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.
Understanding Anisole Bromination
The question asks to identify the major product when anisole undergoes a reaction with bromine (Br₂) in ethanoic acid (CH₃COOH). This type of reaction is classified as electrophilic aromatic substitution.
Anisole and the Methoxy Group's Influence
Anisole is an organic compound with the chemical formula C₆H₅OCH₃. It consists of a benzene ring attached to a methoxy group (-OCH₃). The methoxy group plays a crucial role in determining the outcome of electrophilic aromatic substitution reactions due to its electronic properties:
- Activation: The lone pair of electrons on the oxygen atom of the methoxy group can be donated into the benzene ring through resonance. This delocalization of electrons increases the electron density of the aromatic ring, making it more reactive towards electrophiles. Consequently, the methoxy group is considered an activating group.
- Directing Effect: Resonance also concentrates the increased electron density at specific positions on the ring. For the methoxy group, this increased electron density is most significant at the positions immediately adjacent (ortho) and directly opposite (para) to it. This makes the methoxy group an ortho, para director in electrophilic aromatic substitution reactions.
Predicting the Major Product
In the reaction of anisole with bromine in ethanoic acid, the electrophile is the bromonium ion (Br⁺), which is generated from bromine. This electrophile attacks the electron-rich benzene ring.
Based on the directing effect of the methoxy group:
- Substitution is expected to occur at the ortho and para positions.
- While both positions are activated, the para position is generally favoured as the major product. This preference is often attributed to steric factors. The methoxy group, although not extremely bulky, can cause some steric hindrance to attack at the nearby ortho positions. The para position is sterically unhindered.
- Therefore, p-bromoanisole is formed as the major product, while o-bromoanisole is formed as a minor product. m-bromoanisole is not a significant product because the methoxy group directs substitution to the ortho and para positions, not meta.
The reaction can be summarized as:
C₆H₅OCH₃ + Br₂ $\xrightarrow{CH_3COOH}$ p-C₆H₄(Br)OCH₃ (Major) + o-C₆H₄(Br)OCH₃ (Minor)
Thus, the major product in the reaction of anisole with bromine in ethanoic acid is p-bromoanisole.
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 ____________.
