Chapter 35 : Carbonyl Compounds 羰基化合物

Chpt. 35: p.1

L.S.T.LeungChikWaiMemorialSchool

F.7 Chemistry

I.INTRODUCTION

Carbonyl compounds 羰基化合物(羰音湯)refer to the two families of compounds known as aldehydes醛and ketones酮, both of which contain the carbonyl functional group (-C=O):

aldehydesketones

This carbonyl group has characteristic properties that are exhibited by both classes of compounds, so that the two homologous series are more conveniently considered together. However, the attachment of a hydrogen to the carbonyl group of an aldehyde does give it certain properties which ketones do not share, and which enables the two families of organic compounds to be distinguished from one another.

The carbonyl carbon is sp2 hybridised, with its three attached atoms lying in the same plane. The bond angles between the three attached atoms are what is expected for a trigonal planar structure: approximately 1200.

Since the carbonyl oxygen is more electronegative than the carbonyl carbon, the carbonyl oxygen bears a substantial partial negative charge, whereas the carbonyl carbon bears a substantial partial positive charge.

This charge distribution is resulted from two effects:

(a)inductive effect of the electronegative oxygen; and

(b)the resonance contribution of the second canonical form shown below, due to mesomeric effect of the electronegative oxygen.

resonance structure

In both aldehydes and ketones, such a carbonyl function is found attached to a variety of saturated or unsaturated carbon skeletons:

Examples of aldehydes (named by replacing `-e' of alkane by `-al')

Example of Ketones: ( naming ‘e’ of alkane by ‘-one’ )

II.SIMPLE METHODS OF FORMATION OF ALDEHYDES AND KETONES

A.Preparation of Aldehydes備製醛

1.Oxidation of Primary Alcohols

Aldehydes may be prepared by the controlled oxidation of primary alcohols, using an acidified solution of potassium dichromate or potassium permanganate:

The alcohol is added dropwise to the oxidising solution and the reaction mixture is kept at a temperature below the boiling point of alcohol but above that of the aldehyde. This allows the aldehyde formed to be distilled from the reaction mixture once it is formed, and avoids its further oxidation to carboxylic acid.

B.Preparation of Ketones

Oxidation of Secondary Alcohols

Ketones can also be obtained from oxidation of secondary alcohols, most commonly by the use of an acidified solution of dichromate:

The ketones formed in the reaction mixture can be similarly distilled out as it is formed, since carbonyl compounds are relatively volatile (no hydrogen bonding) and a good separation is easily obtained.

2.Friedal-Crafts Acylation

Many aromatic compounds react with acid chlorides in the presence of aluminium trichloride to give aromatic ketones. This Friedal-Crafts acylation has been discussed in chapter 11.

3.Ozonalysis
III. REACTIONS OF ALDEHYDES AND KETONES

The main reactions of aldehydes and ketones can be grouped into the following types:

(1)Nucleophilic addition.

(2) Condensation.

(3)Reactions with halogens or compounds of halogens.

(4)Oxidation and reduction.

IV. NUCLEOPHILIC ADDITION

Owing to the high electronegativity of oxygen, the carbonyl group is strongly polarised, with the electrons in the and  bonds shifted toward the oxygen atom. The carbonyl carbon is therefore electron-deficient or electrophilic, whereas the oxygen is electron-rich or nucleophilic.

As a result, the carbonyl carbon is readily attacked by an electron-rich nucleophile, and reactions of nucleophilic agents at the carbonyl carbon dominate the reactivity of carbonyl compounds.

The nucleophilic agent may be neutral or negatively charged, but it should have at least one extra pair of electrons for co-ordinating with the carbonyl carbon. Once a new bond is formed from the nucleophilic agent to the carbonyl carbon, the carbonyl oxygen gains an unshared electron pair. This electron rich oxygen can transfer its electron pair to a proton, thus completing the overall addition of Nu-H to the carbonyl group.

The relative reactivities of aldehydes and ketones toward nucleophilic addition depend on two factors:

(1)electronic influence of the groups attached to the carbonyl carbon.
The more electron-releasing are the groups, the less electron-deficient is the carbonyl carbon, and the less reactive towards nucleophiles.

(2)steric bulkiness of the groups.

Alkyl groups are electron-releasing relative to hydrogen and are also much bulkier.

Hence for both electronic and steric reasons ketones with the C=O group flanked by two alkyl or aryl groups, are in general less reactive than aldehydes. The general order of reactivity is therefore:

To illustrate these features of nucleophilic addition, the following reactions will be discussed: - Addition of Hydrogen Cyanide

- Addition of Sodium Hydrogen Sulphate(IV)

- Addition of Grignard Reagents

- Addition of Alcohols

Addition of Hydrogen Cyanide與氰化氫的加成反應

Hydrogen cyanide adds to the carbonyl groups of aldehydes and most ketones to form hydroxynitriles (cyanohydrins) 氰醇(羥(襁)基烷)

Hydrogen cyanide is a very weak acid but the cyanide ion is a strong nucleophile. The mechanism of cyanohydrin formation is initiated by a nucleophilic attack by the cyanide ion on the carbonyl carbon.

As hydrogen cyanide is very toxic and volatile, it is usually prepared in situ by treating sodium cyanide with dilute acid.

The reaction can be base-catalysed, because the base can increase the CN- ion concentration:

The mechanism of nucleophilic addition of hydrogen cyanide does have some stereochemical consequence in some carbonyl compounds. Treatment of butanal, say, with hydrogen cyanide gives a mixture of two isomeric products which cannot be separated by careful distillation, as they are enantiomers that are non-superimposable on each other.

Addition of Sodium Hydrogen Sulphate(IV) (Bisulphate)與亞硫酸氫鈉的加成反應

On shaking the aldehyde or ketone with excess 40% aqueous sodium hydrogen sulphate(IV) at room temperature, colorless crystals called bisulphate adducts are obtained.

Mechanism

This is again a nucleophilic addition with the attack initiated by the –SO3H nucleophile.

This reaction is very sensitive to steric hindrance and is limited to sterically unhindered aliphatic aldehydes and ketones (methyl ketones). It can be used for the separation and purification of aldehydes and ketones from other compounds, as they can be regenerated by treating the adducts with aqueous alkali or dilute acids, which reverse the above equilibria to the left.

Furthermore, these bisulphite products may be used to prepare hydroxynitriles, thus eliminating the production of toxic hydrogen cyanide:

Addition of Grignard Reagents

Grignard reagents react with carbonyl compounds to produce alcohols. The reaction is again a nucleophilic addition, with R- serving as the nucleophile:

Addition of Alcohols - Acetal and Ketal Formation

In the presence of a little dry hydrogen chloride, aldehydes and ketones react with excess anhydrous alcohols to form acetals and ketals respectively.

Aldehydes react with alcohols in two stages, in the presence of dry hydrogen chloride, to give hemiacetals and then acetals, e.g.

The essential structural features of a hemiacetal are an -OH and an -OR group attached to the same carbon; whereas in the acetal there are two -OR groups attaching to the same carbon.

All the above steps are reversible. If the acetal is then placed in water with a small amount of acid, all the above steps would reverse. Under such conditions (an excess of water) the equilibrium favors aldehyde formation. The acetal is said to undergo hydrolysis.

The acetals or ketals have the structure of an ether and can serve as a protecting group to the aldehydes or ketones.

For example, if we are to convert

Direct reduction by LiAlH4/anhydrous ether would certainly convert the -COOH group into -CH2OH group, but the keto group", C=O would also be reduced into >CHOH as well:

To protect the keto group from being reduced, we can convert it to a cyclic ketal first, then reduce the COOH group to CH2OH group, and hydrolyse the cyclic ketal to restore the keto group. The ether structure of cyclic ketal is unaffected by the reducing agent and can be hydrolysed back to keto group by subsequent acid hydrolysis.

Exercise:

Arrange the following group of compounds in order of increasing reactivity towards nucleophilic addition reaction. Explain briefly.

V.CONDENSATION (ADDITION-ELIMINATION) REACTIONS縮合反應

This involves combination of two molecules with the elimination of simple molecules such as water. The reaction is quite similar to nucleophilic addition reactions as discussed before. An example of this kind of reaction involves the reaction of aldehydes or ketones with a number of derivatives of ammonia in the following way:

1.Reaction with Hydroxylamine (NH2OH) 與羥胺的反應

Aldehydes and ketones react with hydroxylamine to form oximes月丏, known as aldoximes and ketoximes respectively. They are all crystalline solids:

Simple aliphatic oximes are too soluble to be precipitated. To get the solid form of them requires careful crystallization.

2. Reactions with Hydrazine, Phenylhydrazine, and2,4-Dinitrophenylhydrazine班特尼試劑

Aldehydes and ketones react with hydrazine and its derivatives to form products such as hydrazone, phenylhydrazone and 2,4-dinitrophenylhydrazone2,4-二硝基苯月宗.

All these condensation products have sharp characteristic melting points and they can thus be used for purposes of identification of the original aldehyde or ketone.

VII.OXIDATION AND REDUCTION

A.Oxidation

Aldehydes are oxidized to carboxylic acids readily by a number of different oxidizing agents such as acidified KMnO4, K2Cr2O7, HNO3 or even very mild oxidizing agents such as ammoniacal silver nitrate, Fehling's solution and Schiff's reagent.

1.Reaction with acidified KMnO4, K2Cr2O7, and HNO3

Unlike aliphatic aldehydes, aromatic counterparts do not undergo oxidation readily, but benzaldehyde undergoes light-catalysed auto-oxidation to benzoic acid when it is exposed to air.

Ketones do not undergo oxidation readily. It requires more drastic conditions to bring about the cleavage of the carbon-carbon single bond.

Reaction with Tollen's reagent托倫斯試劑(the Silver Mirror test銀鏡測試)

Mixing aqueous silver nitrate with aqueous ammonia produces a solution known as Tollen's reagent. It is a weak oxidizing agent but when heated gently it can oxidize aldehydes to carboxylate ions with itself reduced to metallic silver, which deposits on the wall of the reaction vessel as silver mirror.

The Tollen's reagent is prepared in the laboratory by adding aqueous ammonia solution to silver(I) nitrate solution drop by drop until the precipitate is just dissolved.

2Ag+ + 2OH-Ag2O(s) + H2O

Ag2O(s) + 4NH3 + H2O  2Ag(NH3)2OH(aq)

There may be an explosive hazard if the mixture is heated to dryness. Furthermore, if the test-tube is not clean, a silver mirror cannot be formed and a black solid deposit is obtained instead.

Tollen's reagent gives a negative result with all ketones and therefore can serve as a specific test for distinguishing aldehydes from ketones.

Reaction with Fehling's reagent (Fehling's test斐令試驗)

Aliphatic aldehydes reduce the copper (1I) ion in Fehling's reagent to the reddish-brown copper (I) oxide precipitate.

Ketones and aromatic aldehydes give negative result to this test, so this reagent can also serve as a means to distinguish aldehydes from ketones.

N.B. Fehling's reagent is a solution mixture of copper (II) sulphate and sodium potassium tartrate in excess sodium hydroxide.

4.Haloform Reaction

Methyl ketones or those containing a group react with halogens in aqueous sodium hydroxide, ultimately producing a carboxylate ion and a haloform (CHX3). Among these, iodoform will be more useful as it is a yellow precipitate and thus can be used to distinguish the methyl ketone from other compounds:

B.Reduction

1.With lithium aluminium hydride四氫合鋁酸鋰(LiAIH4) or sodium borohydride (NaBH4) 四氫合硼酸鈉

Aldehydes and ketones are reduced to the primary and secondary alcohols by the two reducing agents.

Exercise:

Outline simple chemical tests, together with the expected observations, which would enable you to distinguish

a)CH3CH2CHO and CH3COCH3

b)CH3COCH3 and C2H5COC2H5

VIII.USES OF CARBONYL COMPOUNDS

1.Plastics and Resins

Carbonyl compounds are important in the manufacture of plastics. Methanal, for example, is used in the manufacture of tough plastics like Bakelite and Delrin whereas propanone is used in the manufacture of perspex.

2.Solvents

Propanone with a boiling point 56°C is widely used as an inexpensive solvent in both the laboratory and industry. In industry, for example, propanone is used as a solvent for plastics, varnishes and grease. Organic Syntheses

Carbonyl compounds are also used in the laboratory and in industry to synthesize a range of chemical intermediates and products. Ethanal, for example, is used as a starting point in the manufacture of ethanoic acid and chloral hydrate - a hypnotic drug. The main use of chloral is, however, in the manufacture of insecticide DDT.

3.Other Uses

Methanal is used as a 40% aqueous solution, known as formalin, for the preservation of biological specimens. The solid trimethanal (paraformaldehyde) is a convenient source of methanal.

Benzaldehyde, which has a characteristic almond odor, is used to flavor foods.