Polynuclear Hydrocarbons, Photochemistry, Pericyclic Reactions, and Spectroscopy of Organic Compounds — DU B.Sc (hons) Chemistry Previous Year Question Papers

DSC: Polynuclear Hydrocarbons, Photochemistry, Pericyclic Reactions, and Spectroscopy of Organic B.Sc. (Hons) Chemistry Attempt six questions in all. Attempt sub-parts of each question together. 1. (a) Sulphonation of naphthalene with cone. H2S04 at 40°C and l60°C yields two different products Explain using a suitable energy diagram. (b) (i) Anthracene is less aromatic and more reactive than benzene and naphthalene. Explain. (ii) How will you prepare anthracene from o-bromobenzyl bromide? (c) Carry out the following conversions : (Any two) (i) Anisole(methoxybenzene) to 7-methoxy-1-methylnaphthalene (ii) Naphthalene to anthracene 2. (a) (b) Define €max and explain why the intensity of 7t i 7i:* transitions are 10 to 100 times higher than n i 7t* transitions. Differentiate between a bathochromic and a hypsochromic shift using a suitable example. (c) (d) (e) Using Woodward-Fieser rules, calculate the ^max values for the following compounds : ock ock Hetero annular Dienes Ci,, a-unsaturated ketones (six-membered) Alkyl group (dienes and polyenes) Exocyclic Double bond Increments for extended conjugation Ch, a, y, 8 ring residues or alkyl groups for Ch, a-unsaturated carbonyl compound: + 10,12,18,18 nm respectively. Give the frequency ranges for the functional and fingerprint regions in the IR spectrum and explain their importance to IR spectroscopy, taking a suitable example. With the help of suitable diagrams, explain why alkenes absorb in the range 4.5 to 6.5 ppm, and aromatic compounds absorb in the range 6.5-8.0 ppm in the NMR spectrum, when both have c=C bonds. (3X5=15) 3. (a) What do you mean by the terms IR-active and IR-inactive? Give reasons and explain which of the following compounds would be IR active and which would be IR inactive? (b) How will you differentiate between the following compounds using IR spectroscopy? (i) CH2C02H and CH3COCH3 (c) Define chemical shift in ppm and give its mathematical expression. Explain why all protons in a molecule do not absorb at the same frequency in the NMR region? (5,5,5) 4. (a) (b) (c) Define coupling constant. Explain how you will distinguish between cz.s and frcz73s-crotonic acid (MecH=CHCOOH) by their NMR spectra? Discuss the correlation diagram for conrotatory and disrotatory cyclization of butadiene .to cyclobutene. Predict the product(s) of the following reactions : (a) The combination of a carbonyl compound and an alkene leads to the formation of a photoadduct. Give the name and discuss the mechanism of this reaction in detail. a) Complete the following reaction : Draw the aromatic transition state for each reaction and illustrate the product formation using curved arrow notation. Mention the name of the reactions. (c) Thermal [4+2] cycloaddition of 1,3-butadiene and ethylene is symmetry-allowed. Explain. (5 ,5 ,5) 6. (a) a)) Calculate DBE and write the structural formulae for the compounds with the following molecular formulae, showing the mentioned signals in their NMR spectra :