Types of Reactions of Alcohols with Mechanisms and Conditions
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 and Their Chemical Behavior
1. What are the main reactions of alcohols in organic chemistry?
The main reactions of alcohols include oxidation, dehydration, substitution, esterification, and reaction with metals. These reactions depend on whether the alcohol is primary, secondary, or tertiary.
- Oxidation: Primary alcohols form aldehydes or carboxylic acids; secondary form ketones; tertiary resist oxidation.
- Dehydration: Alcohols lose water to form alkenes using concentrated H2SO4.
- Substitution: –OH group is replaced by halogen using HX, PCl5, or SOCl2.
- Esterification: Alcohol reacts with a carboxylic acid to form an ester.
- Reaction with metals: Alcohol reacts with Na to form alkoxide and H2 gas.
2. How do alcohols undergo oxidation?
Alcohols undergo oxidation by gaining oxygen or losing hydrogen, typically using oxidizing agents like acidified K2Cr2O7 or KMnO4.
- Primary alcohol: R–CH2OH → R–CHO → R–COOH
- Secondary alcohol: R2CHOH → R2C=O
- Tertiary alcohol: Do not oxidize easily due to absence of α-hydrogen.
3. What happens when alcohols are heated with concentrated H2SO4?
When alcohols are heated with concentrated H2SO4, they undergo dehydration to form alkenes or ethers depending on temperature.
- At about 170°C: Alkene formation (intramolecular dehydration).
- At about 140°C: Ether formation (intermolecular dehydration).
4. How do alcohols react with sodium metal?
Alcohols react with sodium metal to form sodium alkoxide and hydrogen gas. The reaction shows the weakly acidic nature of alcohols.
- General reaction: 2R–OH + 2Na(s) → 2R–ONa + H2(g)
- Effervescence is observed due to H2 gas evolution.
5. What is esterification of alcohols?
Esterification is the reaction in which an alcohol reacts with a carboxylic acid in the presence of concentrated H2SO4 to form an ester and water.
- It is a reversible reaction.
- Produces fruity-smelling esters.
6. How do alcohols react with hydrogen halides (HX)?
Alcohols react with hydrogen halides (HCl, HBr, HI) to form alkyl halides by substitution of the –OH group.
- General reaction: R–OH + HX → R–X + H2O
- Tertiary alcohols react fastest; primary react slowest.
- ZnCl2 is often used with HCl (Lucas reagent).
7. What is the difference between oxidation of primary, secondary, and tertiary alcohols?
The key difference is that primary alcohols form aldehydes or acids, secondary form ketones, and tertiary resist oxidation under normal conditions.
- Primary (1°): R–CH2OH → R–CHO → R–COOH
- Secondary (2°): R2CHOH → R2C=O
- Tertiary (3°): No reaction with mild oxidizing agents.
8. How are ethers formed from alcohols?
Ethers are formed from alcohols by intermolecular dehydration at about 140°C using concentrated H2SO4.
- Two alcohol molecules combine and lose one molecule of water.
- Common with primary alcohols.
9. Can alcohols undergo combustion reactions?
Yes, alcohols undergo combustion reactions in oxygen to produce carbon dioxide and water with release of heat.
- Combustion is an exothermic reaction.
- Alcohols are used as fuels (e.g., ethanol).
10. Why do tertiary alcohols not oxidize easily?
Tertiary alcohols do not oxidize easily because they lack a hydrogen atom on the carbon bearing the –OH group (α-hydrogen). Oxidation requires removal of this hydrogen to form a carbonyl group.
- No α-hydrogen means no formation of C=O without breaking C–C bonds.
- They only oxidize under very strong conditions with bond cleavage.
