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.
The question asks about the specific reaction pathway followed when an alkoxide ion attacks an alkyl halide during the Williamson synthesis. Let's break down the process based on the provided information and general chemical principles.
Williamson Synthesis Pathway Explained
Williamson synthesis is a crucial method for preparing ethers, as mentioned in the provided text. It involves the reaction between an alkyl halide and an alkoxide ion (typically derived from sodium alkoxide).
- Reactants: In this synthesis, the alkoxide ion (R-O-) acts as a strong nucleophile, and the alkyl halide (R'-X) acts as the substrate or electrophile.
- Mechanism: The core of this reaction involves the nucleophilic attack of the alkoxide ion on the carbon atom bonded to the halogen (the leaving group) in the alkyl halide.
Understanding Nucleophilic Substitution Pathways
Nucleophilic substitution reactions can occur via different mechanisms, primarily $S_N1$ and $S_2$.
- $S_N1$ (Substitution Nucleophilic Unimolecular): This is a two-step process. First, the leaving group departs, forming a carbocation intermediate. Second, the nucleophile attacks the carbocation. This mechanism is favored by tertiary substrates and weak nucleophiles in protic solvents.
- $S_N2$ (Substitution Nucleophilic Bimolecular): This is a one-step, concerted process. The nucleophile attacks the substrate from the backside simultaneously as the leaving group departs. This mechanism is favored by primary and secondary substrates, strong nucleophiles, and polar aprotic solvents.
Alkoxide Ion Attack in Williamson Synthesis
In the context of Williamson synthesis:
- The alkoxide ion (R-O-) is a potent, strong nucleophile.
- Williamson synthesis is typically performed using primary or secondary alkyl halides, which are suitable substrates for the $S_N2$ mechanism. While the text mentions substituted alkyl groups can be used, tertiary alkyl halides often lead to elimination (E2) or $S_N1$ reactions rather than the desired ether formation via $S_N2$.
- The strong nucleophilicity of the alkoxide ion makes it highly likely to perform a backside attack on the alkyl halide's carbon atom. This concerted attack, where bond-breaking and bond-making happen simultaneously, is characteristic of the $S_N2$ pathway.
- An $S_N1$ pathway involving a carbocation intermediate is less likely because the alkoxide is a strong nucleophile that would readily attack before a carbocation could form significantly, and carbocations are also prone to side reactions like elimination, especially in the presence of a strong base like an alkoxide.
Therefore, the alkoxide ion predominantly attacks the alkyl halide via the $S_N2$ pathway.
Important Questions from Alcohols, Phenols And Ethers
Among the following statements, choose the correct statements.
A. Boiling point of alcohols increases with increase in the number of carbon atoms.
B. In alcohols, boiling points increases with increase of branching in carbon chain.
C. Boiling points of alcohols are lesser in comparison to haloalkanes of comparable molecular mass.
D. Boiling points of alcohols are higher in comparison to hydrocarbons of comparable molecular mass.
E. The high boiling points of alcohols are mainly due to the presence of intramolecular hydrogen bonding.
Choose the correct answer from the options given below:
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 ____________.
