Is carboxylic acid electrophilic
10. The carbonyl group: aldehydes and ketones - nucleophilic addition
The C = O double bond of the Carbonyl group is the most important functional group in organic chemistry. This chapter deals with the chemistry of Aldehydes and Ketones.
10.1 Types of carbonyl groups
The following table shows the most important classes of carbonyl compounds:
It can be useful to divide the carbonyl compounds into two classes:
| Aldehydes and ketones have either one |
H atom or an alkyl or aryl group.
Such groups cannot have a negative charge
wear. You can therefore not be used as an outgoing
Aldehydes and ketones have similar chemical properties
Properties, but differ in their
Chemistry of carboxylic acid derivatives.
10.2 Structure and chemical properties of the carbonyl group
Both the oxygen and carbon atoms of the carbonyl group are sp2-hybridized and are therefore in the same plane as the other two neighboring atoms of the carbon atom; the bond angles at the carbon atom are about 120O :
A comparison with the electronic structure of the double bond in alkenes shows two major differences; The oxygen atom has two free electron pairs and is also more electronegative than C. The latter influences the π electron cloud in such a way that a noticeable polarization of the C = O bond can be observed:
In this way, the carbon atom becomes electrophilic and the oxygen atom becomes nucleophilic and slightly basic:
10.3 Nomenclature of aldehydes and ketones
The representatives of this class of compounds are named with systematic and with common names.
The systematic names of the aldehydes are derived from those of the corresponding alkanes by adding the suffix -al from. The position of the C = O group is not specified. By definition, its carbon atom is C1. As long as the aldehyde function belongs to the longest carbon chain, the numbering of the other carbon atoms is also clearly defined:
Systems that are not so easy to use because of the ending -al can be named as Carbaldehydes designated.
According to the IUPAC rules, ketones are called Alkanones, where the ending becomes -on appended to the name of the corresponding alkane. The position of the carbonyl group in the longest chain is determined by numbering in such a way that the carbon atom of the carbonyl group receives the lowest possible number:
In the case of complicated structures, the designation oxo used to indicate the presence of a carbonyl group. Many substituents that contain a carbonyl group have special names, for example:
10.4 Reactions of Aldehydes and Ketones: Nucleophilic Addition Mechanisms
The carbonyl group is highly polarized, so electrophiles attack the oxygen atom and nucleophiles attack the carbon atom. One of the most important reactions aldehydes and ketones enter into is that Nucleophilic addition on the carbonyl group:
We can write the following general mechanisms:
10.4.1 Nucleophilic addition of water
Water can behave as a nucleophile, although its nucleophilicity is not that strong (see chapter 8.4.2). In the water solution, there is an equilibrium between the carbonyl compound and the corresponding one geminal diol, which is also called Carbonyl hydrate referred to as :
Carbonyl hydrates arise only slowly in water at pH 7, but are formed considerably faster in the presence of acids or bases. The reaction is therefore catalyzed by acid or base - the equilibrium constant remains identical, but the reaction reaches the equilibrium position more quickly. Why?
In the case of base catalysis, at pH> 7, it is OH- present, which is a much better nucleophile than water:
In the base-catalyzed mechanism, the hydroxide ion acts as a nucleophile. Water then traps the intermediate adduct, forming the geminal diol, and the catalyst is released again.
In the acid-catalyzed mechanism, protonation to the Lewis basic carbonyl-O takes place first. This makes the C = O group more polarized and the carbonyl C atom group a stronger electrophile. Now a water molecule, although still a weak nucleophile, attacks very quickly:
Acid Catalyzed Addition
It should be noted that the base-catalyzed process is faster because the OH--Ion is a much better nucleophile than a water molecule. The acid-catalyzed process, on the other hand, is faster because the protonation of the carbonyl group creates a better electrophile for the nucleophile attack.
The reaction equations show that the hydration of aldehydes and ketones is reversible. The equilibrium is usually on the left for ketones and on the right for formaldehydes and aldehydes with electron-withdrawing substituents. Aldehydes have equilibrium constants around 1, e.g .:
In general, additions to C = O groups take place faster, the more electrophilic the carbonyl C atom is:
Order of reactivity of C = O groups in a nucleophilic addition:
One can roughly correlate the electrophilic power of this center with the stability of the carbenium ion formulated in the dipolar resonance structure. The more highly substituted (with alkyl groups) carbenium ion is the more stable, and its reactivity decreases in that order. Electron-withdrawing substituents destabilize the positively polarized carbon atom, which is why it is more reactive in the event of a nucleophilic attack.
10.4.2 Nucleophilic addition of alcohols
Aldehydes and ketones react with alcohols in the presence of an acid catalyst and give Acetals and ketals as products, e.g .:
It should come as no surprise that alcohols also add to aldehydes and ketones, with the mechanism practically similar to that of hydration. The products obtained in this way are called Half acetals or Half-ketals :
These addition reactions are also governed by an equilibrium that is normally on the side of the carbonyl compound. In the presence of an excess of alcohol, the acid-catalyzed reaction with aldehydes or ketones goes beyond the hemiacetal stage:
This second step can be used as an SN1 substitution reaction (sp3 Center):
Every step is reversible. The entire reaction sequence from the carbonyl compound to the acetal is an equilibrium process. By manipulating the reaction conditions, the equilibrium can be shifted to the right or to the left:
Acetal / ketal formation requires an excess of alcohol
Acetal / ketal hydrolysis requires an excess of water
1,2-Ethanediol (glycol) and similar diols react with aldehydes and ketones in the presence of catalytic amounts of acid to form cyclic acetals and ketals:
An important property of the acetals and ketals is their relative inertness towards bases, Grignard reagents, hydride reducing agents and other nucleophiles. Not too surprising when you think of them as ethers. One can think of acetals and ketals as "masked" aldehydes or ketones. The cyclic systems in particular are used as protective groups for the carbonyl function in aldehydes and ketones.
Half-acetals and half-ketals cannot usually be isolated. However, half-acetals or half-ketals can be isolated from hydroxy aldehydes or hydroxy ketones if ring closure leads to the formation of relatively stress-free five- or six-membered rings:
This can be explained with the help of entropy. In intermolecular reactions, 2 molecules combine to form a new structure. This is entropically unfavorable. Correspondingly, the reverse reaction is entropically favored. In contrast, the intramolecular reaction transforms one molecule into a new one. Now the change in entropy is much smaller, and accordingly the equilibrium is more on the product side, since the enthalpy balance is favorable.
The intramolecular formation of hemiacetals has in the chemistry of sugar (carbohydrates) big meaning.
10.4.3 Nucleophilic addition of Grignard reagents
Grignard reagents add to aldehydes and ketones in the same way as other nucleophiles to form alcohols (see Section 9.5):
This mechanism explains all of the reactions between Grignard reagents and aldehydes or ketones given in Section 9.5.
10.4.4 The nucleophilic addition of amines to aldehydes and ketones
Condensation to imines, oximes and hydrazones
One can consider amines as the N-analogues of alcohols. However, the N atom is more nucleophilic than the O atom, and therefore amines add very effectively to carbonyl groups of aldehydes and ketones, first with the formation of hemiaminals and then of Imines:
The formation of an imine is also reversible and upon treatment with aqueous acid the imine is hydrolyzed again to an aldehyde or ketone.
A number of other imine-like derivatives can also be produced by the reaction between an aldehyde or ketone and an amine derivative (H.2N-X):
10.4.5 The addition of carbon nucleophiles to aldehydes and ketones
In addition to alcohols and amines, numerous other nucleophiles can attack the carbonyl group. Carbon nucleophiles are particularly important, as new C-C bonds can be formed in this way.
HCN, for example, adds to carbonyl compounds, thereby forming hydroxyalkanenitriles, which too Cyanohydrins to be named:
The mechanism of cyanohydrin formation begins with a nucleophilic attack by the cyanide ion and ends with a protonation of the O atom:
The reaction can easily be reversed by bases, since they shift the equilibrium to the side of the free cyanide ions by withdrawing the protons.
Since the nitrile group can be converted through further reactions, cyanohydrins are important intermediates:
10.4.6 The nucleophilic addition of hydride - the REDUCTION of aldehydes and ketones
We saw in Chapter 9.5 how aldehydes and ketones form alcohols reduced can be:
Metal hydrides such as NaBH4 and LiAlH4 transfer one equivalent of hydride (H.-) on the C = O double bond. An aldehyde or ketone is reduced to an alcohol like this:
LiAlH4 is more reactive than NaBH4 and can only be used in aprotic solvents such as ether. In total, all four hydrogen atoms are made by AlH4- transferred to four carbonyl groups. With lithium aluminum hydride, hydride ions can also be added nucleophilically to carbonyl groups:
10.5 Oxidation of alcohols and aldehydes with CrO3 - mechanism
We saw earlier that alcohols containing CrO3 can be oxidized. Primary alcohols produce aldehydes first. Further oxidation leads to carboxylic acids. The mechanisms for such oxidation processes can be formulated as follows:
Starting from an aldehyde, a carbonyl hydrate can be formed in the presence of water, which can then be oxidized via an analogous mechanism. Secondary alcohols can only be oxidized to ketones with this process.
10.6 Examples from biological chemistry
Many of the reactions used in the laboratory also occur in nature.
A biosynthetic pathway to certain α-amino acids runs, for example, via a nucleophilic addition of ammonia to an α-ketocarboxylic acid (here pyruvic acid): (see Chapter 6.2)
Other examples often occur in carbohydrate chemistry. Glucose, for example, is a hexose and can be written as an aldehyde in a Fischer projection. But if you examine glucose with spectroscopic methods, you will find that there is no aldehyde group present! The reason for this is that glucose acts as a cyclic hemiacetal is present.
Cyanohydrins also occur in nature. They play an interesting role in a chemical defense mechanism that certain millipedes use, e.g. Apheloria corrugata. When attacked, mandonitrile and an enzyme that catalyzes the breakdown of mandonitrile into benzaldehyde and HCN are secreted.
HCN is "shot at the attackers"!
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