Replacement of a hydrogen atom in a hydrocarbon by an alkoxy or carboxyl group yields a class of compounds known as ethers. Ethers are classified as symmetrical or unsymmetrical on the basis of groups attached to the oxygen atoms. Diethyl ether, a symmetrical ether, has been widely used as an inhalation anesthetic. Ethers can be prepared by acid catalyzed intermolecular dehydration of alcohols and Williamson's synthesis. Acid catalyzed dehydration of alcohols is not generally preferred as it gives a mixture of elimination and substitution products. In Williamson's synthesis, an alkyl halide is allowed to react with sodium alkoxide. Ethers containing substituted Alkyl groups may also be prepared by this method. The C-O bond in ether is weakly polar and is cleaved under drastic conditions with excess of hydrogen halides. In electrophilic substitution, the alkoxy group deactivates the aromatic ring and directs the incoming group to ortho and para positions.
Ether Cleavage Hydrogen Halide Reactivity
Ethers are organic compounds containing an oxygen atom connected to two alkyl or aryl groups (R-O-R'). The C-O bond within an ether molecule can be broken or 'cleaved' when treated with strong acids, particularly hydrogen halides (HX).
Hydrogen Halide Reactivity Factors
The effectiveness of a hydrogen halide in cleaving the C-O bond in ethers is determined by key chemical properties:
- Hydrogen-Halogen (H-X) Bond Strength: This refers to the energy required to break the bond between hydrogen and the halogen atom. A weaker bond breaks more readily. The general trend for bond strength is:
This means the H-I bond is the weakest.H-F > H-Cl > H-Br > H-I - Nucleophilicity of the Halide Ion (X⁻): After the hydrogen atom protonates the ether's oxygen, the halide ion acts as a nucleophile, attacking the carbon atom. Stronger nucleophiles facilitate this attack. The general trend for nucleophilicity in polar protic solvents is:
The iodide ion (I⁻) is the strongest nucleophile in this series.F⁻ < Cl⁻ < Br⁻ < I⁻
Ether Cleavage Reaction Mechanism
The cleavage typically proceeds in two main steps:
-
Protonation of Ether Oxygen: The hydrogen halide donates a proton (H⁺) to the lone pair of electrons on the ether oxygen atom. This forms a protonated ether (oxonium ion), making the attached alkyl groups more vulnerable to attack.
R-O-R' + HX \rightleftharpoons R-\overset{+}{O}H-R' + X^- -
Nucleophilic Attack by Halide Ion: The halide ion (X⁻) attacks one of the carbon atoms adjacent to the positively charged oxygen. This nucleophilic attack breaks the C-O bond, yielding an alkyl halide and an alcohol.
If the alcohol produced is secondary or tertiary, it can react further with the hydrogen halide to form another molecule of alkyl halide.R-\overset{+}{O}H-R' + X^- \rightarrow R-X + R'-OH
Hydrogen Halide Reactivity Order
Considering both the bond strength and nucleophilicity factors, hydrogen iodide (HI) emerges as the most reactive hydrogen halide for ether cleavage. This is because it possesses both the weakest H-X bond and the strongest nucleophilic halide ion (I⁻), which collectively enhance the rate and efficiency of the C-O bond breaking process.
The overall order of reactivity for hydrogen halides in ether cleavage reactions is established as:
HI > HBr > HCl > HF
Most Reactive Hydrogen Halide
Based on the analysis of bond strength and nucleophilicity, HI is the most reactive hydrogen halide for cleaving ethers.
Important Questions from Alcohols, Phenols And Ethers
Which of the following compounds is most acidic in character?
Isomer of diethyl ether is
What is aspirin?
The reactivity of primary, secondary and tertiary hydrogen for bromination is ____________.
Diethyl ether is _________.
