The hydroxyl group is absent in the molecules. Functional groups of organic compounds. Role in biology

Functional groups are called groups of atoms that determine the characteristic chemical properties of a given class of substances.

Alcohols

The structure of alcohol molecules R-OH. The oxygen atom, which is part of the hydroxyl group of alcohol molecules, differs sharply from hydrogen and carbon atoms in its ability to attract and hold electron pairs. Alcohol molecules contain polar S-O connections and he.
Given the polarity O-N connections and a significant positive charge on the hydrogen atom, the hydrogen of the hydroxyl group is said to be “acidic” in nature. In this way, it differs sharply from the hydrogen atoms included in the hydrocarbon radical. The oxygen atom of the hydroxyl group has a partial negative charge and two lone pairs of electrons, which allows alcohol molecules to form hydrogen bonds.

Phenols

By chemical properties phenols differ from alcohols, which is caused by the mutual influence of the hydroxyl group and the benzene ring (phenyl - C 6 H 5) in the phenol molecule. This effect comes down to the fact that the π-electrons of the benzene ring partially involve lone electron pairs of the oxygen atom of the hydroxyl group into their sphere, as a result of which the electron density of the oxygen atom decreases. This decrease is compensated by the large polarization of the O-H bond, which in turn leads to an increase in the positive charge on the hydrogen atom:

Consequently, the hydrogen of the hydroxyl group in the phenol molecule is acidic in nature.
The influence of atoms in the molecules of phenol and its derivatives is mutual. The hydroxyl group affects the density of the π-electron cloud in the benzene ring. It decreases at the carbon atom associated with the OH group (i.e. at the 1st and 3rd carbon atoms, meta position) and increases at the neighboring carbon atoms - 2, 4, 6 - ortho- And pair provisions.
Hydrogen atoms of benzene and ortho- And pair positions become more mobile and are easily replaced by other atoms and radicals.

Aldehydes

Aldehydes have a general formula , Where — C=O — carbonyl group. The carbon atom in the carbonyl group sp 2 is hybridized. The atoms directly connected to it are in the same plane. Due to the large electronegativity of the oxygen atom compared to the carbon bond C=O is strongly polarized due to a shift in the electron density of the π bond towards oxygen:

Under the influence of the carbonyl carbon atom in aldehydes, the polarity of the C–H bond increases, which increases the reactivity of this H atom.

Carboxylic acids

Carboxylic acids contain a functional group , called carboxyl group, or carboxyl. It is so named because it consists of a carbonyl group -C=O and hydroxyl -HE.
In carboxylic acids, the hydroxyl group is bonded to a hydrocarbon radical and a carboxyl group. The weakening of the bond between oxygen and hydrogen in the hydroxyl group is explained by the difference in electronegativity of the carbon, oxygen and hydrogen atoms. The carbon atom acquires some positive charge. This carbon atom attracts an electron cloud from the oxygen atom of the hydroxyl group. Compensating for the shifted electron density, the oxygen atom of the hydroxyl group attracts the electron cloud of the neighboring hydrogen atom. The O–H bond in the hydroxyl group becomes more polar, and the hydrogen atom becomes more mobile.

As a result of chemical transformations, phenolic hydroxyl produces the same products as alcoholic hydroxyl: phenolates, ethers and esters, etc. The benzene ring has practically no effect on the course and direction of the corresponding reactions. But the reactivity of phenolic hydroxyl is significantly reduced due to the aromatic nucleus. An example is the negative result obtained when trying to replace hydroxyl with chlorine. Concentrated hydrohalic acids do not replace hydroxyl in phenols, phosphorus pentachloride causes chlorination into the nucleus, phosphorus trichloride forms triphenyl phosphate. At the same time, it should be noted that sometimes phenolic hydroxyl can still be replaced by chlorine. This happens with phenols containing electron-withdrawing substituents in the ring, in addition to hydroxyl. With these phenols the reaction can be carried out as a bimolecular nucleophilic substitution

14.1.2.1. Acidity. Like alcohols, phenols exhibit a certain acidity. Given this feature of phenol itself, it is sometimes called carbolic acid, carbolic acid. In order to be able to judge the acidity of phenols, let us compare the acidity constants K a some related compounds.

Connections K a

alcohols 10 -16 – 10 -18

phenols 10 -10

carboxylic acids 10 -5

P-cresol 0.67 · 10 -10

O-chlorophenol 77 · 10 -10

O-nitrophenol 600 · 10 -10

pyrocatechin 1·10 -10

resorcinol 3 · 10 -10

hydroquinone 2 · 10 -10

From these data it is clear that the acidity of phenols is many orders of magnitude higher than that of alcohols. This is explained by the fact that the phenoxide anion resulting from the deprotonation of phenol is largely stabilized due to the delocalization of the negative charge with the participation of the benzene ring

The stability of the phenoxide anion, and therefore the acidity of phenol, is also affected by substituents in the aromatic ring. This effect depends on the nature of the substituent, their number and position in the benzene ring. In general, electron-donating substituents reduce the acidity of phenols, and electron-withdrawing substituents increase it.

Being acids, phenols with bases give salts called phenolates

When ferric chloride is added to phenols in dilute aqueous or alcoholic solutions, a violet (phenol) or blue color (cresols) appears. The appearance of color in these cases is associated with the formation of ferric iron phenolates, which absorb light in the visible region.

14.1.2.2. Formation of ethers. Phenol ethers cannot be obtained simply by reacting phenols with alcohols. This is only possible when using strong alkylating agents (dimethyl sulfate) or using the Williamson reaction. In both cases, the reaction is carried out in an alkaline environment in which phenol exists as a phenolate anion. This nucleophile, which is much stronger than the phenol itself, attacks the halide or sulfate to form an ether (reaction S N 2)

It is easy to see that in the Williamson reaction, to obtain the same ether, another pair of reagents can be used - an aryl halide and an alcohol alkoxide. However, an aromatically bound halogen is unable to participate in this reaction. This is only possible if the aromatic ring, in addition to the halogen, contains activating groups - electron-withdrawing groups. In this case, the reaction proceeds as a normal bimolecular substitution reaction

The Williamson reaction is used not only as a laboratory method, but also for the preparation of some ethers in industrial scale. A well-known example is the synthesis of 2,4-dichlorophenoxyacetic acid (2,4-D) by the reaction of sodium 2,4-dichlorophenolate with the sodium salt of monochloroacetic acid

As a result of the Williamson reaction, the oxygen atom of phenol receives an alkyl substituent, so they say what happens ABOUT-alkylation. In this case, practically nothing happens WITH-alkylation, i.e. entry of the deputy into the ring. This is explained by the fact that of the two competing reactions ABOUT- And WITH-alkylation of the first goes faster. Moreover, in many cases the product ABOUT-alkylation is thermodynamically more stable. However, this is not always the case. At 200 0 C allylphenyl ether isomerizes into O-allylphenol

This reaction is characteristic only of allyl ethers and is called Claisen rearrangements(1912). It is assumed that the reaction proceeds through a cyclic transition state

In the Claisen reaction, the allyl group migrates to O-position with simultaneous allylic rearrangement of this group. If both O-positions are occupied, then the migrating allylic group can occupy P-position. Experiments with labeled carbon showed

that in this case the movement of the allyl group into the ring does not occur as in the previous case. It looks like when P-migration, the allylic group is split off from the ether and, in the form of an allylic cation, attacks the free position of the benzene ring. This is reminiscent of the Fries rearrangement involving phenolic esters.

Peculiar ethers of phenols are the so-called ethoxylated alkylphenols, which turned out to be good nonionic detergents. They are obtained by the reaction of alkylphenols with ethylene oxide in an alkaline medium at 180 0 C

Phenolic ethers also include epoxy resins obtained from bis-phenol and epichlorohydrin.

Let's denote the middle fragment bis-phenol via R

Then the reaction bis-phenol with two molecules of epichlorohydrin can be written as follows

The resulting diepoxide undergoes an opening reaction of the epoxy ring

When these reactions (Williamson and epoxy ring opening) are repeated many times, we get epoxy resin

Resin can be cured—turned into a polymer with a three-dimensional structure—in several ways. Most often, trifunctional amines are used, in particular diethylenetriamine.

During curing, each amino group acts as a nucleophile on the epoxy group

Once the opening of the epoxy rings is complete, a cross-linked polymer is obtained.

14.1.2.3. Formation of esters. As hydroxyl-containing compounds, phenols would be expected to participate in a Fischer reaction (esterification) with acids to form esters. However, this does not happen. For phenols, carboxylic acids are too weak acylating agents. Therefore, to obtain phenol esters it is necessary to use anhydrides and acid halides of carboxylic acids in an alkaline medium (Schotten-Bauman method)

Phenol esters have an interesting property - when heated with aluminum chloride, they undergo rearrangement with migration of the acyl part of the ester into the free O- And P-position of the benzene ring ( Fries regrouping, 1908)

It is assumed that the Fries rearrangement occurs as an intramolecular acylation reaction: first, the acylium cation RCO is generated + , which further attacks the benzene ring.

Some phenolic esters have found use as polyester-type polymers.

Back in 1953, Germany received ester bis-phenol and carbonic acid – poly-, having unique properties. The polymer (Lexan, Merlon, polycarbonate) turned out to be transparent like glass and durable like steel. Polycarbonate is usually obtained by the reaction bis-phenol with phosgene

14.1.2.4. Removal of hydroxyl group. In phenols, the hydroxyl group is linked to the benzene ring quite tightly. One can even draw an analogy with an aromatically bound halogen. However, conditions for the elimination of hydroxyl phenols have been found. This occurs when phenols are heated with zinc powder.

HYDROXYL, HYDROXYL GROUP

[cm. hydr(ogenium) + ox(ignenium)]

he group (oxygen-hydrogen); is part of water non, bases, for example. naoh, alcohols, e.g. cjh6oh, and other compounds.

Dictionary of foreign expressions. 2012

Hydroxyl radical

Hydroxyl radical is a highly reactive and short-lived OH radical formed by the combination of oxygen and hydrogen atoms. It is usually formed during the decomposition of hydroperoxides, in atmospheric chemistry, by the interaction of excited oxygen molecules with water or under the action of ionizing radiation.

Role in biology

The hydroxyl radical is a reactive oxygen species and is the most active component of oxidative stress. It is formed in the cell primarily by the reduction of hydrogen peroxide in the presence of a transition metal (such as iron). Half-life t 1/2 of hydroxyl radical in vivo- very short - about 10 −9 s, which, together with its high reactivity, leads to the fact that it is one of the most dangerous agents formed in the body. Unlike superoxide, which can be detoxified by superoxide dismutase, there is no enzyme that eliminates the hydroxyl radical because the lifetime is too short for it to diffuse into the active site of the enzyme. The only defense of the cell against this radical is high level low molecular weight antioxidants such as glutathione. The resulting hydroxyl radical instantly reacts with any oxidizable molecule in its immediate environment. Of the most biologically important components of the cell, the hydroxyl radical is capable of oxidizing carbohydrates, nucleic acids (which can lead to mutation or damage to genes), lipids (causing lipid peroxidation), and amino acids.

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An excerpt characterizing the hydroxyl group

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The reaction is called xanthogen test into primary and secondary hydroxyl groups. Primary and secondary alcohols in the presence of alkalis react with carbon disulfide, forming water-soluble salts of alkyl xanthogenates-1:

Salts of alkyl xanthates react with solutions of divalent copper salts to give brown cuprous xanthates:

Xanthates of tertiary alcohols are unstable and decompose to mineral compounds, as a result of which this reaction is unsuitable for the determination of tertiary alcohols.

Methodology: dissolves a drop of the test substance in 1 cm 3 of diethyl ether, add a drop of carbon disulfide and a few grains of sodium hydroxide. The mixture is shaken in a test tube and slightly heated in a water bath. Add a drop of a solution of 2% CuSO 4 solution. If there is an alcohol group in the substance, a brown precipitate of copper xanthate precipitates. In the absence of hydroxyl groups, the color of the precipitate is blue.

Reaction to phenols

Most phenols give an intense color with a solution of iron (III) chloride:

The usual color of the solution is blue or purple. But for a number of complex phenols it is green or red. The reaction is carried out in aqueous solutions or in chloroform to distinguish phenols from enols . The latter give intense coloration in methanol or ethanol.

Methodology: Several crystals or one drop of a substance are dissolved in a test tube in 1 cm 3 of water or chloroform. Shake, add 1 drop of 1% aqueous solution of FeCl 3. In the presence of phenolic hydroxide, an intense color immediately appears. Enols under these conditions give only weak coloring. Phenols react more clearly in the presence of water.

Reaction to glycols and polyhydric alcohols

Most polyhydric alcohols containing hydroxy groups at adjacent carbon atoms form chelated copper glycolates, soluble in water and colored bright blue:

Glycolates are stable in an alkaline environment, but decompose into their parent compounds (copper salts and glycols) in an acidic environment.

Methodology: 10 drops of a 3% solution of CuSO 4 and 1 cm 3 of 5% sodium hydroxide are poured into a test tube. Three drops of the test solution are added to the mixture. If a polyhydric alcohol is present in it, the blue precipitate of freshly precipitated copper hydroxide dissolves and the solution takes on an intense blue color. -amino acids and -amino alcohols behave in the same way.

Carbonyl group
1. Reaction with hydroxylamine hydrochloride

The reaction of hydroxylamine with an unhindered carbonyl group is also very general:

Since hydroxylamine hydrochloride has an almost neutral reaction, and the resulting oxime is not a strong base, the progress of the reaction can be easily controlled by increasing the acidity of the medium due to the release hydrogen chloride.

Methodology: to 2 cm 3 of 3% hydroxylamine hydrochloride in a test tube, add a solution of 0.1 g of the test substance in 0.5 cm 3 of ethanol. Heat the mixture in a water bath. Add one drop of methyl orange indicator. If the test substance contains a carbonyl group, a distinct reddening of the indicator is observed. The reaction is interfered with by carboxylic acids that react with hydroxylamine. It is easy to verify their absence by testing the test solution for litmus. Instead of the indicated indicators, it is permissible to use universal indicator paper.