Alcohols: Properties, Preparation & Reactions | Chemistry Class 12 - NEET

What are the Properties of Alcohols?

• Alcohols are organic compounds in which a hydrogen atom of an aliphatic carbon is replaced with a hydroxyl group. Thus an alcohol molecule consists of two parts; one containing the alkyl group and the other containing functional group hydroxyl group. 
  Alcohol
• They have a sweet odour. They exhibit a unique set of physical and chemical properties.

The physical and chemical properties of alcohols are mainly due to the presence of hydroxyl group.

Physical Properties

1. Boiling Point

• Alcohols show an increase in boiling point with an increase in molecular weight amongst homologues.
• Alcohols have a higher boiling point than hydrocarbons of the same molecular weight. The reason for the higher boiling point is the intermolecular H-bonding present in alcohols.

Ethanol has a higher Boiling Point than Ethane

Hydrogen Bonding in Ethanol

2. Solubility in Water

• As molecular weight increases solubility in water decreases.
• The lower alcohols are miscible with water. This is due to intermolecular hydrogen bonding between alcohol and water molecules.

Intermolecular H bond between water & alcohol molecules

Tables showing Solubility of various Alcohols

• Formation of carbocation intermediate (Markovnikov addition, rearrangement possible).

The reaction of Alkenes with Acids follow Markovnikov's Rule

General Reaction:

e.g.     

(b) By Oxymercuration - Demercuration Process

• Oxymercuration involves an electrophilic attack on the double bond by the positively charged mercury species. The product is a mercurinium ion, an organometallic cation containing a three-membered ring.

• In the second step, water from the solvent attacks the mercurinium ion to give (after deprotonation) organomercurial alcohol. 
• The third step is demercuration to remove the Hg. Sodium borohydride (NaBH₄, a reducing agent) replaces the mercuric acetate fragment with hydrogen.

General reaction:

  + OH^- + NaBH₄

e.g.

(c) By Hydroboration-oxidation process

(Forms anti-Markovnikov alcohol, no rearrangement).

General reaction: R-Ch=CH₂R-CH₂-CH₂-OH

e.g. 

Question: Give the major product of the following reaction:

Ans. Major product is

(a)
because 3^o carbocation is more stable.

(b)

(c)

2. From Alkyl Halides

By nucleophilic substitution reactions.

(a) By SN² Mechanism (Second-Order Substitution)

• It is given by primary (and some secondary) halides.

General reaction: R-CH₂-Br  R-CH₂-OH

e.g. (CH₃)₂CHCH₂CH ₂-Br  (CH₃)₂CHCH₂CH ₂- OH

e.g.

(b) By SN¹ mechanism (first-order substitution)

• It is given by tertiary and some secondary halides.

General reaction :

e.g.

3. From Grignard's Reagent

(a) From air

• A Grignard reagent may be used to synthesize an alcohol by treating it with dry oxygen and decomposing the product with acid :

General reaction:

RMgX  RO₂MgX  2ROMgX  2ROH

e.g. C₂H₅MgBrC₂H₅O₂ MgX2C₂H₅OMgX 2C₂H₅OH MgBr(OH)

(b) From ethylene oxide

• The addition of Grignard reagent to ethylene oxide gives primary alcohol (with two carbon atoms added).

General reaction:

  RCH₂CH₂OMgX  RCH₂CH₂OH MgX(OH)

e.g.  C₂H₅MgBr  C₂H₅CH₂CH₂ OMgX  MgBr(OH)

(c) From carbonyl compounds

• Nucleophilic addition to the carbonyl groups by Grignard's reagent.

General reaction:
 Mg(OH)X

(i) Addition of formaldehyde gives primary alcohol

General reaction:
= O RMgX RCH₂-OH

(ii) Addition to an aldehyde (other than formaldehyde) gives secondary alcohol

General reaction:

 (sec-alcohol)

e.g.  +  

(iii) Addition to a ketone gives tertiary alcohol. 

General reaction:

e.g. CH₃CH₂MgCl +  

(iv) Addition to an acid halide or an ester gives tertiary alcohol.

• Esters on treatment with Grignard's reagent first form ketones which then react with the second molecule of Grignard's reagent and form tertiary alcohol.

General reaction:

4. By Reduction of Carbonyl Compounds(a) Catalytic hydrogenation of aldehydes and ketones

General reaction: 

e.g. CH₃CHO + 2H  CH₃CH₂OH

e.g.  CH₃CH₂CH₂OH

_e.g.  + 4H

(b) Lithium aluminium hydride reduction of aldehydes and ketones

General reaction:

(i)

(ii)

(iii) RCOOH+ 4H  RCH₂OH + H₂O

(iv)  RCH₂OH+ HCl

(v)  R-CH₂OH +R'OH

e.g.  CH₃CH₂OH

e.g.  C₂H₅CH₂OH +HCl

(c) By NaBH4 (sodium borohydride) 

• It is insoluble in ether and is used in an aqueous ethanolic solution to reduce carbonyl compounds. It does not reduce esters and acids.

 R-CH₂-OH

e.g. 

e.g. CH=CHCHO +4H  CH₃CH=CHCH₂OH

(i) 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.

General reaction:

(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.

Mechanism:

e.g. CH₃CHO +2H CH₃CH₂OH

e.g. CH₃COOC₂H₅ +4H2CH₃CH₂OH

e.g. 

Ex. Identify (X) in the following reaction:

X

Ans.

Ex. What are the product A, B, C, D and E in the following reactions?

A

D E

Ans. A :  (EtOH)

B :  Ester part is not affected by NaBH₄

C :  Ester part and keto parts are affected by LiAlH₄

D :

E :

5. By Reaction of Nitrous Acid on Aliphatic Primary Amines

General reaction: R-NH₂ + HONO  R-OH +N₂  + H₂O

Mechanism:

R-NH₂ (R)  ROH +N₂ +

e.g. (i) C₂H₅NH₂ +HNO₂ C₂H₅OH+ N₂ + H₂O

(ii) CH₃-CH₂- +HONO  CH₃-CH₂-+ N₂ +H₂O

Mechanism:

6. Hydroxylation: Forms vicinal diols (glycols)

• 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

General reaction :

e.g. 

(b) Anti hydroxylation, using peracids

Chemical Reactions of Alcohols

1. Reaction with Hydrogen Halides

General reaction:

R - OH + HX  R - X + H₂O (R may rearrange)

Reactivity of HX : Hl > HBr > HCl

Reactivity of ROH: allyl > benzyl > 3^o > 2^o > 1^o

Mechanism:
R - OH  R- R-X

e.g.

e.g.

e.g.

2. Reaction with Phosphorus Trihalides

• Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr₃, PCl₃, & PCl₅ work well and are commercially available.
• Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with ter. alcohols. The two phosphorus halides used most often are PBr₃ and the P₄ / I₂ combination.

General reaction:

3R - OH + PX₃ 3R - X + H₃PO₃ 

Mechanism:

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

e.g.  

e.g.  

3. Reaction with Thionyl Chloride

 R-Cl SO₂ HCl

4. Dehydration of Alcohols

 H₂O (Rearrangement may occur)

Mechanism 

Step 1 :

Step 2 :

Step 3 :

Reactivity of ROH : 3º > 2º > 1º

5. Reaction with Metals

e.g.  

e.g. 

e.g. 

6. Ester Formation

General reaction: 

  +  + H₂O

e.g. CH₃CH₂O - H +   + H₂O

e.g.  +

7. Oxidation Reactions 

(a) Oxidation of primary alcohols

• Oxidation of primary alcohol initially forms an aldehyde. Obtaining the aldehyde is often difficult since most oxidizing agents are strong enough to oxidize the aldehydes formed. CrO₃ acid generally oxidizes primary alcohol all the way upto the carboxylic acid

(b) Oxidation of secondary alcohols

• Sec. alcohols are easily oxidized to give excellent yields of ketones. The chromic acid reagent is often best for laboratory oxidations of secondary alcohols. The active species in the mixture is probably chromic acid, H₂CrO₄, or the acid chromate ion,

(c) Resistance of tertiary alcohols to oxidation

• Oxidation of tertiary alcohol is not an important reaction in organic chemistry. Ter-alcohols have hydrogen atoms on the carbonyl carbon atom, so oxidation must take place by breaking C-C bonds. These oxidations require severe conditions and result in mixtures of products.

➤ Primary

➤ Secondary

➤ Tertiary

no reaction

e.g. 

e.g. 

e.g. 

Question: A

Identify A

Ans.

Some Commercially Important Alcohols

Methanol and Ethanol

• Methanol (CH3OH) and ethanol (C2H5OH) are the simplest members of the primary alcohol family and have a wide range of applications in the fuel industry. Some important uses of methanol and ethanol are listed in this article.

Uses of Ethanol

• Owing to its antibacterial and antifungal properties, ethanol (also known as ethyl alcohol) is used in many hand sanitizers and medical wipes.
• Ethanol is also used as an antiseptic and as a disinfectant.
• In cases of ethylene glycol poisoning or methyl alcohol poisoning, ethanol is often administered as an antidote.

• Several medications that are insoluble in water are often dissolved in ethanol. For example, ethanol (in concentrations ranging from 1% to 25%) is used as a solvent for some analgesics and mouthwashes.
• Ethanol is the primary ingredient in many alcoholic drinks that are orally consumed for recreational purposes. It acts as a psychoactive drug by reducing anxiety and creating a feeling of euphoria in Humans. However, it also impairs cognitive and motor functions and acts as a central nervous system (CNS) depressant.
• Ethanol is used industrially in the production of ethyl esters, acetic acid, diethyl ether, and ethyl amines.
• This compound is widely used as a solvent due to its ability to dissolve both polar and nonpolar compounds.
• Since it has a melting point of -114.1^oC, ethanol is used as an ingredient in cooling baths in several laboratories. It also serves as the active fluid in many spirit thermometers.

Uses of Ethanol as a Fuel

• Ethanol is widely used as a fuel additive and as an engine fuel. Some forms of gasoline are known to contain up to 25% ethanol. 
• This compound has also been used as rocket fuel in some bipropellant rockets. When used in fuel, ethanol is believed to reduce carbon monoxide and nitrogen oxide emissions. 
• Since it is widely available and has low toxicity and cost, ethanol is used in direct-ethanol fuel cells (or DEFCs). However, commercially used fuel cells generally use methanol, hydrogen, or natural gas.

Uses of Methanol

• Methanol is widely used in the production of acetic acid and formaldehyde.
• In order to discourage the recreational consumption of ethanol, methanol is often added to it as a denaturant.
• This compound is also used as an antifreeze (an additive that is used to lower the freezing point of a liquid) in many pipelines.
• It is also used in sewage treatment plants since it serves as a carbon-based food source for denitrifying bacteria.
• The polyacrylamide gel electrophoresis (PAGE) technique involves the use of methanol as a destaining agent.
• A mixture of water and methanol is used in high-performance engines in order to increase power.
• Methanol is used in the production of hydrocarbons, olefins, and some aromatic compounds.
• It is also used in the production of methyl esters and methylamines.

Uses of Methanol in Fuel

• Methanol can be used as a fuel in several internal combustion engines. The chemical equation for the burning of methanol is given by:
• 2CH₃OH + 3O₂ → 4H₂O + 2CO₂
• However, the primary disadvantage of methanol as a fuel is that it has a tendency to corrode aluminum and some other metals.
• Another shortcoming of methanol as a fuel is that its energy density is approximately half of the energy density offered by gasoline. An advantage of methanol as a fuel is that it is relatively easy to store.
• The storage of liquid methanol is much easier than the storage of hydrogen gas or natural gas. Other merits of this compound include its biodegradability and its short half-life in groundwater.