What are Alcohols, Phenols, and Ethers?

Alcohols are hydroxyl (-OH) group or moiety containing organic aliphatic molecule. They are produced when a hydrogen atom of an aliphatic compound is substituted with hydroxyl moiety. The general expression of alcohol is R-OH, where the R is an aliphatic group. Their general formula is C_nH_2n+2O, where n = number of carbon atoms. If n = 1, then CH₄O = CH₃OH or methanol, n = 2, C₂H₆O = C₂H₅OH ethanol.

Phenols are organic hydrocarbon molecules containing hydroxyl (-OH) group directly attached with an aromatic ring such as benzene ring. Ethers are special classes of organic hydrocarbon molecules, in which alkoxy (-OR) or aryloxy (-OAr) moieties are directly attached with an aliphatic moiety. 

Classification of alcoholic compounds based on the number of alpha H atoms 

They are classified according to the number of C atoms attached to the alpha-carbon of the C atom which is directly attached with -OH moiety. In all of them, the alpha C atom is SP³ hybridized.

Primary or 1^o 

The alpha C atom is attached with only one C atom in these molecules. The other two hands of alpha C atoms are occupied with two H atoms. Examples of primary alcohols are ethanol (CH₃CH₂OH), propanol (CH₃CH₂CH₂OH) etc.

Secondary or 2^o 

In these molecules, the alpha C atom is attached to two other C atoms and an H atom. Examples are propan-2-ol or (CH₃)₂CH-OH.

Tertiary or 3^o 

The alpha C atom is directly attached with three C moieties and one OH moiety in these molecules. Examples are tert- Butanol or (CH₃)₃C-OH.

Classification of alcohols with respect to the number of '-OH' moiety

Alcohols may be monohydric, dihydric and trihydric when the number of one, two or three OH moieties are attached to it respectively. When more than three OH moieties are present molecule is called polyhydric. Examples, ethanol (monohydric), ethylene glycol (dihydric), glycerol (trihydric) and sorbitol (polyhydric).

Physical properties 

• The lower homologues are generally liquids and higher ones having more than twelve C atoms are solids.
• When fewer C atoms are present they are readily soluble in water, e.g. ethanol is highly soluble in water. But with increasing the number of C atoms or molecular weight the solubility decreases. Waxy solid nature increases in long alkyl chain alcohols. Their solubility in water can be explained due to the formation of -H bonds with water, with increasing molecular weight due to the presence of long alkyl chains hydrophobicity increases. Hence solubility in water decreases.
• Their boiling and melting points are much higher than those of corresponding alkanes due to the formation of intermolecular H-bonding.
• Their boiling points (b.p), melting points (b.p) and specific gravity increase with the increase in weight. Among their isomeric molecules, b.p and m.p are in the order primary > secondary > tertiary.

Synthesis of Alcoholic Compounds

From Alkyl halide

They are synthesized with the reaction of an alkyl halide in presence of aqueous NaOH or KOH.

R-CH₂-X + KOH (aq.) → K-X + R-CH₂-OH

where X = halogen atom

From Ester

They are synthesized along with acids salts from the alkaline hydrolysis of esters. An aqueous solution of NaOH is used as the alkaline medium.

R-COO-CH₂-R + NaOH → R-CH₂-OH + R-COO-Na

From Primary amines

When primary amine reacts with NaNO₂ and HCl above 5⁰C, it produces primary alcohol. During this reaction oxidation of primary amine by NaNO₂ and HCl produces an alcoholic molecule and nitrogen gas is released. Here substitution of -NH₂ takes place by -OH.

R-CH₂-NH₂ + NaNO₂ + HCl → R-CH₂-OH + N₂↑ + H₂O 

From Ethers

Hydrolysis of ethers produces 2 molecules of alcohol.

CH₃-O-CH₃ + H₂O → 2 CH₃OH

From Grignard reagents

Alcohols are produced by reacting Grignard reagents with carbonyls (Aldehydes or Ketones) followed by acid hydrolysis. Formaldehyde will give 1°, any other aldehydes give 2°, and ketones give 3° products.

H-CHO + CH₃-Mg-Cl + H₃O⁺ → CH₃-CH₂-OH + Mg(OH)(Cl)

From Aldehydes and Ketones

Aldehydes and ketones are reduced by LiAlH4 or NaBH4 to produce alcohols.

CH₃-CHO + 2[H] → CH₃-CH₂-OH

Chemical Properties 

Their reactivity in terms of cleavage of the O-H bond decreases with an increase in molecular weight. The order of cleavage of the O-H bond is primary > secondary > tertiary. In the case of reactivity of cleavage of C-O bonding from R-OH, the order is just reversed. This is due to the inductive effect of alkyl moieties. The order of reactivity of C-O bond breaking is tertiary > secondary > primary.

Chemical reactions of alcohols

Alcohols can react with various organic molecules.

With Metallic sodium

They react with metallic Na to produce sodium alkoxide molecule and H₂ gas. This process is used for drying ethanol.

R-CH₂-OH + 2Na → R-CH₂-ONa

With Grignard reagent

They react with Grignard reagents (R-MgX) to produce organic compounds like alkane.

R-CH₂-OH + R-MgX → R-H + MgX-O-CH₂-R

where X = halogen atom

Uses

• Methanol is used as a solvent in paints, varnishes, and for making formaldehyde.
• Ethanol is mainly used in alcoholic beverages.

Synthesis of Ethers

Ethers can be synthesized from various organic molecules.

From Alcohols

1° alcoholic molecules undergo intermolecular dehydration by heating with concentrated sulfuric acid at 140°C to produce ethers.

2R-CH₂-OH + Conc. H₂SO₄ → R-CH₂-O-CH₂-R 

From Alkyl halides

Alkyl halides are heated with silver oxide to produce a precipitate of silver halide and ethers. Substitution of halogen occurs with the oxygen atom and produces AgX as precipitation.

2R-CH₂-X + Ag₂O → 2AgX↓ + R-CH₂-O-CH₂-R

Williamson's synthesis

When sodium or potassium alkoxides are heated with alkyl halides, ethers are produced.

R-CH₂-ONa + R-CH₂-X → NaX + R-CH₂-O-CH₂-R

Physical Properties

• They are polar but insoluble in water.
• They have low boiling points than alcohols (having comparable molecular masses) because ethers do not form hydrogen bonding between the molecules.
• Low molecules (up to three C atoms) are soluble in water. Their solubility resembles those of alcoholic molecules of the same molecular masses.
• They should be kept in amber bottles under a blanket of inert gas in order to prevent its oxidation and production of peroxides. 

Chemical Properties

The reactions of these molecules are mainly due to lone pairs of ethereal O, cleavage of C-O bonding, and R moieties.

Reaction with Hydrogen iodide

One molecule of ether reacts with cold and dilute HI to produce one molecule of alkyl iodide and one molecule of alcohol. For unsymmetrical ethers, longer C-chains will produce alcoholic molecules and shorter C-chains will produce iodides. For tertiary ethers, iodide is formed with the tertiary C atoms. Ethers react with hot and concentrated HI to produce alkyl iodides only.

R-CH₂-O-CH₂-R + HI (cold and dil.) → R-CH₂-OH + R-CH₂-I

R-CH₂-O-CH₂-R + HI (hot and conc.) → 2R-CH₂-I

Reaction with phosphorus pentachloride

They react with PCl₅ to produce alkyl or aryl chlorides.

R-CH₂-O-CH₂-R + PCl₅ → 2R-CH₂-Cl + POCl₃

Uses

• They are used as solvents for oils and fats.
• They are used as general anesthetics.
• They provide an inert and moisture-free medium for various reactions.

Synthesis of Phenols

From Diazonium salt

They are prepared from the hydrolysis of benzene diazonium chloride (C₆H₅-N=N-Cl). During the reaction, the temperature kept at 100⁰C and N₂ and HCl formed as side products.

C₆H₅-N=N-Cl + H₂O → C₆H₅-OH + N₂ + HCl

From Dow's process

By heating chlorobenzene with NaOH at 360⁰C and 320 atm pressure, sodium phenoxide is produced. Further acidic hydrolysis of sodium phenoxide produces phenol. This is the commercial method and involves a nucleophilic substitution reaction.

 C₆H₅-Cl + NaOH → C₆H₅-ONa

C₆H₅-ONa + HCl → C₆H₅-OH

Physical characteristics of phenols

• Pure phenols the types of organic compounds that are generally colorless, solid or liquid organic molecules.
• Phenols in general, insoluble in water. Their solubility increases with an increase in temperature.
• Phenols have a relatively high boiling point due to the presence of intermolecular H-bonding.

Chemical Characteristics of Phenols

Acidic character

Unlike alcoholic molecules, phenols are acidic in nature and react with alkali metals and their hydroxide to form salts known as phenoxides.

C₆H₅-OH + 2Na → 2C₆H₅-ONa + H₂

C₆H₅-OH + NaOH → C₆H₅-ONa + H₂O

Reduction with zinc dust

Phenol reacts with Zn dust to produce benzene. In this reaction oxidation of Zn occurs and produces ZnO. Oxidation of Zno takes place.

C₆H₅-OH + Zn → C₆H₆ + ZnO

With acetyl chloride and acetic anhydride

Phenol reacts with acetyl chloride (CH₃-CO-Cl) and acetic anhydride (CH₃-CO-O-CO-CH₃) to produce phenolic ester phenylacetate (C₆H₅-COO- CH₃).

C₆H₅-OH + CH₃-CO-Cl → C₆H₅-COO- CH₃ + HCl

C₆H₅-OH + CH₃-CO-O-CO-CH₃ → CH₃-COOH + C₆H₅-COO- CH₃

Uses

• Phenol is commonly used in the preparation of soaps lotions and mainly bakelite.
• It is also used as a precursor for the synthesis of several drugs like salol, phenacetin, aspirin etc.
• Derivatives of phenols like bisphenol are the classes of organic compounds having potential use as an antiseptic.

Context and Applications

This topic is significant in the professional exams for both graduate and postgraduate courses, especially for,

• B.Sc. in Chemistry
• B.Sc. in Biochemistry
• M.Sc. in Biochemistry

Related Concepts

• Halo-hydrocarbons
• Biomolecules
• Carbonyls