Organic Chemistry — Class 11 Chemistry Solved Exercises

Q1. Choose the Correct Answer

i.. Catenation is most extensive for:

ii.. The functional group in alcohols is:

iii.. Members of a homologous series differ by:

iv.. Carbon in ethene is:

v.. Cis–trans isomerism requires:

vi.. Lassaigne’s test is used to detect:

vii.. Which technique separates liquids with different boiling points?

viii.. Tetravalency of carbon means carbon forms:

ix.. Ethanol and methoxymethane are:

x.. Organic compounds are generally:

Q2. Give Short Answers

2(i). What is catenation? Why is it important for carbon?

Catenation is the ability of an element to form bonds with atoms of the same element, making chains and rings. Carbon shows exceptional catenation because C–C bonds are strong and carbon is tetravalent. This allows millions of organic compounds with diverse structures. Silicon catenates much more weakly, so its chemistry is less extensive in this sense. Multiple bonding (C=C, C≡C) further expands organic structural variety. Catenation is a foundation of organic chemistry.

2(ii). Define functional group and homologous series.

A functional group is an atom or group of atoms that gives a compound its characteristic reactions. Examples include −OH (alcohol), −COOH (carboxylic acid), and C=C (alkene). A homologous series is a family of compounds with the same functional group and general formula, differing by CH₂ units. Members show gradual trends in physical properties and similar chemical reactions. Alkanes, alcohols and carboxylic acids are familiar series. Functional groups organise the study of organic reactions.

2(iii). Differentiate sigma and pi bonds in organic molecules.

A σ bond forms by end-on overlap and is the framework of every single bond. A π bond forms by side-on overlap and appears in double and triple bonds alongside σ bonds. Alkenes have one σ and one π in the double bond; alkynes have one σ and two π. π electrons are higher in energy and more reactive toward electrophiles. Restricted rotation about π bonds causes cis–trans isomerism in suitably substituted alkenes. Bonding type controls geometry and reactivity.

2(iv). What is isomerism? Give structural and stereoisomerism briefly.

Isomers are compounds with the same molecular formula but different arrangements of atoms. Structural (constitutional) isomers differ in connectivity, e.g. chain, position or functional-group isomers. Stereoisomers have the same connectivity but different spatial arrangement. Geometric (cis–trans) isomerism occurs in alkenes and some rings; optical isomerism involves chiral centres. Isomerism greatly increases the number of distinct organic compounds. Identifying isomer type is a core organic skill.

2(v). State the tetravalency of carbon and hybridisation in alkanes, alkenes and alkynes.

Carbon has four valence electrons and forms four covalent bonds (tetravalency). In alkanes, carbon is sp³ hybridised with tetrahedral geometry. In alkenes, doubly bonded carbons are sp² hybridised and trigonal planar. In alkynes, triply bonded carbons are sp hybridised and linear. Hybridisation explains bond angles of about 109.5°, 120° and 180° respectively. It links electronic structure to molecular shape in organic chemistry.

2(vi). Why are organic compounds generally covalent and often insoluble in water?

Carbon bonds to H, C and many non-metals by sharing electrons, producing covalent molecules. Many organic compounds are non-polar or only weakly polar, especially hydrocarbons. Water is highly polar and hydrogen-bonded, so it dissolves ionic and polar solutes preferentially. “Like dissolves like” explains why hydrocarbons dissolve in non-polar solvents. Compounds with −OH or −COOH groups may be water-soluble if the carbon chain is short. Solubility trends follow polarity and hydrogen-bonding ability.

3. Explain the unique position of carbon in the periodic table that leads to a vast organic chemistry.

Carbon is a small Group 14 atom with four valence electrons, allowing four strong covalent bonds. It catenates extensively, forming long chains, branched structures and rings. It forms stable single, double and triple bonds to itself and multiple bonds to O, N and others. Intermediate electronegativity lets carbon bond to metals and non-metals in diverse ways.

sp³, sp² and sp hybridisation create tetrahedral, trigonal and linear geometries. Isomerism multiplies possible structures for one molecular formula. Together these features produce an enormous number of stable compounds—far more than for any other element.

Organic chemistry therefore underpins fuels, plastics, medicines, biomolecules and materials. Understanding carbon’s bonding is the starting point for all later functional-group chemistry.

4. Classify organic compounds by functional groups and describe homologous series with examples.

Hydrocarbons include alkanes, alkenes, alkynes and aromatics. Compounds with heteroatom functional groups include haloalkanes, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, amines and amides. Each functional group has characteristic reagents and tests.

In a homologous series such as alcohols CₙH₂ₙ₊₁OH, each member differs by CH₂. Boiling points rise with chain length due to greater London forces. Chemical reactions remain similar because the functional group dominates reactivity.

IUPAC naming combines the parent chain, unsaturation and functional-group suffixes/prefixes. Classification by functional group organises an otherwise overwhelming subject into learnable patterns.

5. Discuss structural isomerism and stereoisomerism with suitable examples.

Chain isomerism: butane and 2-methylpropane share C₄H₁₀ but differ in branching. Position isomerism: propan-1-ol and propan-2-ol differ in −OH position. Functional-group isomerism: ethanol and methoxymethane both are C₂H₆O. Metamerism historically described isomers differing in alkyl groups about a functional atom.

Geometric isomerism: but-2-ene has cis and trans forms because π bonding prevents free rotation. Optical isomerism: lactic acid has enantiomers that rotate plane-polarised light oppositely when a chiral carbon is present.

Isomers can differ markedly in physical properties, biological activity and reaction rates. Drawing clear structures is essential to avoid confusing isomers with identical compounds.

6. Explain detection of common elements in organic compounds (C, H, N, S, halogens).

Carbon and hydrogen are detected by heating with dry CuO: CO₂ turns limewater milky and H₂O turns anhydrous CuSO₄ blue. Nitrogen, sulfur and halogens are detected by Lassaigne’s (sodium fusion) test, converting them to ionic NaCN, Na₂S or NaX. Prussian blue confirms nitrogen; lead acetate or sodium nitroprusside tests confirm sulfur; silver nitrate after acidification detects halides.

Careful removal of cyanide/sulfide interference is needed before halide tests. Quantitative analysis (Liebig, Dumas, Carius methods) gives percentage composition for formula determination.

These classical tests remain pedagogically important even when modern spectroscopy is available.

7. Describe the purification of organic compounds: crystallisation, distillation, chromatography.

Crystallisation purifies solids by dissolving in a hot solvent and cooling so pure crystals form while impurities stay dissolved. Distillation separates liquids by boiling-point differences; fractional distillation resolves closer-boiling mixtures. Steam distillation isolates steam-volatile compounds immiscible with water. Sublimation purifies some solids that vapourise without melting.

Chromatography (paper, TLC, column, GC) separates components by differential partitioning between stationary and mobile phases. Rf values aid identification on TLC. Purity is checked by sharp melting/boiling points or chromatographic homogeneity.

Choosing a method depends on physical state, thermal stability and scale. Purification precedes reliable characterisation and synthesis work.

Q1. Choose the Correct Answer

i.. Catenation is most extensive for:

ii.. The functional group in alcohols is:

iii.. Members of a homologous series differ by:

iv.. Carbon in ethene is:

v.. Cis–trans isomerism requires:

vi.. Lassaigne’s test is used to detect:

vii.. Which technique separates liquids with different boiling points?

viii.. Tetravalency of carbon means carbon forms:

ix.. Ethanol and methoxymethane are:

x.. Organic compounds are generally:

Q2. Give Short Answers

Q1. Choose the Correct Answer

i.. Catenation is most extensive for:

ii.. The functional group in alcohols is:

iii.. Members of a homologous series differ by:

iv.. Carbon in ethene is:

v.. Cis–trans isomerism requires:

vi.. Lassaigne’s test is used to detect:

vii.. Which technique separates liquids with different boiling points?

viii.. Tetravalency of carbon means carbon forms:

ix.. Ethanol and methoxymethane are:

x.. Organic compounds are generally:

Q2. Give Short Answers

2(i). What is catenation? Why is it important for carbon?

2(ii). Define functional group and homologous series.

2(iii). Differentiate sigma and pi bonds in organic molecules.

2(iv). What is isomerism? Give structural and stereoisomerism briefly.

2(v). State the tetravalency of carbon and hybridisation in alkanes, alkenes and alkynes.

2(vi). Why are organic compounds generally covalent and often insoluble in water?

Long Questions

3. Explain the unique position of carbon in the periodic table that leads to a vast organic chemistry.

4. Classify organic compounds by functional groups and describe homologous series with examples.

5. Discuss structural isomerism and stereoisomerism with suitable examples.

6. Explain detection of common elements in organic compounds (C, H, N, S, halogens).

7. Describe the purification of organic compounds: crystallisation, distillation, chromatography.

Q1. Choose the Correct Answer

i.. Catenation is most extensive for:

ii.. The functional group in alcohols is:

iii.. Members of a homologous series differ by:

iv.. Carbon in ethene is:

v.. Cis–trans isomerism requires:

vi.. Lassaigne’s test is used to detect:

vii.. Which technique separates liquids with different boiling points?

viii.. Tetravalency of carbon means carbon forms:

ix.. Ethanol and methoxymethane are:

x.. Organic compounds are generally:

Q2. Give Short Answers

2(i). What is catenation? Why is it important for carbon?

2(ii). Define functional group and homologous series.

2(iii). Differentiate sigma and pi bonds in organic molecules.

2(iv). What is isomerism? Give structural and stereoisomerism briefly.

2(v). State the tetravalency of carbon and hybridisation in alkanes, alkenes and alkynes.

2(vi). Why are organic compounds generally covalent and often insoluble in water?

Long Questions

3. Explain the unique position of carbon in the periodic table that leads to a vast organic chemistry.

4. Classify organic compounds by functional groups and describe homologous series with examples.

5. Discuss structural isomerism and stereoisomerism with suitable examples.

6. Explain detection of common elements in organic compounds (C, H, N, S, halogens).

7. Describe the purification of organic compounds: crystallisation, distillation, chromatography.