Ethers: Preparation & Properties
Module 3 | CBSE Class 12 Chemistry | Organic Chemistry
1. Preparation of Ethers
1.1 By Dehydration of Alcohols
Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether, depends strictly on the reaction conditions, specifically temperature.
At 413 K: 2 CH3CH2OH →(H2SO4) CH3CH2-O-CH2CH3 (Ethoxyethane - Substitution)
The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of an alcohol molecule on a protonated alcohol molecule.
1.2 Williamson Synthesis (Highly Tested Name Reaction)
This is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide.
Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method. The reaction involves an SN2 attack of an alkoxide ion on primary alkyl halide.
Better results are obtained if the alkyl halide is primary and the alkoxide is tertiary.
What if a tertiary alkyl halide is used?
If a tertiary alkyl halide is used, an elimination reaction occurs exclusively, and no ether is formed. This is because alkoxides are not only nucleophiles but also strong bases. They react with 3° alkyl halides to abstract a proton, forming an alkene instead.
Example: Reaction of CH3ONa with (CH3)3C-Br yields 2-Methylpropene, not tert-butyl methyl ether.
Preparation of Phenolic Ethers (Anisole): Phenols are also converted to ethers by this method. Phenol is used as the phenoxide moiety (because aryl halides are unreactive towards nucleophilic substitution).
C6H5ONa (Sodium phenoxide) + CH3I → C6H5-O-CH3 (Anisole) + NaI
2. Physical Properties of Ethers
- Boiling Points: The C-O bonds in ethers are polar and thus, ethers have a net dipole moment. However, the weak polarity does not appreciably affect their boiling points. The boiling points of ethers are much lower than the boiling points of alcohols of comparable molecular masses. Reason: Ethers lack intermolecular hydrogen bonding.
- Solubility: The solubility of lower ethers in water is comparable to that of alcohols of the same molecular mass. (e.g., ethoxyethane and butan-1-ol are both miscible to a similar extent). Reason: Like alcohols, oxygen of ether can form hydrogen bonds with water molecules.
3. Chemical Reactions of Ethers
Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides.
3.1 Cleavage of C-O Bond by Hydrogen Halides (HX)
Dialkyl ethers react with concentrated HX at high temperatures to yield two molecules of alkyl halides.
R-OH + HX → R-X + H2O
The order of reactivity of hydrogen halides is: HI > HBr > HCl.
When one alkyl group is primary or secondary, and the other is also primary/secondary, the reaction occurs by SN2 mechanism. Due to steric hindrance, the smaller alkyl group forms the alkyl iodide, and the larger one forms the alcohol.
Example: CH3-O-CH2CH3 + HI → CH3I (Methyl iodide) + CH3CH2OH (Ethanol).
Exception: If one of the alkyl groups is Tertiary (3°), the reaction proceeds by an SN1 mechanism. The cleavage occurs to form the highly stable 3° carbocation. Therefore, the tertiary group forms the alkyl iodide.
Example: (CH3)3C-O-CH3 + HI → (CH3)3C-I (tert-Butyl iodide) + CH3OH.
Cleavage of Alkyl Aryl Ethers (e.g., Anisole):
Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond (which has partial double bond character due to resonance). The reaction yields phenol and alkyl halide.
C6H5-O-CH3 + HI → C6H5OH (Phenol) + CH3I (Methyl iodide).
(Note: Iodobenzene is never formed).
3.2 Electrophilic Substitution in Aromatic Ethers
The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.
- Halogenation: Phenyl alkyl ethers undergo usual halogenation in the benzene ring. For example, anisole undergoes bromination with bromine in ethanoic acid (even in the absence of iron(III) bromide catalyst). It yields p-bromoanisole as a major product.
- Friedel-Crafts Reaction: Anisole undergoes Friedel-Crafts alkylation and acylation. The alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide respectively, in the presence of anhydrous AlCl3.
- Nitration: Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole (para is major).
4. NCERT Solved Examples (Step-by-Step)
NCERT Example 11.6: The following is not an appropriate reaction for the preparation of t-butyl ethyl ether.
C2H5ONa + (CH3)3C-Cl → (CH3)3C-O-C2H5
(i) What would be the major product of this reaction?
(ii) Write a suitable reaction for the preparation of t-butylethyl ether.
(i) The given reactants are a tertiary alkyl halide and sodium ethoxide (a strong base). Instead of substitution, an elimination reaction takes place. The major product is an alkene: 2-Methylprop-1-ene.
(ii) For Williamson synthesis to work, the alkyl halide must be primary. Therefore, to prepare t-butyl ethyl ether, we must use ethyl chloride and sodium t-butoxide.
Equation: (CH3)3C-O-Na+ + CH3CH2-Cl → (CH3)3C-O-CH2CH3 + NaCl
NCERT Example 11.7: Give the major products that are formed by heating each of the following ethers with HI.
(i) CH3-CH2-CH(CH3)-CH2-O-CH2-CH3
(ii) CH3-CH2-CH2-O-C(CH3)2-CH2CH3
(iii) Benzyl phenyl ether
(i) The ether has primary alkyl groups on both sides of the oxygen. The cleavage follows SN2 mechanism, so the smaller alkyl group forms the iodide.
Products: CH3CH2I and CH3CH2CH(CH3)CH2OH.
(ii) One of the alkyl groups is tertiary. The cleavage follows SN1 mechanism due to the formation of a stable tertiary carbocation. The tertiary group forms the iodide.
Products: CH3CH2CH2OH and CH3CH2C(I)(CH3)2.
(iii) Benzyl phenyl ether (C6H5-CH2-O-C6H5). The O-C6H5 bond has partial double bond character due to resonance. Cleavage occurs at the benzyl-oxygen bond, generating a highly stable benzyl carbocation.
Products: C6H5OH (Phenol) and C6H5CH2I (Benzyl iodide).
5. Previous Year Questions (PYQs) & Exhaustive Question Bank
Part A: Conceptual (1-2 Marks)
Q1. Why is the boiling point of ethanol much higher than that of methoxymethane (dimethyl ether)?
Q2. Write the equation of the reaction of hydrogen iodide with anisole.
C6H5-O-CH3 + HI →(heat) C6H5OH (Phenol) + CH3I (Methyl iodide)
Part B: Assertion-Reason Type (1 Mark)
Q3. Assertion (A): Ethers can be prepared by Williamson synthesis by reacting a tertiary alkyl halide with a primary alkoxide.
Reason (R): Alkoxides are not only nucleophiles but also strong bases.
Williamson synthesis fails if a tertiary alkyl halide is used. Because alkoxides are strong bases (as stated correctly in the reason), they cause the tertiary alkyl halide to undergo an elimination reaction, forming an alkene instead of an ether.
Part C: Application Based (3 Marks)
Q4. Give reasons for the following:
(a) Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method.
(b) Anisole reacts with bromine in ethanoic acid even in the absence of iron(III) bromide catalyst.
(a) For secondary and tertiary alcohols, the competing elimination reaction is much faster than the substitution reaction. Instead of forming ethers, they readily form alkenes.
(b) The methoxy group (-OCH3) in anisole is a strongly activating, electron-donating group (+R effect). It increases the electron density on the benzene ring to such an extent that the ring becomes highly reactive towards electrophilic substitution. Hence, the bromination can occur without the need for a Lewis acid catalyst like FeBr3.
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