Consider the following alcohols 1\. 1-phenyl-1-propanol 2\. 3-phenyl-1-propanol 3\. I-phenyl-2-propanol The correct sequence of the increasing order of reactivity of these alcohols in their reaction with \(\mathrm{HBr}\) is (a) \(1,3,2\) (b) \(2,3,1\) (c) \(2,1,3\) (d) \(1,2,3\)

In organic chemistry, carbocation stability is a critical factor in many reactions, including the reaction of alcohols with hydrobromic acid (HBr). A carbocation is a positively charged carbon atom (C⁺), and its stability influences how easily a reaction progresses. Carbocations can be ranked based on their degree: primary, secondary, and tertiary, with tertiary being the most stable due to the inductive effect from surrounding alkyl groups.

However, there's an additional stability factor: resonance. Resonance occurs when electrons are delocalized over adjacent atoms, providing extra stabilization. For instance, in the reaction of 1-phenyl-1-propanol with HBr, a primary carbocation forms, which is stabilized by resonance from the adjacent benzene ring. This stability makes the alcohol more reactive with HBr compared to a non-resonance stabilized primary carbocation.

When determining carbocation stability, consider the following aspects:

• The number of alkyl groups attached to the positively charged carbon.
• The presence of resonance structures that can delocalize the positive charge.
• Hyperconjugation effects that can also contribute to stabilization.

These aspects help predict the outcome of substitution reactions, such as those involving alcohols and HBr.

Substitution reactions are at the heart of organic chemistry's transformative processes. In these reactions, an atom or group of atoms in a molecule is replaced by another atom or group of atoms. The reactivity of alcohols with HBr is an example of a nucleophilic substitution reaction where the hydroxyl group (OH) of the alcohol is replaced by a bromide ion (Br^-).

The overall mechanism typically proceeds through two main pathways: SN1 and SN2. The SN1 reaction involves the formation of a carbocation intermediate and is favored in tertiary alcohols due to their stable carbocations. On the other hand, the SN2 mechanism involves a backside attack by the nucleophile and is favored in primary and secondary alcohols.

Factors affecting substitution reactions include:

• The steric accessibility of the carbon undergoing substitution.
• The strength of the leaving group (hydroxyl group in the case of alcohols).
• The stability of the intermediate states (like carbocations in SN1 reactions).
• The nature of the nucleophile (bromide ion in HBr reactions).

Understanding these factors aids in predicting product formation and reaction rates.

Reactions involving hydrobromic acid (HBr) showcase the fascinating dynamics between electrophiles and nucleophiles in chemistry. HBr, a strong acid, readily dissociates in solution to give protons (H⁺) and bromide ions (Br^-). In the context of alcohol reactivity, HBr provides the necessary components for substitution: a good leaving group (H⁺) and a strong nucleophile (Br^-), resulting in the replacement of the hydroxyl group with a bromine atom.

The reaction proceeds as follows:

1. The alcohol is protonated by H⁺, turning the hydroxyl group into water, a better leaving group.
2. The subsequent loss of water creates a carbocation intermediate.
3. Finally, the nucleophilic bromide ion attacks the carbocation, completing the substitution.

The efficiency of these steps depends on the stability of the carbocation that forms, with more stable carbocations leading to faster reaction rates and higher yields.

Resonance stabilization is key when discussing carbocation intermediate durability during substitution reactions. A resonance-stabilized carbocation has its positive charge spread over several atoms, which reduces the charge density on a single carbon atom and makes the intermediate less demanding in terms of electron density.

For example, in the case of 1-phenyl-1-propanol reacting with HBr, the resulting carbocation is adjacent to a benzene ring. This proximity allows the positive charge to resonate over the benzene ring's pi electrons, greatly enhancing the stability of the carbocation. Carbocations can engage in resonance when they are positioned next to conjugated pi systems, such as double bonds or aromatic rings.

When evaluating possible resonance structures, remember that:

• The more resonance structures a carbocation has, the more stable it will be.
• Structures where the positive charge is spread over more electronegative atoms are generally more stable.
• Resonance effects can compete with and often surpass inductive effects from alkyl groups in stabilizing carbocations.

This understanding of resonance stabilization helps explain the reactivity patterns of different alcohols with HBr and allows for better prediction of reaction outcomes.