Organic acid
Organic acids are organic chemical compounds that have a functional group which undergoes an equilibrium reaction with water or other solvents can be protonated. Here, in the case of water creates the oxonium ion H3O , which can be acidic the solution, while the acid releases a proton to the solvent water:
Most of the term organic acid is equated with carboxylic acid, but there are also organic acids which carry no carboxy group. As a measure of the acid strength is considered the acidity constant (or pKa ) of a substance. These values indicate the extent to which an acid on the equilibrium reaction protolyzed with water, that is, how far the equation (1) on the right side is moved. Please note: the smaller the pKa value, the greater the acid strength of the fabric.
Groups of organic acids
Members of the group of carboxylic acids having one or more carboxyl (-COOH ), which are acidic. However, other groups such as the hydroxyl group ( -OH) sulfanyl -SH ( thiols ) or -SO 3 H (sulfonic acids ) may be present which can eliminate protons. Reaction of a compound an acid will be easier if the resulting anion is stabilized by resonance. Thus, ascorbic acid (vitamin C) has no carboxyl group, but acts through its acidic enol groups. The acidity of the phenol and ethyl acetoacetate are the result of stabilization of the anions.
Carboxylic acids
Carboxylic acids have a significantly more acidic character than, for example alcohols in general. The acid strength increases when close to the carboxy group ( in α -, β - or γ - position) more electron-withdrawing groups are present. Trifluoroacetic acid or trichloroacetic acid are stronger acid than lactic acid, which in turn is a stronger acid than acetic acid. Carboxylic acids with 1-3 carbon atoms smell pungent, corrosive to human skin and highly soluble in water. Carboxylic acids with 4-9 carbon atoms strong smell rancid, as the best known example is butyric acid. Carboxylic acids having 10 or more carbon atoms are odorless and waxy solids ( fatty acids).
Alcohols, phenols, enols
Unsubstituted alcohols have a very low acidity, while phenols and naphthols are easily deprotonated by the resonance stabilization of the resulting phenolate to phenol is the pK a value of 9.9 by a factor of 106 more acidic than alkanols such as ethanol ( with a pKa of 16 ). Substituents on the compounds which exert a - I or M- effect, also increase the acid strength by stabilizing the negative charge of the resulting anion. This leads to a strongly acidic compounds such as picric acid ( -M effect) or pentachlorophenol (-I effect). In some substituents such as halogen atoms or nitro groups occur both effects - sometimes even in a different direction - up. The enediol of ascorbic acid has a pKa of 4.2, making it more acidic than acetic acid ( pK a 4.8 ).
Thiols
Thiols have a higher acidity than the corresponding alcohols; the resulting thiolates are more stable than the Alkololate. This shows a comparison of the analog connections ethanol ( pKa 16) and ethanethiol ( ethyl mercaptan and, pKa 12).
Sulfuric acid esters and sulfonic acids
The monoesters of sulfuric acid and sulfonic acids have one of the sulfuric acid strength comparable. The I effect of long alkyl chains stabilizes the resulting anions. P- toluene sulfonic acid has a pKa of about 0.7, the ester of lauryl alcohol with sulfuric acid, lauryl the value -0.09.
Phosphoric acid esters and phosphonic acids
The mono-and diesters of phosphoric acid (phosphoric acid esters ), and the organic derivatives of phosphonic acid (commonly known as phosphonic acids ) react as strong acids.
CH- and NH -acidic compounds
And hydrocarbons without carboxy, hydroxy or sulfanyl group can act as acids due to their CH- NH or acidity. In this case, the presence of multiple bonds and neighboring functional groups or hetero atoms, is the cause of the acidity. The most famous CH -acidic compound is ethyne (acetylene ), which due to the triple bond a pKa of 25 has ( Ethan: 50). Other examples are the nitroethane ( pKa 8.6 ) and the Nitro cyclopentadiene, which is more acidic ( pKa 3.3 ) and formic acid (3.75 ). Wherein results from the deprotonation nitro cyclopentadiene derivative of the aromatic cyclopentadienyl anion. Again, determine mesomeric effects together with inductive effects, the acidity of the compounds. Acetonitrile is a further example of a simple C-H acidic compound. Known NH -acidic compounds are phthalimide ( pKa = 8.3 ) and sulfonylureas ( tolbutamide has a pKa of 5.16 ).