Organic oxygen-containing compounds, among which are various alcohols, are important functional derivatives of hydrocarbons. They are monatomic, diatomic and polyatomic. Monohydroxy alcohols are, in fact, hydrocarbon derivatives, in the molecular component of which there is one hydroxyl group (denoted by "-OH") associated with saturated carbon atoms.
Monohydric alcohols are widely distributed in nature. For example, methyl alcohol in small quantities is contained in the juice of a number of plants (for example, hogweed). Ethyl alcohol, being a product of alcoholic fermentation of organic compounds, is contained in acidified fruits and berries. Cetyl alcohol is found in whale oil. Beeswax includes ceryl, myricyl alcohols. 2-phenylethanol was found in the petals of roses. Terpene alcohols in the form of fragrant substances are represented in many aromatic cultures.
The alcohols are divided by the molecular number of hydroxyl groups. First of all on:
- monohydric alcohols (for example, ethanol);
- diatomic (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 alcohols in 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 IUPAC nomenclature, the name of the alcohol is formed from the corresponding alkane with the addition of the ending "-ol". For example:
- methanol - methyl alcohol;
- methyl propanol-1-2 - isobutyl (tert-butyl);
- ethanol - ethyl;
- butanol-1-2 - butyl (V-butyl);
- propanol-1-2 - propyl (isoprapyl).
Numbering according to the rules of IUPAC is classified by the position of the hydroxyl group, it gets a smaller number. For example: pentanediol-2-4, 4-methylpentanol-2, etc.
Limit monohydric alcohols have the following types of structural and spatial isomerism. For example:
- Carbon skeleton.
- Isomeric ethers.
- The position of the functional group.
Spatial isomerism of alcohols is represented by optical isomerism. Optical isomerism is possible when an asymmetric carbon atom is present in the molecule (containing four different substituents).
Methods for producing monohydric alcohols
To obtain the ultimate monohydric alcohol by several methods:
- Hydrolysis of halogen-alkanes.
- Hydration of alkenes.
- Recovery of aldehydes and ketones.
- Organomagnesium synthesis.
Hydrolysis of halogen-alkanes is one of the most common laboratory methods for producing alcohols. Water treatment (alternatively - an aqueous solution of alkali) alcohols are primary and secondary:
Tertiary halogenoalkanes are hydrolyzed even more easily, but they have an easier side reaction of elimination. Therefore, tertiary alcohols get other methods.
Alkenes are hydrated by addition of water to alkenes in the presence of acid-containing catalysts (H3 PO4). The method underlies the industrial production of alcohols such as ethyl, isopropyl, tert-butyl.
The carbonyl group is reduced by hydrogen in the presence of a hydrogenation catalyst (Ni or Pt). In this case, secondary alcohols are formed from ketones, and aldehydes - primary terminal monohydric alcohols. The formula of the process:
Addition of magnesium-organic compounds to aldehydes and ketones of alkyl magnesium halides. 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 for this reaction, ketones - tertiary alcohols.
Industrial methanol synthesis
Industrial methods, as a rule, are continuous processes with multiple recirculation of large masses of reactants 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 under 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 ethanol synthesis
A common production method for the synthesis of technical ethanol is the hydration of ethylene. The formula of monohydric alcohol ethanol will receive the following form:
The process is carried out under a pressure of 6-7 MPa in the gas phase, passing ethylene and water vapor over the catalyst. The catalyst is phosphoric or sulfuric acid supported on silica gel.
Food and medical ethyl alcohol is obtained by enzymatic hydrolysis of sugars contained in grapes, berries, cereals, potatoes, followed by fermentation of glucose formed. Fermentation of sugary substances caused by yeast, belonging to the group of enzymes. For the process, the most favorable temperature is 25-30 ° C. At industrial enterprises, ethanol is used, obtained by fermentation of wood and pulp and paper production of carbohydrates formed during the hydrolysis of wood.
Physical properties of monohydric alcohols
In the molecules of alcohols there are hydrogen atoms associated with the electronegative element - oxygen, almost devoid of electrons. Intermolecular hydrogen bonds are formed between these hydrogen atoms and oxygen atoms that have lone electron pairs.
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 usual covalent bond. The hydrogen bond energy is in the range of 4-60 kJ / mol; for alcohol molecules, it is 25 kJ / mol. It differs from ordinary s-bonds in a longer length (0.166 nm) in comparison with the O – H bond length (0.107 nm).
The chemical reactions of monohydric alcohols are determined by the presence of a hydroxyl group in their molecules, which is functional. The oxygen atom is in the sp3 hybrid state. The valence angle is close to tetrahedral. Two sp3-hybrid orbitals go to form bonds with other atoms, and the other two orbitals are lone pairs of electrons. Accordingly, a partial negative charge is concentrated on the oxygen atom, and partial positive charges on the atoms of hydrogen and carbon.
C-O and C-H bonds are covalent polar (the latter is more polar). Heterolytic rupture of the O – H bond with the formation of H + causes the acidic properties of monohydric alcohols. A carbon atom with a partial positive charge may be subject to attack by 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. The oxidation of monohydric alcohols occurs when interacting with active metals (alkali and alkaline-earth) with the release of hydrogen and the formation of alcoholates:
2ROH + 2Na → 2RONa + H2.
Alkali metal alkoxides are substances with an ionic bond between oxygen and sodium, in a solution of a monohydric alcohol they dissociate to form alkoside 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:
Will ethanol react with alkali? Hardly ever. Water is a stronger acid than ethyl alcohol, so equilibrium is established here. With an increase in the length of the hydrocarbon radical in the alcohol molecule, the acidic properties decrease. Also limit monohydric alcohols are characterized by a decrease in acidity in the series: primary → secondary → tertiary.
Nucleophilic substitution reaction
In alcohols, the C – O bond is polarized, and a partial positive charge is concentrated on the carbon atom. As a result, the carbon atom is attacked by nucleophilic particles. In the process of breaking the C-O bond, another nucleophile replaces a hydroxyl group.
One of these reactions 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 into exchange reactions using the SN mechanism.2. Secondary monohydric alcohols, like secondary halogenoalkanes, react with hydrohalic acids. The conditions of interaction of alcohols are subject to the nature of the reacting components. The reactivity of alcohols is subject to the following laws:
Under mild conditions (neutral or alkaline solutions of potassium permanganate, chromic 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 an oxidation process to ketones. Tertiary oxidized in the presence of acid in very harsh conditions (for example, chromic mixture at a temperature of 180 ° C). The oxidation reaction of tertiary alcohols goes through the dehydration of alcohol with the formation of alkene and the oxidation of the latter with a double bond breaking.