Some Basic Principles of Organic Chemistry
- Among the given compounds I-III, the correct order of bond dissociation energy of C-H bond marked with * is : Answer: 1. II > I > III Key idea: the more s-character in the carbon, and the less stable the radical left behind, the stronger the C-H bond... 1
- Total number of possible isomers (both structural as well as stereoisomers) of cyclic ethers of molecular formula C_4H_8O is : Answer: 3. 10 Key idea: list all rings of formula C₄H₈O containing O, and count enantiomers/cis-trans. Oxolane (THF): 1. Oxetane with a meth... 1
- Which one of the following compounds can exist as cis-trans isomers? Answer: 4. 1,2-Dimethylcyclohexane Key idea: cis-trans isomerism needs a double bond with different groups on each carbon, or a ring with tw... 1
- The most stable carbocation among the following is: Answer: 1. [structure: methylcyclohexyl carbocation, see figure] Key idea: carbocation stability: tertiary > secondary > primary, and more h... 1
- A compound with a molecular formula of C_6H_14 has two tertiary carbons. Its IUPAC name is: Answer: 4. 2,3-dimethylbutane Key idea: a tertiary carbon is bonded to three other carbons (one C-H). 2,2-dimethylbutane: one quaternary C,... 1
- The correct IUPAC name of the following compound is :[skeletal structure with Cl, OH, Br and a methyl branch] Answer: 2. 1-bromo-5-chloro-4-methylhexan-3-ol Key idea: choose the longest chain containing the -OH group, give -OH the lowest locant, then... 1
- The incorrect statement regarding chirality is : Answer: 4. Enantiomers are superimposable mirror images on each other. Key idea: enantiomers are non-superimposable mirror images. Statement... 1
- Which statement regarding polymers is not correct ? Answer: 1. Thermosetting polymers are reusable. Key idea: thermosetting polymers are cross-linked and cannot be remoulded. Thermosetting pol... 1
- Match List - I with List - II.List - I : (a) 2SO_2(g) + O_2(g) → 2SO_3(g) (b) HOCl(g) h OH + Cl (c) CaCO_3 + H_2SO_4 → CaSO_4 + H_2O + CO_2 (d) NO_2(g) h NO(g) + O(g)List - II : (i) Acid rain (ii) Smog (iii) Ozone depletion (iv) Tropospheric pollutio… Answer: 3. (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii) Key idea: identify each reaction with its environmental context. (a) SO₂ oxidised to SO₃ i... 1
- The correct structure of 2,6-Dimethyl-dec-4-ene is : Answer: 1. [skeletal structure - see figure] Key idea: build the chain from the name: 10 carbons (dec), a double bond between C-4 and C-5, a... 1
- The compound which shows metamerism is : Answer: 4. C₄H₁₀O Key idea: metamerism occurs between compounds with the same functional group (usually ethers, ketones, secondary amines) b... 1
- Which one of the following polymers is prepared by addition polymerisation ? Answer: 1. Teflon Key idea: addition polymers form from unsaturated monomers by chain addition; condensation polymers lose small molecules.... 1
- A tertiary butyl carbocation is more stable than a secondary butyl carbocation because of which of the following ? Answer: 4. Hyperconjugation Methyl groups attached to the positive carbon stabilise it by hyperconjugation: C-H sigma electrons delocalise i... 1
- Elimination reaction of 2-Bromo-pentane to form pent-2-ene is :(a) β-Elimination reaction(b) Follows Zaitsev rule(c) Dehydrohalogenation reaction(d) Dehydration reaction Answer: 1. (a), (b), (c) 2-Bromopentane CH₃CHBrCH₂CH₂CH₃ loses HBr with alcoholic KOH. (a) H is removed from the carbon beta to the carbon b... 1
- The biodegradable polymer is : Answer: 2. nylon 2-nylon 6 Nylon 2-nylon 6 is a copolymer of glycine and amino caproic acid joined by peptide (amide) linkages that enzymes... 1
- The number of sigma (σ) and pi (π) bonds in pent-2-en-4-yne is : Answer: 1. 10 σ bonds and 3 π bonds Count every bond in pent-2-en-4-yne, CH₃-CH=CH-Cequiv CH . A single bond is one σ ; a double bond is 1 σ... 1
- Which of the following carbocations is expected to be most stable ? Answer: 2. cyclohexadienyl cation with NO ₂ at top carbon, H and Y on the sp ³ carbon, positive charge adjacent (picture) In the intermediat... 1
- Which of the following is correct with respect to -I effect of the substituents ? (R = alkyl) Answer: 3. - NR ₂ < - OR < - F The -I effect increases with the electronegativity of the atom attached to the carbon chain: N < O < F. Since... 1
- Which of the following molecules represents the order of hybridisation sp^2, sp^2, sp, sp from left to right atoms ? Answer: 3. CH ₂ =CH - C equiv CH A carbon in a double bond is sp² , one in a triple bond is sp . mathrmCH₂=CH-CequivCH has carbons sp²,sp²,s... 1
- The correct statement regarding electrophile is : Answer: 3. Electrophile can be either neutral or positively charged species and can form a bond by accepting a pair of electrons from a nucl... 1
- The IUPAC name of the compound OHC-CH(CH_3)-C(=O)-CH=CH-CH_3 (drawn in skeletal form) is ______. Answer: 4. 3-keto-2-methylhex-4-enal Key idea: the principal functional group is the aldehyde (-CHO), which gets C-1. Chain: OHC(1)-CH(CH₃)(... 1
- Which one of the following structure represents nylon 6,6 polymer? Answer: 3. [structure shown in figure] Key idea: nylon 6,6 is made from hexamethylenediamine and adipic acid, linked by amide bonds. Repeat... 1
- In pyrrole [figure: five-membered ring, N1 with H, carbons 2,3,4,5] the electron density is maximum on Answer: 3. 2 and 5 Key idea: in pyrrole the N lone pair is part of the aromatic sextet, so the ring carbons are electron-rich, most of all t... 1
- Which among the given molecules can exhibit tautomerism? Answer: 4. III only Key idea: keto-enol tautomerism needs an alpha-H, and the enol must be able to form (a double bond at a bridgehead is no... 1