Introduction:

Ethers are organic compounds containing -C-O-C- linkage known as ether linkage. The general formula of the ethers C_nH_2n+2O is similar to that of alcohols and are thus isomeric with the alcohol. Ethers are better represented by the formula R-O-R, Ar-O-R, Ar-O-Ar.

Classification

1. Symmetrical: When two alkyl or aryl groups in ether are same, the ether is said to be symmetrical or simple e.g. CH₃-0-CH₃, diethyl ether.
2. Unsymmetrical: When the two alkyl or aryl groups are different, the ether is said to be the unsymmetrical or mixed ether. eg.CH₃-O-C₂H₅

Ether

Nomenclature of Ethers:

1. Common or trivial system: In this nomenclature system, the ethers are named according to the alkyl or the aryl groups attached to the oxygen atom and the word ether is placed after it.

2. IUPAC system: According to this system the ethers are named as hydrocarbons in which the hydrogen atom is replaced by an alkoxy group. The larger carbon chain is chosen as the parent alkane.

Physical Properties:

1. The lower members are gases or volatile liquids and their net dipole vapours are highly inflammable.
2. The C-0-C bond angle in ethers is not 180^o. The dipole moments of the two C-O bonds do not cancel each other and hence ethers possesses a small net dipole moment e.g. diethyl ether μ = 1.18 D.

3. The boiling points of ethers are much lower than those of alcohols containing the same number of carbon atoms because ethers can not associate through hydrogen bondings, their boiling points are more comparable with alkanes having nearly same molecular weight.

Solubility: Ethers are sparingly soluble in water but they are soluble in concentrated acids. The water solubility of ether arise due to hydrogen bonding between water molecules and ether molecules. However as the size of 'R' group increases the extent of hydrogen bonding and hence solubility decreases.

Industrial Sources of Ethers:

Dehydration of alcohols: A number of symmetrical ethers containing the lower ethers (e.g. diethyl ether) are prepared by dehydration of ethyl alcohols, which are chiefly used as solvent. Alcohols are heated with concentrated H₂SO₄ at 140^oC to form alkyl hydrogen sulphate which reacts with excess of alcohol molecules to form symmetrical ether.

Laboratory Preparations:

1. Williamson synthesis: This is the most convenient method for preparation of symmetrical (simple) as well as unsymmetrical (mixed) ethers. When an alkyl halide is allowed to react with sodium alkoxide or sodium phenoxide gives ether.

2. Alkoxy mercuration-demercuration: Alkenes react with mercuric trifluoroacetate in the presence of an alcohol to give alkoxy mercurial compounds which on reduction yield ethers.

Advantages of this method:

1. This reaction requires less time, gives higher yield and is more convenient.
2. There is absence of rearrangement.
3. Compared with Williamson's synthesis, there is no competing elimination reaction and hence be used for the synthesis of nearly every kind of alkyl ethers, except di-tertiary alkyl ethers, due to steric interactions.

3. Diazomethane method: This is a special method for preparation of methyl ethers only. When an alcohol or a phenol is heated with diazomethane (CH₂N₂) in presence of fluoroboric acid (HBF₄) methyl ether is formed.

Reaction of Ethers ( cleavage by acids ) and analysis of ethers

Ethers are unreactive compounds. The ether linkage is quite stable towards bases oxidising agents and reducing agents. They undergo only cleavage reaction by acids under quite vigorous conditions, like concentrated acids (HI or HBr) and high temperature.

1. Reaction with conc. Hydro iodic acid (HI)
1. Cold conditions: Simple ether like diethyl ether reacts with conc. HI in cold to give ethyl alcohol and ethyl iodide.

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Mixed ether like ethyl methyl ether reacts with conc. HI in cold condition to give methyl iodide and ethyl alcohol. (In mixed ethers higher group form alcohol)

b. Hot conditions: In hot condition conc. HI reacts with simple as well as mixed ether always forms alkyl iodide. (The alcohol formed in the first step is finally converted to iodide).

A dialkyl ether yields initially an alkyl halide and an alcohol, which may react further to form a second mole alkyl halide.

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An alkyl aryl ether undergoes clearage of the alkyl oxygen bond and yields alkyl halide and phenol due to the low reactivity at the bond between oxygen and aromatic ring.

2. Reaction with dil. H₂SO₄ (Hydrolysis)

When steam is passed over ether under pressure in presence of dil. H₂SO₄ hydrolyse to alcohol.

Phenols are the organic compounds containing a benzene ring bonded to a hydroxyl group. phenols are the simplest hydroxy derivative of the benzene ring or aromatic ring. They are also known as carbolic acids. Thus, a phenol molecule consists of two parts one aryl group part and the other hydroxyl group part.

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On the basis of the number of hydroxyl groups attached to the aryl group, it can be classified into mono-, di-, tri- or polyhydric phenols.

Phenols

Phenols (Phenol case):

Introduction and IUPAC nomenclature,

Preparation: Cumene hydroperoxide method, from diazonium salts. Reactions: Electrophilic substitution: Nitration, halogenation and sulphonation. Reimer-Tiemann Reaction, Gattermann-Koch Reaction, Houben–Hoesch Condensation, Schotten–Baumann Reaction. Ethers (aliphatic and aromatic): Cleavage of ethers with HI.

Learning Outcome:

After studying the Alcohols and Phenols student will able to

1. Identify and draw the structures alcohols / phenols from their names or from structure name can be assigned.
2. Able to differentiate between alcohols and phenols
3. Explain / discuss synthesis of alcohols / phenols.
4. Write / discuss the mechanism of various reactions involved.
5. Explain /Discuss important reactions of alcohols / phenols.
6. To correlate reagent and reactions of alcohols / phenols
7. Give synthesis of expected alcohols / phenols.

Nomenclature of phenols:

1. Simple method: In this method the substituent attached on the aromatic is designated as ortho, meta or para as prefix to the name of phenol.

2. Numbering method: In this method, the carbon atom of the aromatic ring to which -OH group is attached is given number 1. Then the carbon atom of the ring to which the other substituent is attached is given lowest possible number.

3. Polysubstituted derivations: If more than two groups are attached to the aromatic ring. only number method is used to indicate their relative positions w.r.t. - OH group.

Physical properties

1. Acidic nature: Although alcohols and phenols have hydroxyl (-OH) as a functional group, alcohols are neutral compounds but phenols are acidic in nature. This is because phenoxide ion formed after the loss of proton gets stabilized due to resonance effect (-R effect) of the ring, on the other hand the alkoxide ion is destabilized due to inductive effect (+I effect).

Secondly, phenols react with bases and form salt which dissolve in aqueous medium.

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The salt on treatment with acid gives phenol back which are generally insoluble

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Alcohols do not react with acid and base to form salt, hence neutral in nature (Lower members are soluble in all aqueous reagents).

2. Physical constants: Phenols show high boiling points than hydrocarbons of comparable molecular weight due to presence of hydrogen bonding in phenolic compounds.

m-nitrophenol and p-nitrophenol show intermolecular hydrogen bonding. While o-nitrophenol show intra molecular hydrogen bonding. Hence, m and p-nitrophenols have higher physical constant than o-nitro phenol.

Due to hydrogen bonding phenols also dissolve in water to a considerable amount. In fact hydrogen bonding between water and phenol is much stronger than between water and alcohol.

Colour: Simple phenols are colourless but if chromophoric groups (NO₂, -NH₂) are introduced on the aromatic ring then they show colour. e.g. nitrophenols and nitroamines are yellow to brown coloured compounds .

Preparation of Phenols

1. Cumene hydroperoxide method

The starting material cumene, can be obtained from benzene and propene by Friedel Craft alkylation reaction.

2. Oxidation of Cumene: When cumene is treated with oxygen at elevated temperature gives Cumene hydroperoxide which on treatment with 10 % H₂SO₄ undergoes rearrangement and yields phenol and acetone. The acetone byproduct obtained from the reaction is also an important solvent .

3. From Diazonium salts: In this process first the arylamine is treated with nitrous acid (NaNO₂) to obtain the diazonium salt, which on acid catalysed hydrolysis yields the phenol.

Reactions of Phenol

phenol is an aromatic compound it undergoes electrophilic substitution reactions. As the hydroxyl group is electron donating group (activating group), the product obtained is ortho - para substituted phenol.

Some of the ring substituted reactions are

1. Nitration
2. Sulphonation
3. Halogenation
4. Nitrososation
5. Kolbe's reaction (carbonation)
6. Reimer-Tiemann reaction

1. Nitration

A. With dil. HNO₃: Phenol reacts with dil. HNO₃ and produces a mixture of O-nitrophenol and p-nitrophenol. The mixture can be easily separated by steam distillation method. Due to presence of intra molecular hydrogen bonding in o-nitrophenol it is steam volatile and passes over along with steam; while p-nitrophenol remains in the distillation flask.