What are Alcohols?
Alcohols are organic molecule that contain one or more hydroxyl groups attached to the aliphatic or aromatic carbon group with the covalent Bond. The compounds obtained by replacing one hydrogen atom from aliphatic hydrocarbons by hydroxyl group are alcohols whereas those obtained by replacing the hydrogen atom of aromatic hydrocarbons are phenols.
In this article, we have discussed the various alcohol reactions.
Methods of Formation of Alcohol
The important methods of formation of alcohols are given below:
1. Preparation From Haloalkanes- Haloalkanes, when boiled with aqueous NaOH or KOH or moist silver oxide (AgOH), give alcohols. General reaction for the preparation of alcohols by this method is given below:
R-X + KOH (aq) → R-OH + KX
C2H5Br + KOH → C2H5-OH + KBr
Primary haloalkanes give a good yield of alcohol. However, tertiary haloalkanes in this reaction give mainly alkenes due to dehydrohalogenation. Secondary haloalkanes give a mixture of alcohol and alkenes.
2. By Reduction of Aldehydes and Ketones- alcohols can be prepared by the reduction of aldehyde and ketones. The reduction is carried by common reducing agents such as hydrogen in the presence of a catalyst (platinum, palladium, and nickel), sodium in the presence of the alcohol, and lithium aluminium hydride.
Chemical Reaction of Alcohol
In alcohols reactions, alcohol can act both as nucleophiles as well as electrophiles. Alcohols behave as nucleophiles in the reactions in which the bond between O-H is broken. Alcohols can behave as electrophiles in the reactions in which the bond between C-O is broken.
1. Oxidation of Alcohols- The oxidation of alcohols involves the formation of carbon-oxygen double bond (C=O) with cleavage of O-H and C-H bonds. This type of cleavage and formation of bonds occur in oxidation reactions. These reactions are also called dehydrogenation reactions because they involve the loss of hydrogen from alcohol.
Oxidation of primary alcohols- A primary alcohol is easily oxidised to form first an aldehyde and then a carboxylic acid. Both the aldehyde and the acid formed to contain the same number of carbon atoms as the parent alcohol.
Oxidation of Secondary Alcohols- A secondary alcohol is easily oxidised to form a ketone with chromic anhydride. The ketone may be further oxidised under strong conditions to form a mixture of acids. While the ketone contains the same number of carbon atoms as the parent alcohol, the acids formed, contain a lesser number of carbon atoms.
Oxidation of Tertiary Alcohol- The oxidation of tertiary alcohol is very difficult because it does not have hydrogen in the carbon-bearing hydroxyl group (OH). However, the oxidation of tertiary alcohol can be possible when treated with acidic oxidising agents under very strong conditions at elevated temperatures, cleavage of various C-C bonds takes place. They form mixtures of ketones and carboxylic acids. Both the ketones and acids contain a lesser number4 of carbon atoms than the starting alcohols.
2. Reduction of Alcohol- The Reduction of alcohol is not an easy step. The hydroxyl group (OH) is a poor leaving group. Therefore, the direct reduction of alcohol is not possible at normal room temperature and pressure. The alcohol can be first converted into any other compound by an oxidation reaction and then it can be reduced to the alkane form. Only indirect methods of reduction of alcohols are possible.
3. Acidic Reaction of Alcohol- Alcohols are weakly acidic in nature. Therefore, it reacts with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.
Ethanol Reaction
Ethanol is lower alcohol that gives all the general acidic reactions. Therefore, it is called ethanol acid.
Reaction with Active Metal
C2H5OH + 2M → C2H5OM + H2
Reaction with Metal Hydrides
C2H5OH + NaH → C2H5O- Na + H2
Reaction with Carboxylic Acid
C2H5OH + ROH → RCOOC2H5
4. Dehydration Reaction of Alcohols- The most common question asked by the student in organic chemistry is primary alcohols undergo what reaction to form alkenes? The answer to this question is the dehydration reaction. When the alcohols are heated with a protonic acid such as conc.H2SO4 or H3PO4 at 443 K, they get dehydrated to form alkenes. This reaction mechanism involves three steps:
Protonation of alcohol mechanism
Elimination of Water molecule (dehydration)
Elimination of hydrogen ion (deprotonation)
5. Hydrolysis of Alcohol
The hydrolysis reaction of alcohol is a kind of oxidation reaction. In this reaction, the water molecule acts as a catalyst. Aldehydes and ketones are formed as the main products in this hydrolysis reaction.
CH3CH2OH + H2O → CH3CHO + H2O + H2
Did You Know?
Biological oxidation of methanol and ethanol occurs in the body.
If an alcoholic person, by mistake, drinks denatured alcohol the methanol is oxidised in the body first to methanal and then to methanoic acid, which can cause blindness and death.
FAQs on Reactions of Alcohols
1. What are the main types of reactions that alcohols undergo based on bond cleavage?
Alcohols are highly versatile and undergo reactions primarily involving the cleavage of two different bonds:
- Reactions involving cleavage of the O-H bond: In these reactions, alcohols act as nucleophiles. Examples include their reaction with active metals to show their acidic character and the esterification reaction with carboxylic acids.
- Reactions involving cleavage of the C-O bond: In these reactions, alcohols act as electrophiles, typically after the -OH group is protonated. Examples include reactions with hydrogen halides (Lucas Test), and dehydration to form alkenes or ethers.
2. Why are alcohols considered weakly acidic, and how do they demonstrate this property?
Alcohols are weakly acidic due to the polar nature of the O-H bond, caused by the high electronegativity of oxygen. This polarity allows them to donate a proton (H⁺) to a strong base. They demonstrate this acidic nature by reacting with active metals like sodium (Na), potassium (K), or aluminium (Al) to liberate hydrogen gas and form a metal alkoxide. For example: 2R-OH + 2Na → 2R-O⁻Na⁺ + H₂.
3. What is the major product when butan-2-ol is dehydrated using concentrated H₂SO₄?
When butan-2-ol is heated with concentrated sulphuric acid (conc. H₂SO₄) at 443 K, it undergoes dehydration. According to Saytzeff's Rule, in an elimination reaction, the more substituted alkene is the major product. Therefore, the removal of a hydrogen atom from the adjacent carbon with fewer hydrogen atoms is preferred, leading to the formation of But-2-ene as the major product and But-1-ene as the minor product.
4. How can one distinguish between primary, secondary, and tertiary alcohols using a chemical test?
The Lucas test is the standard chemical method to distinguish between the three classes of alcohols. This test uses the Lucas reagent, which is a mixture of concentrated HCl and anhydrous ZnCl₂. The distinction is based on the rate of formation of a cloudy precipitate (alkyl chloride).
- Tertiary alcohols: React almost instantly to produce turbidity.
- Secondary alcohols: React within 5-10 minutes to produce turbidity.
- Primary alcohols: Show no reaction at room temperature; turbidity appears only upon heating.
5. Why does the oxidation of primary, secondary, and tertiary alcohols yield different products?
The difference in oxidation products is due to the structure of the alcohol, specifically the number of hydrogen atoms attached to the carbinol carbon (the carbon bearing the -OH group).
- Primary alcohols have two hydrogens and can be oxidised first to an aldehyde and then further to a carboxylic acid.
- Secondary alcohols have one hydrogen and are oxidised to form a ketone. Further oxidation is difficult and requires harsh conditions.
- Tertiary alcohols have no hydrogens on the carbinol carbon, making them resistant to oxidation. Under forceful conditions, C-C bonds break, yielding a mixture of smaller acids and ketones.
6. What is esterification, and what makes it a significant reaction of alcohols?
Esterification is the reversible reaction where an alcohol reacts with a carboxylic acid in the presence of an acid catalyst (like conc. H₂SO₄) to form an ester and water. This reaction is significant because esters are valuable compounds known for their characteristically pleasant, fruity smells. They are widely used in industrial applications, including the manufacturing of perfumes, artificial flavourings, solvents, and soaps.
7. Under what conditions do alcohols act as nucleophiles versus electrophiles?
The role of an alcohol depends on which bond is broken during the reaction:
- Alcohols act as nucleophiles when the O-H bond breaks. The lone pairs of electrons on the oxygen atom can attack an electron-deficient species. This occurs during esterification, where the alcohol attacks the carbonyl carbon of the carboxylic acid.
- Alcohols act as electrophiles when the C-O bond breaks. This requires the hydroxyl group to be protonated first (forming -OH₂⁺), making it a good leaving group. The carbon atom then becomes electrophilic and can be attacked by a nucleophile, as seen in the reaction with hydrogen halides.
8. What is the chemical basis for the severe toxicity of methanol when consumed?
The toxicity of methanol is not caused by the alcohol itself but by its metabolic byproducts. Inside the liver, enzymes oxidise methanol first into methanal (formaldehyde) and then into methanoic acid (formic acid). Methanoic acid leads to severe metabolic acidosis and inhibits cellular respiration, which can cause permanent blindness by destroying the optic nerve. Methanal is also highly toxic as it reacts with proteins and enzymes, leading to widespread cellular damage and, in severe cases, death.
