Question

The correct order of reactivity of the following alcohols with hydrobromic acid is 1-phenylpropan-1-ol \(\underset{(\mathrm{II})}{\text { 1-phenylpropan-2-ol }}{\text { 3-phenylpropan-l-ol }}{(\mathrm{III})}\) (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\) (c) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Short answer

Question: Arrange the following alcohols in order of reactivity with hydrobromic acid: 1-phenylpropan-1-ol (I), 1-phenylpropan-2-ol (II), and 3-phenylpropan-1-ol (III). Answer: The correct order of reactivity is III > I > II.

Step-by-step solution

Identify the Reaction Type

The given reaction is an acid-catalyzed substitution reaction where hydrobromic acid (HBr) reacts with the alcohols. In this type of reaction, the oxygen atom in the alcohol donates a lone pair of electrons to form a bond with a proton from HBr. This results in a protonated alcohol, which undergoes a nucleophilic substitution reaction with the bromide ion (Br-) to form an alkyl bromide product.

Look at the Structure of Each Alcohol

To determine the reactivity of each alcohol, we need to examine their structures: (I) 1-phenylpropan-1-ol: There is no steric hindrance at the alcohol site, and the phenyl group is connected to the carbon atom next to the alcohol site. The oxygen in the alcohol site does not participate in resonance with the benzene ring in the phenyl group. (II) 1-phenylpropan-2-ol: There is a moderate amount of steric hindrance at the alcohol site, and the phenyl group is connected to the carbon atom two positions away from the alcohol site. The oxygen in the alcohol site can participate in resonance with the benzene ring in the phenyl group through the alkene-like intermediate formed during the reaction. (III) 3-phenylpropan-1-ol: There is no steric hindrance at the alcohol site, and the phenyl group is connected to the carbon atom three positions away from the alcohol site. The oxygen in the alcohol site does not participate in resonance with the benzene ring in the phenyl group.

Consider Steric Hindrance and Resonance Stabilization

Now that we have looked at the structure of each alcohol, we can assess their reactivity with HBr based on steric hindrance and resonance stabilization: 1. Steric hindrance: In general, greater steric hindrance decreases the reactivity of alcohols with HBr. Therefore, (II) would be the least reactive alcohol among the three due to steric hindrance at the alcohol site. 2. Resonance stabilization: Alcohols that have their oxygen atom participating in resonance with the phenyl ring (benzene ring) are more stable and, thus, less reactive. The only alcohol in this case with resonance stabilization is (II). Taking both factors into account, the correct order of reactivity should be (III) > (I) > (II), as (III) and (I) have no steric hindrance and no resonance stabilization, while (II) has a moderate amount of steric hindrance and is resonance stabilized. Therefore, the correct answer is (c) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\).

Key concepts

Reactivity of Alcohols

When dealing with alcohols and their reactions, understanding their reactivity is crucial. Alcohols are known to undergo substitution reactions, especially with strong acids like hydrobromic acid (HBr). These reactions typically follow a pathway where the alcohol group (OH) is replaced by another group, such as a halide ion like bromide (Br-). The reactivity of a particular alcohol in these reactions can depend on several factors.

• Protonation Step: The first step involves protonation of the alcohol by the acid, making it a better leaving group. The hydroxyl group turns into water, which is a good leaving group.
• Factors Affecting Reactivity: Structural features of the alcohol influence its reactivity. These include the presence of electron-donating or withdrawing groups, the position of functional groups, and the stability of the resulting carbocation.

Aliphatic alcohols, like those involved in the exercise, can have varying degrees of reactivity based on their structure and the nature of the substituents attached to the carbon atom bearing the hydroxyl group.

Steric Hindrance

Steric hindrance occurs when the size of groups around the reactive site of a molecule restricts the approach of reactants, often reducing the reactivity of that molecule. In alcohols, steric hindrance can significantly impact their susceptibility to nucleophilic substitution reactions.
For example, an alcohol with large substituents or many such groups close to the hydroxyl group can experience steric hindrance.

• This makes it harder for reactants, like HBr, to approach and form bonds efficiently.
• In the case of 1-phenylpropan-2-ol, moderate steric hindrance is observed, reducing its reactivity as compared to 1-phenylpropan-1-ol and 3-phenylpropan-1-ol, where steric hindrance is minimal.
• As a result, alcohols with less steric hindrance tend to react faster with acids like HBr in substitution reactions.

Thus, designing a reaction sequence or predicting outcomes of a reaction involving alcohols requires careful consideration of steric effects.

Resonance Stabilization

Resonance stabilization is another critical concept influencing the reactivity of alcohols in substitution reactions. Resonance involves the delocalization of electrons across adjacent atoms in a molecule, providing extra stability to molecular structures.

• This effect is particularly relevant when a molecule, like the alcohol in our task, can form intermediates that allow the participation of resonance.
• For example, in 1-phenylpropan-2-ol, the proximity of the oxygen to the phenyl group allows some resonance stabilization, albeit limited. This reduces its reactivity with HBr.

In general, alcohols with resonance stabilization across the reactive site tend to react slower in substitution reactions, as the delocalized electrons confer additional stability.
On the contrary, 1-phenylpropan-1-ol and 3-phenylpropan-1-ol lack such resonance effects directly involving the hydroxyl group, making them more reactive compared to the resonance-stabilized structure.
Understanding resonance can help in predicting the stability and reactivity of intermediate species in chemical reactions.