why would concentrated hydrobromic acid be an inappropriate catalyst for the dehydration of alcohols?
The correct answer and explanation is:
Correct Answer:
Concentrated hydrobromic acid would be inappropriate as a catalyst for the dehydration of alcohols because it tends to promote substitution reactions rather than elimination reactions.
Explanation:
The dehydration of alcohols is a chemical reaction that involves the removal of a water molecule from an alcohol to produce an alkene. This reaction typically occurs under acidic conditions and is catalyzed by strong acids such as concentrated sulfuric acid or phosphoric acid. These acids serve primarily as proton donors, activating the hydroxyl group of the alcohol and facilitating its departure as a water molecule.
Concentrated hydrobromic acid (HBr), although a strong acid, behaves differently. In aqueous or concentrated form, HBr provides not only protons (H⁺) but also a high concentration of bromide ions (Br⁻). These bromide ions are good nucleophiles. In the presence of alcohols, instead of promoting elimination to form alkenes, the bromide ion often attacks the carbocation intermediate that forms after protonation and loss of water. This results in a substitution reaction, where the hydroxyl group is replaced by a bromine atom, forming an alkyl bromide instead of an alkene.
For example, if ethanol is treated with concentrated HBr, the expected major product is bromoethane (C₂H₅Br), not ethene. The presence of a good nucleophile like Br⁻ drives the reaction toward substitution via the SN1 or SN2 mechanism depending on the structure of the alcohol.
In contrast, sulfuric acid or phosphoric acid do not have nucleophilic anions that interfere. These acids allow the carbocation intermediate to undergo elimination (E1 mechanism), leading to the formation of an alkene.
Therefore, concentrated hydrobromic acid is not suitable for dehydration reactions, because it promotes substitution rather than elimination, leading to the wrong type of product.