aldol condensation

(i) With hydroxylamine : Aldehydes and ketones react with hydroxylamine to give corresponding oximes containing (> C = N - OH) group. The reaction is carried in the presence of a base like NaOH.(a) Acetaldehyde with hydroxylamine gives acetaldoxime. h hI ________ICH3 - C = Jo + _H2]nOH ) CH3 - cf NOH + HzO *acetaldehyde acetaldoximeCH3 CH3 ....CH3 - c =€f +~H21NOH ) CH3 - c = NOH + H20i____.Z'i ; - acetone acetoxime (ii) With hydrazine : Aldehydes and ketones react with hydrazine to give corresponding hydrazones. h h 1 r-___„ * IH3C-C = jO + H2|- N-NH2-> H3C-C =N-NH2 + H2O acetaldehyde acetaldehyde hydrazone ch3 ch3 I .1 h3c - c =jo_ + JH2_j- n - nh2—> h3c - c = n - nh2 + h2oacetone acetone hydrazone(iii) With phenylhydrazine : Aldehydes and ketones react with phenylhydrazine to give the corresponding phenylhydrazone.(a) Acetaldehyde with phenylhydrazine gives acetaldehyde phenylhydrazone.H HI Ich3 - c = jo + H2j nnhc6h5 -4 ch3 - c = nnhc6h5 + h2oacetaldehyde acetaldehyde phenylhydrazone(b) Acetone with phenylhydrazine gives acetone phenylhydrazone.CH3 CH3I ICH3- C=iO + H^N -NH-CgHs —» CH3-C = N-NH_C6H5 +H20acetone acetone phenylhydrazone(iv) Aldol condensation : Aldehydes or ketones containing active a-hydrogen atom, i.e. hydrogen on the carbon atom adjacent to carbonyl group (a-carbon atom) give P-hydroxy aldehyde, i.e. aldol or p-hydroxy ketone respectively on warming with dilute solution of sodium hydroxide (10%) or sodium carbonate. This reaction is known as aldol condensation. Formaldehyde and benzaldehyde do not give this reaction as they have no a-hydrogen atom. Aldol contains an aldehydic group (-cho) and also alcoholic group (-oh). (Aid for aldehyde group and -ol for alcohol, hence the name aid + ol = aldol.)(a) This is an addition reaction. This addition takes place in such a way that a-carbon atom of one aldehyde molecule is attached to the carbonyl carbon of the second molecule and a-hydrogen of one aldehyde is attached to carbonyl oxygen of the other to give P-hydroxy aldehvde.Aldol loses one water molecule on heating with an acid to form unsaturated aldehyde.When aldol condensation takes place between two molecules of different aldehydes, then it is known as cross aldol condensation. h h I „ Na2C03 I „ch3ch2c =o+hch2c = o-> ch3ch2-cp-ch2-c = o| solution ( |h oh hpropionaldehyde acetaldehyde propionaldol(b) Certain ketones like acetone in presence of barium hydroxide or sodium hydroxide give diacetone alcohol (p-hydroxy ketone). CH3 H O CH3 H O I I II Ba(OH)2 J J II CH3 -C + H-c«-c-CH3---^ cha-pc-aC -C-CH3Oh oh hacetone acetone diacetone alcohol(4-hydroxy, 4 -methyl, pentan-2-one)