Methods of preparing alcohols (Methanol and Ethanol)

Hydrolysis of Alkyl Halides

This is a nucleophilic substitution reaction.

R-X + KOH_aq → R-OH

The method is not satisfactory as olefins are also formed as by-products. However better yields is obtained by using moist Ag₂O or aqueous K₂CO₃. Tertiary butyl halides mainly gives alkene due to dehydrohalogenation.

Hydration of Alkenes

This is electrophilic addition of H₂O to alkenes.

Mechanism of Hydration of alkenes:

Protonation of alkene to form carbocation by electrophilic

Nucleophilic attack of water on carbocation.

Deprotonation to form an alcohol.

Except ethyl alcohol no other primary alcohol can be obtained by this method, however hydroboration of terminal alkenes give primary alcohols.

Oxymercuration and Demercuration of Alkanes

Alkenes react with mercuric acetate in presence of H₂O and tetra hydrofuran to give alkyl mercury compounds.

Examples:

Hydroboration Oxidation From Grignard Reagents

All the three types of monohydric alcohols (primary, secondary and tertiary alcohols) are obtained by the use of Grignard reagents and carbonyl compounds. The addition of RMgX on carbonyl compounds followed with hydrolysis yields alcohols.

The Grignard reagent : an organometallic compound

When a solution of an alkyl halide in dry ethyl ether, (C₂H₅)O, is allowed to stand over turnings of metallic magnesium a vigorous reaction takes place: the solution turns cloudy, begins to boil, and the magnesium Metal gradually disappears. The resulting solution is known as a Grignard reagent, after Victor Grignard (of the University of Lyons) who received the Nobel prize in 1912 for its discovery. It is one of the most useful and versatile reagents known to the organic chemist.

CH₃I + Mg CH₃MgI

H₃CH₂Br + Mg CH₃CH₂MgBr

Ethyl bromide Ethylmagnesium bromide

The Grignard reagent has the general formula R MgX, and the general name alkylmagnesium halide. The carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic. R^–Mg⁺X

Since magnesium becomes bonded to the same carbon that previously held halogen, the alkyl group remains intact during the preparation of the reagent. Thus n-propyl chloride yields n-propylmagnesium chloride, and isopropyl chloride yields isopropylmagnesium chloride.

CH₃CH₂CH₂Cl + Mg CH₃CH₂CH₂MgCl

n-Propyl chloride n-Propylmagnesium chloride

CH₃CHClCH₃ + Mg CH₃CHMgClCH₃

Isopropyl chloride Isopropylmagnesium chloride

The Grignard reagent is the best-known member of a broad class of substances, called organometallic compounds, in which carbon is bonded to a metal: lithium potassium, sodium, zinc, mercury, lead, thallium-almost any metal known. Each kind of organometallic compound has, of course, its own set of properties, and its particular uses depend on these. But whatever the metal, it is less elctronegative than carbon, and the carbon-metal bond-like one in the Grignard reagent – is highly polar. Although the organic group is not a full-fledged carbanion–an anion in which carbon carries negative charge–it nevertheless has considerable carbanion character. As we shall see, organometallic compounds owe their enormous usefulness chiefly to one common quality: they can serve as a source from which carbon is readily transferred with its electrons.

The Grignard reagent is highly reactive. It reacts with numerous inorganic compounds including water, carbon dioxide, and Oxygen, and with most kinds of organic compounds; in many of these cases the reaction provides the best way to make a particular class of organic compounds.

The reaction with water to form an alkane is typical of the behaviour of the Grignard reagent–and many of the more reactive organometallic compounds–toward acids. In view of the marked carbanion character of the alkyl group, we may consider the Grignard reagent to be the magnesium salt, R MgX, of the extremely weak acid,

R–H. The reaction

R MgX + HOH → R–H + Mg(OH)X

Stronger Weaker

acid acid

is simply the displacement of the weaker acid, R–H, from its salt by the stronger acid, HOH.

R MgX + NH₃ → R–H + Mg(NH₂)X

Stronger Weaker

acid acid

An alkane is such a weak acid that it is displaced from the Grignard reagent by compounds that we might ordinarily consider to be very weak acids themselves, or possibly not acids at all. Any compound containing hydrogen attached to oxygen or nitrogen is tremendously more acidic than an alkane, and therefore can decompose the Grignard reagent: for example, ammonia or methyl alcohol.

RMgX + CH₃OH → R–H + Mg(OCH₃)X

Stronger Weaker

acid acid

Grignard Synthesis of Alcohols

The Grignard reagent, we recall, has the formula RMgX, and is prepared by the reaction of metallic magnesium with the appropriate organic halide. This halide can be alkyl (1^o, 2^o, 3^o), allylic, aryl alkyl (e.g., benzyl), or aryl (phenyl) or substituted phenyl. The halogen may be –Cl, –Br or –I, (Arylmagnesium chlorides must be made in the cyclic ether tetrahydrofuran instead of ethyl ether.)

Aldehydes and ketones resemble each other closely in most of their reactions. Like the carbon-carbon double bond, the carbonyl group is unsaturated, and like the carbon-carbon bond, it undergoes addition. One of its typical reactions is addition of the Grignard reagent.

Since the electrons of the carbonyl double bond hold together atoms of quite different electronegativity, we would not expect the electrons to be equally shared; in particular, the mobile p cloud should be pulled strongly towards the more electronegative atom, oxygen. Whatever the mechanism involved, addition of an unsymmetrical reagent is oriented so that the nucleophilic (basic) portion attaches itself to carbon, and the electrophilic (acidic) portion attaches itself to oxygen.

The carbon-magnesium bond of the Grignard reagent is a highly polar bond, carbon being negative relative to electropositive magnesium. It is not surprising, then, that in the addition to carbonyl compounds, the organic group becomes attached to carbon and magnesium to oxygen. The product is the magnesium

salt of the weakly acidic alcohol and is easily converted into the alcohol itself by the addition of the stronger acid, water. Since the Mg(OH)X thus formed is a gelatinous material difficult to handle, dilute mineral acid (HCl, H₂SO₄) is commonly used instead of water, so that water-soluble magnesium salts are formed.

Products of the Grignard Synthesis

The class of alcohol that is obtained from a Grignard synthesis depends upon the type of carbonyl compoud used: formaldehyde, HCHO, yields primary alcohols; other aldehydes, RCHO, yield secondary alcohols; and ketones, R₂CO, yield tertiary alcohols.

This relationship arises directly from our definitions of aldehydes and ketones, and our definitions of primary, secondary, and tertiary alcohols. The number of hydrogens attached to the carbonyl carbon defines the carbonyl compound as formaldehyde, higher aldehyde or ketone. The carbonyl carbon is the one that finally bears the –OH group in the product; here the number of hydrogen defines the alcohol as primary, secondary, or tertiary.

For example:

A related synthesis utilized ethylene oxide to make primary alcohols containing two more carbons than the Grignard reagent.

Here, too, the organic group becomes attached to carbon and magnesium to oxygen, this time with the breaking of a carbon-oxygen s bond in the highly strained three-membered ring. For example:

Reduction of Carbonyl Compounds

Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, either by catalytic hydrogenation or by use of chemical reducing agents like lithium aluminum hydride, LiAlH₄. Such reduction is useful for the preparation of certain alcohols that are less available than the corresponding carbonyl compounds, in particular carbonyl compounds that can be obtained by the aldol condensation. For example

Reduction of ketones gives secondary alcohol.

Note : tertiary alcohols can be obtained by this method.

Sodium borohydride, NaBH₄, does not reduce carbon-carbon double Bonds, not even those conjugated with carbonyl groups, and in thus useful for the reduction of such unsaturated carbonyl compounds to unsaturated alcohols.Let us look a little more closely at reduction by metal hydrides. Alcohols are formed from carbonyl compounds, smoothly and in high yield, by the action of such compounds as lithium aluminum hydride, LiAlH₄. Here again, we see

Nucleophilic addition : this time the nucleophile is hydrogen transferred with a pair of electrons-as a hydride ion, H:^––from the metal to carbonyl carbon:

Reduction of acids to alcohols: Lithium aluminum hydride, LiAlH₄, is one of the few reagents that can reduce an acid to an alcohol; the inital product is an alkoxide from which the alcohol is liberated by hydrolysis:

4RCOOH + 3LiAlH₄ → 4RCH₂OH 1^oalcohol

Because of the excellent yields it gives, LiAlH₄ is widely used in the laboratory for the reduction of not only acids but many other classes of compounds. As an alternative to direct reduction, acids are often converted into alcohols by a two-step process: esterification, and reduction of the ester.

Reduction of esters: Like many organic compounds, esters can be reduced in two ways: (A) by catalytic hydrogenation using molecular hydrogen, or (B) chemical reduction. In either case, the ester is cleaved to yield (in addition to the alcohol or phenol from which it was derived) a primary alcohol corresponding to the acid portion of the ester.

RCOOR’ RCH₂OH + R’OH

Ester 1^oalcohol

Hydrogenolysis (cleavage by hydrogen) of an ester requires more severe conditions than simple hydrogenation of (addition of hydrogen to) a carbon-carbon double bond. High pressures and elevated temperatures are required: the Catalyst used most often is a mixture of oxides known as copper chromite, of approximately the composition CuO.CuCr₂O₄. For example:

CH₃(CH₂)₁₀COOCH₃ CH₃(CH₂)₁₀CH₂OH + CH₃OH

(Methyl dodecanoate) (1-Dodecanol)

Chemical reduction is carried out by use of sodium metal and alcohol, or more usually by use of lithium aluminium hydride

By the reduction of acids and their Derivatives :

RCOOH RCH₂OH

(RCO₂)O RCH₂OH

RCOCI RCH₂OH

RCOOR’ RCH₂OH + R’OH

Note : If C₂H₅OH + Na is used as reducing agent, the reduction is known as Bouveault-Blane reaction.

By the action of nitrous acid on primary amines :

R-NH₂ + HNO₂ → R-OH + N₂ + H₂O

However under similar conditions CH₃NH₂ gives CH₃-O-N=O or CH₃OCH₃

CH₃NH₂ + 2HNO₂ → CH₃-O-N=O + 2H₂O + N₂

or 2CH₃NH₂ + 2HNO₂ → CH₃OCH₃ + 2N₂ + 3H₂O

Preparation of Methanol: Methanol can also be prepared as

Hydroxylation of Alkenes

By Fermentation-2/”>Fermentation :

Fermentation is the slow decomposition of complex organic compounds into simpler organic compounds by the activity of ENZYMES. Enzymes are complex, nitrogenous (proteins), non living macro Molecules of high molecular weight derived from living organisms. These are also known as biological catalysts.

Fermentation process is generally accompanied with evolution of gases like CO₂ & CH₄ and are exothermic in nature.

The alcoholic fermentation involves conversion of sugar into ethyl alcohol by yeast.

The starting material for alcoholic fermentation is starch (potato, rice, barley, maize). The source of starch depends upon its availability in that country. In India, alcoholic fermentation is made by molasses i.e. the dark coloured syrupy liquid left after crystallization of sugar from sugar cane juice. Molasses contains about 50% sugar left after crystallization of sugar from cane juice.

Conditions Favourable for Fermentation

1.Optimum temperature range for fermentation s 25-30^oC. At higher temperature enzymes are coagulated.

2.Certain inorganic substances, (NH₄)₂SO₄, phosphate etc are added as food for ferment cells.

3.Solution to be fermented should be dilute.

4.Substances like boric acid, mercury slats etc. should not be present as they retard fermentation.

5.Proper aeration should be maintained in fermentation.

Note : The name fermentation has been derived from Latin word ferver meaning to boil, because during fermentation there is lot of frothing due to evolution of CO₂ and this gives the appearance of boiling liquid.

Alcohols are a class of organic compounds that contain a hydroxyl group (-OH) attached to a carbon atom. They are a major component of many natural products, including fats, oils, and waxes. Alcohols are also used in a variety of industrial and commercial applications, such as solvents, fuels, and antifreeze.

There are many different ways to prepare alcohols. Some of the most common methods include:

Synthesis from carbon monoxide and hydrogen: This is the most common method for producing methanol, the simplest alcohol. In this process, carbon monoxide and hydrogen are reacted over a catalyst to form methanol.
Synthesis from wood: Wood is a complex mixture of organic compounds, including cellulose, hemicellulose, and lignin. These compounds can be converted to alcohols by a process called hydrolysis. Hydrolysis is the process of breaking down a compound by adding water. In the case of wood, hydrolysis is used to break down the cellulose, hemicellulose, and lignin into sugars. These sugars can then be fermented to produce alcohol.
Synthesis from ethylene: Ethylene is a gas that is produced from natural gas or petroleum. It can be converted to alcohol by a process called hydration. Hydration is the process of adding water to a compound. In the case of ethylene, hydration is used to add water to the ethylene molecule to form ethanol.
Synthesis from syngas: Syngas is a mixture of carbon monoxide and hydrogen that is produced from natural gas or coal. It can be converted to alcohol by a process called Fischer-Tropsch synthesis. Fischer-Tropsch synthesis is a process that uses a catalyst to convert syngas into hydrocarbons, including alcohols.

In addition to these methods, there are a number of other ways to prepare alcohols. Some of these methods include:

From iodomethane: Iodomethane is a compound that contains iodine and methane. It can be converted to methanol by a process called the Wurtz reaction. The Wurtz reaction is a reaction that uses sodium metal to react alkyl halides to form alkanes. In the case of iodomethane, the Wurtz reaction is used to react iodomethane with sodium metal to form methanol.
From Grignard reagents: Grignard reagents are compounds that contain magnesium and an alkyl halide. They can be used to prepare alcohols by a process called the Grignard reaction. The Grignard reaction is a reaction that uses Grignard reagents to react with carbonyl compounds to form alcohols.
From hydroboration: Hydroboration is a reaction that uses borane to react with alkenes to form alcohols.
From oxidation of alkenes: Alkenes are compounds that contain carbon-carbon double bonds. They can be oxidized to alcohols by a process called the alkene oxidation. The alkene oxidation is a reaction that uses a catalyst to oxidize alkenes to alcohols.
From hydration of alkenes: Alkenes can also be hydrated to alcohols by a process called the hydration of alkenes. The hydration of alkenes is a reaction that uses water to react with alkenes to form alcohols.
From reduction of carbonyl compounds: Carbonyl compounds are compounds that contain a carbon-oxygen double bond. They can be reduced to alcohols by a process called the reduction of carbonyl compounds. The reduction of carbonyl compounds is a reaction that uses a reducing agent to reduce carbonyl compounds to alcohols.

The methods that are used to prepare alcohols depend on the specific alcohol that is desired. For example, methanol is typically prepared by the synthesis from carbon monoxide and hydrogen, while ethanol is typically prepared by fermentation.

Methanol

What is methanol?
Methanol is a colorless, flammable liquid that is used as a solvent, fuel, and antifreeze. It is also known as wood alcohol or methyl alcohol.
How is methanol produced?
Methanol is produced by the catalytic conversion of natural gas or coal to synthesis gas, followed by the catalytic hydrogenation of synthesis gas to methanol.
What are the uses of methanol?
Methanol is used as a solvent, fuel, and antifreeze. It is also used in the production of formaldehyde, acetic acid, and other chemicals.
What are the hazards of methanol?
Methanol is a toxic substance that can cause blindness and death if ingested. It is also a flammable substance that can cause fires and explosions.
How is methanol treated?
Methanol poisoning is treated with ethanol, which competes with methanol for Metabolism in the liver. Ethanol is also a toxic substance, but it is less toxic than methanol and can help to protect the body from the effects of methanol poisoning.

Ethanol

What is ethanol?
Ethanol is a colorless, flammable liquid that is produced by the fermentation of sugars. It is also known as grain alcohol or ethyl alcohol.
How is ethanol produced?
Ethanol is produced by the fermentation of sugars, such as those found in corn, sugarcane, or potatoes. The sugars are converted into ethanol by yeast, which is a type of fungus.
What are the uses of ethanol?
Ethanol is used as a fuel, solvent, and in the production of alcoholic beverages. It is also used in the production of other chemicals, such as acetaldehyde and acetic acid.
What are the hazards of ethanol?
Ethanol is a toxic substance that can cause intoxication and death if ingested in large amounts. It is also a flammable substance that can cause fires and explosions.
How is ethanol treated?
Ethanol intoxication is treated with supportive care, such as intravenous fluids and monitoring of vital signs. In severe cases, dialysis may be necessary.

Question 1

Which of the following is not a method of preparing alcohols?

(A) Fermentation
(B) Oxidation of alkenes
(C) Reduction of carbonyl compounds
(D) Hydrolysis of esters

Answer
(D) Hydrolysis of esters

Explanation
Alcohols can be prepared by fermentation, oxidation of alkenes, and reduction of carbonyl compounds. However, they cannot be prepared by hydrolysis of esters.

Question 2

Which of the following is the most common method of preparing methanol?

(A) Fermentation
(B) Oxidation of alkenes
(C) Reduction of carbonyl compounds
(D) Synthesis gas reaction

Answer
(D) Synthesis gas reaction

Explanation
The most common method of preparing methanol is by the synthesis gas reaction. This reaction involves the catalytic reaction of carbon monoxide and hydrogen to produce methanol.

Question 3

Which of the following is the most common method of preparing ethanol?

(A) Fermentation
(B) Oxidation of alkenes
(C) Reduction of carbonyl compounds
(D) Synthesis gas reaction

Answer
(A) Fermentation

Explanation
The most common method of preparing ethanol is by fermentation. This process involves the anaerobic conversion of sugars to ethanol by yeast.

Question 4

What is the chemical formula of methanol?

(A) CH3OH
(B) C2H5OH
(C) C3H7OH
(D) C4H9OH

Answer
(A) CH3OH

Explanation
The chemical formula of methanol is CH3OH. It is a colorless, flammable liquid with a strong odor. It is used as a solvent, fuel, and antifreeze.

Question 5

What is the chemical formula of ethanol?

(A) CH3OH
(B) C2H5OH
(C) C3H7OH
(D) C4H9OH

Answer
(B) C2H5OH

Explanation
The chemical formula of ethanol is C2H5OH. It is a colorless, flammable liquid with a characteristic odor. It is used as a solvent, fuel, and in the production of alcoholic beverages.