Production of alcohols, application, properties. Methods for producing alcohols
Alcohols are common in nature. Most people are familiar with - the active ingredient in alcoholic beverages, but it is only one of a family of organic compounds known as alcohols. Getting them, above all ethyl (wine as a result of the enzymatic fermentation of grape juice, was one of the first chemical processes mastered by mankind.
Nomenclature of alcohols
Alcohols are organic compounds of the hydroxyl (OH) functional group with an aliphatic carbon atom. Since OH is a member of the molecules of all alcohols, they are often represented as derivatives of water with the general formula ROH, where R is an alkyl group.
Production of methanol alcohols (CH3HE) and ethanol (CH3CH2HE), being the first two members of their homologous series, is an important task of the chemical industry in many countries.When containing from one to four carbon atoms, they are often called common names, in which the name of an alkyl group is followed by the word alcohol:
It can be seen that all four (the last two are isomers of one substance) the above alcohol molecules contain one hydroxyl group. On this basis, they all belong to the class of monohydric alcohols (there are also two-, three, four-, and polyatomic). In addition, they are all derivatives of saturated hydrocarbons from a number of alkanes: methane, ethane, propane (the names of alcohols are obtained by adding to the name of the alkene the ending "-ol"). Therefore, they are also called marginal monohydric alcohols.
The production, properties (both physical and chemical) of these compounds depend on the number of carbon atoms attached to its atom, directly related to the OH group. Therefore, monohydric alcohols can be grouped into three classes on this basis.
- Primary alcohols have a molecule in which one carbon atom linked to an OH group is attached to another atom C. Their general formula is RCH2HE. For example, ethanol is the primary alcohol.
- Secondary alcohols have one carbon atom in the molecule with an OH group attached to two other C atoms. Their general formula is R2SNON. These include propyl and isopropyl alcohols.
- Tertiary alcohols contain a carbon atom in the molecule with an OH group attached to three other C atoms. Their general formula is R3SLEEP.
Obtaining monohydric alcohols in the industry is possible in a number of ways, which will be discussed below.
Methanol as a natural gas product
Methanol is produced by mixing a gas of hydrogen and carbon monoxide at high temperatures and pressures (200 at, 350°C)in the presence of a catalyst consisting of zinc oxide (ZnO) and chromium oxide (Cr2O3) as a catalyst: 2H2+ CO → CH3OH.
In this case, the raw material for the production of reagents is natural gas and water vapor, mixing which produces synthesis gas, which is a mixture of CO and H2.
Methanol is an important solvent and is used as an automobile fuel, either as a pure liquid - in some racing cars, or as a high-octane additive to gasoline. The production and use of alcohols in the world, and in particular methanol, is measured in millions of tons. At the end of 2013, 66 million tons of methanol were consumed worldwide, of which 65% were in Asia, 17% in Europe and 11% in the USA.
Getting the ultimate alcohols from alkenes
Many simple weight alcohols of industrial importance are produced by hydration (by adding water) of alkenes (ethylene, propylene, butene). Ethanol, isopropanol, butanol (secondary and tertiary) are obtained by this reaction.
Known direct and indirect methods of producing alcohols by hydration. Direct allows you to avoid the formation of stable intermediate products, usually with the help of acid catalysts.
The catalyst is usually phosphoric acid adsorbed on a porous carrier such as silica gel or kieselguhr. This catalyst was first used for large-scale ethanol production in the United States by Shell in 1947. The reaction is carried out in the presence of high-pressure steam at 300 ° C, with the ratio 1.0: 0.6 being maintained between ethylene and steam.
A similar reaction for the production of isopropyl alcohol from catalysts in the form of sulfuric acid is as follows
Indirect method of ethylene hydration
In an indirect way, in practice for the first time applied on an industrial scale in 1930, but today considered to be almost completely obsolete, the reaction to produce alcohols is to convert the alkene to sulfate esters, which are then hydrolyzed.Traditionally, alkene is treated with sulfuric acid to produce alkyl sulfate esters. In the case of ethanol production, this step can be written as: H2SO4+ C2H4→ C2H5-O-SO3H
Subsequently, this sulfate ester is hydrolyzed until sulfuric acid is regenerated and ethanol is liberated: C2H5-O-SO3H + H2O → H2SO4+ C2H5HE.
Methods for producing alcohols are extremely diverse, but the process described below is probably known, at least by hearsay to every reader.
It is a biological process in which molecules, such as glucose, fructose and sucrose, are converted to cellular energy with the parallel production of ethanol and carbon dioxide as products of metabolism. Fermentation is catalyzed by enzymes contained in yeast and proceeds through a complex multi-step mechanism, which generally involves the conversion (at the first stage) of starch contained in vegetable grains into glucose, followed by the production of ethanol from it. Since yeast performs this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process.
The reaction of obtaining fermentation of alcohols can be represented as follows:
Ways to get alcoholic beverages
All ethanol contained in alcoholic beverages is produced by fermentation caused by yeast.
Wine is made by fermentation of the natural sugars present in the grapes; Cider is obtained by analogous fermentation of natural sugar in apples and pears, respectively; and other fruit wines produced by the fermentation of sugars in any other types of fruit. Brandy and brandy spirits (for example, plum brandy) are produced by distilling beverages obtained by fermentation of fruit sugars.
Honey drinks are produced by fermentation from the natural sugars present in honey.
Beer, whiskey, and vodka are produced by fermentation of starch grains, which are converted to sugar by the action of the enzyme amylase present in grain kernels that have undergone malting germination. Other sources of starch (for example, potatoes and non-malted grains) can be added to the mixture, since amylase will also act on their starch.
Rice wines (including sake) are produced by fermentation of cereal starch, converted into sugar by Asugi Aspergillus ogugae.
Rum and some other drinks are obtained by fermentation and distillation of sugar cane. Rum is usually made from sugar cane product - molasses.
In all cases, fermentation must occur in a vessel that allows carbon dioxide to escape, but prevents the entry of outside air. This is necessary because the effect of oxygen prevents the formation of ethanol, and the accumulation of carbon dioxide creates the risk of rupture of the vessel.
Nucleophilic substitution reaction
Alcohols are produced in laboratories by methods that use chemical substances of a wide variety of classes, from hydrocarbons to carbonyl compounds, as starting products for reactions. There are several ways that boil down to a few basic reactions.
Primary halogenoalkanes react with aqueous solutions of alkali NaOH or KOH, forming, forming, mainly, primary alcohols in the reaction of nucleophilic aliphatic substitution. When, for example, methyl bromide reacts with sodium hydroxide solution, the hydroxyl groups formed during alkali dissociation replace bromine ions to form methanol.
Several reactions that allow for the production of alcohols in laboratories are listed below.
Grignard reagents (magnesium compounds with alkyl halides - iodides or bromides), as well as organometallic compounds of copper and lithium react with carbonyl groups (C = O) of aldehydes to form primary and secondary alcohols, depending on the addition mechanism. Similar reactions with ketones lead to tertiary alcohols .
The Barbier reaction proceeds between a halogen-alkane and a carbonyl group as an electrophilic substrate in the presence of magnesium, aluminum, zinc, indium, tin or its salts. The reaction product is a primary, secondary, or tertiary alcohol. The mechanism of its occurrence is similar to the Grignard reaction with the difference that the Barbier reaction is a synthesis in one vessel, whereas the Grignard reagent is prepared separately before the addition of the carbonyl compound.Being a nucleophilic addition reaction, it occurs with relatively inexpensive and water-resistant metals or their compounds, unlike Grignard or organolithium reagents. For this reason, it is possible in many cases to run it in water, which makes the process part of green chemistry.The Barbier Reaction is named after Philip Barbier, the teacher of Victor Grignard.
Aldehydes or ketones are reduced to alcohols with sodium borohydride (NaBH4) or (after acid treatment) with lithium aluminum hydride (LiAlH).
In the reaction of Meerwein-Pondorf-Wehrli (MPV), the production of alcohols by reducing them from ketones and aldehydes takes place using an aluminum alkoxide catalyst. The advantages of MPV are its high chemoselectivity and the use of a cheap, environmentally friendly metal catalyst. The reaction was discovered by Meerwein and Schmidt, and independently Verli in 1925. They found that a mixture of aluminum ethoxide and ethanol can reduce aldehydes to their alcohols. Ponndorf applied the reaction to ketones and updated the catalyst to aluminum isopropylate (Al (O-i-Pr)3where i-Pr means an isopropyl group (CH (CH3)2). in order to obtain isopropanol.
The general equation for obtaining alcohol by MPV-reduction of ketones to alcohols is as follows:
This, of course, is not all that can be said about alcohols and their properties, but we hope you managed to get a general idea of them.