Page 1 of 9. Sessional Examination (November 2017) Max Marks: 20 Date: Time: One Hour. Model Answers

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1 Page 1 of 9 Sessional Examination (November 2017) Class: B. Pharm-II yr (III sem) Subject: Pharma Org. Chem-II Max Marks: 20 Date: Time: One Hour Model Answers Q. 1. Solve the following (ANY SIX) 06 marks a) Draw the resonance structure of naphthalene b) Define isomer and enantiomer with one example Answer: Isomers are compounds that have the same molecular formula, but different structural formula. e. g : Propene, C3H6, has 2 isomers, prop-1-ene and prop-2-ene. Enantiomer are the compounds which are mirror images of each other and non super imposable e.g. (R) Lactic acid and (S) lactic acid c) Give the position of electrophilic substitution in naphthalene and pyridine Answer: Electrophilic substitution in naphthalene takes place at alpha (1) position where as in pyridine it takes place at 3-position d) What is Wolff Kishner reduction? Give its general reaction. Answer: The reduction of aldehydes and ketones to alkanes in the presence of reducing agent hydrazine hydrate and base is known as wolff krishner reduction. The general reaction is given below

2 Page 2 of 9 e) Identify erythro, threo and meso compound from following Answer: Compound A is erythro, compound B is threo and compound C is Meso f) What is Diel s Alder reaction? Give one example Answer: It is the [4+2]-cycloaddition of a conjugated diene and a dienophile (an alkene or alkyne), that involves the 4 π-electrons of the diene and 2 π-electrons of the dienophile. The driving force of the reaction is the formation of new σ-bonds, which are energetically more stable than the π-bonds. The reaction is given below: g) Mention any four reducing and oxidizing agents Answer : Oxidizing agents 1. Potassium permanganate 2. Potassium dichromate 3. Hydrogen peroxide 4. Peroxy acid Reducing agents: 1. Sodium borohydrate 2. Lithium aluminum hydride 3. Hydrogen in the presence of Matals 4. Hydrazine hydrate h) Give R and S nomenclature to following compounds

3 Page 3 of 9 Answer: Compound I is (S) and compound II is (S) i) Draw the structures of pyrazole, pyridine, Benzo[b]thiophen, thiophene Answer: Q.2. Solve the following (ANY TWO) 06 Marks a) I) Differentiate between conformation and configuration Conformation Conformation is any of the spatial arrangements which the atoms in a molecule may adopt and freely convert between, especially by rotation about individual single bonds. Position of atoms or groups may change It is given by eclipsed, guache, anti One form of conformation can change into another form Example configuration Configuration is an arrangement of parts or elements around chiral carbon or C=C double bond Position of atoms or groups is fixed It is given by L /D and R/S and E/Z One form of configuration can not change into another form Example

4 Page 4 of 9 II) Give E/Z configuration to following:. Answer: A) E, B) E and C) E b) Identify the reaction and give its mechanism: Answer : The reaction is Beckmann rearrangement. It is the migration of group towards electron deficient nitrogen. An acid-induced rearrangement of oximes to give amides. Mechanism: The Beckmann rearrangement is achieved by converting the oxygen of the oxime to a good leaving group, and then heating. Treatment of the ketone with NH2OH forms the oxime, bwhich on heating with acid leads to the Beckmann rearrangement The first step in the process is formation of an oxime from the aldehyde or ketone, which occurs in a sequence similar to formation of imines and hydra zones. Then, the lone pair of nitrogen displaces the hydroxide in an elimination reaction and then nitrogen is deprotonated by base to give the oxime. The mechanism is given below: Amide

5 Page 5 of 9 c) What is resolution? Enlist various methods of resolution and explain any one method Answer: Resolution is any process by which a mixture called a racemate is separated into its two constituent enantiomers. Method of resolution are given below 1. Resolution by Salt formation 2. Mechanical method 3. Biological method 4. Chromatographic method 5. Chemical method Chemical method (i.e. by formation of diasteriomers: ) A common way of separation of enantiomers uses the conversion into diastereomers, that are not mirror images of each other. The principle of method is that the enantiomer in the form of racemic mixture are converted into the diasteriomer by using resolving agent, which lead to formation of diasteriomer and then it is separated on the basis of solubility (diasteriomer have different solubility). The enantiomer are liberated from it by suitable method. Example: The two enantiomers 1-phenylethylamine (S-(-), R-(+)) are separated using (L)-(+)-tartaric acid (also known as the (R,R)-form) as resolving agent. The two salts formed possess different cation ions are therefore not enantiomers of each other anymore. The (S)-(1)-phenylethyl ammonium- (R,R)-tartrate salt crystallizes faster than the (R)-(1)-phenylethyl ammonium-(r,r)-tartrate salt. The reactions are shown below.

6 Page 6 of 9 d) Identify the reaction and explain the mechanism of it. Answer: The above reaction is Skraup quinoline synthesis. The mechanism is given below: The Skraup synthesis is a chemical reaction used to synthesize quinolines. It is named after the Czech chemist Zdenko Hans Skraup. In the reaction, aniline is heated with sulfuric acid, glycerol, and an oxidizing agent such as nitrobenzene to yield quinolines. The glycerol undergo the dehydration to form acrolein. The attack og nucleophilic amine group on the acrolein lead to formation of intermediate which undergo further reaction and cyclization to form quinioline. Q.3. Solve the following (ANY ONE) 08 Marks A) i) What is Grignard reagent? How it is prepared? Explain application of it with reactions.

7 Page 7 of 9 Answer: The Grignard reaction is an organometallic chemical reaction in which alkyl, vinyl, or arylmagnesium halides add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon carbon bonds. The alkyl, vinyl, or arylmagnesium halides is known as Grignard reagents Preparation of Grignard reagents: It is form via the reaction of an alkyl or aryl halide with magnesium metal. The reaction is conducted by adding the organic halide to a suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound. During the reaction the dry condition and dry reagent should be used otherwise the reaction will lead to from magnesium hydroxide instead of Grignard reagent. The reaction is R X + Mg RMgX Ar X + Mg ArMgX Applications: (Write one reaction in each of the following application) 1. Addition of Grignard Reagents To Epoxides 2. Reaction of Grignards With Aldehydes and Ketones 3. Reaction of Grignard Reagents With Esters 4. Formation of alkanes 5. Formation of carboxylic acid ii) What is pinacol-pinacolone rearrangement? Discuss its mechanism and migratory aptitude. Pinacol Rearrangement is also called as Pinacol-Pinacolone rearrangement. It is an acid catalyzed organic chemical reaction in which 1, 2-diols are converted to carbonyl compounds. The name comes from the reactant and the product that is pinacol to pinacolone. Mechanism: This reaction takes place in variety of fully substituted 1,2-diols, through formation of carbocation intermediate that subsequently undergoes a rearrangement by 1,2-alkyl/aryl shift to produce the carbonyl compound. If two of the substituent form a ring, the Pinacol Rearrangement can constitute a ring-expansion or ring-contraction reaction. Example:

8 Page 8 of 9 In the first step, the hydroxide ion gets protonated in the presence of acid. Carbocation is formed after the removal of water molecule and it is stable because it is tertiary carbocation. Rearrangement takes place and the methyl group gets shifted to the positive carbon. Hydrogen ion is removed so that the pinacolone is generated. Migratory aptitude: In this case the molecule is symmetric and alkyl migration is the only reaction pathway available. Alkyl migration can 1) help stabilize the developing carbocation center and 2) once fully migrated, produces a resonance stabilized hydroxyl-carbocation. If one or more of the methyl groups is replaced with some other substituent (for example, alkyl, hydrogen, aryl), then our reaction possibilities become more complex, which will depend on the the ability of the migrating group to stabilize positive charge plays a role. The relative order of migratory aptitude of groups in pinacol-pinacolone rearrangement is: H>Ph>Me3C>MeCH2>Me B) i) Write a note on Paal Knorr synthesis The Paal Knorr Synthesis used for the synthesis of furans, pyrroles, or thiophenes from 1,4- diketones. It is a synthetically valuable method for obtaining substituted furans, thiophenes and pyrroles. Write the three reactions and give their explanation with examples

9 Page 9 of 9 ii) Why chair conformation of cylcohexane is more stable than boat conformation? Discuss briefly conformational analysis of n-butane. Chair conformation of cylcohexane is more stable than boat conformation The chair is the most stable conformation of cyclohexane because in the chair conformation torsional strain and other strain are not present, which is available in the boat conformation due to crowding of hydrogen atom which come close in boat geometry. conformational analysis of n-butane: Explain the gauche, staggered and anti conformation in butane. Show that anti is more stable than other. Draw the Newman projection formula for all three conformations. Also support your answer with energy diagram