Aldehydes And Ketones

Aldehydes and Ketones Carbonyle Group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom : C=O. However the carbonyl group is best described as a hybrid of the following resonance structures.

Carbonyl group characterizes the following types of compounds (where -CO denotes a C=O carbonyl group)

Oxygen of carbonyl group has a slight negative charge and partial positive charge on carbon.

The partial positive charge on carbon makes it venerable to nucleophile and partial negative charge on oxygen is attractive to electrophile.

Carbonyl group has sp3 hybridization and a trigonal planer structure. The planer structure leaves open space above and below making it susceptible to attack from both sides. Aldehydes and ketones follow nucleophilic addition while other compounds prefer nucleophilic substitution. The partial negative charge on oxygen makes it attractive for protons and it can be easily protonated.

Aldehydes and Ketones General structures and structures of 1st member of aldehyde and ketones family are shown below:

Physical properties a) Boiling point increases as molecular weight increases and follows the same trend as followed by alkane and alkene. Formaldehyde is gas at room temperature. Acetaldehyde boils at 20ºC. Rests of all the members are liquids. b) Density of aldehyde and ketones is lesser than water. c) Aldehydes and ketones are polar compounds. d) They don’t have hydrogen bonding but form hydrogen bonding with the compound which have hydrogen bonding. e) First four members are soluble in water. f) They are excellent organic solvents. Chemical Properties Aldehydes and ketones shows nucleophile addition reaction or acts as Bronstred-lowry acid by donating its ∀-hydrogen. The carbon which is attached to carbonyl carbon is called ∀-carbon. The next carbon is called ∃-carbon and another in sequence is called (-carbon. The hydrogen which is attached to these carbon is called ∀-hydrogen, ∃hydrogen and (-hydrogen respectively.

The ∀-hydrogen of ketones and aldehyde is acidic in nature. The ionization of a ∀-hydrogen forms a carbanion. The carbanion is resonance hybrid of the two structures. This type of resonance is possible only through participation by carbonyl group. This type of resonance is not possible for carbanion formed by ionization of (-hydrogen or ∃-hydrogen from saturated carbonyl compound. The carbonyl group thus effects the acidity of ∀-hydrogen by helping to accommodate negative charge on it. This carbanion is called enolate anion.

The ∀-carbon of enolate ion is negatively charged. It can act as a nucleophile. When ∃-carbon is also a carbonyl (called a ∃-dicarbonyl), the enol formed is more stabilized due to internal hydrogen bonding and resonance.

Aldehydes are more reactive than ketones as well as more acidic than ketones. The reason is same for both. Aldehyde has only one alkyl group while ketones have two alkyl groups. Alkyl group is electron donating group. Hence more electrons are released from ketones than from aldehyde. This intensifies negative charge on carbonyl oxygen. Thus intensification of negative charge due to ketones will be more than aldehyde. The excess negative charge can not be distributed easily hence ketones are less reactive and less acidic than aldehydes. Ketones show tautomerism with enol. Tautomerism refers to a state of equilibrium between two different structures of the same compound. Usually the tautomers differ in the point of attachment of a hydrogen atom. Tautomerism is a reaction at equilibrium and not a resonance. In resonance structure neither atom moves nor structure actually exists. Therefore tautomerism is not resonance. A structure with -OH attached to a doubly bonded carbon is called enol. There is equilibrium between the two structures. Equilibrium almost always lies in favor of ketones and tends to move toward ketones. The mechanism is shown below.

Acetal Preparation The aldehyde is dissolved in excess of an anhydrous alcohol with a small amount of anhydrous acid. The result is formation of acetal. The reaction is reversible with water and small quantity of acid. Alcohol reacts as a nucleophile and follows addition reaction mechanism. Ketones also react in the same pattern. The product formed is called ketal. Initially hemiacetal and hemiketal are formed from aldehyde and ketones respectively. Acetal and ketal are formed subsequently.

The aldehyde products can be easily recognized by the presence of lone hydrogen which is not present in ketones products. Hemiacetal and hemiketal

can be distinguished by the presence of alcohols, while acetal and ketal both do not have alcohol. The ‘hemi’ products can be catalyzed either by base or an acid. However in formation of acetal and ketal from ‘hemi’ the presence of acid is must because hydroxyl group must be protonated. Hence this part of reaction has acid as the catalyst.

Although acetal is hydrolyzed to aldehyde and ketones in aqueous acid, they are stable in basic solution. This property is utilized to protect aldehyde and ketones.

We can convert an aldehyde or ketones to acetal, carry out a reaction on some other part of the molecule and then hydrolyze acetal by aqueous acid.

In a similar reaction aldehyde or ketones are dissolved in aqueous solution and establish equilibrium with their hydrate a geminal diol.

Aldol condensation A molecule which contains a function group of alcohol and aldehyde in general is called aldol.An Aldol condensation is an organic reaction in which an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone. When acetaldehyde reacts with dilute sodium hydroxide it produces 3 hydroxy butanal. It is the compound containing alcohol and aldehyde function groups. The general common name for the product is Aldol.

The mechanism for Aldol condensation shows two important properties of carbonyl compounds. First is acidity of their ∀-hydrogen and second is the tendency of the carbonyl group to accept a nucleophile. Step 1: The reaction starts with an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen from ∀-carbon of one of the molecules giving the resonance stabilized reactive enolate ion.

Step 2: The nucleophilic enolate attacks the ketone at the electrophilic carbonyl carbon of second molecule in a nucleophilic addition type process giving an intermediate alkoxide anion.

Step 3: This step involves an acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide and the β-hydroxyketone, which is the aldol product.

Although the reaction is conventionally called condensation, no condensation has taken and the reaction is purely an addition reaction. The next step from aldol to enal is condensation. The step may also be called dehydration. In some reaction dehydration occurs so rapidly that product in aldol form can not be isolated and the product formed is enal (alkene aldehyde).

Halogenation Halogens react by substitution mechanism with ketones in presence of base as well as acid. The substitution takes place exclusively at ∀-carbon. This behavior is due to acidity of ∀-hydrogen and tendency of ketones to form enol.

In presence of a base, reaction is slow and forms either enolate anion or enol followed by rapid reaction of enolate ion or enol with halogen. The base is consumed forming water. In the presence of acid, enol is formed and enol reacts with halogen. The acid is recovered at the end of reaction. Thus in true sense it is acid catalyzed reaction.

Haloform reaction When methyl ketones are treated with the halogen in basic solution, polyhalogenaton followed by cleavage of the methyl group occurs. The products are the carboxylate ion and trihalomethane, otherwise known as haloform. The reaction proceeds via successively faster halogenations at the α-position until all the three hydrogen have been replaced. The halogenations get faster since the halogen stabilizes the enolate negative charge and makes it easier to form. Then a nucleophilic acyl substitution by hydroxide displaces the anion CX3 as a leaving group that rapidly protonates. This reaction is often performed using iodine and as a chemical test for identifying methyl ketones. Iodoform is yellow and precipitates under the reaction conditions.

The Wittig Reaction The Wittig reaction is a chemical reaction of an aldehyde or ketone with a triphenyl phosphoniumylide to give an alkene and triphenylphosphine oxide. Ketones behaves normal by initially undergoing nucleophilic addition to form betine which easily breaks down to triphenylphosphine oxide and the alkene.

∀-∃ Unsaturated Carbonyls α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure Cβ=Cα−C=O. In these compounds, the carbonyl group is conjugated with an alkene (hence the adjective unsaturated), from which they derive special properties. Examples of unsaturated carbonyls are acrolein, mesityl oxide, acrylic acidand maleic acid. α,β-Unsaturated aldehyde and ketone may react with nucleophile reagent in two ways. a) simple additioin process in which a nucleophile add across the double bond of the carbonyl group. b) A conjugate addition

In some cases both the additions occur in the same mixture. The major product formed depends upon nucleophile. Strong nucleophile favors simple addition while weak nucleophile favors conjugate addition

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