3) The general characteristic to reactionary ability of carbonyl compounds.

Aldehydes and ketones undergo a variety of reactions that lead to many different products. The most common reactions are nucleophilic addition reactions, which lead to the formation of alcohols, alkenes, diols, acetal and etc. In aldehydes, the relatively small hydrogen atom is attached to one side of the carbonyl group, while a larger R group is affixed to the other side. In ketones, however, R groups are attached to both sides of the carbonyl group. Thus, steric hindrance is less in aldehydes than in ketones.

Electronically, aldehydes have only one R group to supply electrons toward the partially positive carbonyl carbon, while ketones have two electron-supplying groups attached to the carbonyl carbon. The greater amount of electrons being supplied to the carbonyl carbon, the less the partial positive charges on this atom and the weaker it will become as a nucleus.

4) Nucleophilic addition reaction of aldehydes and ketones.

a) Formation of a hydrate via a nucleophilic addition(addition of water)

b) Aldol condensation reaction

c) Formation of acetals or hemiacetals via a nucleophilic substitution

The main reactions of the carbonyl group are nucleophilic additions to the carbon-oxygen double bond. As shown below, this addition consists of adding a nucleophile and a hydrogen across the carbon-oxygen double bond.

Due to differences in electronegativity, the carbonyl group is polarized. The carbon atom has a partial positive charge, and the oxygen atom has a partially negative charge.

a) Addition of water

The addition of water to an aldehyde results in the formation of a hydrate.

The formation of a hydrate proceeds via a nucleophilic addition mechanism.

1. Water, acting as a nucleophile, is attracted to the partially positive carbon of the carbonyl group, generating an oxonium ion.

2. The oxonium ion liberates a hydrogen ion that is picked up by the oxygen anion in an acid-base reaction.

Small amounts of acids and bases catalyze this reaction. This occurs because the addition of acid causes a protonation of the oxygen of the carbonyl group, leading to the formation of a full positive charge on the carbonyl carbon, making the carbon a good nucleus. Adding hydroxyl ions changes the nucleophile from water (a weak nucleophile) to a hydroxide ion (a strong nucleophile). Ketones usually do not form stable hydrates.

b) Aldol condensation

Aldehydes that have α hydrogens react with themselves when mixed with a dilute aqueous acid or base. The resulting compounds, β-hydroxy aldehydes, are referred to as aldol compounds because they possess both an aldehyde and alcohol functional group.

The aldol condensation proceeds via a carbon ion intermediate. The mechanism of base-catalyzed aldol condensation follows these steps:

1. The base removes an α-hydrogen.

2. The carbon ion undergoes nucleophilic addition with the carbonyl group of a second molecule of ethanal, which leads to formation of the condensation product.

3.A reaction with water protonates the alkoxide ion.

Cross-aldol condensation

An aldol condensation between two different aldehydes produces a cross-aldol condensation. If both aldehydes possess α hydrogens, a series of products will form. To be useful, a cross-aldol must be run between an aldehyde possessing an α hydrogen and a second aldehyde that does not have α hydrogens.

Ketonic aldol condensation

Ketones are less reactive towards aldol condensations than alde-hydes. With acid catalysts, however, small amounts of aldol product can be formed. But the Aldol product that forms will rapidly dehydrate to form a resonance-stabilized product. This dehydration step drives the reaction to completion.

The acid-catalyzed aldol condensation includes two key steps: the conversion of the ketone into its enolic form, and the attack on a protonated carbonyl group by the enol. The mechanism proceeds as follows:

1. The oxygen of the carbonyl group is protonated.

2. A water molecule acting as a base removes an acidic α hydrogen, which leads to an enol.

3. The enol attacks a protonated carbonyl group of a second ketone molecule.

c) Addition of alcohol

Reactions of aldehydes with alcohols produce either hemiacetals (a functional group consisting of one —OH group and one —OR group bonded to the same carbon) or acetals (a functional group consisting of two —OR groups bonded to the same carbon), depending upon conditions. Mixing the two reactants together produces the hemiacetal. Mixing the two reactants with hydrochloric acid produces an acetal. For example, the reaction of methanol with ethanal produces the following results:

A nucleophilic substitution of an OH group for the double bond of the carbonyl group forms the hemiacetal through the following mechanism:

1. An unshared electron pair on the alcohol's oxygen atom attacks the carbonyl group.

2. The loss of a hydrogen ion to the oxygen anion stabilizes the oxonium ion formed in Step 1

The addition of acid to the hemiacetal creates an acetal through the following mechanism:

1. The proton produced by the dissociation of hydrochloric acid protonates the alcohol molecule in an acid-base reaction.

2. An unshared electron pair from the hydroxyl oxygen of the hemiacetal removes a proton from the protonated alcohol.

3. The oxonium ion is lost from the hemiacetal as a molecule of water.

4. A second molecule of alcohol attacks the carbonyl carbon that is forming the protonated acetal.

5. The oxonium ion loses a proton to an alcohol molecule, liberating the acetal.