AlDeHyDeS AnD KeToNeS

Aldehydes and ketones contains the same functional group, the carbonyl group (> C = O).
        
                           

Aldehydes behave as reducing agents due to presence of reducing hydrogen atom where as ketones have no such property so aldehydes easily reacts with oxidising agent. Even HCOOH shows some properties of carbonyl compounds due to presence of – CHO group. HCOOH behaves as reducing agent while in other acids no reducing hydrogen atom, so no reaction with oxidising agent. 

Preparation

(i) Oxidation of alcohols

(a) From PCC or pyredeniumchlorochromate. Which is pyridine, CrO₃ and HCl, in equal ratio

By Jones reagent (CrO₃ and aq. CH₃COCH₃) reaction goes till acids as it is a strong oxidising agent where as by PCC and Collins reagent reaction stops at aldehyde.
(ii) Rosenmund’s reduction
Reduction of acid halide into aldehydes by Pd and BaSO₄ is known as rosenmund reduction, only aldehydes can be prepared by this reaction.

HCOCl break down into CO + HCl so the first aldehyde from rosenmund reduction is CH₃CHO. Here Pd act as catalyst and BaSO₄ as catalytic poison to prevent conversion of aldehyde into alcohol by further reduction. 

(iii) Stephan’s Reduction

When Cyanides are partially reduced by means of stannous chloride and hydrochloric acid followed by hydrolysis yield aldehyde. Ketones can not be prepared by this process.

Here [H₂SnCl₄] is formed which works as powerful reducing agent.

(iv) Oxidation of alkyl halide by dimethyl sulphoxide

(v) From Grignard Reagent                       (vi) Dry distillation of calcium salts of acids                       (vii) Oxidation of vicinal glycols by periodic acid        This reaction can also be done by lead tetrs acetate. We have already studied the mechanism of this reaction in last chapter.       (viii) Action of MnO on acids at 300oC temprature                             (ix) By Hydrolysis of Acetoacetic ester                         (x) Catalytic Dehydrogenetion by Cu at 300oC    (xi) Oppenauer oxidation   This is specific method for oxidation of secondary alcohol to ketone. Secondary alcohol is heated with aluminium iso-propoxide in a ketone (usually acetone)- secondary alcohol is oxidised to ketone whereas ketone is reduced to secondary alcohol.                      Actually this is a reversible reaction and equilibrium can be shifted in either direction by appropriate manipulation.  For example, by taking an excess of secondary alcohol, ketone can be reduced; this reduction is called Meerwein Ponndorf - Verley reduction which has been discussed later.  The uniqueness of Oppenauer oxidation lies in the fact that it selectively oxidises hydroxy group which means that if the compound besides hydroxy group has some other oxidisable functionality, the latter will remain unaffected under these conditions.    For example :  (xii) Hydrolysis of gemdihalide   Properties  (i) Nucleophilic addition reaction  Six electron system of carbocation is formed if primary attack of electrophile takes place, otherwise it is a eight electron system which is more stable when primary attack of nucleophile takes place. So Primary attack here is of the nucleophile due to more stable intermediate oxygen anion of eight electron system. The reactivity of different carbonyl compounds towards formation of nucleophilic addition decreases in the following order : This order arises due to the following two factors : (i) Electronic factor : We have seen above that carbon of the carbonyl group is susceptible to the attack of nucleophile due to the presence of positive charge on it (i.e. on carbon). As the intensity of positive charge on carbon decreases, the nucleophilic attack would occur less readily. On the other hand, if intensity of positive charge increases nucleophilic attack would occur more readily. Alkyl group exerts + I effect, due to which it reduces the intensity of positive charge on carbonyl carbon, which decreases in the following order : Intensity of positive charge on carbonyl carbon decreases. Hence the reactivity of these compounds towards nucleophilic addition also decreases in this order. (ii) Steric factor : The change in C - C - O angle as a result of nucleophilic addition may be noted. It is obvious that nucleophilic addition causes decrease in C – C – O bond angle and hence groups are pushed closer. The bigger groups oppose coming closer, hence the reactivity of carbonyl group decreases. Size of methyl group is much larger than hydrogen, hence acetaldehyde is less reactive than formaldehyde. Similarly, acetone will be less reactive than acetaldehyde. (a) Reaction with HCN In the presence of alkali the dissociation of HCN increases because HO- of alkali trap H+ of HCN so that ionisation of HCN increases. But in presence of acid the dissociation is suppresed due to common ion effect. Thus the reaction shifts to backward direction. (b) Reaction with NaHSO3 (sodiumbisulphite) This reaction is most versatile test for separating carbonyl compounds from noncarbonyl compounds, it first form white crystalline solid which when passed with dry HCl gas re-form carbonyl compound. Here the carbonyl compounds are reobtained by reaction with dry HCl, so these reaction are used for separation test. But always remember that for separation of aldehydes & ketones is done by Tollen’s reagent (ammonical AgNO3 ) i.e. [Ag(NH3)2+], where aldehydes reacts to form white metallic silver. (c) Reaction with sodio - derivatives of 1 - alkynes                                      (d) Reaction with C2H5SH (Ethanethiol)     (ii) Aldol Condensation   Aldehydes or ketones with  when treated with dilute alkali like NaOH,K2CO3 etc. undergo nucleophilic addition by active intermediate carbanion to form  known as aldols.  Various basic reagents such as potassium hydroxide, aqueous alkali carbonate, alkali metal alkoxides, etc., may be used. The reaction is not favourable for all type of ketones but applicable in some special cases like CH3COCH3. Aldol condensation are of different type and it can occur between (i) two identical or different aldehydes, (ii) two identical or different ketones and (iii) an aldehyde and a ketone. When the condensation is between two different carbonyl compounds, it is called crossed aldol condensation. (a) Simple Aldol   Above reaction is also called as Claisen Schimdt Condensation where one is aromatic carbonyl compound and other is aliphatic carbonyl compound. In cross aldol condensation possibility of more than one product is always there so these reactions are not very much useful for synthatic chemistry. For example :    Acetone can also undergo aldol type reaction in the presence of dilute alkali (base catalyst) or dry HCl gas.  Intraaldol condensation: Dicabonyl compounds, having two carbonyl groups within the same molecule, undergo intramolecular aldol condensation reactions. Even bases as weak as sodium carbonate are adequate in these reactions.  Intramolecular aldol condensations proceed best when five- or six- membered rings result because these are of minimum strain ring.    (iii) Cannizaro’s Reaction  Aldehydes which do not have any ,  when treated with concentrated solution of NaOH or KOH, undergo simultaneous oxidation and reduction (disproportionation) forming a salt of carboxylic acid and alcohol. Here 'R' should be –H or phenyl but not alkyl group with presence of . The reaction follows third-order law (second order in aldehyde and first order in base),/ i.e., rate . This suggests the reaction between the first-formed anion (from base and aldehyde) and another molecule of aldehyde in the rate-determining step. In the presence of a high concentration of base, the reaction follows fourth-order law (second order in both two molecules of base), i.e., rate The hydride ion is directly transferred from one molecule of the aldehyde to the other, and does not become free in solution has been proved by the observation that the recovered alcohol does not contain deuterium when the reaction is performed in the presence of D 2 O.       Cannizaro Reaction of HCHO Transfer of hydride ion due to back donation of oxygen charged electron give out hydride ion which is used for reduction of other carbonyl molecule so oxidation of one molecule gives acid salt while other by reduction forms alcohol.   (a) Simple cannizaro's reaction Mechanism of Cannizaro's of benzaldehyde (b) Cross Cannizaro's Reaction Reaction of carbonyl compounds (does not having ) with alkali forms each molecules of acid and alcolol.   Whenever HCHO is present, there are only two main products otherwise there will be four products in all other cases. Because HCHO contains maximum reducing hydrogen atom so easily undergo oxidation process. One of the most important applications is the crossed Cannizzaro reaction between formaldehyde and other aldehydes containing (iv) Bimolecular Reduction or Pinacol Reaction            Two molecules of acetone undergo reduction in the presence of Mg/Hg to form Pinacol. Upon treatment with mineral acids, 2,3-dimethyl 2,3- butane diol (pinacol) is converted into methyl ter-butyl ketone (pinacolone). The 1,2-diol undergo dehydration in such a way that rearrangement of the carbon skeleton occurs. Other 1,2 diols undergo analogous reactions, which are known as pinacol pinacolone type rearrangement. Mechanism  Pinacol–Pinacolone type Rearrangement As the migrating group migrates with its electron pair, the more nucleophilic group might be expected to migrate. Thus, the order of migratory attitude amongst the aryl groups is p -anisyl > p -tolyl > phenyl > p -chlorophenyl, etc.            Remember, electron-attracting groups will retard the migration. The migratory aptitude amongst the alkyl groups is Me 3 C > Me 2 CH > Me. However, the stability of the initially formed carbocation may offset the migratory attitude order. Thus, in the compound 1, 1-dimethyl-2, 2-diphenyl glycol, the resonance-stabilized carbocation (I) is formed instead of (II) and so it is the methyl group and not the phenyl group which migrates, contrary to the above sequence. Steric hinderance may affect the rate of migration– p -anisyl group migrates 1000 times faster than o -anisyl group.         Migrating group attacks from the trans side or back side of to the leaving group. This has significant aspect in cyclic systems. Thus, the two isomers of 1, 2-dimethyl-cyclohexane-1, 2-diol give different products due to different orientations of the methyl and hydroxyl groups. The one (III) in which the Me and OH groups are trans  to each other gives 2, 2-dimethylcyclohexanone by methyl shift. The other (IV) in which the Me and OH groups are cis to each other undergoes ring methylene group shift instead of Me-shift with consequent ring contraction to give 1-acetyl-1-methylcyclopentane (V). Applications Ketones from cyclic diols        Pinacol rearrangement has been also applied to prepare ketones which are very difficult to prepare by other method. (v) Reaction with NH3             When carbonyl compounds reacts with ammonia, nucleophilic addition reaction occurs but products are formed according to praportions between ammonia and carbonyl compound. For example (vi) Reaction with Ammonia Derivatives  NH2 –          Z is an example of nucleophilic addition elemination reaction where – Z groups are following in nature. First attack from nucleophilic site of nitrogen atom to electrophilic carbon atom then elimination of water occurs simultaneously.  These reactions are usually carried out in weakly acidic medium because weak acid catalyses the reaction by protonating carbonyl oxygen but in presence of excess of acid nucleophile is also protonated which reduces its reactivity, so optimum pH is necessary. When zine compounds reacts with carbonyl compound they form crystalline solid as zone derivatives. The reaction with 2,4–DNPH is used for sepration of carbonyl compound with noncarbonyl compound. Always remember, presence of traces of creates better polarity in the carbonyl compound and more electrophilic nature of carbon atom but on the other hand presence of excess H + in the ammonia derivative makes it a bad nucleophile. For example when NH 2 NH 2 (hydrazine) in presence of excess of acid converted into NH 2 N + H 3 cation (hydrazinium) which is a bad nucleophile. Hence optimum pH is a very important condition for this reaction,only tracer amount of H + are required. (vii) Distinction between aldehydes & Ketones              Aldehydes having reducing hydrogen atom whereas ketones don’t, thus only aldehydes reacts with oxidising agent and forms respective product.               Ammonical AgNO3 (Tollen’s reagent)      Aldehydes reacts with silveroxide to form white precipitate of metallic silver while ketones cannot, due to absence of reducing hydrogen atom. Reaction with HgCl2 (Mercuric chloride)         Aldehydes reacts with mercuric chloride to form white precipitate of mercurous chloride which changes into black precipitate of metallic mercury. (c)          Fehling's solution There are two solutions which contains cupric oxide as oxidising agent, when reacts with aldehydes it forms red precipitate of cuprous oxide. C 6 H 5 CHO and their aromatic derivatives do not give test with Fehling's solution because aromatic aldehydes are not good reducing agents. It gives reaction with tollen's reagent due to more oxidising nature of Ag 2 O than CuO. (Ag contains higher reduction potential than copper in electrochemical series) Benedict's Solution Similar chemical reaction and cupric oxide are present in benedict's solution but presence of citrate gives differant complex. It reacts with aldehydes to form red precipitate of Cu 2 O. It cannot reacts with benzaldehyde and its aromatic derivatives. Schiff’s Reagent                                    When dilute solution of p-rosaniline hydrochloride, pink in colour, passed through sulphurdioxide gas forms colourless solution known as schiff's reagent. This restore it colour by reducing nature of aldehyde while ketones gives no response.   (viii) Reformatsky Reaction  When an usually an reacts with carbonyl compound in the presence of zinc metal to produce a . This is known as Reformatsky reaction.  When, a mixture of the carbonyl compound, and zinc in dry solvent benzene is carefully heated under reflux when zinc undergo dissolution. Zinc may be activated by adding traces of iodine, or copper powder. The mixture is then treated with ice-cold dilute sulphuric acid and benzene layer separated. Benzene is distilled off when is obtained. Aldol condensation, Knovengel’s reaction, Perkin reaction and Reformatsky reaction are base catalysed reaction so these reactions are carbanian active process. The advantage of using zinc in place of magnesium is that the organo-zinc compounds are less reactive than the organo-magnesium derivatives of so that they do not normally react with their own ester groups. (ix) Perkin synthesis In Perkin reaction, synthesis has been effected between aromatic aldehydes and aliphatic acid anhydrides in the presence of sodium or potassium salt of the acid corresponding to the anhydride, to yield In this reaction active species also comes in the presence of base as carbanion (C–H2COOCOCH3). Besides simple aromatic aldehydes, their vinyl derivatives, heterocyclic aldehydes and even phathalic anhydride (as the carbonyl component) give this reaction. When carbonyl compounds reacts with acetic anhydride in the presence of base to form active carbanian species, which give nucleophilic addition with carbonyl compounds. But remember, absence of base gives simple fission of anhydride (no carbanian) which on reaction with carbonyl forms stable alkyledene acetates. Toluene oxidation by chromic acid forms benzoic acid while in the presence of acetic anhydride reaction stops at benzaldehyde due to formation of stable intermediate benzeledene di-acetate which on hydrolysis again form –CHO group. So protection of –CH 3 group oxidation at –CHO group, we use chromic acid and anhydride mixture. Following reactions are given below