Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Organic Acid By Tarek . I . Kakhia 1 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 2 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 1 - Introduction To Organic Acid Contents 1 Introduction 2 Characteristics 3 Applications 4 Application in food 5 Application in nutrition and animal feeds 1 - Introduction An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group – COOH. Sulfonic acids, containing the group – SO2OH, are relatively stronger acids. Alcohols, with – OH, can act as acids but they are usually very weak. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: the thiol group –SH, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally referred to as organic acids. 2 - Characteristics In general, organic acids are weak acids and do not dissociate completely in water, where as the strong mineral acids do. Lower molecular mass organic acids such as formic and lactic acids are miscible in water, but higher molecular mass organic acids, such as benzoic acid, are insoluble in molecular (neutral) form. On the other hand, most organic acids are very soluble in organic solvents. p-Toluene sulfonic acid is a comparatively strong acid used in organic chemistry often because it is able to dissolve in the organic reaction solvent. Exceptions to these solubility characteristics exist in the presence of other substituents that affect the polarity of the compound. 3 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 3 - Applications Simple organic acids like formic or acetic acids are used for oil and gas well stimulation treatments. These organic acids are much less reactive with metals than are strong mineral acids like hydrochloric acid (HCl) or mixtures of HCl and hydrofluoric acid (HF). For this reason, organic acids are used at high temperatures or when long contact times between acid and pipe are needed . The conjugate bases of organic acids such as citrate and lactate are often used in biologically- compatible buffer solutions. Citric and oxalic acids are used as rust removal. As acids, they can dissolve the iron oxides, but without damaging the base metal as do stronger mineral acids. In the dissociated form, they may be able to chelate the metal ions, helping to speed removal . Biological systems create many and more complex organic acids such as L-lactic, citric, and D- glucuronic acids that contain hydroxyl or carboxyl groups. Human blood and urine contain these plus organic acid degradation products of amino acids, neurotransmitters, and intestinal bacterial action on food components. Examples of these categories are alpha – ketoisocaproic vanilmandelic, and D-lactic acids, derived from catabolism of L-leucine and epinephrine (adrenaline) by human tissues and catabolism of dietary carbohydrate by intestinal bacteria, respectively. The general structure of a few weak organic acids. From left to right: phenol, enol, alcohol, thiol. The acidic hydrogen in each molecule is colored red. 4 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org The general structure of a few organic acids. From left to right: carboxylic acid, sulfonic acid. The acidic hydrogen in each molecule is colored red. 3 - Application in food Organic acids are used in food preservation because of their effects on bacteria. The key basic principle on the mode of action of organic acids on bacteria is that non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria that we call pH-sensitive, meaning that they cannot tolerate a wide internal and external pH gradient. Among those bacteria are Escherichia coli, Salmonella spp., C. perfringens, Listeria mono cytogenes, and Campylobacter species. Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to situations that will impair or stop the growth of bacteria. On the other hand, the anionic part of the organic acids that cannot escape the bacteria in its dissociated form will accumulate within the bacteria and disrupt many metabolic functions, leading to osmotic pressure increase, incompatible with the survival of the bacteria. It has been well demonstrated that the state of the organic acids (undissociated or dissociated) is extremely important to define their capacity to inhibit the growth of bacteria, compared to undissociated acids. Lactic acid and its salts sodium lactate and potassium lactate are widely used as antimicrobials in food products, in particular, meat and poultry such as ham and sausages. 4 - Application in nutrition and animal feeds Organic acids have been used successfully in pig production for more than 25 years. Although less research has been done in poultry, organic acids have also been found to be effective in poultry production. 5 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Organic acids ( C1 – C7 ) are widely distributed in nature as normal constituents of plants or animal tissues. They are also formed through microbial fermentation of carbohydrates mainly in the large intestine. They are sometimes found in their sodium, potassium, or calcium salts, or even stronger double salts. Organic acids added to feeds should be protected to avoid their dissociation in the crop and in the intestine (high pH segments) and reach far into the gastrointestinal tract, where the bulk of the bacteria population is located. From the use of organic acids in poultry and pigs, one can expect an improvement in performance similar to or better than that of antibiotic growth promoters, without the public health concern, a preventive effect on the intestinal problems like necrotic enteritis in chickens and Escherichia coli infection in young pigs. Also one can expect a reduction of the carrier state for Salmonella species and Campylobacter species. 6 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 2 - Introduction To Carboxylic Acid Carboxylate ion Contents 1 Introduction 2 Physical properties 2.1 Solubility 2.2 Boiling points 2.3 Acidity 2.4 Odor 3 Characterization 4 Occurrence and applications 5 Synthesis 5.1 Industrial routes 5.2 Laboratory methods 5.3 Less - common reactions 6 Reactions 6.1 Specialized reactions 7 Nomenclature and examples 8 Carboxyl radical 1 - Introduction A carboxylic acid is an organic acid characterized by the presence of at least one carboxyl group . The general formula of a carboxylic acid is R- COOH, where R is some monovalent functional group. A carboxyl group (or carboxy) is a functional group consisting of a carbonyl (RR'C= O) and a hydroxyl (R- O -H), which has the formula - C(= O)OH, usually written as - COOH or - CO2H. Carboxylic acids are Brønsted-Lowry acids because they are proton (H+) donors. They are the most common type of organic acid. 7 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Among the simplest examples are formic acid H-COOH, which occurs in ants, and acetic acid CH3- COOH, which gives vinegar its sour taste. Acids with two or more carboxyl groups are called dicarboxylic, tricarboxylic, etc. The simplest dicarboxylic example is oxalic acid (COOH)2, which is just two connected carboxyls. Mellitic acid is an example of a hexacarboxylic acid. Other important natural examples are citric acid (in lemons) and tartaric acid (in tamarinds). Salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its conjugate base forms a carboxylate anion. Carboxylate ions are resonance stabilized and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide; under some circumstances they can be decarboxylated to yield carbon dioxide. 2 - Physical properties 2 – 1 - Solubility Carboxylic acid dimers Carboxylic acids are polar. Because they are both hydrogen- bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), they also participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl. Carboxylic acids usually exist as dimeric pairs in nonpolar media due to their tendency to “self-associate.” Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be rather soluble in less-polar solvents such as ethers and alcohols. 2 – 2 - Boiling points Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their 8 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org tendency to form stabilised dimers. Carboxylic acids tend to evaporate or boil as these dimers. For boiling to occur, either the dimer bonds must be broken, or the entire dimer arrangement must be vaporised, both of which increase enthalpy of vaporization requirements significantly. 2 – 3 - Acidity Carboxylic acids are typically weak acids, meaning that they only partially dissociate into H+ cations and RCOO – anions in neutral aqueous solution. For example, at room temperature, only 0.4% of all acetic acid molecules are dissociated. Electronegative substituents give stronger acids. Carboxylic acid Forula pKa Formic acid H COOH 3.75 Acetic acid CH3 COOH 4.76 Chloro acetic acid CH2 ClCOOH 2.86 Oxalic acid HOOC- COOH 1.27 Benzoic acid C6H5 COOH 4.2 Deprotonation of carboxylic acids gives carboxylate anions, which is resonance stabilized because the negative charge is delocalized between the two oxygen atoms increasing its stability. Each of the carbon - oxygen bonds in carboxylate anion has partial double-bond character.