Hydrocarbons
- How many products (including stereoisomers) are expected from monochlorination of the following compound?(CH_3)_2CH-CH_2-CH_3 Answer: 4. 6 Key idea: count the distinct kinds of hydrogen in 2-methylbutane, then count stereoisomers. Two equivalent CH₃ on C-2: 1-chloro... 1
- Which one of the following compounds does not decolourize bromine water? Answer: 1. cyclohexane Key idea: bromine water is decolourised by alkenes (addition) and by activated aromatics (substitution). Styrene: C=C... 1
- Which one of the following reactions does NOT give benzene as the product ? Answer: 4. benzenediazonium chloride with H₂O , warm Key idea: identify the product of each reaction. Sodium benzoate + soda lime, heat: dec... 1
- For the given reaction:[scheme: ring-C(H)=CH-ring KMnO_4/H^+ 'P' (major product), see figure]'P' is Answer: 3. [structure: ring-COOH] Key idea: hot or acidic KMnO₄ cleaves a C=C bond and oxidises each fragment fully to the carboxylic acid.... 1
- Given below are two statements:Statement I : The boiling point of three isomeric pentanes follows the ordern-pentane > isopentane > neopentaneStatement II : When branching increases, the molecule attains a shape of sphere. This results in smaller sur… Answer: 2. Both Statement I and Statement II are correct. Key idea: among isomers, more branching means a more spherical shape and a lower b... 1
- Consider the following compounds/species :i. naphthalene ii. cyclopentadienyl anion iii. cyclobutadiene iv. cyclopropenyl anion v. cyclopropenyl cation vi. cyclooctatetraene vii. anthracene (all drawn as structures)The number of compounds/species whi… Answer: 2. 4 Key idea: Hückel's rule: a planar, cyclic, fully conjugated species with (4n+2) π electrons is aromatic. i. Naphthalene: 10 π e... 1
- Which amongst the following molecules on polymerization produces neoprene? Answer: 3. H₂C=C(Cl)-CH=CH₂ Key idea: Neoprene is a polymer of chloroprene, 2-chlorobuta-1,3-diene. Chloroprene is CH₂=C(Cl)-CH=CH₂ , which... 1
- Identify the final product [D] obtained in the following sequence of reactions.CH_3CHO [ii)\ H_3O^+]i)\ LiAlH_4 [A] [Δ]H_2SO_4 [B] HBr [C], then [C] + bromobenzene Na/dry\ ether [D] Answer: 2. ethylbenzene (structure) Key idea: Follow the sequence, ending with a Wurtz-Fittig reaction. CH₃CHO with LiAlH₄ gives ethanol [A]... 1
- Weight (g) of two moles of the organic compound, which is obtained by heating sodium ethanoate with sodium hydroxide in presence of calcium oxide is : Answer: 3. 32 Key idea: Sodium ethanoate heated with soda lime (NaOH + CaO) undergoes decarboxylation to give an alkane with one carbon fewe... 1
- Compound X on reaction with O_3 followed by Zn/H_2O gives formaldehyde and 2-methyl propanal as products. The compound X is : Answer: 2. 3-Methylbut-1-ene Key idea: ozonolysis cuts the C=C bond and puts a C=O on each carbon. Working backwards: HCHO and mathrm(CH₃)₂C... 1
- Which compound amongst the following is not an aromatic compound ? Answer: 1. [structure in figure] Key idea: an aromatic compound is cyclic, planar, fully conjugated and has (4n+2) π electrons. (1) Cyclohep... 1
- CH_3CH_2COO^-Na^+ [Heat]NaOH,\ +\ ? CH_3CH_3 + Na_2CO_3.Consider the above reaction and identify the missing reagent/chemical. Answer: 3. CaO Key idea: decarboxylation of a sodium carboxylate with soda lime. mathrmCH₃CH₂COONa+NaOHxrightarrowCaO,\ ΔCH₃CH₃+Na₂CO₃ . The... 1
- Dihedral angle of least stable conformer of ethane is : Answer: 4. 0^° Key idea: the eclipsed conformation of ethane has the highest torsional strain, so it is least stable. In the eclipsed form t... 1
- The major product formed in dehydrohalogenation reaction of 2-Bromo pentane is Pent-2-ene. This product formation is based on ? Answer: 1. Saytzeff's Rule Key idea: in dehydrohalogenation, the more substituted (more stable) alkene is the major product. This is Saytzef... 1
- The major product of the following chemical reaction is :(CH_3)_2CH-CH=CH_2 + HBr (C_6H_5CO)_2O_2 ? Answer: 1. (CH₃)₂CH-CH₂-CH₂-Br Key idea: with peroxide, HBr adds to alkenes by a free-radical path: anti-Markovnikov (Kharasch peroxide effe... 1
- An alkene on ozonolysis gives methanal as one of the product. Its structure is : Answer: 3. Cyclohexyl-CH ₂ -CH=CH ₂ Ozonolysis cleaves C=C and gives a carbonyl on each carbon. Methanal (HCHO) comes only from a terminal =... 1
- Which of the following alkane cannot be made in good yield by Wurtz reaction ? Answer: 3. n-Heptane The Wurtz reaction ( 2RX+2Na→ R-R ) gives good yield only when both alkyl groups are the same, so one alkyl halide is u... 1
- An alkene "A" on reaction with O_3 and Zn-H_2O gives propanone and ethanal in equimolar ratio. Addition of HCl to alkene "A" gives "B" as the major product. The structure of product "B" is : Answer: 3. H ₃ C-CH ₂ -C(CH ₃ )(Cl)-CH ₃ (see figure) Work back from the ozonolysis products. Propanone (CH₃)₂C=O and ethanal CH₃CHO join at... 1
- Among the following, the reaction that proceeds through an electrophilic substitution, is : Answer: 2. (structure 2, see figure): benzene + Cl ₂ xrightarrowAlCl₃ chlorobenzene + HCl (1) Sandmeyer reaction: via aryl radical / diazoni... 1
- The most suitable reagent for the following conversion, is :H_3C-C C-CH_3 → cis-2-butene Answer: 2. H ₂ , Pd/C, quinoline Partial reduction of an alkyne to a cis-alkene needs syn addition of H₂ on a poisoned catalyst, i.e. Lindla... 1
- Hydrocarbon (A) reacts with bromine by substitution to form an alkyl bromide which by Wurtz reaction is converted to gaseous hydrocarbon containing less than four carbon atoms. (A) is Answer: 4. CH ₄ The alkyl bromide undergoes Wurtz coupling, mathrm2R-Br+2Na→ R-R+2NaBr . The product has twice the carbon count of R, and mu... 1
- The compound C_7H_8 undergoes the following reactions :C_7H_8 3\ Cl_2/Δ A Br_2/Fe B Zn/HCl CThe product 'C' is Answer: 1. m-bromotoluene Work through the sequence: C₇H₈ is toluene. 3 Cl₂/Δ : side-chain chlorination gives benzotrichloride, C₆H₅CCl₃ (A)... 1
- Identify the major products P, Q and R in the following sequence of reactions :benzene + CH_3CH_2CH_2Cl Anhydrous AlCl_3 P ; P [(ii) H_3O^+/Δ](i) O_2 Q + R Answer: 4. P = cumene (C ₆ H ₅ CH(CH ₃ ) ₂ ), Q = phenol (C ₆ H ₅ OH), R = CH ₃ - CO - CH ₃ Friedel-Crafts alkylation with n-propyl chloride... 1
- Nitration of aniline in strong acidic medium also gives m-nitroaniline because Answer: 4. In acidic (strong) medium aniline is present as anilinium ion. Aniline is nitrated with a mixture of HNO₃ and H₂SO₄ , which is st... 1