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Iii, a Unit Co I Aldehydes and Ketones Ii Carboxylic Acids

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Iii, a Unit Co I Aldehydes and Ketones Ii Carboxylic Acids STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 UNIT CONTENT PAGE Nr I ALDEHYDES AND KETONES 03 II CARBOXYLIC ACIDS & ACID DERIVATIVES 07 ORGANOMETALLIC COMPOUNDS AND ORGANO III 17 SULPHUR COMPOUNDS IV REACTIVE METHYLENE COMPOUNDS & TAUTOMERISM 22 V ALICYCLIC COMPOUNDS 23 Page 1 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 UNIT - I ALDEHYDES AND KETONES The Carbonyl Group A carbonyl group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom --> [C=O] The simplest carbonyl groups are aldehydes and ketones usually attached to another carbon compound. These structures can be found in many aromatic compounds contributing to smell and taste. Introduction Before going into anything in depth be sure to understand that the C=O entity itself is known as the "Carbonyl group" while the members of this group are called "carbonyl compounds" --> X-C=O. The carbon and oxygen are usually sp2 hybridized and planar. Carbonyl Group Double Bonds The double bonds in alkenes and double bonds in carbonyl groups are very different in terms of reactivity. The C=C is less reactive due to C=O electronegativity attributed to the oxygen and its two lone pairs of electrons. One pair of the oxygen lone pairs are located in 2s while the other pair are in 2p orbital where its axis is directed perpendicular to the direction of the pi orbitals. The carbonyl groups properties are directly tied to its electronic structure as well as geometric positioning. For example, the electronegativity of oxygen also polarizes the pi bond allowing the single bonded substituent connected to become electron withdrawing. Note: Both the pi bonds are in phase (top and botom blue ovals) The double bond lengths of a carbonyl group is about 1.2 angstroms and the strength is about 176-179 kcal/mol). It is possible to correlate the length of a carbonyl bond with its polarity; the longer the bond meaing the lower the polarity. For example, the bond length in C=O is larger in acetaldehyde than in formaldehyde (this of course takes into account the inductive effect of CH3 in the compound). Knoevenagel reaction The Knoevenagel condensation is an organic reaction used to convert an aldehyde or ketone and an activated methylene to a substituted olefin using an amine base as a catalyst. The reaction begins by deprotonation of the activated methylene by the base to give a resonance stabilized enolate. The amine catalyst also reacts with the aldehyde or ketone to form an iminium ion intermediate, which then gets attacked by the enolate. The intermediate compound formed gets deprotonated by the base to give another enolate while the amine of the intermediate gets protonated. A rearrangement then ensures which releases the amine base, regenerates the catalyst, and yields the final olefin product An enol intermediate is formed initially: Page 2 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 This enol reacts with the aldehyde, and the resulting aldol undergoes subsequent base-induced elimination: A reasonable variation of the mechanism, in which piperidine acts as organocatalyst, involves the corresponding iminium intermediate as the acceptor: Page 3 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 The Doebner-Modification in refluxing pyridine effects concerted decarboxylation and elimination: Wolff-Kishner Reduction The reduction of aldehydes and ketones to alkanes. Condensation of the carbonyl compound with hydrazine forms the hydrazone, and treatment with base induces the reduction of the carbon coupled with oxidation of the hydrazine to gaseous nitrogen, to yield the corresponding alkane. The Clemmensen Reduction can effect a similar conversion under strongly acidic conditions, and is useful if the starting material is base-labile. Mechanism of the Wolff-Kishner Reduction Page 4 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 Wittig reaction The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphoniumylide (often called a Wittig reagent) to give an alkene and triphenylphosphine oxide. Mechanism of the Wittig reaction Following the initial carbon-carbon bond formation, two intermediates have been identified for the Wittig reaction, a dipolar charge-separated species called a betaine and a four-membered heterocyclic structure referred to as an oxaphosphatane. Cleavage of the oxaphosphatane to alkene and phosphine oxide products is exothermic and irreversible. 1) Nucleophillic attack on the carbonyl Page 5 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 2) Formation of a 4 membered ring 3) Formation of the alkene MEERWEIN-PONDORF-VERLEY REDUCTION - DEFINITION The reduction of ketones and aldehydes to their corresponding alcohols using Aluminium alkoxide catalyst in the presence of a sacrificial alcohol is called as Meerwein- Pondorf-Verley reaction. Mechanism The MPV reduction is believed to go through a catalytic cycle involving a six-member ring transition state as shown in Figure 2. Starting with the aluminium alkoxide 1, a carbonyl oxygen is coordinated to achieve the tetra coordinated aluminium intermediate 2. Between intermediates 2 and 3 the hydride is transferred to the carbonyl from the alkoxy ligand via a pericyclic mechanism. At this point the new carbonyl dissociates and gives the tricoordinated aluminium species 4. Finally, an alcohol from solution displaces the newly reduced carbonyl to regenerate the catalyst 1. Figure, Catalytic cycle of Meerwein–Ponndorf–Verley reduction Page 6 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 UNIT - II CARBOXYLIC ACIDS & ACID DERIVATIVES Acid Strength The strength of the carboxylic acid (and any other Bronsted acid, for that matter), is related to the 'stability' of its conjugate base, the carboxylate anion. The carboxylic acid and the carboxylate anion are in equilibrium with one another, and the relative acidity of carboxylic acids depends upon the position of this equilibrium. One of the main reasons why carboxylic acids are acidic is due to the ability of the charge to be delocalised around the pi-system: However, in addition to this delocalisation about the pi system, extra stability can be gained by the presence of electron withdrawing groups adjacent to the carboxylate group (i.e. -I groups attached to the R group in the diagram above). These electron withdrawing groups draw electron density away from the carboxylate anion via the sigma system (the single bonds). This is advantageous as the stability increases with the number of atoms it can be spread across (i.e. how diffuse the charge is). Electron releasing groups (+I as you call them) destabilise, by increasing the electron density around the carboxylate group, which of course makes it less favourable for the carboxylic acid to lose a proton and hence makes it less acidic. The +I groups aren't involved in breaking the O-H bond to any great extent, as you allude to. This is because when the proton leaves, it transfers its electron density to the oxygen it was attached to, cleaving the bond and allowing it to leave as H+. If the +I groups were donating into this bond, you would have to have the H leaving as H-, which is rarely observed. Hell-Volhard-Zelinsky Reaction Treatment with bromine and a catalytic amount of phosphorus leads to the selective α- bromination of carboxylic acids. Mechanism of the Hell-Volhard-Zelinsky Reaction Phosphorus reacts with bromine to give phosphorus tribromide, and in the first step this converts the carboxylic acid into an acyl bromide. An acyl bromide can readily exist in the enol form, and this tautomer is rapidly brominated at the α-carbon. The monobrominated compound is much less nucleophilic, so the Page 7 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 reaction stops at this stage. This acyl intermediate compound can undergo bromide exchange with unreacted carboxylic acid via the anhydride, which allows the catalytic cycle to continue until the conversion is complete. Lactic Acid. Lactic acid is the main constituent of milk that has gone sour and hence its name (L. Lactis=milk). Preparation. (1) By the hydrolysis of acetaldehyde cyanohydrin. From Molasses. (2) Lactic acid is manufactured by fermentation of molasses (or milk whey containing lactose) by the microorganism Bacillus acidilactiti (BAL). Page 8 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 A dilute solution of molasses (or whey) is treated with BAL (sour milk). The fermentation is carried at 35-40°C in the presence of CaCO3, As the lactic acid is produced, it reacts with CaCO3, to form calcium lactate. The calcium lactate is filtered off and decomposed with calculated quantity of dilute H2SO4. The insoluble Calcium sulphate is removed and the lactic acid set free in the solution is recovered by distillation in vacuo. The product is D-lactic acid. Physical Properties. Lactic acid is a colourless, crystalline solid, mp 53°C, and has a sour taste. The acid obtained from molasses is D-isomer. The synthetic product is racemic lactic acid, mp 18°C, specific rotation [α] = +3.82, Lactic acid is soluble in water, alcohol, and ether. Chemical Properties. Lactic acid molecule contains a secondary alcohol group (>CHOH) and a carboxyl group (COOH), and gives reactions of both. Reaction Involving COOH Group. (1)Formtion of salt. It reacts with alkalis to form salts. Reaction Involving OH Group. Page 9 of 28 STUDY MATERIAL FOR B.SC CHEMISTRY ORGANIC CHEMISTRY - II SEMESTER - III, ACADEMIC YEAR 2020-21 (2) Reaction with CH3COCl.
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