Introduction
This chapter introduces the fascinating world of organic chemistry — the chemistry of carbon compounds. You will learn why carbon forms such a large number of compounds (catenation and tetravalency), how to name organic compounds using IUPAC nomenclature, the concept of homologous series, and important chemical reactions of carbon compounds including combustion, oxidation, addition, and substitution reactions. Special focus is given to ethanol and ethanoic acid.
Bonding in Carbon: Covalent Bonds, Catenation, and Tetravalency
Carbon has atomic number 6 with electronic configuration 2,4. It has 4 valence electrons and needs 4 more to complete its octet. It cannot form C4+ (losing 4 electrons requires too much energy) or C4- (gaining 4 electrons is not favourable). Instead, carbon shares electrons to form covalent bonds. Carbon's two unique properties make it special: (1) Catenation — the ability to form bonds with other carbon atoms, creating long chains, branched chains, and rings. (2) Tetravalency — carbon forms 4 bonds, allowing it to bond with many different elements (H, O, N, S, Cl, etc.). These two properties together explain why carbon forms the largest number of known compounds (over 10 million). Carbon can form single bonds (C-C), double bonds (C=C), and triple bonds (C-triple-C).
Key Points
- •Carbon has 4 valence electrons and forms 4 covalent bonds (tetravalency)
- •Catenation: carbon bonds with itself to form chains, branches, and rings
- •Single bond: share 1 pair of electrons; Double: 2 pairs; Triple: 3 pairs
- •Saturated compounds: only single bonds (alkanes); Unsaturated: double/triple bonds
- •Silicon also shows catenation but C-C bonds are much stronger than Si-Si bonds
- •Covalent compounds have low melting/boiling points and poor electrical conductivity
Worked Example
Methane (CH4): Carbon shares its 4 electrons with 4 hydrogen atoms. Ethene (C2H4): Two carbons share 2 pairs of electrons (double bond) and each bonds with 2 hydrogens. Ethyne (C2H2): Two carbons share 3 pairs of electrons (triple bond) and each bonds with 1 hydrogen.
Watch Out
Always draw the electron dot structure in board exams when asked about bonding. Show shared pairs clearly between atoms.
Homologous Series and IUPAC Nomenclature
A homologous series is a family of organic compounds with the same general formula, same functional group, and similar chemical properties. Each successive member differs by a CH2 unit (14 atomic mass units). The three main homologous series are: Alkanes (CnH2n+2, single bonds only), Alkenes (CnH2n, one double bond), and Alkynes (CnH2n-2, one triple bond). IUPAC naming follows a systematic approach: (1) Find the longest carbon chain (root word: meth-1, eth-2, prop-3, but-4, pent-5). (2) Identify the functional group (suffix: -ane for alkane, -ene for alkene, -yne for alkyne, -ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid). (3) Number the chain to give the functional group the lowest number. (4) Name substituents with their position numbers.
Key Points
- •Homologous series: same general formula, differ by CH2, similar chemical properties
- •Alkanes: CnH2n+2 (methane CH4, ethane C2H6, propane C3H8)
- •Alkenes: CnH2n (ethene C2H4, propene C3H6)
- •Alkynes: CnH2n-2 (ethyne C2H2, propyne C3H4)
- •Root: meth(1C), eth(2C), prop(3C), but(4C), pent(5C), hex(6C)
- •Functional groups: -OH (alcohol), -CHO (aldehyde), -COOH (carboxylic acid), -CO- (ketone)
Worked Example
Name: CH3-CH2-OH Longest chain: 2 carbons = eth- Functional group: -OH (alcohol) = -ol suffix Name: Ethanol Name: CH3-COOH Longest chain: 2 carbons = eth- Functional group: -COOH (carboxylic acid) = -oic acid suffix Name: Ethanoic acid
Watch Out
In board exams, structural formulae are often given and you must name them. Practice naming at least 10 compounds from different functional groups.
Chemical Properties of Carbon Compounds
Key reactions of carbon compounds include: (1) Combustion: carbon compounds burn in oxygen to produce CO2, H2O, and energy. Complete combustion (sufficient O2) gives CO2; incomplete gives CO (toxic) or carbon (soot). Saturated compounds burn with a clean blue flame; unsaturated with a sooty yellow flame. (2) Oxidation: alcohols can be oxidised to carboxylic acids using oxidising agents like alkaline KMnO4 or acidified K2Cr2O7. (3) Addition reaction: unsaturated compounds add atoms across the double/triple bond. Hydrogenation: adding H2 in the presence of nickel/palladium catalyst converts unsaturated to saturated (vegetable oil to vanaspati ghee). (4) Substitution reaction: saturated hydrocarbons replace H atoms with other atoms (e.g., chlorination of methane in sunlight).
Key Points
- •Combustion: CxHy + O2 -> CO2 + H2O + energy (complete combustion)
- •Saturated: clean blue flame; Unsaturated: sooty yellow flame
- •Oxidation: CH3CH2OH --(alk. KMnO4)--> CH3COOH (ethanol to ethanoic acid)
- •Addition: CH2=CH2 + H2 --(Ni catalyst)--> CH3-CH3 (hydrogenation)
- •Vegetable oil (unsaturated, liquid) --(hydrogenation)--> vanaspati ghee (saturated, solid)
- •Substitution: CH4 + Cl2 --(sunlight)--> CH3Cl + HCl
Worked Example
Hydrogenation of vegetable oil: Vegetable oil has C=C double bonds (unsaturated, liquid at room temperature). Adding H2 with Ni catalyst at 473K converts double bonds to single bonds. Result: vanaspati ghee (saturated, solid at room temperature).
Watch Out
The conversion of vegetable oil to vanaspati ghee (hydrogenation) is one of the most frequently asked applications. Remember: Ni catalyst, 473K.
Ethanol and Ethanoic Acid
Ethanol (C2H5OH): a liquid at room temperature, used as a solvent and in alcoholic beverages. Reacts with sodium to produce hydrogen gas: 2C2H5OH + 2Na -> 2C2H5ONa + H2. Dehydrated by conc. H2SO4 at 443K to form ethene: C2H5OH -> C2H4 + H2O. Ethanoic acid (CH3COOH): also known as acetic acid. A 5-8% solution is vinegar. Pure (glacial) acetic acid freezes at 290K (16.6 degrees C). Reacts like a typical acid with bases, carbonates, and bicarbonates. Esterification: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O (in presence of conc. H2SO4). The reverse reaction (ester + NaOH -> alcohol + sodium salt of acid) is called saponification and is the basis of soap making.
Key Points
- •Ethanol: 2C2H5OH + 2Na -> 2C2H5ONa + H2 (less vigorous than water + Na)
- •Dehydration: C2H5OH --(conc.H2SO4, 443K)--> C2H4 + H2O
- •5-8% acetic acid solution = vinegar; pure form = glacial acetic acid (freezes at 16.6C)
- •Esterification: acid + alcohol --(conc.H2SO4)--> ester + water (fruity smell)
- •Saponification: ester + NaOH -> alcohol + sodium salt (soap making)
- •Soap: sodium/potassium salt of long-chain fatty acid; Detergent: works in hard water too
Worked Example
Esterification reaction: CH3COOH + C2H5OH --(conc.H2SO4)--> CH3COOC2H5 + H2O (ethanoic acid + ethanol -> ethyl ethanoate + water) The ester has a sweet fruity smell. Adding NaOH reverses the reaction (saponification).
Watch Out
Esters have a sweet fruity smell — this is a standard identification test in board exams. Also remember: soap does not work in hard water (forms scum), but detergents do.
Quick Summary
- ✓Carbon forms covalent bonds due to tetravalency (4 bonds) and shows catenation
- ✓Homologous series: same general formula, differ by CH2 (Alkanes CnH2n+2, Alkenes CnH2n, Alkynes CnH2n-2)
- ✓IUPAC naming: root word (chain length) + suffix (functional group)
- ✓Combustion produces CO2 + H2O; saturated burns clean, unsaturated burns sooty
- ✓Oxidation: alcohol -> acid (using KMnO4 or K2Cr2O7)
- ✓Addition: hydrogenation converts unsaturated to saturated (vegetable oil to ghee, Ni catalyst)
- ✓Substitution: H atoms replaced (CH4 + Cl2 -> CH3Cl + HCl in sunlight)
- ✓Esterification: acid + alcohol -> ester + H2O; Saponification: ester + NaOH -> soap
Key Formulas
Alkanes: CnH2n+2; Alkenes: CnH2n; Alkynes: CnH2n-2
Esterification: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O
Saponification: CH3COOC2H5 + NaOH -> C2H5OH + CH3COONa
Dehydration: C2H5OH --(conc.H2SO4,443K)--> C2H4 + H2O
Hydrogenation: vegetable oil + H2 --(Ni,473K)--> vanaspati ghee
Substitution: CH4 + Cl2 --(sunlight)--> CH3Cl + HCl
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