Carbon Compounds Class 10 NCERT

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1. Introduction to Carbon Compounds Carbon is the 17th most abundant element by mass in the Earth's crust and 3rd most abundant element in the human body. It is a non-metal with atomic number 6. Carbon forms the basis of all living organisms and many industrial materials. The branch of chemistry dealing with carbon compounds is called Organic Chemistry. 2. Bonding in Carbon - The Covalent Bond Atomic Structure: Carbon has 6 electrons: 2 in the K-shell and 4 in the L-shell. It needs 4 more electrons to achieve the stable noble gas configuration of Neon (8 valence electrons). Ionic Bond Formation Difficulty: To gain 4 electrons would require the nucleus (with 6 protons) to hold 10 electrons, which would be highly unstable and energetically unfavourable ($C^{4-}$ ion). To lose 4 electrons would require a large amount of energy to remove them from the small, positively charged nucleus ($C^{4+}$ ion). Covalent Bond Solution: Carbon overcomes this by sharing its 4 valence electrons with other atoms. This sharing leads to the formation of covalent bonds. Each shared pair of electrons constitutes one covalent bond. Covalent bonds are strong due to the mutual sharing of electrons. However, compounds formed by covalent bonds generally have relatively weak intermolecular forces, leading to low melting and boiling points compared to ionic compounds. They are generally poor conductors of electricity as they do not form ions. 3. Versatile Nature of Carbon a. Catenation (Self-linking Property) Carbon has the unique ability to form long chains, branched chains, and rings by bonding with other carbon atoms. This property is called catenation. Carbon atoms can be linked by single bonds (C-C), double bonds (C=C), or triple bonds ($C \equiv C$). No other element exhibits catenation to the extent carbon does. Silicon, for example, can form compounds with up to 7 or 8 silicon atoms, but these are highly reactive. b. Tetravalency (Valency of Four) With a valency of four, carbon can bond with four other atoms. It forms stable and strong covalent bonds with a wide range of elements, including hydrogen, oxygen, nitrogen, sulfur, halogens (chlorine, bromine, iodine), etc. This allows carbon to form a vast number of diverse compounds. 4. Saturated and Unsaturated Carbon Compounds Saturated Hydrocarbons (Alkanes): Contain only carbon-carbon single bonds. All valencies of carbon are satisfied by single bonds. They are generally less reactive. General formula: $C_nH_{2n+2}$. Examples: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$). Unsaturated Hydrocarbons: Contain at least one carbon-carbon double bond or triple bond. Alkenes: Contain at least one $C=C$ double bond. General formula: $C_nH_{2n}$. Examples: Ethene ($C_2H_4$), Propene ($C_3H_6$). Alkynes: Contain at least one $C \equiv C$ triple bond. General formula: $C_nH_{2n-2}$. Examples: Ethyne ($C_2H_2$), Propyne ($C_3H_4$). They are more reactive than saturated hydrocarbons due to the presence of pi bonds. 5. Hydrocarbons (Compounds of Carbon and Hydrogen) Alkanes: Methane ($CH_4$): Simplest alkane, major component of natural gas. Ethane ($C_2H_6$): Two carbon atoms, single bond. Propane ($C_3H_8$): Three carbon atoms, single bonds. Alkenes: Ethene ($C_2H_4$): Two carbon atoms, one double bond. Also known as ethylene, used for ripening fruits. Propene ($C_3H_6$): Three carbon atoms, one double bond. Alkynes: Ethyne ($C_2H_2$): Two carbon atoms, one triple bond. Also known as acetylene, used in oxy-acetylene welding. Propyne ($C_3H_4$): Three carbon atoms, one triple bond. 6. Electron Dot Structures (Lewis Structures) Represent the valence electrons of atoms as dots and shared pairs as lines. Methane ($CH_4$): Each H shares 1 electron with C. C shares 1 electron with each H. $$ H \\ \quad | \\ H - \underset{ \cdot \cdot }{C} - H \\ \quad | \\ H $$ Each H achieves a duet, and C achieves an octet. Ethane ($C_2H_6$): $$ H \quad H \\ \quad | \quad | \\ H - C - C - H \\ \quad | \quad | \\ H \quad H $$ Ethene ($C_2H_4$): Carbon atoms share two pairs of electrons (double bond). $$ H \quad H \\ \quad \backslash \quad / \\ \quad C = C \\ \quad / \quad \backslash \\ H \quad H $$ Ethyne ($C_2H_2$): Carbon atoms share three pairs of electrons (triple bond). $$ H - C \equiv C - H $$ 7. Isomers Compounds that have the same molecular formula but different structural formulas (different arrangement of atoms). Example: Butane ($C_4H_{10}$) n-butane: Straight chain structure. $CH_3-CH_2-CH_2-CH_3$ iso-butane (2-methylpropane): Branched chain structure. $$ \quad CH_3 \\ \quad | \\ CH_3 - CH - CH_3 $$ Isomers have different physical properties (e.g., boiling point) but similar chemical properties. 8. Functional Groups An atom or a group of atoms that replaces one or more hydrogen atoms in a hydrocarbon and is responsible for the characteristic chemical properties of the organic compound. Heteroatoms: Atoms other than carbon and hydrogen (e.g., O, N, S, P, halogens). These atoms impart specific properties to the carbon chain. Homologous series: A series of organic compounds in which all members have the same functional group and similar chemical properties. All members can be represented by a general formula (e.g., $C_nH_{2n+1}OH$ for alcohols). Each successive member differs by a $-CH_2-$ group. Physical properties (melting point, boiling point, density) show a gradual change with increasing molecular mass. Chemical properties are similar due to the same functional group. Functional Group Formula Class of Compound Prefix/Suffix Example (IUPAC Name) Haloalkane $-X$ (X=F, Cl, Br, I) Haloalkane halo- Chloromethane ($CH_3Cl$) Alcohol $-OH$ Alcohol -ol Ethanol ($CH_3CH_2OH$) Aldehyde $-CHO$ Aldehyde -al Ethanal ($CH_3CHO$) Ketone $R-CO-R'$ Ketone -one Propanone ($CH_3COCH_3$) Carboxylic Acid $-COOH$ Carboxylic Acid -oic acid Ethanoic acid ($CH_3COOH$) Alkene $C=C$ Alkene -ene Ethene ($CH_2=CH_2$) Alkyne $C \equiv C$ Alkyne -yne Ethyne ($CH \equiv CH$) 9. Nomenclature of Carbon Compounds (IUPAC System) Steps for Naming: Identify the longest carbon chain: This determines the root word (e.g., meth-, eth-, prop-, but-). Identify the functional group: This determines the suffix (e.g., -ol, -al, -one, -oic acid, -ene, -yne). If no functional group, use -ane. Number the carbon chain: Start numbering from the end closest to the functional group or substituent to give it the lowest possible number. Identify substituents (if any): Name them (e.g., methyl, ethyl, chloro, bromo) and indicate their position with a number. Combine them: Prefix (substituents) + Root word (number of carbons) + Suffix (functional group). Examples: $CH_3-CH_2-OH$: Ethanol (2 carbons, alcohol group) $CH_3-CHO$: Ethanal (2 carbons, aldehyde group) $CH_3-CO-CH_3$: Propanone (3 carbons, ketone group) $CH_3-COOH$: Ethanoic acid (2 carbons, carboxylic acid group) $CH_2=CH_2$: Ethene (2 carbons, double bond) $CH_3-CH_2-CH_2-Cl$: 1-Chloropropane (3 carbons, chloro group at position 1) 10. Chemical Properties of Carbon Compounds a. Combustion (Burning) Most carbon compounds burn readily in air (oxygen) to produce carbon dioxide, water, heat, and light. This is an exothermic reaction. General reaction: Carbon compound + $O_2 \to CO_2 + H_2O + Heat + Light$ Examples: $C + O_2 \to CO_2 + Heat + Light$ (e.g., burning charcoal) $CH_4 (g) + 2O_2 (g) \to CO_2 (g) + 2H_2O (l) + Heat + Light$ (burning methane, natural gas) $CH_3CH_2OH (l) + 3O_2 (g) \to 2CO_2 (g) + 3H_2O (l) + Heat + Light$ (burning ethanol) Flame types: Saturated hydrocarbons: Generally burn with a clean, blue, non-sooty flame, indicating complete combustion. Unsaturated hydrocarbons: Often burn with a yellow, sooty flame, indicating incomplete combustion due to a higher percentage of carbon. The unburnt carbon particles glow yellow. Limited air supply: Leads to incomplete combustion, producing carbon monoxide (CO), which is a highly poisonous gas, and soot (carbon). b. Oxidation Oxidation reactions involve the addition of oxygen or removal of hydrogen. Alcohols can be oxidized to carboxylic acids. Oxidizing Agents: Substances that provide oxygen for oxidation. Alkaline potassium permanganate ($KMnO_4$) Acidified potassium dichromate ($K_2Cr_2O_7$) Reaction: $CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 \text{ + Heat}} CH_3COOH$ (Ethanol to Ethanoic acid) The purple colour of $KMnO_4$ disappears during the reaction, indicating its consumption. c. Addition Reaction (Hydrogenation) Characteristic reaction of unsaturated hydrocarbons (alkenes and alkynes). Hydrogen is added across the double or triple bond in the presence of a catalyst (e.g., Palladium (Pd) or Nickel (Ni)). This converts unsaturated hydrocarbons into saturated hydrocarbons. Reaction: $C_2H_4 (g) + H_2 (g) \xrightarrow{Ni \text{ catalyst, Heat}} C_2H_6 (g)$ (Ethene to Ethane) $CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$ Application: This reaction is widely used in the food industry to convert unsaturated vegetable oils (liquid at room temperature, containing double bonds) into saturated vegetable ghee or vanaspati (solid at room temperature, single bonds). This process is called hydrogenation. d. Substitution Reaction Characteristic reaction of saturated hydrocarbons (alkanes). Saturated hydrocarbons are generally unreactive and inert in the presence of most reagents. However, they react with chlorine in the presence of sunlight (UV light). In this reaction, one or more hydrogen atoms of the alkane are replaced (substituted) by chlorine atoms. Reaction: $CH_4 + Cl_2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl$ (Methane to Chloromethane) This is a fast reaction and can lead to the substitution of all hydrogen atoms: $CH_3Cl + Cl_2 \xrightarrow{\text{sunlight}} CH_2Cl_2 + HCl$ (Dichloromethane) $CH_2Cl_2 + Cl_2 \xrightarrow{\text{sunlight}} CHCl_3 + HCl$ (Trichloromethane - Chloroform) $CHCl_3 + Cl_2 \xrightarrow{\text{sunlight}} CCl_4 + HCl$ (Tetrachloromethane - Carbon tetrachloride) 11. Important Carbon Compounds: Ethanol and Ethanoic Acid a. Ethanol ($CH_3CH_2OH$) Also known as ethyl alcohol or grain alcohol. Physical properties: Colourless liquid with a characteristic pleasant smell and burning taste. Miscible with water in all proportions. Volatile liquid with a boiling point of $78^\circ C$ ($351 K$). Neutral to litmus. Reactions: Reaction with Sodium: Ethanol reacts with sodium metal to produce sodium ethoxide and hydrogen gas. $2Na + 2CH_3CH_2OH \to 2CH_3CH_2O^-Na^+ + H_2 (g)$ Dehydration: When ethanol is heated with excess concentrated sulfuric acid at $170^\circ C$ ($443 K$), it loses a water molecule to form ethene. Concentrated sulfuric acid acts as a dehydrating agent. $CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4 \text{ (170°C)}} CH_2=CH_2 + H_2O$ Combustion: Burns with a clean, blue flame. $CH_3CH_2OH + 3O_2 \to 2CO_2 + 3H_2O + Heat + Light$ Oxidation: Oxidized to ethanoic acid by oxidizing agents. $CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 \text{ or acidified } K_2Cr_2O_7} CH_3COOH$ Uses: Used as a solvent for many organic compounds (e.g., in paints, medicines like tincture of iodine, cough syrups, and tonics). Used in alcoholic beverages. Used as a fuel (e.g., in some countries as a blend with petrol). Denatured alcohol: Ethanol made unfit for drinking by adding poisonous substances like methanol, pyridine, or dyes. This is done to prevent its misuse and for industrial purposes. Harmful effects of consuming alcohol: Causes drunkenness, affects metabolism, damages liver (cirrhosis), brain damage, and can lead to death in high doses. Methanol is extremely poisonous and can cause blindness and death. b. Ethanoic Acid ($CH_3COOH$) Commonly known as Acetic Acid. A 5-8% solution of ethanoic acid in water is called Vinegar and is used as a preservative in pickles. Physical properties: Colourless liquid with a sour taste and pungent smell. Boiling point is $118^\circ C$ ($391 K$). Pure ethanoic acid freezes at $17^\circ C$ ($290 K$) during winter, resembling a glacier, hence called glacial acetic acid. Weak acid, partially dissociates in water. Reactions: Esterification: Ethanoic acid reacts with ethanol in the presence of an acid catalyst (conc. $H_2SO_4$) to form an ester (ethyl ethanoate) and water. Esters are sweet-smelling compounds. $CH_3COOH + CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4} CH_3COOCH_2CH_3 + H_2O$ Saponification: The reaction of an ester with a base (like NaOH) to form the sodium salt of the carboxylic acid and alcohol. This is used in soap making. $CH_3COOCH_2CH_3 + NaOH \to CH_3COONa + CH_3CH_2OH$ Reaction with a base: Being an acid, it reacts with bases to form salt and water. $CH_3COOH + NaOH \to CH_3COONa + H_2O$ (Sodium ethanoate) Reaction with Carbonates and Bicarbonates: Ethanoic acid reacts with metal carbonates and bicarbonates to produce carbon dioxide gas, a salt, and water. $2CH_3COOH + Na_2CO_3 \to 2CH_3COONa + H_2O + CO_2 (g)$ $CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2 (g)$ The $CO_2$ gas turns lime water milky. Uses: As vinegar, in rubber coagulation, as a solvent, in the production of esters for perfumes and dyes. 12. Soaps and Detergents a. Soaps Definition: Soaps are the sodium or potassium salts of long-chain carboxylic acids (fatty acids). Preparation: Made by heating animal fat or vegetable oil (esters of long-chain fatty acids) with a strong alkali like NaOH (saponification). Structure: A soap molecule has two main parts: Long hydrocarbon chain: This is the non-polar, hydrophobic (water-repelling) 'tail'. It is soluble in oil/grease. Ionic part ($ -COO^-Na^+$ or $ -COO^-K^+$): This is the polar, hydrophilic (water-attracting) 'head'. It is soluble in water. Cleansing action of soap: When soap is dissolved in water, the hydrophobic tails attach themselves to the oil/grease particles (dirt), while the hydrophilic heads remain pointing outwards towards the water. This forms a spherical structure called a micelle . The oil/grease is trapped in the center of the micelle. The charged heads of the micelles repel each other, preventing them from aggregating and allowing them to remain suspended in water as an emulsion. These micelles, with the dirt encapsulated, can then be easily rinsed away with water. Limitation of Soaps (Hard Water): Hard water contains dissolved salts of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions. When soap is used in hard water, the $Ca^{2+}$ and $Mg^{2+}$ ions react with the soap molecules to form an insoluble precipitate called scum . This scum is sticky, adheres to clothes, and reduces the cleansing ability of soap. It also forms a white layer on bathtubs and sinks. Example reaction: $2C_{17}H_{35}COO^-Na^+ \text{ (Soap)} + Ca^{2+} \to (C_{17}H_{35}COO^-)_2Ca^{2+} \downarrow \text{ (Scum)} + 2Na^+$ b. Detergents Definition: Detergents are generally ammonium or sulfonate salts of long-chain carboxylic acids. They are synthetic cleansing agents. Advantages over soaps: Detergents work effectively in both soft and hard water. The charged ends of detergents do not form insoluble precipitates with $Ca^{2+}$ and $Mg^{2+}$ ions present in hard water. Instead, they form soluble salts, preventing scum formation. Examples: Sodium alkyl sulphates (e.g., Sodium lauryl sulphate) Sodium alkylbenzene sulphonates (e.g., Sodium dodecylbenzenesulphonate) Uses: Commonly used in washing powders, liquid detergents, shampoos, and other cleaning products. Environmental concerns: Some detergents contain phosphates, which can cause water pollution (eutrophication). Biodegradable detergents are preferred.