, which is hydrophobic and dissolves in oil and grease, and the ionic head, which is hydrophilic and dissolves in water.
• Micelle: a ball-like arrangement in which the tails point inwards towards the grease and the heads point outwards towards the water, so the grease is trapped and lifted away.
• Environmental point: soaps are biodegradable, whereas some detergents with a branched hydrocarbon chain are not, and this is the standard comparison asked at the end of the chapter.