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(Sree MEV o Tp, | 0., Ay !, I, , CS, Nut eee RCN \, Ny 2 i of (Ada, 10 6:, - A +:, , R-C-Nu ——* —-R-©, h | u, Step 1: Nucleophilic addition. As in the case of aldehydes and ketones, the carbony,, , i i ivati i i ili k at the carb on, resent in acid res is highly susceptible to nucleophilic attac tbonyi <2, P derivatives is highly P ‘A trons or accommodate negative charge Ty., , This is, ? ili en toacquire e, CE eT nn i the nucleophilic attack on the carbonyl ca elo, , step 1 of nucleophilic acyl substitution is similar to |, sidehydes and analy It involves the addition of nucleophile to the carbonyl! carbon to f a, tetrahedral intermediate. During this process, the trigonal carbon (sp hybridised) of the ,', , (or carbony!) group of the starting compound is converted into tetrahedral carbon (sp’ hybridise), , the intermediate., Step 2: Elimination of leaving, , (Eliming,, , , , na, , group. The tetrahedral intermediate formed instep 1 elimina,, j the group Y (originally present in the acid derivative) in the form of a negative ion, : Y”. Due tot, elimination; the carbon-oxygen double bond is re-formed and the substitution product is generatey, The reason for the occurrence of this step is that the groups originally attached to carbon, , carbon inacid derivatives are all good leaving groups. For example, the chlorine atom of acid chloride, , is an excellent leaving group and is readily eliminated as a chloride ion (: Cl) as soon as tetrahedrd, addition intermediate is formed., , In general, the ease with which a leaving group is eliminated depends upon its basicity: tk, , weaker the base, the better it would be as a leaving group. In the nucleophilic acyl substitution of adi, chlorides, acid anhydrides, esters and amides, the leaving group (: Y~) is equivalent toCT, RCOO,, R’O and NII} respectively. Now CI’ isa very weak base, RCOO" is a.moderately weak base whit, R‘O and NH, are strong bases. Therefore, the order of ease of leaving groups is:, , Cl > RCOO: > R'O" > NH,, Inconclusion, the complete mechanism of nucleophilic acy! substitution may be summed ups, , , , awe, , , , follows:, i", ?Nuv, R ., S. <6 Addition ARS Elimination ue vs: 0: + Y, AR tt ly Qo 2 ee GH, Acyl . , nt Sees savin,, Anionic Substitution Lea, compound x, intermediate product gro?, sist, , which is rate determining..., , The overall rate of reaction is sometimes affected by the rate of both the steps but generally itis the fi, , Scanned with CamScanner