Aldehydes and Ketones Final

8/3/2019 Aldehydes and Ketones Final

1/67

ALDEHYDES AND KETONES(CARBONYL COMPOUNDS)

Aldehydes and ketones contain the same functional group, the carbonyl group

(>C=O). In aldehydes the carbonyl group is attached either to two hydrogen

atoms (as in formaldehyde) or to one hydrogen atom and one alkyl group;

while in ketones the carbonyl group is always attached to two alkyl groups.

Structure of the carbonyl group Like the carbon-carbon double bond of alkenes,the carbon-oxygen double of the carbonyl group is composed of one and

one bond.

In the carbonyl group, carbon atom is in a state of sp2 hybridization. The

C-O bond is produced by overlap of a sp2 orbital of carbon with a p-orbital of

oxygen. On the other hand, the C-O bond is formed by the sideways overlap

of p-orbitals of carbon and oxygen. The remaining two sp2

orbitals of carbon

form bond with the s-orbital of hydrogen or sp2 orbitals of carbon of the alkyl


2/67

group. Now since the three bonds of the carbonyl carbon utilize sp2 orbitals,

they lie in one plane and are 1200 apart (similarity with C=C).)

However, it is important to note that the carbon oxygen double bond is

different from carbon-carbon double bond. Due to greater electronegativity of

oxygen atom, the -electron cloud is attached towards oxygen. Consequently

oxygen attains a partial negative charge and carbon a partial positive charge.

This polar nature of the carbonyl group cause intermolecular attraction in

aldehydes or ketones and hence accounts for their higher boiling points than

that of hydrocarbons and ethers of comparable molecular weight. Moreover,

polar nature of the carbonyl group also explains the dipole moment in

aldehydes and ketones. However the high values of dipole moments

(2.3-2.8 D) of aldehydes and ketones cannot be accounted for only by

inductive effect; this can be accounted for if carbonyl group is a resonance

hybrid of the two structures.


3/67

Resonance in carbonyl group also explain the shorter C=O bond (and hence

greater bond energy) than the C=C

Other important difference in carbon-oxygen and carbon-carbon double

bonds lies in the fact that the carbonyl group undergoes nucleophilic addition

reaction while olefinic undergoes electrophilic addition reactions

Isomerism: Aldehydes show chain and functional isomerism.Chain isomers:Functional Isomers: Aldehydes and ketones are functional isomers of oxiranes(cyclic ethers), unsaturated alcohols and unsaturated ethers.

Ketones show chain, functional and mesmerism. Examples, of functionalisomerism are given above in aldehydes.

Chain isomers:

Metamers:


4/67

Preparation 1. By oxidation of alcohols. Primary alcohols form aldehydes whilesecondary alcohols form ketones.

Controlled oxidation can be carried out by using CrO3-pyridine (Collin reagent).

Controlled oxidation of alcohols can also be done by pyridinium dichromate

(PDC) or pyridinium chlorochromate PCC which is a mixture of pyridine CrO3

and HCl in 1:1:1 ration. This reagent also does not attack double bonds.

2. By the catalytic dehydrogenation o alcohols


5/67

Oppenauer oxidation. Secondary alcohols can be oxidized to ketones byaluminum 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.

3. By dry distillation of calcium salts of fatty acids. Calcium formate, onpyrolysis, 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.


6/67

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 3000C.

It is believed that here carboxylic are first converted into manganese salts which

decomposes to form aldehyde or ketone.

4. By the hydration of alkynes.


7/67

(ii) Hydroboration of alkynes. Aldehydes are formed when the triple bondpresent on the terminal carbon atom, however, ketones are formed when the

triple bond is present on non-terminal carbon.

However, remember that vinyl boranes formed from terminal alkynes (used for

preparing) still have one hydrogen atom that react with fresh molecule of

diborance to low yield of aldehyde. Thus, it is advisable to use sterically

hindered alkyl borane instead of diborane, especially during preparation of

aldehydes. One such sterically hindered alkyl borane is disiamyl borane.

5. By the hydrolysis of gem-dihalides. Gem-Dihalides having two halogenatoms on the terminal carbon atom give aldehydes, while gemdihalides, having

two halogen atoms on non-carbon give ketones.


8/67

This method is not used much because aldehydes are affected by alkali and

dihalides are usually prepared from the carbonyl compounds themselves.

6. From Grignard reagents. Hydrogen cyanide gives aldehydes, while alkylcyanides give ketones.

From acid chlorides, ketones can best be prepared by using weaker

organometallic reagent, e.g. lithium dialkycuprate or dialkyl cadmium.

7. By the reduction of acid chlorides and esters. Acid chlorides can bereduced into aldehydes with hydrogen in boiling xylene using palladium as a

catalyst-supported on barium sulphate. This reaction is called Rosenmundreducation and used for preparing only aldehydes, but not ketones.


9/67

Barium sulphate acts as a poison for Pd catalyst and prevents reeducation

RCHO to RCH2OH. Quinoline and sulphur are better poisoning agents for Pd

catalyst. Formaldehyde cannot be prepared method since formyl chloride is

unstable at room temperature.

Acid chlorides are readily reduced to aldehydes by lithium tri-ter

butoxyaluminum hydride, LiAl(OCMe3

)3

H or tri-n-butyl tin hydride, Sn(C4

H9

)3

H.

Esters can be reduced easily to aldehydes by sodium aluminium hydride

NaAlH4 or di-isobutyl alumunium hydride (DIBAL-H), Al[(CH3 )2 CH2CH2]2H.

8. From nitriles (Stephen reduction). Nitriles when reduced by means ofstannous chloride and hydrochloric acid in absolute ether followed by hydrolysis

yield aldehydes. This reaction is known Stephen reduction.


10/67

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.

9. From alkenes (i) Oxo process:

The net reaction appears to be an addition of formaldehyde through anti-

Markownikoff rule; this reaction in known as hydroformylation or carbonylationand applied for the preparations of aldehydes only.

(ii) Wacker process:

10. From acetoacetic ester. Ketones (but not aldehydes) are prepared by theketonic hydrolysis of acetoacetic ester or its alkyl derivatives by heating with dil.

aq. acid or dil. alcoholic solution of alkali.


11/67

11. For aromatic aldehydes and ketones. Aromatic aldehydese.g. benzaldehydecan be prepared by

(i) Boiling benzyl chloride with a solution of cupric or head nitrate (Laboratorymethod).

(ii) Oxidizing toluene with chromium trioxide in presence of acetic anhydride to

trap benzaldehyde acetate and thus avoid its oxidation to benzoic acid

(Laboratory method).

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

complex precipitated with water (Etard reaction).


12/67

Remember that side chains bigger that CH3 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 AlCl3

(Gattermann-Koch synthesis).


13/67

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 AlCl3 or ZnCl2 (Gattermann aldehydesynthesis)

(vi) Vilsmayer reaction this reaction involves the conversion of aromaticcompounds to aldehydes in the presence of a 20 amine and formic acid.


14/67

Aromatic ketones 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.


15/67

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

Ketones from B-keto acids. The extreme ease with which B-keto acids undergodecarboxylation is applied for the preparation of ketones (aliphatic as well as

aromatic).

Properties. 1. Lower aldehydes and ketones are soluble in water due tohydrogen 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.


16/67

2. Aldehydes and ketones have higher boiling points as compared to

corresponding alkenes. This is due to dipole-dipole attraction between the twocarbonyl 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.


17/67

4. Nucleophilic addition. Addition of HCN, NaHSO3 Grignard reagent etc. Allthese 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 the typical reaction of aldehydes and ketones is nucleophilic addition.

(Not the presence of charge on O which it can easily carried)Note that in the transition state, oxygen has started acquiring negative charge

which it will have bear in the product. Actually, it is tendency of oxygen toacquire electrons (i.e. its ability to carry a negative charge) which is responsiblefor the reactivity of the carbonyl group towards nucleophiles. The polarity of thecarbonyl group is not the cause of reactivity; t is simply another manifestation of

the electronegativity of oxygen.


18/67

The reactivity of the carbonyl group towards the nucleophilic addition of

the reactions depends upon the magnitude of the positive charge on thecarbonyl carbon atom (electrophilic character of carbonyl carbon) and also oncrowding around the carbonyl carbon atom (steric effect), the site wherenucleophile 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,


19/67

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 aromatic aldehydes the positive charge on the carbonyl carbon atomcan delocalize over benzene nucleus, these are less reactive than aliphaticaldehydes.

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 Eact for nucleophilic attack, since it permits

oxygen to accept electrons without having a negative charge.


20/67

(Undergoes nucleophilic attack more readily)

(i) Addition of water (gem-diol formation). Aldehydes and ketonesreact with water in presence of acid or base to form hydrate.

Like the general nucleophilic additions, hydrate formation follows

the following order.

(ii) Addition of alcohols (acetal formation). Aldehydes react withalcohols 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.


21/67

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.

(iii) Addition of HCN. HCN is a weak acid thus a poor source of CN- (thenucleophile). However addition of a base that generates CN- from HCN

furnishes ample supply of CN-. Thus NaCN in presence of H2SO4 generally

used as a source of CN- .


22/67

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 -hydroxyl acids, -amino acids (Strecker synthetic) and , -unsaturatedcarboxylic acid.

(iv) Addition of sodium busulphite


23/67

Higher aliphatic ketones and aromatic ketones do not react with NaHSO3The 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.

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

ketones give tertiary alcohols

(vi) Addition of organozine compounds. (Reformatsky reaction). Reaction ofaldehydes and ketones with -bromoesters in the presence of metallic zinc and

ether to give -hydroxy ester is known as Reformatsky reaction. The -hydroxyesters are easily dehydrated to unsaturated esters having stable conjugated

system.


24/67

Mechanism. An organozinc compound is first formed which then adds on thecarbonyl 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.


25/67

(vii) Addition of ylides (Wittig reaction). An ylide is a neutral molecule having anegative 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.


26/67

Mechanism. The ylide acting as a nucleophile attacks the carbonyl carbon ofthe aldehyde or ketones to form an unstable intermediate betainefollowed by

oxaphosphetanewhich 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 -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 (Cannizzaro reaction), e.g.

Since acetaldehyde (CH3CHO) has hydrogen atoms, it does not undergo

Cannizzaro reaction; while trichloroacetaldehyde, CCl3CHO having no -

hydrogen atom, undergoes Cannizzaro reaction.


27/67

Remember that isobutyraldehyde [2 methypropnal, (CH3)2CH. CHO] althoughcontains -hydrogen atom, it undergoes Cannizzaro reaction. This exceptionalbehavior is probly due to + Ieffect to the two alkyl groups.

Mechanism. 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.


28/67

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 D2O instead of H2O, no C D bond is

formed indicating that the hydrogen comes forms the aldehyde and not form the

solvent.

Crossed Cannizzaro reaction: Cannizzaro reaction, between two differentaldehydes each having -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.

Intramolecular or internal Cannizzaro reaction. Here half-part of the molecule isoxidized and other half part is reduced.


29/67

Aldehydes having -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 Tischenko reaction.

(ix) Addition of terminal alkynes. The reaction takes place in presence ofalkoxide and forms alkynol ( and alkynediol with CHCH); this reaction is

known as ethinylation.


30/67

5. Replacement of carbonyl oxygen. Ammonia and some ammonia derivativeslike hydroxylamine (NH2OH), hydrazine (H2N NH2), phenylhydrazine (H2N NHC

6

H5

) and semicarbazide (H2

N NHCONH2

) react with aldehydes and

ketones in weakly acidic medium.

These derivatives are crystalline solids and used for the identification of

carbonyl compounds


31/67

(i) Reaction with hydroxylamine (oxime formation)

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,


32/67

Ketoximes when treated acid catalysts like conc. H2SO4, PCl5, H3PO4, SOCl2 or

C6H5SO2Cl, undergo rearrangement to form substituted amides. This reaction is

known as Backmann rearrangement.

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.


33/67

(ii) Reaction with hydrazine, H2N.NH2 (hydrazone formation).

(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) Reaction with semicarbazide.


34/67

(iv) In addition to ammonia derivatives, thioalochols and PCl5 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.

(vi) Reaction with phosphorus pentachloride.

(B) Acidity of -hydrogens o carbonyl compound. In addition to nucleophilicaddition reactions carbonyl compound exhibit the unusual acidity of-hydrogen

atoms. Actually, in the nucleophilic additions carbonyl group acts as a functional

group, while in the acidity of-hydrogen atoms, it acts as a substituent and

exerts on the adjacent (alpha) carbon atoms.


35/67

The unusual acidity of the -hydrogen is due strong electron-withdrawing

nature of carbonyl group which in turn makes -carbon also electron -

withdrawing. Hence, in presence of base, it easily loses hydrogen as proton and

itself converted into carbanion which is stabilized by resonance.

Note that the two important properties of a carbonyl group viz., susceptibility to

nucleophilic attack and acidity of-hydrogen is due to the ability of oxygen toaccommodate the negative charge.

Important reactions of carbonyl compounds due to acidic hydrogen, i.e. due to

enols and enolate are discussed here under.

6. Aldol condensation. Aldehydes and ketones containing at least one -hydrogen atom (i.e., a hydrogen atom attached to the -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


36/67

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

same or different ketones.

(a) Condensation between two same aldehyde molecules.

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

Since formaldehyde, trichloroactcetalehyde (Cl3C.CHO) and benzaldehyde

(C6 H5CHO) do not have any -hydrogen atom, they do not undergo Aldol

condensation in presence of dilute base.

(b) Condensation between two different aldehydes. Although here, all possibleproducts are obtained, yet by using different catalysts, one product may be

made to predominate. In presence of base, -hydrogen atom of lower aldehyde

is more acidic and so migrates, while in presence of an acid, -hydrogen atom

the higher aldehyde is more acidic.


37/67

(c) Condensation between aldehyde and ketone. It is the -hydrogen atom ofthe ketone which is involved in Aldol condensation.

(d) Condensation between formaldehyde molecules. Although formaldehydedoes not have any -hydrogen atom, it undergoes Aldol condensation on

treatment with a strong base.

(e) Aldol condensation between acetone molecules. Aldol condensation ofacetone molecules produce different product under different condition.

(i) Two molecules of acetone condense together in the presence of barium

hydroxide to form diacetone alcohol.


38/67

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.


39/67

Mechanism of base catalysed aldol condensation. Let us take the example ofthe 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 -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.


40/67

Mechanism of acid catalyzed Aldol condensation: The of the aldehydemolecules undergoes enolisation which then attacks the protonated carbonyl

group another aldehyde molecule.

-Hydroxyaldehyde or ketone so, formed undergoes dehydration easily forming

a double bond at -carbon atom leading to ,-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-hydroxycarbonly compound. For example:


41/67

Intramolecular aldol condensation (Cyclization via aldol condensation).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.

Other reactions releated to aldol condensation.Claisen Schmidt reaction Perkin reaction, knoevenagel reaction (all discussed

futher in aromatic aldehydes), halogenations and haloform reaction.

7. -Halogenation. Aldehydes and ketone having -hydrogen atom, treatedwith Cl2 or Br2 in solvents like water, chloroform, acetic acid or ether lead to -

mono di-or tri-halogenated product.


42/67

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

Mechanism: Base catalyzed


43/67

8. Haloform reaction. Acetaldehyde and methyl ketones (CH3.CO.R) reactrapidly with halogens (Cl2, Br2 or I2) in the presence of alkali to form haloform.

This reaction is usually known as haloform reaction since haloform (CHX3) isthe main product. It involves the formation of carbanion.

Diethyl ketone (C2H5CO.C2H5) has no COCH3 group, hence it does not

undergo haloform reaction. Holoform reaction is used as a diagnostic test for

detecting the presence of COCH3 group in a compound.


44/67

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 (CH3CH2OH) 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 (CH3CHO) or methyl ketone

respectively, which in turn gives the haloform reaction because of the presence

of CO. CH3 grouping.


45/67

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,

Mechanism. The reaction consists mainly of two important : (A) halogenationsof COCH3 grouping to form COCX3 and (B) elimination of CX3 part a :C

-X3

anion.

(A) Halogenations of COCH3 grouping. This part of the reaction involves

following steps:


46/67

(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 COCH3

are replaced by halogen atoms. Note that the removal of hydrogen from

-COCH2 X is easier than from COCH3 because the presence of halogen atom

increase the acidity of the hydrogen atoms. Similarly, removal of hydrogen from

COCHX2 is easier that from COCH2X.

Thus, the three steps of halogenations of COCH3 may be represented as


47/67

(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 (CH3COO-) and

haloform (HCX3

).

9. Oxidation. Aldehydes are easily oxidized to the corresponding acid and thusact as strong reducing agents.

Aldehyde can also be oxidised by much milder oxidising agents like Tollensreagent (ammonical silver nitrate), Fehling solution [blue colored alkaline


48/67

solution of cupric ion (Fehling solution 1) complexed with sodium potassium

tartrate (Fehling soluation 2)] and Benedict solution (alkaline solution of cupricion complexed with citrate ions). Thus these regents are reduced by aldehydes.

Benzadehyde (aromatic aldehydes) although reduces Tollens regent, it doesnot reduce Fehling and Benedict solutions.

On the other hand, ketones are not oxidized by milder oxidizing agents and

thus they do not reduce Tollens reagent Fehling and Benedict solution

(difference from aldehydes). However, stronger oxidizing agents like aciddichormate alk. KMnO4 and hot conc. HNO3 oxidise ketones to carboxyclic

acids.


49/67

In case ketones is unsymmetrical, cleavage takes place in such a way that

carbonyl group is retained by smaller alkyl group (Popoffs rule). For example.

Aldehydes and ketones with a methyl or methylene group adjacent to the

carbonyl group are oxidized by SeO2 to give dicarbonyl compounds. For

example,

Hypohalites (-OX where X=Cl, Br or I) oxidize CH3CHO and methyl ketone to

acid salt along with formation of haloform (haloform reaction).


50/67

Ketones are also oxidized by Caros acid (H2SO5) or perbenzoic acid

(C6H5CO3H) or peracetic acid (CH3CO3H) to form esters.

The reaction is called Baeyer Villiger oxidation In case of aliphatic ketonesoxygen is inserted between carbonyl carbon and the alkyl group. However, in

case of aromatic ketones both products are formed.

10. Reeducation. Aldehydes and ketones are reduced to the primary andsecondary alcohols respectively by catalytic hydrogenation (H2 in presence of

Ni or Pt), nascent hydrogen (sodium amalgam and acid or sodium and alcohol)

lithium aluminium hydride (LiAlH4) sodium borohydride or aluminium

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


51/67

isopropoxide is known as Meerwein-Ponndorf-Verley (MPV) reeducation; MPVreduction does not reduce NO2, -CH=CH2, -CC-, etc.

(a) Reduction by lithium aluminium hydride and sodium borohydride. Both theseregents reduce aldehydes and ketones to 10 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 NaBH4 does

not affect these groups.

(ii) The hydride ion in LiAlH4 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. NH4Cl solution.


52/67

(b) Catalytic hydrogenation reduces the carbonyl group as well as C=C bond,but not esters.

(c) MPV reduction.

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

Oppenauer oxidation.

(d) Aldehydes and ketones are reduced to the corresponding alkanes by meansof amalgamated zinc and hydrochloric acid (Clemmensen reduction) or alkalinehydrazine solution (Wolf-Kishner reduction or Hung Milnon reaction).


53/67

(e) The same conversion can be made by heating aldehydes and ketones with

red phosphorus and hydroiodic acid

(f) Bimolecular reduction or Pinacol reduction. Two molecules of ketonesundergo reduction in prances of Mg/Hg to form which is converted into

pinacolone when treated with mineral acids.

Conversion of pinacol to pinacolone is known as pinacol-pinacolonerearrangement.


54/67

Mechanism:

11. Polymerisation. Lower aldehydes undergo polymersation to form differentproducts under different conditions.

12. Shifts test. Aldehydes restore the pink colors of Schiffs reagent (Schiffsreagent is a dilute solution of rosaniline hydrochloride in water whose red colour

has been discharged by passing sulphur dioxide). Ketones do not restoreSchiffs reagent colour.

Special reactions of Aromatic Aldehydes and Ketones

(i) Reaction with ammonia. Benzaldehyde reacts with ammonia to formhydrobenzamide. Aldehydes other than HCHO give aldehyde ammonia, while

HCHO forms urotropine.


55/67

(ii) Reaction with amines. Benaldehyde reacts with primary aliphatic or aromaticamines to form Shifts base.

(iii) Crossed Cannizzaro reaction.

In a crossed Cannizzaro reaction, if one of the aldehydes is formaldehyde, it is

always oxidized (and not reduced) to formic acid.

(iv) Benzoin condensation. Benzaldehyde when heated with aqueous ethanolicNaCN (or KCN) undergoes self-condensation to form benzoin.


56/67

(v) Knoevenagal reaction. Condensation between an aldehyde or ketones withcompounds containing active methylene group in the presence of ammonia (its

derivative (amines, pyridine, piperidine etc.) to form unsaturated compound is

known as kenoevengel reaction.

(vi) Perkin reaction: Condensation of an aromatic aldehyde with acid anhydridein presence of sodium salt of the acid from which anhydride is derived to form,

, -unsaturated acid in known as Perkin reaction.

Note that in the second example it is -carbon atom of the propionic anhydride

that reacts with the aldehydic group.

(vii) Claisen-schmidt reaction (crossed Aldol type condensations). This is anexample ofcrossed Aldol condensation in which aromatic aldehydes or ketones


57/67

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

-hydrogen atoms in the presence of dilute alkali to form , -unsaturated

carbonyl compounds.

The reaction may also take place between two ester molecules, at least one of

which has -hydrogen atom

(viii) Reaction with chlorine.


58/67

Phenacyl chloride is a relatively harmless but powerful lachrymator or tear gasand is used by police to disperse mobs.

(ix) Reaction with phosphorus pentchloride. Like aliphatic aldehydes andketones, aromatic aldehydes and ketones react with PCl5 to give dichloro

derivative. For example,

(x) Reactions of benzene nucleus. Aromatic aldehydes and ketones undergoelectrophilic 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.

Individual members of Aldehydes and Ketones

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


59/67

(ii) passing water gas (CO+H2) at low pressure through an electric discharge,

and (iii) oxidation of methanol with air over a heated catalyst (Cu or Ag).

Properties. It is a colorless, pungent smelling gas extremely soluble in water.Since it is a gas, it is marketed as 40% aqueous solution under the name offormalin or in the form of solid polymers, paraformaldehyde (polymer) andmetaformaldehyde (trimer) which on heating liberate HCHO. Chemically, itgives most of the chemical properties of aldehydes discussed earlier like

reaction with HCN, NaHSO3, Grignard reagent, NH2OH, 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.

1. Reaction with ammonia

Hexamethylene tetramine is used as a urnary antiseptic under the trade name

of urotropine.

2. Condensation with phenol: formation of bakelite


60/67

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.

Uses: As mentioned above, formaldehyde is used as its 40% aqueous solutionunder 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.

ACETALDEHYDE , Ethanal CH3CHOIt 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).


61/67

3. From ethylence (Wacker process).

Properties: Acetaldehyde is a colourless liquid with strong pungent andirritating 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. H2SO4.

Uses: Acetaldehyde is used

(i) as an antiseptic inhalant in nose troubles.,

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


62/67

(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.

3. Acetone Dimethyl Ketone, Propanone : CH3.CO.CH3. It is manufactured (i)by the oxidation of isopropyl alcohol with oxygen at 5000C, (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.


63/67

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

(bimolecular reduction).

Use. 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.


64/67

4. Benzaldehyde (Oil of bitter almonds), C6H5CHO. It occurs as glucoside(amygdalin) in bitter almonds and hence it is commonly named as oil of bitteralmonds. 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.

5. Acetophenone, Acetylbenzene, Methylphenyl ketone C6H5COCH3.Commercially it is prepared by the oxidation of ethylbenzene with air in the

presence of V2O5 or oxides of Mn, Zn, etc. at about 5000C.

It is used in perfumery and in medicine as hypnotic (sleep producing drug)

under the name of hypnone. Two molecules of acetophenone condense inpresence of aluminum ter-butoxide to form dypnone.


65/67

On oxidation with perbenzoic acid, it forms phenyl acetate (Baeyer-villigeroxidation).

6. Quinones. Quiones are unsaturated cyclic diketones. Two quinones ofbenzene 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 are highly coloured substance.

Benzoquinone, being the most important is commonly known as quinone. It is

prepared by the oxidation of hydroquione or aniline.


66/67

, UNSATURATEDCARBONYL COMPOUNDSAs 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.


67/67

Two important addition reactions of, -unsaturated carbonyl compounds

are Michael reaction and Diels-Alder reaction.

Date post:	06-Apr-2018
Category:	Documents
Upload:	anil-kumar-verma
View:	295 times
Download:	1 times

Aldehydes and Ketones Final

Documents