|Table of contents|
|What are Alcohols?|
|Types of Alcohols: Classification and Descriptions|
|What are the Properties of Alcohols?|
|Physical Properties of Alcohols|
|Chemical Properties of Alcohols|
|Preparation of Alcohols or Alcohol Synthesis|
|1. Preparation of Alcohols From Alkenes|
|2. Preparation of Alcohols From Alkyl Halides|
|3. Preparation of Alcohols From Grignard Reagent|
|4. By Reduction of Carbonyl Compounds|
|5. By Reaction of Nitrous Acid on Aliphatic Primary Amines|
|6. Formation of diols by Hydroxylation|
|Other Important Chemical Reactions of Alcohols|
|Some Commercially Important Alcohols|
Alcohols are a class of organic compounds characterized by the presence of one or more hydroxyl (-OH) functional groups attached to a carbon atom. They are commonly used as solvents, fuels, and beverages.
Alcohols can be classified into different types based on various factors such as the structure of the hydrocarbon group, the number of hydroxyl groups, and the position of the hydroxyl group in the carbon chain.
Here are some common types of alcohols and their classifications:
Depending on the number of carbon atoms which are directly attached to the carbon that is bonded with the -OH group, alcohols can be classified into three types.
Tertiary alcohols: These alcohols have one hydroxyl group (-OH) attached to a tertiary carbon atom (a carbon atom directly bonded to three other carbon atoms). Example: tert-Butanol
Mono, Di &Trihydric Alcohols
Depending on the number of hydroxyl groups attached, phenols can be classified into three types.
Classification of Phenols
Example: Allyl alcohol (CH2=CHCH2OH).
Example: Benzyl alcohol (C6H5CH2OH).
The physical and chemical properties of alcohols are mainly due to the presence of hydroxyl group.
Some important physical and chemical properties of alcohols are given below:
1. The Boiling Point of Alcohols
Alcohols generally have higher boiling points in comparison to other hydrocarbons having equal molecular masses.
Ethanol has a higher Boiling Point than Ethane
2. Solubility of Alcohols in Water
The solubility of alcohol in water is governed by the hydroxyl group present. The hydroxyl group in alcohol is involved in the formation of intermolecular hydrogen bonding.
1. Oxidation Reactions of Alcohol
2. Acidity of alcohols and their ability to undergo acid-base reactions.
Alcohols, in their pure form, are considered weak acids compared to strong mineral acids such as hydrochloric acid or sulfuric acid. The acidity of alcohols arises from the presence of the hydroxyl (-OH) group, which can donate a proton (H+) in acid-base reactions.
The reaction involves the deprotonation of the alcohol's hydroxyl group, resulting in the formation of an alkoxide ion and water.
3. Dehydration of Alcohols:
4. Esterification Reaction:
5. Reaction with Metals:
6. Reaction with Acid Chlorides:
7. Nucleophilic Substitution:
The preparation of alcohols refers to the methods or processes used to produce alcohol compounds. There are several common methods for the preparation of alcohols, each with its own specific conditions and reagents. Another term often used to describe the preparation of alcohols is "alcohol synthesis" or "alcohol formation."
Here are some common methods for the preparation of alcohols:
(a) By acid-catalyzed hydration of alkenes
Hydration of Alkenes with Acid Catalyst
Mechanism of addition of water to alkenes to from Alcohols
(b) By Oxymercuration - Demercuration Process
Alkenes react with mercuric acetate in presence of H2O and tetrahydrofuran to give alkyl mercury compounds. This is one of the most common types of methods to prepare alcohols. This method always leads to anti Markovnikov’s addition of water to alkenes.
Formation of Alcohol by Oxymercuration - Demercuration Process
(c) By Hydroboration-oxidation process
Alkenes, when treated with diborane, give alkyl boranes, R3B. Alkylboranes on oxidation with alkaline hydrogen peroxide give alcohol. It is significant to note that this method always leads to anti Markovnikov’s addition of water to alkenes.
Formation of Alcohol by Hydroboration-Oxidation process
(d) Hydroformylation of Alkenes (Oxo Reaction)
Lower molecular weight alkenes react with carbon monoxide and hydrogen in the presence of a catalyst in a reaction called hydroformylation or the oxo reaction. The resulting aldehyde is subsequently hydrogenated to form an alcohol.
Hydroformylation of Alkenes
(a) Hydrolysis of Halides
The hydrolysis of alkyl halides involves the reaction of the alkyl halide with an aqueous solution of an alkali hydroxide, typically sodium hydroxide (NaOH) or potassium hydroxide (KOH). This reaction is a nucleophilic substitution mechanism, where the halide ion (X-) is replaced by the hydroxide ion (OH-), resulting in the formation of an alcohol.
The general reaction equation for the hydrolysis of alkyl halides is:
R-X + KOH → R-OH + KX
e.g. (CH3)2CHCH2CH 2-Br (CH3)2CHCH2CH 2- OH
Here, R represents the alkyl group, and X represents the halide (such as chloride, bromide, or iodide).
Nucleophilic substitution reactions for the formation of alcohols can take place through 2 mechanisms:
(a) By SN2 Mechanism (Second-Order Substitution)
(b) By SN1 mechanism (first-order substitution)
Here is a brief explanation of both of these:
SN1 Mechanism: It's a two-step process observed in reactions with tertiary and some secondary alkyl halides. First, the alkyl halide forms a carbocation intermediate. Then, a nucleophile attacks the carbocation, resulting in a mixture of stereoisomers. Given below is the detailed explanation:
SN1 Mechanism for formation of alcohols
SN2 Mechanism: It's a single-step process observed in reactions with primary and some secondary alkyl halides. The nucleophile simultaneously attacks the electrophilic carbon and displaces the leaving group, causing inversion of configuration at the chiral center.
(a) From air
RMgX RO2MgX 2ROMgX 2ROH
e.g. C2H5MgBrC2H5O2 MgX2C2H5OMgX 2C2H5OH MgBr(OH)
(b) From ethylene oxide
RCH2CH2OMgX RCH2CH2OH + MgX(OH)
e.g. + C2H5MgBr C2H5CH2CH2OMgBr + MgBr(OH)
(c) From carbonyl compounds
(i) Addition of formaldehyde gives primary alcohol
= O RMgX RCH2-OH
(ii) Addition to an aldehyde (other than formaldehyde) gives secondary alcohol
(iii) Addition to a ketone gives tertiary alcohol.
e.g. CH3CH2MgCl +
(iv) Addition to an acid halide or an ester gives tertiary alcohol.
(a) Catalytic hydrogenation of aldehydes and ketones
One method of synthesizing alcohols from carbonyl compounds is through catalytic hydrogenation of aldehydes and ketones. This process involves the use of a catalyst, typically a transition metal catalyst, and hydrogen gas (H2) to reduce the carbonyl group to an alcohol.
e.g. CH3CHO + 2H CH3CH2OH
e.g. + 4H
(b) Lithium Aluminium Hydride reduction of aldehydes and ketones
Another method for synthesizing alcohols from carbonyl compounds is through the reduction of aldehydes and ketones using lithium aluminum hydride (LiAlH4). LiAlH4 is a powerful reducing agent that can selectively convert carbonyl groups into alcohols.
(iii) RCOOH+ 4H RCH2OH + H2O
(iv) RCH2OH+ HCl
(v) R-CH2OH +R'OH
e.g. C2H5CH2OH +HCl
Question. Identify (X) in the following reaction:
(c) NaBH4 (Sodium Borohydride) reduction of aldehydes and ketones
e.g. CH=CHCHO +4H CH3CH=CHCH2OH
Note: Reduction of a ketone gives secondary alcohol
(d) Bouveault-Blanc Reduction
The reduction of aldehydes, ketones or esters by means of excess of sodium and ethanol or n-butanol as the reducing agent.
(i) Aldehyde RCHO RCH2OH
(ii) Esters R'CO2R'' R'CH2OH R''OH
(iii) Ketones R2CO R2CHOH
The Bouveault-Blanc reduction is believed to occur in steps involving transfer of one electron at a time.
e.g. CH3CHO +2H CH3CH2OH
e.g. CH3COOC2H5 +4H2CH3CH2OH
The reaction of nitrous acid (HNO2) with aliphatic primary amines (R-NH2) leads to the formation of alcohols. This process is known as the nitrous acid reaction or nitrosation reaction.
General reaction: R-NH2 + HONO R-OH +N2 + H2O
R-NH2 (R) ROH +N2 +
e.g. (i) C2H5NH2 +HNO2 C2H5OH+ N2 + H2O
(ii) CH3-CH2- +HONO CH3-CH2-+ N2 +H2O
Converting an alkene to a glycol requires adding a hydroxy group to each end of the double bond. This addition is called hydroxylation of the double bond.
(a) Syn hydroxylation, using KMnO4 / NaOH or using OsO4/H2O2
Syn hydroxylation is a chemical reaction that introduces hydroxyl (-OH) groups onto an unsaturated compound, typically an alkene. It is commonly achieved using two different reagent systems: potassium permanganate (KMnO4) with sodium hydroxide (NaOH), or osmium tetroxide (OsO4) with hydrogen peroxide (H2O2).
General reaction :
(b) Anti hydroxylation, using peracids
Anti-hydroxylation is a chemical reaction that introduces hydroxyl (-OH) groups onto an unsaturated compound, specifically an alkene, in an anti-addition manner. One common method for achieving anti-hydroxylation is by using peracids, which are peroxy acids
When alcohols react with hydrogen halides (such as hydrogen chloride, HCl, or hydrogen bromide, HBr), they undergo a substitution reaction known as an alcohol halogenation reaction. The reaction involves the replacement of the hydroxyl (-OH) group of the alcohol with a halogen atom (-X).
Here is the general reaction and mechanism involved:
R - OH + HX R - X + H2O (R may rearrange)
Reactivity of HX : Hl > HBr > HCl
Reactivity of ROH: allyl > benzyl > 3o > 2o > 1o
R - OH R- R-X
3R - OH + PX3 3R - X + H3PO3
The mechanism for the reaction involves attachment of the alcohol group on the phosphorus atom, displacing a bromide ion and forming a protonated alkyl dibromophosphite (see the following reaction).
In a second step a bromide ion acts as a nucleophile to displace HOPBr2, a good leaving group due to the electronegative atoms bonded to the phosphorus.
RCH2X + HOPX2
When alcohols react with thionyl chloride (SOCl2), they undergo a substitution reaction known as an alcohol chlorination reaction. The reaction involves the replacement of the hydroxyl (-OH) group of the alcohol with a chlorine atom (-Cl) and the formation of sulfur dioxide gas (SO2) as a byproduct.
The general equation for the reaction of alcohols with thionyl chloride is:
ROH + SOCl2 → RCl + SO2 + HCl
where ROH represents the alcohol, SOCl2 represents thionyl chloride, RCl represents the alkyl chloride product, SO2 represents sulfur dioxide, and HCl represents hydrogen chloride.
Dehydration of alcohols is a chemical reaction in which water (H2O) is removed from the alcohol molecule, resulting in the formation of an alkene or an alkyne. This process involves the loss of a hydroxyl (-OH) group from the alcohol and a hydrogen atom from an adjacent carbon atom.
The general equation for the dehydration of an alcohol is
ROH → R-alkene + H2O
Step 1 :
Step 2 :
Step 3 :
The reaction mechanism for the dehydration of alcohols can vary depending on the type of alcohol involved: primary (1°), secondary (2°), or tertiary (3°) alcohol.
Reactivity of ROH : 3º > 2º > 1º
The reaction of alcohols with metals can vary depending on the specific metal and conditions involved. In general, the reaction between alcohols and metals can result in the formation of metal alkoxides and hydrogen gas. Here are a few examples:
Ester formation is a common reaction that involves the reaction between an alcohol and a carboxylic acid or its derivative. This reaction is known as esterification and results in the formation of an ester compound, along with the production of water as a byproduct.
+ + H2O
e.g. CH3CH2O - H + + H2O
Oxidation reactions of alcohols involve the conversion of alcohol functional groups (-OH) into carbonyl groups (C=O). The degree of oxidation depends on the type of alcohol and the oxidizing agent used. There are three main types of alcohols: primary (1°), secondary (2°), and tertiary (3°), and each reacts differently with oxidizing agents.
(a) Oxidation of primary alcohols
Primary alcohols can undergo two levels of oxidation, depending on the reaction conditions and the oxidizing agent used.
(b) Oxidation of secondary alcohols
Secondary alcohols can be oxidized to ketones by a variety of oxidizing agents, including chromic acid (H2CrO4), potassium dichromate (K2Cr2O7) in acidic conditions, or pyridinium dichromate (PDC). The reaction is typically performed under acidic conditions to facilitate the formation of the carbonyl group without further oxidation to carboxylic acids.
The general equation for the oxidation of secondary alcohols is:
R2-CHOH → R2-CO + H2O
(c) Resistance of tertiary alcohols to oxidation
Tertiary alcohols do not undergo oxidation reactions under typical conditions because they lack a hydrogen atom on the carbon attached to the hydroxyl group. Since oxidation reactions involve the removal of a hydrogen atom from the alcohol, tertiary alcohols are not oxidized to aldehydes or carboxylic acids.
Below are some examples of Oxidation Reactions of primary alcohols
|1. What are the physical properties of alcohols?|
|2. How are alcohols prepared from alkenes?|
|3. What are some common chemical reactions of alcohols?|
|4. What are some commercially important alcohols?|
|5. What are the uses of methanol?|