Organic oxygen-containing compounds, including various alcohols, are important functional derivatives of hydrocarbons. They are monatomic, two- and polyatomic. Monohydric alcohols are, in fact, derivatives of hydrocarbons, in the molecular component of which - one hydroxyl group (designated "-OH"), associated with saturated carbon atoms.
Monohydric alcohols are widely distributed in nature. Thus, methyl alcohol in small amounts is contained in the juice of a number of plants (for example, cow's). Ethyl alcohol, being a product of alcoholic fermentation of organic compounds, is found in acidified fruits and berries. Cetyl alcohol is found in whale fat. Bee wax includes ceric, miracil alcohols. In the petals of roses found 2-phenylethanol. Terpenic alcohols in the form of fragrant substances are represented in many spicy aromatic cultures.
Alcohols are divided by the molecular number of hydroxyl groups. First of all on:
- monohydric alcohols (eg ethanol);
- dihydric (ethanediol);
- polyatomic (glycerin).
The nature of the hydrocarbon radical, alcohols are divided into aromatic, aliphatic, cyclic. Depending on the type of carbon atom connected with a hydroxyl group, alcohols are considered as primary, secondary and tertiary. The General formula of Monohydric alcohol in use to the limit of monatomic alcohols is expressed by the value: n H2n +2 O.
The name of the alcohol according to the radical-functional nomenclature is formed from the name associated with the hydroxyl group of the radical, and the word "alcohol". According to the systematic nomenclature of IUPAC, the name of alcohol is formed from the corresponding alkane with the addition of the ending "-ol". For example:
- methanol-methyl alcohol;
- methylpropanol-1-2-isobutyl (t-butyl);
- ethanol - ethyl;
- butanol-1-2-butyl (sec-butyl);
- propanol-1-2-propyl (isopropyl).
Numbering according to IUPAC rules is classified by the position of the hydroxyl group, it gets a smaller number. For example: pentanediol-2-4, 4-methylpentanol-2, etc.
The limiting monohydric alcohols possess the following types of structural and spatial isomerism. For example:
- Carbon skeleton.
- Isomeric to ethers.
- The positions of the functional group.
Spatial isomerism of alcohols is represented by optical isomerism. Optical isomerism is possible if there is an asymmetric carbon atom in the molecule (containing four different substituents).
Methods for the preparation of monohydric alcohols
One can obtain the limiting monohydric alcohol by several methods:
- Hydrolysis of haloalkanes.
- Hydration of alkenes.
- Reduction of aldehydes and ketones.
- Magnesium-organic synthesis.
Hydrolysis of haloalkanes is one of the most common laboratory methods for obtaining alcohols. Treatment with water (as an alternative - an aqueous solution of alkali), alcohols are primary and secondary:
Tertiary halogenoalkanes hydrolyze even more easily, but they have an easier side-reaction of elimination. Therefore, tertiary alcohols are obtained by other methods.
Hydration of alkenes is carried out by adding water to alkenes in the presence of acid-containing catalysts (H3 PO4). The method underlies the industrial production of such alcohols as ethyl, isopropyl, tert-butyl.
The reduction of the carbonyl group is carried out with hydrogen in the presence of a hydrogenation catalyst (Ni or Pt). From ketones, secondary alcohols are formed, of the aldehydes, primary monohydric alcohols. The formula of the process is:
Addition to aldehydes and ketones of alkyl magnesium halides gives magnesium-organic compounds. The reaction is carried out in dry diethyl ether. Subsequent hydrolysis of organomagnesium compounds forms monohydric alcohols.
Primary alcohols are formed by the Grignard reaction only from formaldehyde and any alkyl magnesium halides. Other aldehydes give secondary alcohols in this reaction, ketones are tertiary alcohols.
Industrial synthesis of methanol
Industrial methods, as a rule, are continuous processes with repeated recirculation of large masses of reacting substances, carried out in the gas phase. Industrially important alcohols are methanol and ethanol.
Methanol (its production volume is the biggest among alcohols) until 1923, received dry distillation (heating without access of air) of wood. Today it is generated from synthesis gas (mixture of CO and H2 ). The process is carried out at a pressure of 5-10 MPa using oxide catalysts (ZnO + Cr2 O3. CuO + ZnO + Al2 O3 and others) in the temperature range 250-400C, received as a result of the limiting Monohydric alcohols. The formula of the reaction: CO 2H2 → CH3 OH.
In 80-e years in the study of the mechanism of this process it was found that methanol is not formed from carbon monoxide and carbon dioxide, resulting in the interaction of carbon monoxide with traces of water.
Industrial synthesis of ethanol
A common production method for the synthesis of technical ethanol is the hydration of ethylene. The formula of monohydric alcohol ethanol will be as follows:
The process is carried out at a pressure of 6-7 MPa in the gas phase, passing ethylene and a pair of water over the catalyst. The catalyst is phosphoric or sulfuric acid, deposited on silica gel.
Food and medical ethyl alcohol is obtained by enzymatic hydrolysis of sugars contained in grapes, berries, cereals, potatoes and subsequent fermentation of the formed glucose. Fermentation of sugary substances is caused by yeast fungi belonging to the group of enzymes. The most favorable temperature for the process is 25-30 ° C. Industrial enterprises use ethanol, obtained by fermentation of wood formed by hydrolysis and waste products of pulp and paper production of carbohydrates.
Physical properties of monohydric alcohols
In molecules of alcohols, there are hydrogen atoms associated with the electronegative element - oxygen, practically devoid of electrons. Between these hydrogen atoms and oxygen atoms, which have unshared electron pairs, intermolecular hydrogen bonds are formed.
The hydrogen bond is due to the specific features of the hydrogen atom:
- When pulling the bonding electrons to the more electronegative atom - the hydrogen atom "laid bare", and formed other electrons unshielded proton. During ionization of any other atom is still electron shell, shielding the nucleus.
- The hydrogen atom has a small size compared to other atoms, so that it is able to penetrate deep enough into the electron shells of neighboring negatively polarized atom not being connected with it by a covalent bond.
The hydrogen bond is about 10 times weaker than the ordinary covalent bond. The hydrogen bond energy is in the range of 4-60 kJ / mol, for molecules of alcohols it is 25 kJ / mol. It differs from ordinary s-bonds with a longer length (0.166 nm) than the O-H bond length (0.107 nm).
Chemical reactions of monohydric alcohols are determined by the presence in their molecules of a hydroxyl group, which is functional. The oxygen atom is in the sp3-hybrid state. The valence angle is close to the tetrahedral angle. Two sp3-hybrid orbitals go for the formation of bonds with other atoms, and on the other two orbitals there are lone pairs of electrons. Accordingly, a partial negative charge is concentrated on the oxygen atom, and partial positive charges on hydrogen and carbon atoms.
C-O and C-H bonds are covalent polar (the latter is more polar). The heterolytic cleavage of the O-H bond to the formation of H + causes the acidic properties of monohydric alcohols. A carbon atom with a partial positive charge can be an object of attack of a nucleophilic reagent.
Alcohols are very weak acids, weaker than water, but stronger than acetylene. They do not cause a change in the color of the indicator. Oxidation of monohydric alcohols is manifested by interaction with active metals (alkaline and alkaline-earth) with the evolution of hydrogen and the formation of alcoholates:
2ROH + 2Na → 2RONa + H2.
Alkali alkali metal alkoxides are substances with an ionic bond between oxygen and sodium, in a solution of a monohydric alcohol they dissociate to form alkoxide ions:
CH3 ONa → CH3 O - + Na + (methoxide ion).
The formation of alcoholates can also be carried out by the reaction of alcohol with sodium amide:
And will the reaction of ethanol with alkali? Hardly ever. Water is a stronger acid than ethanol, so equilibrium is established here. As the length of the hydrocarbon radical increases in the alcohol molecule, the acid properties decrease. Also, the limiting monohydric alcohols are characterized by a decrease in acidity in the series: primary → secondary → tertiary.
The nucleophilic substitution reaction
In alcohols, the C-O bond is polarized, a partial positive charge is concentrated on the carbon atom. As a consequence, the carbon atom is attacked by nucleophilic particles. During the cleavage of the C-O bond, another hydroxyl group is replaced by another nucleophile.
One such reaction is the interaction of alcohols with hydrogen halides or their concentrated solutions. Reaction equation:
To facilitate cleavage of the hydroxyl group is used as a catalyst concentrated sulfuric acid. She profaniruet an oxygen atom, thereby activating the molecule of a Monohydric alcohol.
Primary alcohols, like primary halogenoalkanes, enter the exchange reaction according to the SN mechanism2. Secondary monohydric alcohols, like secondary haloalkanes, react with hydrohalic acids. The conditions for the interaction of alcohols are subordinated to the nature of the reacting components. The reactivity of alcohols obeys the following regularity:
Under mild conditions (neutral or alkaline solutions of potassium permanganate, a chrome mixture at a temperature of 40-50 ° C), primary alcohols are oxidized to aldehydes, when heated to a higher temperature, to acids. Secondary alcohols undergo oxidation to ketones. Tertiary acids are oxidized in the presence of acid under very stringent conditions (for example, a chrome mixture at a temperature of 180 ° C). The reaction of oxidation of tertiary alcohols goes through the dehydration of alcohol to form an alkene and oxidation of the latter with a break in the double bond.