Active Methylene Compounds
Lecture Notes on CARBOHYDRATE – 9 for B.Sc. (Hons. & Sub.) Degree PERIOD 18
Active Methylene Compounds
Methylene group having easily replacable H-atom is called active methylene group. compounds which contain such active methylene group are called active methylene compound.
H
C group
H flanked with two electron with drawing groups e.g >C=O , CN etc, becomes active and compound bearing is called active methylene compound. e.g. O H O
i. C2 H5 O&C&C& C&O &C2H5 Diethyl malonate (DEM)
H
H O
ii. NC&C& C&O &C2H5 Cyano acetic acid ester or, ethyl cyano acetate.
H
O H O
iii. H3C&C& C& C&O &C2H5 Ethyl acetoacetate (EAA)
H Reactivity of active methylene compounds are increased due to following electronic displacement:- O H O O H O¯ O H O¯ O H O¯
&C& C& C& &C& C= C& &C& C& C& &C& C& C&
+ +
H H H + H +
O¯ H O¯ O H O¯ O H O
&C = C= C& &C& C = C& &C& C& C&
+ H + H + H
Ques:- Active methylene compounds show nucleophilic reaction. why? In active methylene compounds H ion (neucleophile+ + ) is available. Any reagent which wants to Notes compiled by Dr. B. G. Thakur, Dept. of Chemistry, C. M. Sc. College, LNMU Darbhanga Lecture Notes on CARBOHYDRATE - 9 for B.Sc. (Hons. & Sub.) Degree PERIOD 18 combine with H ion are nucleophile. so, active methylene compound shows necleophilic reaction.
O H O O H O O H O¯ O H O¯
&C& C& C& &C& C& C& &C& C& C& &C& C= C&
+ H + H H + H +
+ O¯ H O¯ O H O O H O
&C = C C& &C& C = C& = &C& C& C&
+ H H H
Keto-enol toutomerism in EAA:- EAA shows the properties of ketonic and enolic group with one unsaturation point simultaneously. for example, it gives sobisulfite addition product with
NaHso3 and a cyanohydrin with HCN. These reaction show presence of a ketonic group in EAA.
On the other hand, it adds Br2 to its unsaturated centre and evolves hydrogen n with alkali-metal. These rk shows presence of >C= CH3-C-CH2COOC2H5 CH3-C=CH-COOC2H5 O (ketonic form-I) (enolic form-II) Both forms were separately isolated by knorr in 1911. Etherial solution of EAA on cooling upto 780C gives colourless crystals. This crystal corrosponds property to only ketone. But on passing dry HCl gas to the sod derivative of EAA at -780 enol form was obtained. Both forms are stable kept in quartz - vessel at -780C. They however at ordinary temp. rapidly mix-up and behave like ordinary EAA. In solution or in Liq. state it exists as a on equilibrium mixture of both forms. O O OH H O Notes compiled by Dr. B. G. Thakur, Dept. of Chemistry, C. M. Sc. College, LNMU Darbhanga Lecture Notes on CARBOHYDRATE - 9 for B.Sc. (Hons. & Sub.) Degree PERIOD 18 CH3-C-CH2-C-OC2H5 CH3-C=C-C-OC2H5 (I. 79% - 92%) (II. 21% - 8%) Both form are estabilished due to rapid substitution of a proton from one atom to another with a shift to double bond from >C=O to the adjacent -C-C bond (also called prototropy). The methyline group flanked with electron withdrawing group >C=O is activated. The enol form is stable due to interamolecular hydrogen bonding with respect to the keto tautomer. (K.H.Mayer 1920). Interconversion of these forms is greatly catalysed by acids and bases even in small amounts. Mechanism for keto-enol-tautomerism:- (a) Catalysis of acids :- O H H- O + OH &C& C& +H+ &C& + &C = C & -H H C& Keto form H enol form (b) Catolysis of base:- O O + &C& C& +:B BH + &C & C& H +:B en olate ion O OH + H &C & C& &C = C < enolic form From Composition of Keto - enol tautomeric mixture since, only the enolic form reacts rapidly with Br2 in ethanolic medium, benifit of the reaction is taken in determining the proportion of it i in mixture. Thus, we see and have known about this perticular type of to utomerism which involves dyanamic equtibrium between keto-form and enol form of a compund. Notes compiled by Dr. B. G. Thakur, Dept. of Chemistry, C. M. Sc. College, LNMU Darbhanga Lecture Notes on CARBOHYDRATE - 9 for B.Sc. (Hons. & Sub.) Degree PERIOD 18 Notes on - 1. Ketonic Hydrolysis- When EAA is heated with dil, HCl, or heated with dil aqueous or ethanolic alkali, followed by acidification and heating, the net result is the formation of ketone (acetone). O O O (H2O) CH3 - C - CH2 - C - O - C2H5 CH3-C- CH3 +C02 +C2H5 OH H O H This type of hydrolysis which proceeds by cleavage of the molecule so as to produce a ketone, is called Ketonic Hydrolysis or Ketone Cleavage. The hydrolysis of ester group first forms acetoacetic which decarboxylates rapidly upon heating to yield ketone. O O O O CH3 - C - CH2 - C - O - C2H5 + H2O CH3-C- CH2 C0H + C2H5OH acetoacetic ester O O O CH3 - C - CH2 - C - O - H CH3-C- CH3 +C02 acetone Mechanism :- Under the mild conditions, the nucleophile OH attacks only the more electropositive carbonyl carbon and the other carboxyl carbon remains practically unattacked. O O CH3 C CH2 C OC2H5 Notes compiled by Dr. B. G. Thakur, Dept. of Chemistry, C. M. Sc. College, LNMU Darbhanga Lecture Notes on CARBOHYDRATE - 9 for B.Sc. (Hons. & Sub.) Degree PERIOD 18 less electro (+)ve more electro (+)ve O O O O (i) CH3 - C - CH2 - C - OC2H5 + OH CH3-C- CH2 - C - C2H5 O H O O O O (ii) CH3 - C - CH2 - C - OC2H5 CH3-C- CH2 - C - O O H O O + H CH3-C- CH2 - C - OH H O O OH O -CO2 (iii) CH3 - C C =O CH3-C = CH2 CH3-C- CH3 C acetone H2 Notes compiled by Dr. B. G. Thakur, Dept. of Chemistry, C. M. Sc. College, LNMU Darbhanga