Ultraviolet and Visible Spectra Reactions of Aldehydes and Ketones Aldehydes and ketones undergo a variety of reactions that lead to many different products. Due to differences in electronegativities, the carbonyl group is polarized. The carbon atom has a partial positive charge, and the oxygen atom has a partially negative charge. Aldehydes are usually more reactive toward nucleophilic substitutions than ketones because of both steric and electronic effects.
Mechanisms[ edit ] The aldol reaction may proceed by two fundamentally different mechanisms. Carbonyl compounds, such as aldehydes and ketones, can be converted to enols or enol ethers. This is the 'enol mechanism'. Carbonyl compounds, being carbon acidscan also be deprotonated to form enolates, which are much more nucleophilic than enols or enol ethers and can attack electrophiles directly.
The usual electrophile is an aldehyde, since ketones are much less reactive.
This is the 'enolate mechanism'. If the conditions are particularly harsh e. Although the aldol addition usually proceeds to near completion under irreversible conditions, the isolated aldol adducts are sensitive to base-induced retro-aldol cleavage to return starting materials.
In contrast, retro-aldol condensations are rare, but possible. The acid also serves to activate the carbonyl group of another molecule by protonation, rendering it highly electrophilic.
This usually dehydrates to give the unsaturated carbonyl compound.
The scheme shows a typical acid-catalyzed self-condensation of an aldehyde. Acid-catalyzed aldol mechanism Enolate mechanism[ edit ] If the catalyst is a moderate base such as hydroxide ion or an alkoxidethe aldol reaction occurs via nucleophilic attack by the resonance-stabilized enolate on the carbonyl group of another molecule.
The product is the alkoxide salt of the aldol product. The aldol itself is then formed, and it may then undergo dehydration to give the unsaturated carbonyl compound. The scheme shows a simple mechanism for the base-catalyzed aldol reaction of an aldehyde with itself. In this case, enolate formation is irreversible, and the aldol product is not formed until the metal alkoxide of the aldol product is protonated in a separate workup step.
Zimmerman—Traxler model[ edit ] More refined forms of the mechanism are known. InHoward Zimmerman and M. Traxler proposed that some aldol reactions have "six-membered transition states having a chair conformation.
E-enolates give rise to anti productswhereas Z-enolates give rise to syn products. The factors that control selectivity are the preference for placing substituents equatorially in six-membered transition states and the avoidance of syn-pentane interactionsrespectively.
In reality, only some metals such as lithium reliably follow the Zimmerman—Traxler model. Thus, in some cases, the stereochemical outcome of the reaction may be unpredictable. Crossed-aldol reactant control[ edit ] The problem of "control" in the aldol addition is best demonstrated by an example.
Consider the outcome of this hypothetical reaction: In this reaction, two unsymmetrical ketones are being condensed using sodium ethoxide. The basicity of sodium ethoxide is such that it cannot fully deprotonate either of the ketones, but can produce small amounts of the sodium enolate of both ketones.
This means that, in addition to being potential aldol electrophiles, both ketones may also act as nucleophiles via their sodium enolate. Two electrophiles and two nucleophiles, then, have potential to result in four possible products: Acidity[ edit ] The simplest control is if only one of the reactants has acidic protons, and only this molecule forms the enolate.
For example, the addition of diethyl malonate into benzaldehyde would produce only one product. Double-activation makes the enolate more stable, so not as strong a base is required to form it. If one partner is considerably more acidic than the other, the most acidic proton is abstracted by the base and an enolate is formed at that carbonyl while the carbonyl that is less acidic is not affected by the base.
This type of control works only if the difference in acidity is large enough and no excess of base is used for the reaction. A typical substrate for this situation is when the deprotonatable position is activated by more than one carbonyl-like group.
Common examples include a CH2 group flanked by two carbonyls or nitriles see for example the Knoevenagel condensation and the first steps of the Malonic ester synthesis.
Order of addition[ edit ] One common solution is to form the enolate of one partner first, and then add the other partner under kinetic control. For this approach to succeed, two other conditions must also be satisfied; it must be possible to quantitatively form the enolate of one partner, and the forward aldol reaction must be significantly faster than the transfer of the enolate from one partner to another.
Formation[ edit ] The enolate may be formed by using a strong base "hard conditions" or using a Lewis acid and a weak base "soft conditions": In this diagram, B: The dibutylboron triflate actually becomes attached to the oxygen only during the reaction.
Geometry[ edit ] Extensive studies have been performed on the formation of enolates under many different conditions. It is now possible to generate, in most cases, the desired enolate geometry: For estersmost enolization conditions give E enolates.Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones.
Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. Aldehydes and ketones can be starting materials for a range of other functional groups. We will be learning about the nomenclature and reactions of aldehydes and ketones, including how to use acetals as protecting groups.
Small Aldehydes and Ketones are easily dissolved in water but as the chain increases in length, its solubility decreases. (1) In this experiment, the Chromic Anhydride (Jones’s Test), Tollen’s Reagent and the Iodoform reaction were used to test for the presence of aldehydes and ketones.
Aldehydes and ketones can be starting materials for a range of other functional groups. We will be learning about the nomenclature and reactions of aldehydes and ketones, including how to use acetals as protecting groups. The ketone carbon is often described as "sp 2 hybridized", a description that includes both their electronic and molecular regardbouddhiste.coms are trigonal planar around the ketonic carbon, with C−C−O and C−C−C bond angles of approximately °.
Ketones differ from aldehydes in that the carbonyl group (CO) is bonded to two carbons within a carbon skeleton. Aldehydes and Ketones are organic compounds that consist of the carbonyl functional group, C=O.
The carbonyl group that consists of one alkyl substituent and one hydrogen is the Aldehyde and those containing two alkyl substituents are called Ketones.