Moctezuma I

The term carboxylic acid indicates a Molécule including/understanding a grouping carboxyl. They are acids and their combined bases are called carboxylate.

In Organic chemistry, a group carboxyl is a functional Groupe composed of a Atome of Carbone, bound by a double connection to a first atom of Oxygène, and by a simple connection with a second oxygen atom, itself connected to an atom of Hydrogen.

General information

In Chemistry, the carboxylic acid constitute with the sulfonic acid (R-SO₃H), the two types of Acide S of the Organic chemistry. They are found in an abundant way in nature, in the shape of fatty-acid (lipid) and they are very important in industrial Chimie. For example, the acetic Acid (or ethanoic acid) is not only one important brick for the Molécule S complexes which one finds in Biologie, but is also an industrially produced molecule and which one finds in the vinegar. One of most known is the Acetylsalicylic acid , more known under the name of aspirine. The building block of proteins, the amino-acid are carboxylic acids.

The functional Groupe (characteristic) is the carboxyl group:

(or R is a hydrogen or an organic group)

The carboxylic acids have as an empirical formula C_nH_2nO₂ when the R is an alkyl group. the calculation of the number of non-saturation gives: $\backslash \; frac\; \{2n+0-2n+2\}\; \{2\}\; =1~$. This non-saturation translates the double linking carbon-oxygen.

One often writes the groupings carboxyls in the reduced form: - COOH (form not ionized of the grouping). The ionized form of the grouping is: - COO^-.

This one is always located at the end of the carbonaceous chain. The addition of a grouping carboxyl has a organic Composé is a Carboxylation, the elimination of this same grouping is a decarboxylation .

Carboxylic ions

They are rather weak bases. The negative charge on the molecule is carried by an oxygen. This molecule shares its negative charge between its two oxygens by resonance. What explains their relative stability.

Nomenclature

• Systematic: one adds the suffix “- oïque” in the name of alkane corresponding (having the same number of carbon atoms), and by making it precede to precede by " acid ".

• Usual: like many organic compounds, the carboxylic acids have usual names frequently used in the literature and pointing out the source since which they were initially insulated.

Physical properties and structural

State

The carboxylic acids are liquid under the normal conditions as long as their carbonaceous chain presents less than 9 carbon atoms. They sonts solid beyond that.

The low-weight acids molecular have a strong odor, for example the butanoïque acid is responsible for the odor of rancid butter.

Polarity, solubility

The carboxylic acid function is strongly polar and is at the same time donor and acceptor of hydrogen bonds. This allows the creation of hydrogen bridges for example with a Solvant Polaire like water, alcohol, and other carboxylic acids. From this property the carboxylic acids of small size (to the butanoïque acid) are completely water soluble . The molecules of acids are also able to form Dimère S stable by hydrogen bridge, which makes it possible to explain why them boiling point is more high that of corresponding alcohols.

Acidity

In solution in water, the acid dissociates partially in carboxylate ion, according to the equation-assessment:

They are weak acids in water (pK_A between 4 and 5).

Like alcohols, the carboxylic acids show an acid and basic character: the deprotonation in carboxylate ions is easy, but the protonation is more difficult. They thus have a pK_A weaker than that of alcohols. In fact the acidity of the carboxylic acids is explained by the inductive Effet in the grouping carboxyl: connection C=O is very polarized (electronegativity oxygen higher than that of carbon) with the result that carbon is électrophile, and it thus attracts the electrons of other oxygen. However this other oxygen is itself related to a hydrogen, and this connection is also polarized, therefore the electron of the hydrogen which approached oxygen is attracted in its turn by carbon électrophile. This hydrogen thus becomes very easily mobile, from where the acidity of the grouping carboxyl.

The solubility of the carboxylic acid believes with pH.

Spectroscopy

Into infra-red (IR), the carboxylic acid presents 2 valence bands.

Structure

According to theory VSEPR:

• the geometry around the carbon atom (connections) is of trigonal type.
• the geometry around the oxygen atom of the function carbonyl (connection doubles + free doublets) is also of type trigonal
• the geometry around the oxygen atom of the foncition hydroxyl (free connections + doublets) is of tétragonale type (AX₂E₂).

The carboxylic acid has several forms mésomères.

Reactivity

Like the watch, inter alia, the various formulas mésomères of the carboxylic acid:

• the oxygen atoms are nucleophilic sites as well as bases of Lewis
• central carbon is électrophile
• the atom of hydrogen électrophile and acid.

Derived

The carboxylic acids count many derivatives:

• the chlorides acyle:

• the alcanoïques anhydrides

• the Ester S:

• the Amide S

• the Nitrile S

In term of group therefore (nucléofuge) the order of facility is:

Cl^- (acyle chloride), RCOO^- (anhydride), RO^- (ester), ^-NH₂ and ^-NR¹R² (amides).

Synthesis of the carboxylic acids

Synthesis by oxidation

• of alcohols or the aldehydes

The carboxylic acids can be obtained by oxidation of the Aldéhyde S, therefore in fact of a double oxidation of the primary education alcohols.

$RCH\_2OH\; +\; KMnO\_4\; \backslash \; longrightarrow\; RCHO$

$RCHO\; +\; KMnO\_4\; \backslash \; longrightarrow\; RCOOH$

• of the olefinic hydrocarbons

$RCH=CH\_2\; +\; KMnO\_4\; \backslash \; longrightarrow\; RCOOH\; +\; CO\_2$

ex: Synthesis of the acetic Acid by oxidation of propene

$CH\_3-CH=CH\_2\; +\; KMnO\_4\; \backslash \; longrightarrow\; CH\_3-COOH\; +\; CO\_2$

Synthesis starting from a derivative of acid

They are simply the hydrolyzes of the various derivatives of acids

• ester

• hydrolysis in acid medium: retroesterification.

$RCO-O-R\text{'}\; +\; H\_2O\; \backslash \; Longleftrightarrow\; RCOOH\; +\; R\text{'}\; OH$

• hydrolysis in basic medium: Saponification of ester. $RCO-O-R\text{'}\; +\; OH^-\; \backslash \; longrightarrow\; RCOO-\; +\; R\text{'}\; OH$ then $RCOO-\; +\; H^+\; \backslash \; Longleftrightarrow\; RCOOH$

• nitrile

Synthesis by reaction of a Organomagnésien on the Carbon dioxide

Reaction

Conditions

The synthesis takes place at low temperature (- 40°C). Carbon dioxide is then in form solid, known as Carboglace. It is put in excess. after reaction, one carries out acid hydrolysis in mileiu to obtain the carboxylic acid

Mechanism

• First stage : addition of the organomagnésien on CO₂

• Second phase : hydrolize in acid medium

Malonic synthesis

It is about another type of synthesis of the carboxylic acids, starting from the malonate of diéthyle at the end of several reactions.

The malonate of diéthyle

Below the formula (levelled) of the malonate of diéthyle. It is acted in fact of an di-ester (2 functions esters connected by a methyl group).

Moreover, the malonate of diéthyle can be obtained by a double esterification of the malonic acid by ethanol:

In order to simplify the diagrams of the mechanism, and for better illustrating its properties used here, we will note the malonate diéthyle:

Because of the attractile effects inductive and mésomère (- I, - M) of the groups esters, the atoms of hydrogen on central carbon are relatively acid (pK_A=13, to compare with acidity hydrogen on an alkyl group, about 40-50). It is what allows the first stage of the synthesis

Mechanism

• First stage : one makes react the éthanolate in ethanol on the malonate of diéthyle

(reaction acid-bases relatively easy: $\backslash \; Delta\; pK\_A\; \backslash \; approx\; 5~$). The éthanolate is obtained by reaction of sodium on ethanol.

This reaction thus forms a carbanion and ethanol.

This carbanion is stabilized by mesomery:

Note:: it is impossible here that the malonate reacts one second time. The formed product would be too unstable.

However one puts in presence an di-ester and a strong base: that thus implies a reaction of the saponification type of ester. That does not pose a problem, since the selected base is in fact the same one as the group therefore, the ion éthanolate. A reaction of " saponification" one of esters thus does not modify this one:

• Second phase : nucleophilic Substitution of the carbanion obtained previously on a halogenous Derivative. The alkyl part of this derivative will be that of the synthesized carboxylic acid. It is thus with this stage which one decides of the acid that one wants. One can however choose only one derived even secondary primary education, a tertiary derivative, under the conditions of the synthesis (presence of a relatively strong base) would react by a reaction of elimination (=> formation of an olefinic hydrocarbon, and reprotonation of the malonate).

Note:: at this moment, one can again make react the éthanolate and make the one second alkylation.

• Third stage : Saponification (mechanism) of the two functions ester. One uses this time as bases soda (Na⁺, HO^-), or potash (K⁺, HO^-).

• Fourth stage : Double reaction acid-bases.

• Fifth stage : Decarboxylation of diacid.

Diacid " pseudo-cyclise". This conformation is all the more stable as a hydrogen bond is created between the hydrogen of a function hydroxyl and the oxygen of the carbonyl part of the other acid function, and by the fact that this " pseudo-cycle" comprise 6 atoms. An increase in the temperature then allows the departure of a molecule of Carbon dioxide:

A balance of tautomerism comes to finish this reaction.

Cett synthesis is all the more interesting as a priori , it makes it possible to synthesize any carboxylic acid, since, separately a tertiary group, it seems that one can put what one wants in the place of R .

Reactions starting from a carboxylic acid

Transformation into derivative

to see the articles corresponding:

• Chloride acyle.

• alcanoïque Anhydride
• ester
• Amide
• Nitrile

Reduction

• in Aldehyde

• in primary education alcohol

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