Introduction - Carbonyl Compounds
The systematic nomenclature of carboxylic acids is easy to understand. The ending - oic acid is added to the name of the parent alkane to indicate the presence of the CO 2 H functional group. Unfortunately, because of the long history of their importance in biology and biochemistry, you are more likely to encounter these compounds by their common names. Formic acid and acetic acid have a sharp, pungent odor. As the length of the alkyl chain increases, the odor of carboxylic acids becomes more unpleasant. Butyric acid, for example, is found in sweat, and the odor of rancid meat is due to carboxylic acids released as the meat spoils.
The solubility data in the table above show that carboxylic acids also become less soluble in water as the length of the alkyl chain increases. The CO 2 H end of this molecule is polar and therefore soluble in water. As the alkyl chain gets longer, the molecule becomes more nonpolar and less soluble in water. Dicarboxylic and Tricarboxylic Acids. Compounds that contain two CO 2 H functional groups are known as dicarboxylic acids. A number of dicarboxylic acids see table below can be isolated from natural sources. Tartaric acid, for example, is a by-product of the fermentation of wine, and succinic, fumaric, malic, and oxaloacetic acid are intermediates in the metabolic pathway used to oxidize sugars to CO 2 and H 2 O.
Several tricarboxylic acids also play an important role in the metabolism of sugar. The most important example of this class of compounds is the citric acid that gives so many fruit juices their characteristic acidity. Acetic acid, for example, reacts with ethanol to form ethyl acetate and water. This isn't an efficient way of preparing an ester, however, because the equilibrium constant for this reaction is relatively small K c 3.
Chemists tend to synthesize esters in a two-step process. They start by reacting the acid with a chlorinating agent such as thionyl chloride SOCl 2 to form the corresponding acyl chloride. They then react the acyl chloride with an alcohol in the presence of base to form the ester. As might be expected, esters are named as if they were derivatives of a carboxylic acid and an alcohol. The ending - ate or - oate is added to the name of the parent carboxylic acid, and the alcohol is identified using the "alkyl alcohol" convention.
This is because of the resonance stabilized structures which form due to the donation of an electron by this group. The stronger the contribution of this resonance structure, the stronger the stability of the carbonyl. An example of this can be seen in carboxylic acids which upon deprotonation create a degenerate structure and thus increases the acidity of the proton.
This decreases the reactivity of peptide bonds considerably and thus requires much energy or a protease to catalyze the bond. Carbonyls by themselves are very stable bonds and the energy of their formation are usually very high. This makes the formation of carbonyls in organic synthesis to be highly thermodynamically favorable and usually the creation of a carbonyl bond as the end product will drive a reaction to formation.
The carbonyl group has a short, strong, and very polar double bond. Its reactivity of its double bond is very different from the double bond of the alkenes because of their oxygen's electronegativity along with the lone pair of electrons. The carbonyl carbon is also electron withdrawing since it is so close to the highly electronegative oxygen. The polarization of aldehydes and ketones also alters the physical constants.
The polarization of the carbonyl group is the reason why their boiling points are higher than those of the hydrocarbons of similar molecular weight. Carbonyl compounds with more than six carbons are considered large, which is insoluble in solution. The larger the compound, the larger its hydrocarbon chain, the more hydrophobic the molecule is, so its solubility would then decrease. Inductive Effect Take important notice that the electrophilicity of the carbon is highly dependent on the nearby atoms and the atoms it is directly bounded to.
Carbonyl Definition in Chemistry
For example, the carbonyl carbon of a carboxylic acid will not be as electrophilic as a carbonyl carbon of a ketone because of resonance stabilization. In contrast, both the endothermic addition of water to a carbonyl function, and the exothermic elimination of water from the resulting geminal -diol are fast. The inherent polarity of the carbonyl group, together with its increased basicity compared with alkenes , lowers the transition state energy for both reactions, with a resulting increase in rate. Acids and bases catalyze both the addition and elimination of water.
Proof that rapid and reversible addition of water to carbonyl compounds occurs is provided by experiments using isotopically labeled water. If a carbonyl reactant composed of 16 O colored blue above is treated with water incorporating the 18 O isotope colored red above , a rapid exchange of the oxygen isotope occurs. This can only be explained by the addition-elimination mechanism shown here. It has been demonstrated above that water adds rapidly to the carbonyl function of aldehydes and ketones. In most cases the resulting hydrate a geminal-diol is unstable relative to the reactants and cannot be isolated.
Exceptions to this rule exist, one being formaldehyde a gas in its pure monomeric state. Here the weaker pi-component of the carbonyl double bond, relative to other aldehydes or ketones, and the small size of the hydrogen substituents favor addition. Thus, a solution of formaldehyde in water formalin is almost exclusively the hydrate, or polymers of the hydrate.
Aldehydes and Ketones
Similar reversible additions of alcohols to aldehydes and ketones take place. The equally unstable addition products are called hemiacetals. Stable Hydrates and Hemiacetals To see examples of exceptional aldehydes and ketones that form stable hydrates or hemiacetals Click Here. Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents of an alcohol and elimination of water. Ketone derivatives of this kind were once called ketals, but modern usage has dropped that term. The following equation shows the overall stoichiometric change in acetal formation, but a dashed arrow is used because this conversion does not occur on simple mixing of the reactants.
In order to achieve effective acetal formation two additional features must be implemented. First, an acid catalyst must be used; and second, the water produced with the acetal must be removed from the reaction. The latter is important, since acetal formation is reversible. Indeed, once pure acetals are obtained they may be hydrolyzed back to their starting components by treatment with aqueous acid. The mechanism shown here applies to both acetal formation and acetal hydrolysis by the principle of microscopic reversibility. Some examples of acetal formation are presented in the following diagram.
Two equivalents of the alcohol reactant are needed, but these may be provided by one equivalent of a diol example 2. Intramolecular involvement of a gamma or delta hydroxyl group as in examples 3 and 4 may occur, and is often more facile than the intermolecular reaction. Thiols sulfur analogs of alcohols give thioacetals example 5. In this case the carbonyl functions are relatively hindered, but by using excess ethanedithiol as the solvent and the Lewis acid BF 3 as catalyst a good yield of the bis-thioacetal is obtained. Thioacetals are generally more difficult to hydrolyze than are acetals.
The importance of acetals as carbonyl derivatives lies chiefly in their stability and lack of reactivity in neutral to strongly basic environments. As long as they are not treated by acids, especially aqueous acid, acetals exhibit all the lack of reactivity associated with ethers in general.
Among the most useful and characteristic reactions of aldehydes and ketones is their reactivity toward strongly nucleophilic and basic metallo-hydride, alkyl and aryl reagents to be discussed shortly. If the carbonyl functional group is converted to an acetal these powerful reagents have no effect; thus, acetals are excellent protective groups, when these irreversible addition reactions must be prevented.
Water is eliminated in the reaction, which is acid-catalyzed and reversible in the same sense as acetal formation. An addition-elimination mechanism for this reaction was proposed, and an animation showing this mechanism is activated by the button.
Imines are sometimes difficult to isolate and purify due to their sensitivity to hydrolysis. Some of these reagents are listed in the following table, together with the structures and names of their carbonyl reaction products. An interesting aspect of these carbonyl derivatives is that stereoisomers are possible when the R-groups of the carbonyl reactant are different. Thus, benzaldehyde forms two stereoisomeric oximes, a low-melting isomer, having the hydroxyl group cis to the aldehyde hydrogen called syn , and a higher melting isomer in which the hydroxyl group and hydrogen are trans the anti isomer.
At room temperature or below the configuration of the double-bonded nitrogen atom is apparently fixed in one trigonal shape, unlike the rapidly interconverting pyramidal configurations of the sp 3 hybridized amines.