UNIT 14 ORGANIC COMPOUNDS WITH FUNCTIONAL GROUPS CONTAINING OXYGEN-II (Aldehydes, Ketones, Carboxylic acids and their derivatives)
· Aldehydes and ketones
· Carboxylic acids
· Functional derivatives of Carboxylic acids
In this unit we study a very important oxygen containing organic compounds in which oxygen atom is bonded to a carbon atom through a double bond. The functional unit >C=O , present in these compounds is called carbonyl group. Although a large variety of organic compounds containing a carbonyl group is possible, we limit our study to those compounds in which an acyl group (R-C=O) is bonded to hydrogen, carbon, oxygen halogens and nitrogen. These compounds are known as carbonyl compounds and are grouped into families of aldehydes, ketones, carboxylic acids and their derivatives.The functional derivatives of carboxylic acids are further sub-divided into families of esters, acyl(or acid) halides, acid anhydrides and amides. The general formulae of these families of compounds are shown below.
Acid halides and anhydrides do not occur in nature due to their high reactivity. In contrast , other carbonyl compounds are wide spread in plants and animals. They play important roles in biochemical processes to sustain life. They add fragrance and flavour to Nature and constitute several pharnaceuticals. Some members of these families are manufactured in large quantities for use as solvents and for producing materials like adhensives, paints, fabrics, etc. Aldehydes and ketones belong to an important class of reactive organic compounds and occupy the centre stage of organic synthesis.
They have the general formulaCnH2nO and contain the same functional group( >C=O), the carbonyl group. In aldehydes the remaining two valencies of the carbonyl group are satisfied either by hydrogen (formaldehyde) or hydrogen atom and a hydrocarbon group, while in ketones both the valencies are satisfied by the same or different hydrocarbon groups. When the two hydrocarbon groups are same, the ketone is known as simple ketone, otherwise as a mixed ketone.
The common names for aldehydes are derived from the names of corresponding acids. The suffix -ic of the acid in the common name of the acid is replaced with the word -aldehyde. For example HCHO formaldehyde, CH3CHO acetaldehyde .
Branching in the aldehyde chain , if any is indicated by the Greek letters a, b, g, d, etc. The a-carbon atom being the one directly linked to the aldehyde group, b-carbon the next, and so on.
In IUPAC system, the largest chain carrying -CHO group is selected. Then from the name of corresponding alkane, the suffix -e is replaced by 'al'. Therefore, saturated aliphatic aldehydes are called alkanal. For example, HCHO formaldehyde is derived from methane is called methanal . Since the -CHO group is always present at the end of the chain, it is always assigned number 1. When an aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of hydrocarbon. For example,
The position of the aldehydic group is normally not indicated because it is always 1. A list of the Common and IUPAC names of some aldehydes are given as :
If the compound contains both types of carbonyl groups, then it is named as an aldehyde and the keto oxygen atom is treated as a substituent on the chain denoted by the prefix ‘’oxo’’-.
In aromatic aldehydes, the functional group may be attached either directly to the ring or may be present in the side chain. The simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzaldehyde, which is the common name accepted by IUPAC. Other ring substituted aromatic aldehydes are named as derivatives of benzldehyde.
In common system, aliphatic ketones are named after the alkyl groups attached to the carbonyl group followed by the word ketone. The simplest ketone , CH3COCH3 , is also called acetone.The positions of the substituents are indicated by the Greek letters, a, a’, b , b’ and so on ; a, a’ carbons being the ones directly attached to the carbonyl group.
In IUPAC system, the longest chain carrying >C=O group is selected. Then from the name of the corresponding alkane , the suffix -e is replaced by 'one'. Therefore, saturated aliphatic ketones are called alkanone. In this case , the longest chain containing the carbonyl group is selected and the carbonyl group is assigned the lowest possible number while numbering the chain of carbon atoms. For example,
Ketones with carbonyl group attached to a benzene ring are also named as phenones, some of such names have also been accepted by IUPAC.
A list of the common and IUPAC name of some ketones are given below :
Ethyl methyl ketone
01. Write the structural formulae of all the carbonyl compounds with the molecular formula C5H10O and name them according to the IUPAC system.
STRUCTURE OF THE CARBONYL GROUP
The carbonyl carbon in aldehydes and ketones is sp2 hybridised and forms three s-bonds separated by 120° from each other. One of the s-bonds is formed with the oxygen atom and the other two with carbon and or hydrogen atoms. The fourth valence electron , which remains in the unhybridised p-orbital, overlaps with oxygen p-orbital to form p-bond. The oxygen atom has has two lone pairs of electrons, which occupy its remaining two orbitals. The carbonyl carbon and the three atoms bonded to it lie in one plane and the p-electron cloud is above and below this plane , as shown in Fig.
The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. The electrons, particularly the losely held p-electrons , are pulled strongly towards the oxygen atom to give oxygen a partial negative charge and carbon a partial positive charge indicated by the symbol d-and d+ respectively. Thus , the carbonyl carbon is an electrophilic (Lewis acidic), and carbonyl oxygen a nucleophilic (Lewis basic ) centre. Carbonyl compounds have substantial dipole moments and are more polar than ethers. For example, the dipole moments of ethanal, propanone and diethyl ether are 2.72 , 2.88 and 1.18 D respectively. The high polarity of carbonyl group is explained on the basis of resonance involving a neutral (A) and a dipolar (B) structures as shown below :
Physical properties of aldehydes and ketones
Formaldehyde , the first member of the aldehyde series , is a gas at ordinary temperature while, acetaldehyde boils at 21°C. The next nine members (C3-C11) are liquids and the higher members are solids. Ketones up to C11 are colourless mobile liquids, and higher members are solids.
Lower aldehydes possess unpleasant odour but as we go up in the series the smell becomes more and more fruity. Ketones are generally pleasant smelling liquids. The higher aldehydes (C8 - C13 ) and ketones are used in perfumery.
The first few members of the aldehyde and ketone series are soluble in water, but the solubility falls rapidly with increase in the size of the hydrocarbon groups attached to carbonyl group. Those with five or more carbon atoms are sparingly soluble or insoluble.
The solubility of aldehydes and ketones is accounted for by the electrostatic attractions between the polar carbonyl group and water dipoles.
Beyond C5, however, the electrostatic forces of attraction between the polar group and water dipoles are insufficient to hold the molecule in solution since the large hydrocarbon-like nature of alkyl groups become dominant.
Aldehydes and ketones being quite polar compounds ( acetaldehyde 2.70 D ; acetone 2.85 D), there is a good deal of intermolecular association due to electrostatic attraction between opposite ends of C=O dipoles.
Hence the boiling points of carbonyl compounds are higher than the non-polar alkanes of comparable molecular weight. The dipole-dipole attraction between molecules of aldehydes or ketones , however is weaker than the analogous hydrogen bonding between alcohol molecules. Thus the boiling points of the carbonyl compounds are lower than those of corresponding alcohols. For illustration, acetone, propionaldehyde and butane (having molecular weights 58, 58, 60) show boiling points 329 K, 322K and 272.5 K respectively.
In general, melting point and boiling points of the members of both the aldehyde and ketone series show regular increase as we ascend the respective series.
02. Arrange the following compounds in increasing order of their boiling points : CH3CHO, CH3CH2OH, CH3OCH3, CH3CH2CH3.
General Methods of Preparation
The aldehydes and ketones can be prepared by analogous methods as described below :
1. From Alcohols
Primary alcohols on oxidation with acid potassium dichromate, manganese dioxide or chromic anhydride in glacial acetic acid yield aldehydes.
Aldehydes undergo further oxidation to carboxylic acids. Therefore oxidation of aldehydes is carried out under controlled conditions.
Secondary alcohols on similar oxidation gives ketones.
Ketones can be obtained in good yield, by Openauer's oxidation of secondary alcohols using aluminium-tertiary butoxide [(CH3)3CO]3Al in excess of acetone.
Aldehydes have lower boiling points than the corresponding alcohols, hence they should be removed from the reaction mixture as soon as they are formed to avoid further oxidation to corresponding acids.
Pyridinium chloro-chromate (C5H5NH+CrO3Cl-), abbreviated as PCC , is a milder reagent which oxidises primary alcohols to aldehydes and secondary alcohols to ketones in dichloromethane solvent. Aldehydes are not oxidised further to carboxylic acids. Carbon-carbon double bond also remains unaffected. PCC is prepared by mixing pyridine (C5H5N), CrO3 and HCl in dichloromethane.
(b) By Catalytic dehydrogenation
Aldehydes can be prepared by the dehydrogenation of primary alcohols with hot reduced copper at 573 K
Ketones can be obtained by the dehydrogenation of secondary alcohols with hot reduced copper at 573 K
2. From Carboxylic Acids
(i) Catalytic decomposition of Carboxylic Acids : Both aldehydes and ketones can be prepared by the catalytic decomposition of carboxylic acids. Vapours of fatty acid mixed with formic acid , when passed over heated Thoria (623 K), alumina (673 K) or manganous oxide(573 K) give aldehydes.
Vapours of fatty acid alone gives a ketone under similar conditions.
When a mixture of vapours of different fatty acid (other than formic acid) is used, mixed ketones are obtained in low yield.
(ii) By the Pyrolysis of Calcium salts of Fatty Acids : When calcium formate is heated alone formaldehyde is obtained.
Other aldehydes can be obtained by distilling the calcium salt of the fatty acid with calcium formate :
However, when calcium salt of a fatty acid (other than formaldehyde) is distilled a ketone is obtained.
A mixed ketone can be obtained by heating together a mixture of calcium salts of different fatty acids.
3. From glycols
Oxidation of secondary glycols with lead tetra acetate gives an aldehyde.
Tertiary glycols under similar conditions give ketones :
4. From dihalides
The dihalides containing two halogens on the same carbon at the end of a chain on hydrolysis yield aldehydes. A mild alkali like Ba(OH)2 may be used for hydrolysis.
If, however, two halogen atoms are attached to a carbon in the middle of a chain a ketone is obtained.
When a stream of ozone or ozonised oxygen is passed through a solution of alkene in an inert solvent like ether or carbon tetrachloride, it adds a molecule of ozone at the double bond to give an ozonide.
The ozonides on reduction with hydrogen in presence of a catalyst (Pt or Pd) or on boiling with water containing traces of zinc dust , split to give aldehyde and / or ketones. The fission of the molecule takes place at the position occupied by the double bond.
An alkene of the type RR’C=CHR’ gives a mixture of aldehydes and ketones, whereasRR’C=CRR’ gives ketones.
6. From alkynes
Alkynes add on a molecule of water in presence of sulphuric acid and mercurous sulphate ( or mercury-mercuric sulphate mixture) to yield carbonyl compounds. The addition of water to alkyne initially yields a hydroxy alkene in which hydroxyl group is on doubly bonded carbon atom (structure known as Enolic form or Enol). These simple enols immediately rearrange to form a more stable carbonyl or keto form.
7. From Grignard reagent
Hydrogen cyanide or akyl cyanide can be made to react with Grignard reagent to form an addition product, which when hydrolysed with water gives an aldehyde or a ketone as the case may be.
The alkyl radical in the aldehyde comes from the Grignard reagent and in the case of ketone one of the alkyl radical is contributed by Grignard reagent.
Methods Giving Only Aldehyde or Ketone
8. From Acid chlorides
Acid chloride when reduced with hydrogen in boiling xylene in presence of palladium catalyst suspended over barium sulphate(poisoned by organic sulphur compounds) give aldehydes in good yield. This reaction is known as Rosenmund reduction.
Ketones cannot be prepared by this method.
9. From Grignard reagent
An aldehyde can be prepared by treating Grignard reagent with excess of formic ester.
PREPRATION OF AROMATIC ALDEHYDES AND KETONES
(i) By Controlled Oxidation of Side Chains In Arenes
Aromatic aldehydes are prepared when alkyl side chain in aromatic ring is oxidised using chromium trioxide (CrO3) and acetic anhydride. The aldehyde formed immediately gets acetylated with acetic anhydride and this does not get further oxidised. Therefore, the function of acetic anhydride is to prevent further oxidation of the aldehyde to acid.
Another procedure (Etard Reaction) uses chromyl chloride as the oxidising agent. The intermediate complex thus formed is hydrolysed to give benzaldehyde in good yield.
(ii) Reimer-Tiemann reaction
Aromatic aldehydes containing a hydroxyl group (phenolic aldehyde) either to the ortho or para position can be prepared from phenol by treating it with chloroform in aqueous sodium hydroxide solution at about 343 K. This reaction is called Reimer Tiemann reaction.
(iii) Friedel Craft's reaction
Aromatic ketones can be prepared by Friedel Craft's acylation or Benzoylation by treating aromatic hydrocarbons with acid chlorides in presence of Lewis acids like anhydrous aluminium chloride.
(iv) Distilling of calcium salts of acids
(a) Benzophenone can be prepared by distilling calcium benzoate.
(b) Benzaldehyde can be prepared by distilling calcium benzoate with calcium formate.
(v) Oxidation of benzyl chloride
Benzaldehyde is produced by the oxidation of benzyl chloride by boiling with aqueous copper or lead nitrate
(vi) Hydrolysis of benzal chloride
Benzaldehyde is manufactured by the hydrolysis of benzal chloride.
Benzal chloride is obtained by chlorinating toluene until gain in weight corresponds to the substitution of two chlorine atoms per molecule.
(vii) Gattermann Kotch reaction
When an equimolecular mixture of carbon monoxide and hydrogen chloride is bubbled through a solution of benzene in ether containing anhydrous aluminium chloride, benzaldehyde is produced.
Carbon monoxide and hydrogen chloride behave as formyl chloride(HCOCl) which enters into Friedel Craft’s type reaction with benzene.
(viii) Gattermann Aldehyde reaction
Benzene treated with a mixture of hydrogen cyanide and hydrogen chloride in the presence of anhydrous aluminium chloride. An addition product (formimino chloride) first formed enters into Friedel Craft’s type reaction with benzene to give the imine which is decomposed with water to produce benzaldehyde.
(ix) Rosenmund reduction
Benzaldehyde may be produced by the reduction of benzoyl chloride with H2 in presence of palladium-barium sulphate catalyst.
(x) Grignard reaction
Benzaldehyde may be obtained by producing phenyl magnesium bromide and then reacting with ethyl formate.
03. Give the reagents to bring about the following transformations:
(a) Butan-1-ol to butanal
(b) Cyclohexanol to cyclohexanone
(c) Pent-3-en-2-ol to Pent-3-en-2-one
(d) But-2-ene to Ethanal
(e) But-1-yne to But-2-one
(f) p-Nitrotoluene to p-Nitrobenzaldehyde
Aldehydes and ketones undergo similar reactions because of the presence of a carbonyl functional group in both of them.
1. Nucleophilic addition reactions
Being unsaturated, aldehydes and ketones undergo addition reactions. The carbon atom of carbonyl group is electrophilic, and are therefore , susceptible to attack by nucleophiles. Hence, the most typical reactions of aldehydes and ketones are nucleophilic addition reactions to carbon-oxygen double bond.
Mechanism of Nucleophilic Addition Reactions
The attack by the Nucleophile(Nu-) on carbonyl carbon may occur from above or below the plane of the carbonyl group leading to C-Nu bond formation. This is accompanied by heterolytic cleavage of the weaker carbon-oxygen pi-bond with complete transfer of the electron pair of the pi bond to oxygen atom. The oxygen atom , thus gets a negative charge and holds it easily due to its high electronegativity. During this process , the hybridisation of carbonyl carbon changes from trigonal to tetrahedral and the oxygen atom is pushed out of the plane of the carbonyl group. The negatively charged tetrahedral intermediate is basic and captures a proton from the medium to give the electrically neutral product. The net result is the addition of Nu- and H+ across the carbon-oxygen double bond.
The nucleophilic addition reactions of carbonyl group are catalysed by acids and bases.
(i) Acid catalysed addition : The proton released by the acid combines with the carbonyl oxygen and thus increases the electron deficiency of the carbon atom. In this way the positivity of the carbonyl carbon atom is increased and the attack of the nucleophile is enhanced.
(ii) Base-catalysed addition : Here the nucleophile (:Nu) is generated from the conjugated acid (Nu-H) in the presence of a base (OH-) and the addition takes place as described before.
It may be noted that whether the addition is acid-catalysed or base catalysed, the product obtained is the same. An acid catalyst promotes nucleophilic attack by increasing the nuclephilic character of the carbonyl carbon, while a base does so by increasing the nucleophilicity of the reagent.