Aldehydes and Ketones Final

(CARBONYL COMPOUNDS)

Alde Aldehy hyde dess and and keto keton nes cont conta ain the the same same func functi tion ona al grou group p, the the carb carbon onyl yl grou group p (>C= (>C=O) O).. In aldeh ldehyd ydes es the the carb carbon onyl yl grou group p is atta attach ched ed eith either er to two two hydr hydrog ogen en atom atomss (as (as in form formal alde dehy hyde de)) or to one one hydr hydrog ogen en atom atom and and one one alk alkyl yl grou group; p; whil while e in keto ketone ness the the carb carbon onyl yl grou group p is alwa always ys atta attach ched ed to two two alky alkyll grou groups ps..

Like Like the the carbo carbonn-ca carb rbon on doub double le bond bond of alke alkene nes, s, the carb carbo on-ox -oxyge ygen double of the carb carbo onyl grou roup is comp compo osed of one one

and

bond. In the carbonyl group, carbon atom is in a state of sp 2 hybridization. The

C-O

bond is produced by overlap of a sp 2 orb orbital ital of car carbo bon n with with a p-or p-orb bita ital of 

oxyg xygen. en. On On th the othe ther hand, the the C-O

bond is forme rmed by th the sid sideways over verlap lap

of p-orb -orbit ita als of carbo rbon and oxyg xygen. Th The remain mainin ing g two two sp 2 orbit orbitals als of carbon carbon form

bond with th the ss-orbital of hydrogen or sp 2 orbi orbita tals ls of carb carbon on of the the alky alkyll

grou group. p. Now Now sinc since e the the thre three e

bond bondss of of the the carb carbon onyl yl carb carbon on util utiliz ize e sp 2 orbitals,

they they lie lie in one one plane lane and and are are 120 120 0 apar apartt (sim (simililar arit ityy with with C=C) C=C).) .) Howe Howeve ver, r, itit is impo import rtan antt to no note that that the the carb carbon on oxyg oxygen en doub double le bond bond is differ differen entt from from carbo carbonn-ca carbo rbon n dou doubl ble e bond bond.. Due Due to greate greaterr elect electron roneg egati ativi vity ty of  oxygen oxygen atom, atom, the the π-electron -electron cloud is attached attached towards towards oxygen. Consequen Consequently tly oxyg oxygen en atta attain inss a part partia iall nega negati tive ve char charge ge and and carb carbon on a part partia iall posi positi tive ve char charge ge..

This Th is pola polarr natu nature re of the the carb carbon onyl yl grou group p caus cause e inte interm rmol olec ecul ular ar attr attrac acti tion on in alde aldehy hyde dess or keto ketone ness and and henc hence e acco accoun unts ts for for thei theirr high higher er boil boilin ing g poin points ts than than that that of hydr hydroc ocar arbo bons ns and and ethe ethers rs of comp compar arab able le mole molecu cula larr weig weight ht.. More Moreov over er,, polar polar nature nature of the carbonyl carbonyl group group also explain explainss the dipole dipole moment moment in alde aldehy hyde dess and and keto ketone nes. s. Howe Howeve verr the the high high valu values es of dipo dipole le mome moment ntss (2.3 (2.3-2 -2.8 .8 D) of alde aldeh hydes ydes and and keto ketone ness cann cannot ot be acco accoun unte ted d for for only only by indu induct ctiv ive e effe effect ct;; this this can can be be acco accoun unte ted d for for if carb carbon onyl yl grou group p is a res resonan onance ce hybrid hybrid of the the two two struct structure ures. s.

grou group. p. Now Now sinc since e the the thre three e

bond bondss of of the the carb carbon onyl yl carb carbon on util utiliz ize e sp 2 orbitals,

they they lie lie in one one plane lane and and are are 120 120 0 apar apartt (sim (simililar arit ityy with with C=C) C=C).) .) Howe Howeve ver, r, itit is impo import rtan antt to no note that that the the carb carbon on oxyg oxygen en doub double le bond bond is differ differen entt from from carbo carbonn-ca carbo rbon n dou doubl ble e bond bond.. Due Due to greate greaterr elect electron roneg egati ativi vity ty of  oxygen oxygen atom, atom, the the π-electron -electron cloud is attached attached towards towards oxygen. Consequen Consequently tly oxyg oxygen en atta attain inss a part partia iall nega negati tive ve char charge ge and and carb carbon on a part partia iall posi positi tive ve char charge ge..

This Th is pola polarr natu nature re of the the carb carbon onyl yl grou group p caus cause e inte interm rmol olec ecul ular ar attr attrac acti tion on in alde aldehy hyde dess or keto ketone ness and and henc hence e acco accoun unts ts for for thei theirr high higher er boil boilin ing g poin points ts than than that that of hydr hydroc ocar arbo bons ns and and ethe ethers rs of comp compar arab able le mole molecu cula larr weig weight ht.. More Moreov over er,, polar polar nature nature of the carbonyl carbonyl group group also explain explainss the dipole dipole moment moment in alde aldehy hyde dess and and keto ketone nes. s. Howe Howeve verr the the high high valu values es of dipo dipole le mome moment ntss (2.3 (2.3-2 -2.8 .8 D) of alde aldeh hydes ydes and and keto ketone ness cann cannot ot be acco accoun unte ted d for for only only by indu induct ctiv ive e effe effect ct;; this this can can be be acco accoun unte ted d for for if carb carbon onyl yl grou group p is a res resonan onance ce hybrid hybrid of the the two two struct structure ures. s.

Reso Resona nanc nce e in carb carbon onyl yl grou group p also also expl explai ain n the the sho short rter er C=O C=O bond bond (and (and henc hence e greater bond energy) energy) than the C=C C=C Othe Otherr impo import rtan antt diff differ eren ence ce in carb carbon on-o -oxy xyge gen n and and carb carbon on-c -car arbo bon n doub double le bond bondss lies lies in the the fact fact that that the the carb carbon onyl yl grou group p unde underg rgoe oess nucl nucleo eoph phililic ic addi additi tion on reac reacti tion on whil while e olef olefin inic ic unde underg rgoe oess elec electr trop ophi hililicc addi additi tion on reac reacti tion onss Aldeh Aldehyd ydes es show show

and

.

Alde Aldehy hyde dess and and keto keton nes are are func functi tion onal al iso isomers mers of oxir oxiran anes es (cycli (cyclicc ethers ethers), ), unsa unsatur turate ated d alcoh alcohol olss and and unsat unsatura urated ted ethe ethers. rs.

Ketones show

,

isom isomer eris ism m are give given n ab abov ove e in alde aldehy hyde des. s.

. Examples, of functional 

Primary alcohols form aldehydes while secondary alcohols form ketones.

Controlled oxidation can be carried out by using CrO 3-pyridine

Controlled oxidation of alcohols can also be done by pyridinium dichromate (PDC) or pyridinium chlorochromate PCC which is a mixture of pyridine CrO 3 and HCl in 1:1:1 ration. This reagent also does not attack double bonds.

.

. Secondary alcohols can be oxidized to ketones by aluminum ter-butoxide in presence of acetone.

Secondary alcohols are oxidized to ketones and acetone is reduced to isopropanol (secondary alcohol). Unsaturated secondary alcohols are oxidized to unsaturated ketones.

Calcium formate, on pyrolysis, give formaldehyde calcium formate with calcium salt of any other fatty acid gives aldehydes; calcium salts of fatty acids other than calcium formate yield ketones.

Calcium salts of dibasic acids, on heating gives cyclic ketones

Instead of using calcium salt of an acid, vapours of acid or mixture of acids can be passed over heated MnO at 300 0C.

It is believed that here carboxylic are first converted into manganese salts which decomposes to form aldehyde or ketone.

Alde Aldeh hydes ydes are are form formed ed when when the the tri trip ple bond ond pres presen entt on the the term termin inal al carb carbon on ato atom, howe howeve ver, r, keto ketone ness are form formed ed when when the the trip triple le bond bond is prese present nt on nonnon-te term rmin inal al carb carbon on..

Howe Howeve ver, r, reme rememb mber er that that viny vinyll bora borane ness form formed ed from from term termin inal al alky alkyne ness (use (used d for for prep prepar arin ing g) stil stilll have have one one hydr hydrog ogen en atom atom that that reac reactt with with fres fresh h mole molecu cule le of  dibo dibora ranc nce e to low low yiel yield d of alde aldehy hyde de.. Thu Thus, s, it is advi advisa sabl ble e to use use ste steri rica callllyy hind hinder ered ed alky alkyll bora borane ne inst instea ead d of dibo dibora rane ne,, espe especi cial ally ly duri during ng prep prepar arat atio ion n of  aldeh aldehyde ydes. s. On One e such such steric sterical ally ly hind hindere ered d alkyl alkyl boran borane e is disia disiamyl myl boran borane. e.

Gem -Dih -Dihal alid ides es havi having ng two two halo haloge gen n

atom atomss on the the term termin inal al carb carbon on atom atom give give alde aldehy hyde des, s, whil while e gem   –dihalides, having two two halo haloge gen n atom atomss on nonnon-ca carb rbon on give give keto ketone nes. s.

This Th is meth method od is not not used sed much much beca becaus use e aldeh ldehyd ydes es are are affe affect cte ed by alkal lkalii and and dihal dihalide idess are usu usual ally ly prepa prepared red from from the the carbo carbonyl nyl compo compoun unds ds them themsel selves ves.. Hydr Hydrog ogen en cyan cyanid ide e give givess alde aldehy hyde des, s, whil while e alky alkyll cyan cyanid ides es give give keto ketone nes. s. From From acid acid chlo chlori ride des, s, keto ketone ness can can best best be prep prepar ared ed by usin using g weake eakerr organ organome ometa talli llicc reage reagent, nt, e.g e.g.. lithi lithium um dialk dialkyc ycupr uprate ate or dialk dialkyl yl cadm cadmium ium..

Acid Acid chlo chlori ride dess can can be redu reduce ced d into into aldeh ldehyd ydes es wit with hydr hydrog ogen en in boil boilin ing g xyle xylene ne usin using g pall pallad adiu ium m as a cataly catalystst-sup suppo porte rted d on bariu barium m sulph sulphat ate. e. Th This is reac reactio tion n is calle called d and and used used for for prep prepar arin ing g

but but not ketone ketones. s.

Bar Barium ium sulp sulph hate acts as a poison son for Pd catalys lyst and preve reven nts ree reeducat cation ion RCHO RCHO to RCH RCH2OH. OH. Quin Quinol olin ine e and and sulp sulph hur are are bett better er pois poison onin ing g agen agents ts for for Pd cata cataly lyst st.. Fo Form rmal alde dehy hyde de cann cannot ot be pre prepa pare red d meth method od sinc since e form formyl yl chlo chlori ride de is unst unstab able le at room room temp temper erat atur ure. e. Acid Acid chlor chloride idess are are read readily ily reduce reduced d to aldeh aldehyd ydes es by lithiu lithium m tritri-te terr butoxyaluminum butoxyaluminum hydride, hydride, LiAl(OCMe LiAl(OCMe 3)3H or tritri-nn-bu buty tyll tin tin hydr hydrid ide, e, Sn(C Sn(C 4H9)3H.

Este Esters rs can can be redu reduce ced d easi easily ly to alde aldehy hyde dess by sodi sodium um alum alumin iniu ium m hydr hydrid ide e NaAlH4 or di-isob di-isobuty utyll alumunium alumunium hydride hydride (DIBAL-H (DIBAL-H), ), Al[(CH Al[(CH 3 )2 CH2CH2]2H.

(

). Nitri itrile less when hen redu reduce ced d by mean meanss of 

stannous stannous chloride and hydrochlo hydrochloric ric acid in absolute absolute ether ether followed followed by hydrolysis hydrolysis yield aldehydes. aldehydes. This This reaction reaction is known

Nitriles can also be reduced selectively by di-isobutyl aluminum hydride to imines which upon hydrolysis gives aldehydes.

Ketones cannot be prepared by this method. (i) Oxo process:

The net reaction appears to be an addition of formaldehyde through antiMarkownikoff rule; this reaction in known as and applied for the preparations of aldehydes only. (ii) Wacker process:

Ketones (but not aldehydes) are prepared by the ketonic hydrolysis of acetoacetic ester or its alkyl derivatives by heating with dil. aq. acid or dil. alcoholic solution of alkali.

e.g. benzaldehyde can be prepared by (i) Boiling benzyl chloride with a solution of cupric or head nitrate

(ii) Oxidizing toluene with chromium trioxide in presence of acetic anhydride to  trap benzaldehyde acetate and thus avoid its oxidation to benzoic acid 

(iii) Treating toluene with chloride in carbon tetrachloride and decomposing the complex precipitated with water

Remember that side chains bigger that CH 3 are oxidized by Etard reaction at the end carbon atom. e.g.

Partial oxidation of toluene can also be brought about by (a) manganese dioxide and 65% H2SO4 at 310K (vapour phase oxidation) or catalytic oxidation with air diluted with nitrogen at 770 K in presence of oxides of Mn, Mo or Zr.

These methods constitute industrial methods for the preparation of  benzaldehyde. (iv) Treating benzene (aromatic compound) with mixture of carbon monoxide and dry HCl gas under pressure and in the presence of anhydrous AlCl 3

Since formyl chloride (HCOCl) is unstable, formyl group (-CHO) can be introduced in the form CO+HCl or HCN+HCl.

(v) Treating benzene or an aromatic compound having activating group (like –OH, -OC2H5 etc.) with a mixture of hydrogen cyanide and hydrogen chloride in the presence of anhydrous AlCl 3 or ZnCl2

(vi)

this reaction involves the conversion of aromatic

compounds to aldehydes in the presence of a 2 0 amine and formic acid.

are prepared by Friedel –Craft acylation

(c) For the preparation of the ketones of the type Ar. CO. Ar’ if one of the aryl contains deactivating group, it should be present in the acid chloride moiety. For example,

The alternate reactants i.e. nitrobenzene and benzoyl chloride cannot be used because strongly deactivating nitro group prevents the acylation reaction.

Aromatic ketones (as well as aliphatic ketones) can be prepared by treating the acid chloride with dimethyl cadmium.

The use of dimethyl cadmium is preferred over the use of Grignard reagents because the product (ketones) does not the react with dimethyl cadmium The extreme ease with which B-keto acids undergo decarboxylation is applied for the preparation of ketones (aliphatic as well as aromatic).

Lower aldehydes and ketones are soluble in water due to hydrogen bonding between negative oxygen of carbonyl group and positive hydrogen of water. Higher members (having more than five carbon atoms) are practically insoluble in water, but soluble in organic solvents like alcohol and ether.

2. Aldehydes and ketones have higher boiling points as compared to corresponding alkenes. This is due to

between the two

carbonyl groups which are stronger forces than the van der Waals forces existing in alkanes.

Further, aldehydes and ketones cannot form intermolecular hydrogen bonds with each other which are stronger forces than the dipole-dipole attraction hence they have lower boiling points than the corresponding alcohols which can easily form hydrogen bonds. Thus boiling points of aldehydes and ketones are  higher than hydrocarbons but lower than alcohols of comparable masses.

3. Aldehydes and ketones have larger dipole moments than alkyl halides and ethers confirming that a dipolar structure, C +-O- contributes to the structure of  aldehydes and ketones.

All these are examples of nucleophilic (nucleus loving) additions i.e. addition of  nucleophiles (electron rich species) on electron deficits atoms. Since the mobile electrons of carbons-oxygen double bond are strongly pulled towards oxygen, carbonyl carbon is electron-deficient and carbonyl oxygen is electron-rich. The electron deficient (acidic) carbonyl carbon is most susceptible to attack by electron rich nucleophilic reagents, that is, by bases. Hence

Note that in the transition state, oxygen has started acquiring negative charge which it will have bear in the product. Actually, (i.e. its ability to carry a negative charge) The polarity of the carbonyl group is not the cause of reactivity; t is simply another manifestation of  the electronegativity of oxygen.

The reactivity of the carbonyl group towards the nucleophilic addition of  the reactions depends upon the and also the site where nucleophile attacks. Thus, substituent or factor in the carbonyl compound that increases the positive charge on the carbonyl carbon (i.e. electronegative group) will increase its reactivity towards addition reactions and vice versa. Hence the introduction of alkyl group or any other electron donating group on the carbonyl carbon decreases its reactivity; thus formaldehyde (having no group) is more reactive than other aldehydes (having one alkyl group) which in turn are more reactive than ketones (having two alkyl group), i.e.

Similarly, among substituted aldehydes, having –I group,

Among ketones the reactivity decreases with the increase in + I effect of the alkyl group and also with increase in bulkiness of the alkyl group on carbonyl carbon.

Since in

Nucleophilic additions to aldehydes and ketones are catalyzed by acids (sometimes, by Lewis acids). In presence of acid, carbonyl oxygen gets protonated. This prior protonation increase the electrophilic character of the carbonyl, carbon and thus lowers the E act for nucleophilic attack, since it permits oxygen to accept π electrons without having a negative charge.

(Undergoes nucleophilic attack more readily) Aldehydes and ketones react with water in presence of acid or base to form hydrate.

Like the general nucleophilic additions, hydrate formation follows the following order. Aldehydes react with alcohols in presence of dry HCl gas to acetals, e.g.

Since the reaction is reversible, therefore excess of alcohol is used to shift the equilibrium towards acetals formation. Acetals are readily cleaved by acids and are stable towards base.

Ketones however, do not react with monohydric alcohols. Of course, ketones and aldehydes react with dihydric alcohols to form cyclic ketals and cyclic acetals respectively.

Acetals (cyclic acetals) and ketals (cyclic ketals) are used protecting the carbonyl groups. Since aldehydes are more reactive then ketones, alcohols react preferentially with aldehydes leaving ketones group free.

HCN is a weak acid thus a poor source of CN - (the nucleophile). However addition of a base that generates CN - from HCN furnishes ample supply of CN -. Thus NaCN in presence of H 2SO4 generally used as a source of CN- .

All aldehydes, but only lower ketones (acetone, butasnone, 3- pentanone and pinacolone) form cyanohydrins. Higher ketones do to form cyanohydrins because of steric interference. Cyanohydrins are good synthetic reagents as they can be converted into α-hydroxyl acids, α-amino acids carboxylic acid.

and α, β-unsaturated

The bisulphite addition compounds decompose on heating with dil. acids or bases, to regenerate the carbonyl compound. Hence, this reaction is used for  the purification and separation of carbonyl compounds.

Recall that formaldehyde reacts Grignard reagents (or alkyllithiums) to give  primary alcohols, aldehydes other than HCHO give secondary alcohols and  ketones give tertiary alcohols 

Reaction of  aldehydes and ketones with α-bromoesters in the presence of metallic zinc and ether to give β-hydroxy ester is known as

The β-hydroxy

esters are easily dehydrated to unsaturated esters having stable conjugated system.

An organozinc compound is first formed which then adds on the carbonyl group in a manner analogous to that of a Grignard reagent.

Since organozinc reagents are less reactive than Grignard reagents, they do not react further with the ester group.

An ylide is a neutral molecule having a negative carbon adjacent to a positive hetero atom (e.g. P or S), each atom has an octet of electrons and directly bonded to each other. Aldehydes and ketones react with phosphorus ylides to yield alkenes and triphenylphosphine oxide. The reaction, known as Witting reaction, has proved to be a valuable method for synthesizing alkenes.

Thus, the net result of the reaction is the replacement of carbonyl oxygen, =O, by the group =CRR’. The reaction is carried out under mild conditions and in presence of solvents like tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO).

Writing reaction has a great advantage over most other alkene syntheses in that no ambiguity exits as to the location of the double bond in the product.

The ylide acting as a nucleophile attacks the carbonyl carbon of  the aldehyde or ketones to form an unstable intermediate betaine followed by oxaphosphetane which then spontaneously loses triphenyl phosphine oxide to

form an alkene. The driving force for the Wittig reaction is the formation of very strong P-O bond.

(viii) Cannizaro reaction: Aldehydes which do not have any α-hydrogen atom, when treated with a concentrated solution of NaOH or KOH, undergoes a simultaneous oxidation and reduction (disproportionation) forming a salt of  carboxylic acid and alcohol

e.g.

Since acetaldehyde (CH 3CHO) has hydrogen atoms, it does not undergo Cannizzaro reaction; while trichloroacetaldehyde, CCl 3CHO having no αhydrogen atom, undergoes Cannizzaro reaction.

[2 methypropnal, (CH3)2CH. CHO] Cannizzaro reaction. This exceptional

α

behavior is probly due to + I effect to the two alkyl groups. The reaction is believed to follow the following three steps: First step: The first step is the reversible addition of hydroxide ion (nucleophile) to carbonyl group to form ion I. Second step: The hydroxyalkoxide ion I now transfers its hydride ion directly to another aldehyde molecule, the latter is thus reduced to alkoxide ion and the former (ion I) oxidized to an acid. Third step: The acid and alkoxide ion so obtained exchange their protons to give the more stable pair: acid anion and alcohol.

Note that one molecule of the aldehyde as a hydride donor and the other acts as a hydride acceptor. In other words, Cannizzaro reaction is an example of  self- oxidation and reduction. When the reaction is carried out in D 2O instead of H 2O, no C –D bond is formed indicating that the hydrogen comes forms the aldehyde and not form the solvent.

Cannizzaro reaction, between two different aldehydes each having α-hydrogen atoms.

When one of the aldehydes is formaldehyde, it always undergoes oxidation (rather than other aldehyde) since formaldehyde is more nucleophilic than other aldehydes. Here half-part of the molecule is oxidized and other half part is reduced.

Aldehydes having α-hydrogen atom can also be made to undergo Cannizzaro type of reaction, if reaction is carried out in presence of aluminum ethoxide. But in such case, acid and alcohol react together to form ester as the final product. The reaction is now known is

The reaction takes place in presence of  alkoxide and forms alkynol ( and alkynediol with CH ≡CH); this reaction is known as

Ammonia and some ammonia derivatives like hydroxylamine (NH2

hydrazine (H2N – NH2), phenylhydrazine (H2N –

NHC6H5) and semicarbazide (H2N –NHCONH2) react with aldehydes and ketones in weakly acidic medium.

These derivatives are crystalline solids and used for the identification of  carbonyl compounds

The oximes can be hydrolysed back to the parent aldehydes and ketones on treatment with acids, further, oximes have sharp and specific melting points so oxime formation is used for the separation and identification of aldehydes and ketones. Oximes from all aldehydes and mixed ketones (not simple ketones) can exist in two geometrical isomeric forms For example,

Ketoximes when treated acid catalysts like conc. H 2SO4, PCl5, H3PO4, SOCl2 or C6H5SO2Cl, undergo rearrangement to form substituted amides. This reaction is known as

In case, ketoxime has two different alkyl or aryl groups’ difference amides are formed from different isomeric oximes. For example.

It is the anti-alkyl group that migrates. Cyclohexanone oxime when treated with such reagent undergoes Beckmann rearrangement to form caprolactam, a reagent used for synthesizing nylon.

(ii)

.

(iii)

Simple hydrazones have low melting points hence occasionally used to identify carbonyl compounds (difference from 2, 4-dinitrophenylhydrazones). However, they form the basis for the Wolf-Kishner reduction.

(iv)

In addition to ammonia derivatives, thioalochols and PCl 5 also reacts, in which carbonyl oxygen is replaced by two atoms or groups.

Marcaptals of ketones especially actone, are used for preparing sulphonals, used sedatives.

α

In addition to nucleophilic

addition reactions carbonyl compound exhibit the unusual acidity of  α-hydrogen atoms. Actually, in the nucleophilic additions carbonyl group acts as a functional group, while in the acidity of α-hydrogen atoms, it acts as a substituent and exerts on the adjacent (alpha) carbon atoms.

The unusual acidity of the α-hydrogen is due strong electron-withdrawing nature of carbonyl group which in turn makes α-carbon also electron withdrawing. Hence, in presence of base, it easily loses hydrogen as proton and itself converted into

which is stabilized by resonance.

Note that the two important properties of a carbonyl group viz., susceptibility to nucleophilic attack and acidity of α-hydrogen is due to the

to

accommodate the negative charge. Important reactions of carbonyl compounds due to acidic hydrogen, i.e. due to enols and enolate are discussed here under. Aldehydes and ketones containing at least one αhydrogen atom (i.e., a hydrogen atom attached to the α-carbon atom with respect to the functional group aldehyde or ketone) when treated with dilute base like adding on the aldehydic group. Aldol condensation may take place

between (a) same or different aldehydes (b) an aldehyde and a ketone and (c) same or different ketones.

Aldol when heated loses a molecule of water to from unsaturated compound.

Since formaldehyde, trichloroactcetalehyde (Cl 3C.CHO) and benzaldehyde (C6 H5CHO) do not have any α-hydrogen atom, they do not undergo Aldol condensation in presence of dilute base. Although here, all possible products are obtained, yet by using different catalysts, one product may be made to predominate. In presence of base, α-hydrogen atom of lower aldehyde is more acidic and so migrates, while in presence of an acid, α-hydrogen atom the higher aldehyde is more acidic.

It is the α-hydrogen atom of  the ketone which is involved in Aldol condensation.

Although formaldehyde does not have any α-hydrogen atom, it undergoes Aldol condensation on treatment with a

Aldol condensation of  acetone molecules produce different product under different condition. (i) Two molecules of acetone condense together in the presence of barium hydroxide to form diacetone alcohol.

Diaetone alcohol, on heating loss a molecule of water of form mesitly oxide.

(ii) In the presence of dry hydrogen chloride gas, actone molecules condense to form a mixture of mesityl oxide and phorone.

Note that here the understand compound is isolated and not the Aldol or Ketol. (iii) Acetone forms mesitylene (1,3,5-trimethlbenzene) on distillation with concentrated sulphuric acid

Again here, it is the unsatured compound that is isolated and not the Aldol or Ketol.

Let us take the example of  the condensation of two acetaldehyde molecules. The reaction takes place in the three steps as follows: First step: The baser (OH - ion) removes a hydrogen ion from α-carbon atom of  one of the aldehyde molecule to form resonance stabilized carbanion, I.

Second step: The carbanion I ( enolate ), being a nucleophile, adds to the carbonyl carbon atom of the second molecule of acetaldehyde to form the anion of Aldol.

Third step: The Aldol anion now takes a proton form the solvent (water) forming Aldol.

Note that the catalyst (OH -) is regenerated in the step.

of the aldehyde molecules undergoes enolisation which then attacks the protonated carbonyl group another aldehyde molecule.

Β-Hydroxyaldehyde or ketone so, formed undergoes dehydration easily forming

a double bond at β-carbon atom leading to α,β-unsaturated aldehyde or ketone which is quite stable due to conjugation.

If the double bond is in conjugation with the aromatic ring, the product becomes so stable that unsaturated aldehyde or ketone is isolated as the final product instead of  β-hydroxycarbonly compound. For example:

A dialdehyde, a ketoaldehyde or a diketone undergoes Aldol condensation to form 5-or- 6 membered cyclic compounds.

In the above ketoladehyde, although three different enolates are possible, it is the enolate from the ketone side of the molecule that add to the aldehyde. This is because of greater reactivity of aldehydes towards nucleophilic addition than the ketone due to electronic as well as steric factors.

Claisen –Schmidt reaction Perkin reaction, knoevenagel reaction (all discussed futher in aromatic aldehydes), halogenations and haloform reaction. α

Aldehydes and ketone having α-hydrogen atom, treated

with Cl 2 or Br2 in solvents like water, chloroform, acetic acid or ether lead to αmono di-or tri-halogenated product.

The excess of alkali decomposes the trihalogen compound to give haloform.

Mechanism: Base catalyzed 

Acetaldehyde and methyl ketones (CH 3.CO.R) react rapidly with halogens (Cl 2, Br2 or I 2) in the presence of alkali to form haloform. This reaction is usually known as

since haloform (CHX 3) is

the main product. It involves the formation of carbanion.

Diethyl ketone (C2H5CO.C2H5) has no –COCH 3 group, hence it does not undergo haloform reaction. Holoform reaction is used as a diagnostic test for detecting the presence of –COCH 3 group in a compound.

Thus the haloform reaction may also be used for distinguishing the methyl ketones from other ketones since the former forms haloform while the latter does not form any haloform, e.g.

It is important to note that ethyl alcohol (CH 3CH2OH) and secondary alcohols having one of the alkyl groups as methyl, although does not contain a carbonyl group, also respond haloform reaction. It is due to the fact that such alcohols are first oxidised by halogen to acetaldehyde (CH 3CHO) or methyl ketone respectively, which in turn gives the haloform reaction because of the presence of –CO. CH3 grouping.

Thus in short haloform reaction is given by all compounds containing either of  the following groupings.

Remember that hypohalite does not attack carbon-carbon bond present in the molecule. For example,

The reaction consists mainly of two important : (A) halogenations of –COCH3 grouping to form –COCX 3 and (B) elimination of CX 3 part a :C-X3 anion. (A) Halogenations of –COCH 3 grouping. This part of the reaction involves following steps:

(a) The base :B- takes up the hydrogen atom form the carbonyl compound (recall that hydrogen atoms are acidic in nature.) (b) Electrophilic attack by the halogen at the negatively charged carbon of  carbanion. (c) Repetition of the above two steps till all the three hydrogen atoms –COCH 3 are replaced by halogen atoms. Note that the removal of hydrogen from -COCH2 X is easier than from –COCH 3 because the presence of halogen atom increase the acidity of the hydrogen atoms. Similarly, removal of hydrogen from  –COCHX2 is easier that from –COCH 2X. Thus, the three steps of halogenations of –COCH 3 may be represented as

(B) Elimination of –CX3 part. This part the of reaction again involved the following three steps: (a) Nucleophilic attack of –OH on trihaloacetone. (b) Loss of :C -X3 to form haloform anion and acetic acid. (d) Proton exchange to form a more stable pair of acetate ion (CH 3COO-) and haloform (HCX3).

Aldehydes are easily oxidized to the corresponding acid and thus act as strong reducing agents.

Aldehyde can also be oxidised by much milder oxidising agents like (ammonical silver nitrate),

[blue colored alkaline

solution of cupric ion (Fehling solution 1) complexed with sodium potassium tartrate (Fehling soluation 2)] and

(alkaline solution of cupric

ion complexed with citrate ions). Thus these regents are reduced by aldehydes.

On the other hand, ketones are not oxidized by milder oxidizing agents and  thus they do not reduce Tollen’s reagent Fehling and Benedict solution  ( 

However, stronger oxidizing agents like acid

dichormate alk. KMnO4 and hot conc. HNO 3 oxidise ketones to carboxyclic acids.

In case ketones is unsymmetrical, cleavage takes place in such a way that  carbonyl group is retained by smaller alkyl group (Popoff’s rule). For example.

Aldehydes and ketones with a methyl or methylene group adjacent to the carbonyl group are oxidized by SeO 2 to give dicarbonyl compounds. For example,

Hypohalites ( -OX where X=Cl, Br or I) oxidize CH 3CHO and methyl ketone to acid salt along with formation of haloform

Ketones are also oxidized by Caro’s acid (H 2SO5) or perbenzoic acid (C6H5CO3H) or peracetic acid (CH 3CO3H) to form esters.

The reaction is called

In case of aliphatic ketones

oxygen is inserted between carbonyl carbon and the alkyl group. However, in case of aromatic ketones both products are formed. Aldehydes and ketones are reduced to the primary and secondary alcohols respectively by catalytic hydrogenation (H 2 in presence of  Ni or Pt), nascent hydrogen (sodium amalgam and acid or sodium and alcohol) lithium

aluminium

hydride

(LiAlH 4)

sodium

borohydride

or

aluminium

isopropoxide (Me2CHO)3Al in iso-propanol. Reducation by means of aluminum

isopropoxide is known as

MPV

reduction does not reduce –NO 2, -CH=CH2, -C≡C-, etc. Both these regents reduce aldehydes and ketones to 1 0 and 20 alcohols respectively. Neither of the two reagents reduce the C=C bond.. However the two regents differ in the following respect: (i) LiAlH4 also reduces ester and acid chloride to alcohols, while NaBH 4 does not affect these groups. (ii) The hydride ion in LiAlH 4 is very basic and thus it reacts violently with water, hence it is used in dry solvents like dry ether and THF. Moreover, the product exists as alkoxide ion, so it converted into alcohol by using aqueous HCl or aq. NH 4Cl solution.

reduces the carbonyl group as well as C=C bond, but not esters.

It is important to that the reveres of MPV reduction (i.e. oxidation of secondary alcohols to ketnoes) in presence of aluminium ter-butoxide is known as

Aldehydes and ketones are reduced to the corresponding alkanes by means of amalgamated zinc and hydrochloric acid hydrazine solution

or alkaline

The same conversion can be made by heating aldehydes and ketones with red phosphorus and hydroiodic acid

Two molecules of ketones undergo reduction in prances of Mg/Hg to form which is converted into when treated with mineral acids.

Conversion of pinacol to pinacolone is known as

Lower aldehydes undergo polymersation to form different products under different conditions. Aldehydes restore the pink colors of Schiff’s reagent (Schiff’s reagent is a dilute solution of rosaniline hydrochloride in water whose red colour has been discharged by passing sulphur dioxide).

Benzaldehyde reacts with ammonia to form hydrobenzamide. Aldehydes other than HCHO give aldehyde ammonia, while HCHO forms urotropine.

Benaldehyde reacts with primary aliphatic or aromatic amines to form

In a crossed Cannizzaro reaction, if one of the aldehydes is formaldehyde, it is always oxidized (and not reduced) to formic acid. Benzaldehyde when heated with aqueous ethanolic NaCN (or KCN) undergoes self-condensation to form

Condensation between an aldehyde or ketones with compounds containing active methylene group in the presence of ammonia (its derivative (amines, pyridine, piperidine etc.) to form unsaturated compound is known as

Condensation of an aromatic aldehyde with acid anhydride in presence of sodium salt of the acid from which anhydride is derived to form, α, β-unsaturated acid in known as

Note that in the second example it is α-carbon atom of the propionic anhydride that reacts with the aldehydic group. This is an example of  crossed Aldol condensation in which aromatic aldehydes or ketones 

with or without  α-hydrogen atom react with aldehydes, ketones or ester having  α-hydrogen

atoms in the presence of dilute alkali to form  α,

β-unsaturated 

carbonyl compounds.

The reaction may also take place between two ester molecules, at least one of  which has α-hydrogen atom

is a relatively harmless but powerful lachrymator or tear gas and is used by police to disperse mobs. Like aliphatic aldehydes and ketones, aromatic aldehydes and ketones react with PCl 5 to give dichloro derivative. For example,

Aromatic aldehydes and ketones undergo electrophilic

substitution

reactions,

like

nitration,

sulphonation

and

halogenations, in the m-position . However, these reactions are slow because of the deactivating influence of the carbonyl group on the benzene ring, moreover, certain side reaction like oxidation, etc. make the yield poor.

It can be prepared by general methods. It can be manufactured by (i) the controlled oxidation of methane or natural gas,

(ii) passing water gas (CO+H 2) at low pressure through an electric discharge, and (iii) oxidation of methanol with air over a heated catalyst (Cu or Ag). It is a colorless, pungent smelling gas extremely soluble in water. Since it is a

, it is marketed as 40% aqueous solution under the name of 

or in the form of solid polymers,

(polymer) and

(trimer) which on heating liberate HCHO. Chemically, it gives most of the chemical properties of aldehydes discussed earlier like reaction with HCN, NaHSO 3, Grignard reagent, NH 2OH, H2N.NH2 , oxidation, reduction, Cannizzaro reaction and polymerization. On account of presence of  hydrogen in place of alkyl group, formaldehyde is more reactive than other aldehydes and reacts in different manner with some regents.

Hexamethylene tetramine is used as a urnary antiseptic under the trade name of urotropine. 2. Condensation with phenol: formation of bakelite

3. Polymerisation: When an aqueous solution of formaldehyde is evaporated to dryness, paraformaldehyde is formed. When gaseous formaldehyde is allowed to stand at room temperature, metaformaldehyde (trioxame) is produced. Both of them are white solids and regenerate HCHO on heating. As mentioned above, formaldehyde is used as its 40% aqueous solution under the name of formaline. It is used as a preservative for biological and anatomical specimens, it is used in the preparation of urotropine, in the preparation of bakelite, a synthetic plastic, in silvering of mirror.

It can be manufacture in the following ways : 1. By hydration of acetylene.

2. By the catalytic dehydrogenation of ethanol in presence of heated copper (3000C).

3. From ethylence

Acetaldehyde is a colourless liquid with strong pungent and irritating odour. In water it is hydrated to the extent of 58% forming ethylidene hydroxide. The aqueous solution has an agreeable smell.

Chemically, it gives most of the properties of aldehydes. It does not undergo Cannizzaro reaction. Polymerisation:

Both these polymers give acetaldehyde when distilled with dil. H 2SO4. Acetaldehyde is used (i)

as an antiseptic inhalant in nose troubles.,

(ii)

in the preparations of chemicals like acetic acid, ethyl alcohol, etc.

(iii)

in the preparation of acetaldehyde ammonia, a rubber accelerator.

(iv)

in the preparation of paraldehyde, a hypnotic and sporofic.

(v)

in the preparation of metaldehyde, used as solid fuel in sprit lamp, and

(vi)

in the preparation of days and drugs. .

. It is manufactured (i)

by the oxidation of isopropyl alcohol with oxygen at 500 0C, (ii) by the catalytic dehydrogenation of isopropyl alcohol, and (iii) By Wacker process (from propene).

Acetone (Ketone, in general) condenses with chloroform or bormoform in presence of alkali to form addition product.

Acetone condenses with ammonia to form diacetone amine.

Acetone is reduced by magnesium-amalgam and water to give pinacol

1. It is used for storing acetylene. 2. It is very frequently used a solvent. 3. It is used in the preparation of chloroform, iodoform (antiseptic), chloratone (hypnotic and sedative), etc.

(Oil of bitter almonds), C 6H5CHO. It occurs as glucoside in bitter almonds and hence it is commonly named as oil of bitter almonds. Amygdalin on hydrolysis with dilute acids or the enzyme emulsion gives benzaldehyde, glucose and hydrogen cyanide. Commercially, it is prepared from toluene in the following way.

C6H5COCH3. Commercially it is prepared by the oxidation of ethylbenzene with air in the presence of V 2O5 or oxides of Mn, Zn, etc. at about 500 0C.

It is used in perfumery and in medicine as hypnotic (sleep producing drug) under the name of 

. Two molecules of acetophenone condense in

presence of aluminum ter-butoxide to form

On oxidation with perbenzoic acid, it forms phenyl acetate

Quiones are unsaturated cyclic diketones. Two quinones of  benzene are possible (m-benzoquione is not possible as it is not possible to construct such formula by maintain tetravelency of carbon). Note that quinones are non-aromatic conjugated cyclic diketones , Since they are highly conjugated they

Benzoquinone, being the most important is commonly known as quinone. It is prepared by the oxidation of hydroquione or aniline.

α β UNSATURATEDCARBONYL COMPOUNDS

As the name represent these compounds contain unsaturation between and carbon atoms with respect to carbonyl group i.e. –C=C-C=O-. Such molecules are quite stable due to the presence conjugated system of double bond. Such molecules give properties of the double bond carbonyl group and some additional properties due to the interaction of the two groups. Due to electron withdrawing nature of the >C=O group, the reactivity of C=C towards electrophilic reagents decreases as compared to an isolated double bond, On the other hand, C=C group undergo nucleophilic addition reactions which are uncommon for simple alkenes.