Chemical Properties of Aldehydes and Ketones Preparation and Properties
The chemical reactions of carbonyl compounds can be classified in following groups:
- Nucleophilic addition reaction.
- Nucleophilic addition elimination reaction.
- Oxidation reaction.
- Reduction reaction.
- Reactions with alkalies.
- Miscellaneous reaction.
→ Nucleophilic Addition Reaction : Aldehyde and ketones are unsaturated compounds hence they show nucleophilic addition reactions.
→ Mechanism of Nucleophilic Addition Reactions : The reason of activity of aldehyde and ketone is due to the presence of polar > C = O bond in them. As oxygen is more electronegative hence it attracts electron density towards it therefore partial negative charge develops on it on the other hand partial positive charge develops on carbon atom.
→ For initiating the reaction a nucleophile attacks on positively charged carbon atom and a intermediate is formed which further convert into product after the attack of H+ . A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitais of ca rbon atom as shown in the figure 12.3. given below.
→ The hybridisation of carbon changes from sp2 to sp3 in this process a tetrahedral alkoxide intermediate is formed. This intermediate capture a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond.
→ This reaction is named as nucleophilic addition reation due to attack of nucicephile in slow step of the reaction Relative Reactivity of Aldehyde and Ketone :
→ Aldehydes are generally more reactive than ketones in nucleophiic addition reaction. The more reactivity of aldehyde in compare to ketones can be explained by followoing reasons:
→ Inductive Effect : The attack on carbonyl group is easy when positive charge on carbonyl carbon is more i. e., when carbon is electroii deficient means it is more electrophilic.
→ The alkyl group has +I effect. Therefore, as the number of alkyl groups on the carbonyl carbon increases density on carbonyl carbon a’so increases which decreases the reactivity towards nucleophilic substitution reaction.
→ The number of alkyl groups on ketones are more as compared to aldehydes. Hence, aldehydes are more reactive towards nucleophilic addition reaction as compared to ketones.
→ Steric Effect : No alkyl group is attached to carbonyl carbon in formaldehyde while in other aldehydes one and in ketones two alkyl groups are attached to carbonyl carbon. As the number and size of the alkyl groups increases, the steric hinderance to the attack of nucleophile on the carbon of carbonyl group also increases and hence due to increase in steric effect reactivity decreases.
Some of the important nucleophilic substitution reactions of aldehyde and ketones are as follows:
→ Addition of Sodium bisulphite : Aldehyde and ketone reacts with sodium bisuiphite and forms crystalline bisuiphite addition product.
→ The addition product can be decomposed by mineral acid or sodium carbonate solution to give back the original aldehyde or ketone. Therefore the reaction can be used in separation of aldehyde and ketones from other non-carbonyl compounds.
→ Addition of Hydrogen Cyanide : Aldehydo and ketone reacts with sodium cyanide and dii H2SO4 then one molecule of hydrogen cyanide adds on carbonyl group to form cyanohydrin.
→ Cyanohydrin are important synthetic compound which on hydrolysis gives a-hydroxy compound.
- HCN is a weak nucleophile and it release CN– ion with difficulty.
- Hence reaction is carried out in presence of base which help in generating CN– ion
- HCN is highly toxic and volatile hence proper care must be taken while working on it.
→ Addition of Grignard’s R.ag.nt : Aldehydes and ketones add with Grignard’s reagent and forms additional product which on hydrolysis gives alcohol.
→ Formed alcohol depends on used aldehyde or ketone.
→ Example : Formaldehyde gives primary alcohol whereas other aldehyde gives secondary alcohol and ketone gives tertiary alcohols.
→ Addition of Alcohols-Acetal and Ketal FormatIon : Aldehydes react with alcohols in the presence of dry HCl gas to give acetals. In the reaction an intermediate is formed. This intermediate is alkoxy alcohol which is termed as hemiacetal also. It is a unstable compound and reacts further with another molecule of alcohol to form gem-dialkoxy compound or acetal.
→ Acetal hydrolysed by dilute mineral acids to give back the original aldehydes.
→ Generally ketones do not react with monohydric alcohol. However, they combine with ethylene glycol under similar condition to form cyclic products known as ethylene glycol ketal. During this reaction dry hydrogen chloride gas protonated the oxygen of carbonyl group, so that electrophilic tendency of carbonyl group increases which makes easier nucleophilic attack on ethylene glycol.
→ Nucleophilic Addition-Elimination Reaction : Aldehyde and ketone in the presence of weak acid react with ammonia derivatives likes hydroxylamine (NH2OH), hydrazine (NH2 —NH2), phenyl hydrazine (C6H5NHNH2), 2, 4-dinitrophenyl hydrazine (Bready’s reagent) and semi-carbazide (NH2CONHNH2) and forms product containing C = NZ, where Z is attached substitute of NH2 in attacking reagent group.
→ If we express ammonia derivative as NH2 — Z (Z = OH, NH2, NHC6H5, NH CONH2 etc.) then the formation of product can be represented as follows:
→ The reaction is reversible and catalysed by acids. Some important names and structure of ammonia derivatives and their products are given below :
→ Reaction with Hydroxyl Amine : Aldehyde and ketone reacts with hydroxyl amine and forms oxime.
→ Reaction with Hydrazine : Aldehyde and ketone reacts with hydrazine and form hydrazone.
→ Reaction with Phenyl Hydrazlne : Aldehyde and ketone reacts with phenyl hydrazine and form phenyl hydrazone.
→ Reaction with 2, 4-dinitrophenyl hydrazine (2, 4-DNP) : Aldehyde and ketone reacts with (2, 4-DNP) and form 2, 4-dinitrophenyl hydrazone.
→ Reaction with Semicarbazide : Aldehyde and ketone reacts with semicarbazide to form semicarbazone.
→ There are two NH2, groups in semicarbazide but only one involved in the formation of semicarbazone. The NH2 group closer to carbonyl group is deactivated due to resonance stabilisation
→ In nucleophilic attack of ammonia and its derrivatives pH of the medium must be carefully controlled to about 3.5 (medium is weakely acidie). In strongly acidic condition NH2 — Z will aslo be protonated to form NH2+ – Z and it will not be able to be act as a nucleophile.
→ Oxidation of Aldehydes : Aldehydes are easily oxidised to carboxylic acid of same number of carbon atom. The main oxidising agents for oxidation of aldehydes are KMnO4, K2Cr2O7 (acidic), Br2 water, Ag2+, Cu2+ ion etc. In aldehyde the H.atom attack to carbonyl group oxidises to —OH group.
→ Thus, aldehydes acts as strong reducing agents and it can, Reduces Tollen’s reagent to metallic silver. Reduces Fehiing sblution to red precipitate of Cupprous oxide (Cu2O).
→ Reduction of Tollen’s Reagent : Ammonical silver nitrate solution is called Tollen’s reagent. To prepare the Tollen’s reagent, Ammonium hydroxide (NH4OH) ismixed with silver iitrate and silver oxide is precipitated.
→ This precipitate is dissolved in ammonium hydroxide. On warming any aromatic or aliphatic aldehyde with freshly prepared ammonical silver nitrate solution (Tollen’s reagent), the aldehyde reduces tollen’s reagent to metallic silver and bright silver mirror is produced on the inner side of the test tube. That’s why it is called silver mirror test. Sometimes silver metal does not precipitate as mirror but precipitate as black precipitate. Reaction of the test are as follows:
→ Reduction of Fehling’s Solution : Fehiing reagent is a mixture of two solutions. Fehiing solution ‘A’ and Fehiing solution ‘B’.
→ Fehiing solution A : It is an aqueous solution of copper sulphate. Fehiing solution B : It is colourless solution of sodium hydroxide and sodium potassium tartarate (Rochelle salt).
→ Fehiing solution ‘A’ and Fehiing solution ‘B’ are mixed in equal amounts and precipitate of Cu(OH)2 is formed which readily dissolved in solution and gives blue colour solution.
→ When aliphatic aldehyde is heated with fehlings reagent a reddish brown precipitate is obtained because Cu(OH)9 is converted into red colour Cu9O. The following reactions occurs in this reduction
→ Aromatic aldehyde do not reduce Fehling’s reagent.
→ Haloform Reaction : Acetaldehyde and methyl
→ halogen in aqueous NaOH, then haloform is formed (e.g. chloroform, Bromoform, lodoform). In this reaction ketone and aldehyde oxidised to sodium salt of carboxylic acid with one less carbon atom
→ Bayer Villiger Oxidation : Aldehyde oxidises by per acetic acid, perbenzoic acid and form acids. This reaction is called Bayer Villiger oxidation.
→ Reaction with Shift’s Reagent : Aqueous solution of rosaniline hydrochloride is pink in colour. When sulphur dioxide passed through it. It becomes colourless which is known as Schiffs reagent. When aldehyde reacts with Schiff’s reagent, pink colour again appears.
→ Reduction of Benedict’s Solution : Benedict’s solution is a mixture of copper sulphate, sodium citrate and sothum carbonate. When aldehyde is heated with Benedict’s solution, then red brown precipitate of cuprous oxide is obtained.
→ OxIdation of Ketones : Unlike aldehydes ketone do not have any hydrogen atom attach to > C = O group. Hence, ketone can not oxidise without cleavage of double bond. So ketone can not oxidises with weak oxidising agents like Tollen’s reagent, Fehiing solution. These reagents are used to differenciate between aldehyde and ketones.
→ By Strong Oxidising Agent : Ketone is oxidised by strong oxidising agents only like conc. HNO3, KMnO4 / H2SO4, K2Cr2O2 / H2SO4 etc. Their oxidation involves carbon-carbon bond cleavage to obtain a mixture of carboxylic acids having lesser number of carbon atom than the parent ketone.
→ In assymetric ketone the cleavage of carbon-carbon bond occurs according to Popoff s rule. According to this the keto group is always attached to small alkyl group and cleavage of bond occurs between the keto group and other alkyl group.
→ It can be understnd by following example:
→ Bayer Willinger Oxidation : Ketones are oxidised by per acid and forms ester. This reaction is called Bayer Willinger oxidation.
→ Oxidation by Sodium Hypohalite-Haloform Reaction : lodoform reaction is used as test to distinguish between the molecules in which one
→ Reduction Reaction : Aldehyde and ketone are reduced with hydrogen by reacting different reagents and forms various products.
→ Reduction to Alcohols : In presence of metallic catalyst such as Pt, Pd, Ni aldehyde and ketone reacts with molecular hydrogen to form primary alcohols and secondary alcohols respectively.
→ Aldehyde and ketone can be reduced by LiAlH4 and NaBH4. Unsaturated aldehydes can be reduced to unsaturated alcohols without effecting C = C in presence of reducing agents LiAlH4 in dry ether or NaBH4 in alcohol.
→ Meerwin Pondroff Verley Reduction (MPV) : Reduction of ketones in solution of isopropyl alcohol with aluminium isopropoxide gives alcohols. This reaction is called Meerwrn pondroff verley reduction.
→ Reduction In Hydrocarbon : It has following reactions : Clemmensen’s Reduction : In this reduction carboxyl group (> C = O) is reduced to methylene group (>CH2). It is carried out with zinc amalgam and concentrated hydrochloric acid.
→ Reduction by Red Phosphorus and Hl : Aldehyde and ketones when heated with red phosphorus in the presence of HI at 424 K then alkanes are obtained.
→ Wolf Kishner Reduction or Hung-Milnon Reduction : When hydrazones of aldehyde and ketones are treated with sodium ethoxide at 453 K, the nitrogen gas releases and alkanes are obtained. This reaction is called wolf kishner reduction.
→ The yield of alkanes in this reaction is more than obtained in Clemmenson’s reduction. But both wolf kishner reduction and clemmenson reduction generally effected by sterically hindered ketones.
Hence, Hung Milnon modified the wolf kishner reduction. Accordîng to him when aldehyde and ketone reacts with hydrazine, KOH and diethylene glycol (HOCH2CH2 —O—CH2CH2OH) at 473 K then alkanes are obtained.
→ Reduction to Pinacols : When ketones are reduced with water and magnesium amalgam, pinacols are obtained.
→ Reaction with Alkalies : Reactivity of a-hydrogen of aldehydes and ketones.
→ Acidic Behaviour of a-Hydrogen : In carbonyl compounds the hydrogen atom attach with that carbon which is bonded with carbonyl group is called a-Hydrogen. Carbonyl group have (—I) inductive effect. It attracts the electrons of the adjacent carbon-carbon bond. Due to which a-carbon becomes deficient of electron.
→ To fulfill the deficiency of electron a-carbon atom attracts electron from CαH bond towards itself. Because that a-hydrogen is weakly bonded. When carbonyl compound reacts with strong base then bases easily removes H-atom attached with a-carbon and carbanion is formed. Carbanion gains stability due to resonance.
→ The two reasons for the activity (acidity) of a-hydrogen are as follows: The (—I) effect of carhonyl group which weakens the Cα —H bond and
→ The carbanion formed by elimination of W shows resonance and gains stability. Inductive effect or —I effect decreases along the carbon chain as the distance increases. That’s why the inductive effect (—I) of carbonyl group affect only α—H atom. That means β-, γ-, δ- etc., hydrogen does not show acidic character.
→ Aldol Condensation : Carbonyl compounds (aldehyde, ketone) having atleast one α-hydrogen reacts with dilute bases (dilute NaOH, Na2CO3, Ba(OH)2 etc.), its two molecule undergo self condensation to form β-hydroxy
→ aldehyde or f.hydroxy ketone. As the product have both aldehyde and alcohol group that’s why it is called ‘Aldol’ and this reaction i called as ‘Aldol addition.
→ Formaldehyde, benzaldehyde and benzophenone does not give aldol condensation as they do not contain α-Hydrogen atom.
→ Mechanism : Aldol condensation process is completed in three steps:
→ Step 1 : In the first step the base removes acidic α-hydrogen atom to form an enolate ion. The enolate ion gets stabilized by resonance.
→ Step 2 : The enolate ion being a strong nucleophile attacks the carbonyl group of the second molecule of acetaldehyde and form anion (I).
→ Step 3 : Anion (I) abstract a proton from water to form aldol and hydroxide ion.
→ The product formed from aldol addition on heating with dilute acid forms α-β unsaturated aldehyde and ketone by dehydration. (Now it is known as Aldol condensation reaction)
→ Cross Aldol Condensation: When aldol condensation is carried out between two different aldehyde or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol condensation reaction of a mixture of ethanal and propanal.
→ Ketones can be used in cross aldol condensation.
→ Halogenation : When aldehydes and ketones having a-hydrogen react with halogen in the presence of acid or base, then a-halogen is replaced by halogen atoms. Poly halogenatîon occurs ‘n the presence of a base.
→ Monohalogen derivatives is formed in the presence of acid.
→ In presence of excess of halogens di and tri substituted products are formed.
→ Cannizaro’s ReactIon : Aldehydes which do not have ana-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt
→ Above reactions are disproportionation reactions as one molecule oxidises and other reduces here.
→ Cross Cannizaro’s Reaction : When two different aldehyde which do not have α-hydrogen reacts with conc. NaOH/KOH then this reaction is called cross Cannizaro’s reaction.
→ Reaction with Ammonia : Aldehydes except formaldehyde reacts with etheric solution of ammonia to form aldehyde ammonia adducts. The adduct on warming loses a water molecule and form aldimine.
→ Formaldehyde reacts with ammonia to form hexamethylene tetraamine which is used as a medicine to treat urinary infections and it is konwn as urotropine.
→ Benzaldehyde reacts with ammonia to form complex product hydrobenza mide.
→ Aliphatic ketone reacts with ammonia to form complex condensation products.
→ Reaction with alcoholic Potassium Cyanide : Two malecules of aromatic aldehyde on warming with alcoholic solution of KCN condenses to form Benzoin. this reaction is called Benzoin condensation.
→ Reaction with Chloroform : Ketone condenses with chloroform in the presence of base and forms addition product.
Example – acetone forms chioretone which is used as hipnotic.
→ Reaction with Phosphorous Pentachioride : When aldehyde or ketone reacts with PCl5, the oxygen the oxygen of carbonyl group is substituted by two chlorine atoms and gem dihalide are formed.
→ Reaction with Primary amines : Aldehyde and ketone in presence of catalytic amount of acid reacts with primary amine to form azomethene or schiffs base.
→ Polymerisation Reactions : Aldehyde and ketone polymerises in different conditions of form different product.
Polymerization of formaldehyde.
→ When an aqueous solution (40%) of formaldehyde (formalin) is evaporated to dryness on a water bath, it forms a white solid called paraformaldehyde
(CH2O)n H2O here n 6 to 50
→ On heating paraformaldehyde regenerates formaldehyde.
→ When an aqueous solution (60%) of formaldehyde reacts with few drops of conc. H2SO4 it forms polyoxyxnethylene (CH2O)n, H2O. Here n > 100. It is insoluble solid in water and on heating gives formaldehyde.
→ When gaseous formaldehyde is allowed to stand at room temperature it gives trioxane or metaformaldehyde.
→ 6 molecules of formaldehyde when reacts with Ca(OH)2 solution combiiies to form formose (C6H12O6) which is sugar and isomer of glucose.
→ Polymerization of Acetaldehyde : When acetaldehyde is treated with a few drops of cone. H2SO4 at room temperature (298 K), rapid exothermic reaction occurs with the formation of a cyclic trimer called paraldehyde. Paraldehyde is sweet smelling liquid (B.P.-410 K) and is used in medicines as hypnotic.
→ When acetaldehyde reacts with few drops of conc. H2SO4 or dry HCl gas at 273K it forms cyclic tetramer known as metaidehyde.
→ It is white solid of melting point 519K which again forms acetaldehyde on reacting with dii H2SO4.
→ Ring substitution reactions of aromatic aldehyde and Ketones : Aromatic aldehyde and ketone shows general electrophilic substitution reaction of benzene nucleus like—Halogenation, Nitration and Suiphonation etc.
→ Since aldehyde group (—CHO) and ketonic group (r—COR or —COAr) are electron withdrawing so they are deactivating and rn-directing. In other words the electrophilic substitution reactions in aldehyde and ketones occurs at m-position.
→ Halogenation : Nuclear halogenation is hard in aromatic aldehyde and ketones but side chain halogenation occurs very rapidly. so when Cl2 is passed over benzaldehyde in cold, benzoyl chloride is formed.
→ Generally, when acetone reacts with Br2 in ether in pressence of catalytic amount of AlCl3 at 273 K then phenacyl bromide (Bromoacetophenone) is formed.
→ when acetophenone reacts with Br2 in excess of anhydrousAlC3, m-bromoacetophenone is formed.
→ Nitration : The yield of m-nitrobenzaldehyde from nitration of benzaldehyde is low (50%) because some part of benzaldehyde is oxidises with HNO3 in benzoic acid.
→ Sulphonation : Aldehydes and ketones both on suiphonation gives m-derivatives.
→ Tischenko Reaction : When an aldehyde reacts in the presence of aluminium ethoxide, [Al(OC2H5)3] acid and alcohols are obtained which react with each other to form ester. This reaction is called Tischenko reaction.
→ Distinction between Aldehydes and Ketones
|With Schiffs reagent||Pink colour is obtained||No reaction|
|With’Fehling’s Solution||Red ppt is obtained||No reaction|
|With Tollen’s Reagent||Silver mirror is obtained||No reaction|
|Oxidation||Easily converts into acid with equal number of carbon atom||Hardly converts into acid having less number of carbon atom|
|With LiAlH4||On reduction forms primary alcohol||On reduction forms secondary alcohol|
|Reaction with sodium hydroxide||Brown ppt is obtained||No ppt|
|Reaction with sodium nitro prusside and few drops of sodium hydroxide||Deep red colour (Formaldehyde do not give this test)||Red colour appears which change to orange colour|
|Reaction with m-dinitrobenzene in the presence of NaOH||No reaction||Red violet colour appears|
→ Similarities between Aldehydes and Ketones : Aldehyde and ketones both have carbonyl group. Thus, they both show some type of nucleophiic addition and nucleophilic elimination reaction which have been described in chemical properties.