Question

When allowed to react with aqueous HBr, 3 -buten \(-2-\circ 1\) \(\left(\mathrm{CH}_{3} \mathrm{CHOHCH}=\mathrm{CH}_{2}\right)\) yields not only 3 -bromo-1-butene \(\left(\mathrm{CH}_{3} \mathrm{CHBrCH}=\mathrm{CH}_{2}\right)\) but also 1 -bromo-2-butene \(\left(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{Br}\right)\) (a) How do you account for these results? (b) Predict the product of the reaction between \(\mathrm{HBr}\) and 2 -buten-1-o1 \(\left(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{OH}\right)\).

Short answer

The reaction of 3-buten-2-ol with aqueous HBr results in the formation of two products, 3-bromo-1-butene and 1-bromo-2-butene, due to electrophilic addition at two possible sites in the molecule. The reaction proceeds through intermediates, leading to the two products depending on which carbon is attacked by HBr. For the reaction between HBr and 2-buten-1-ol, the main product will be 1-bromo-1-butene due to the formation of a more stable secondary carbocation intermediate.

Step-by-step solution

: Electrophilic addition is a reaction that occurs between an electrophile and a nucleophile, usually a double or triple bonded compound. In this case, the double bond in the alcohol (3-buten-2-ol) acts as the nucleophile, while the HBr acts as the electrophile. #Step 2: Analyzing the reaction of 3-buten-2-ol with HBr#

: When 3-buten-2-ol reacts with HBr, the double bond in the alcohol can undergo electrophilic addition at two possible sites: the carbons that the hydroxyl group is bonded to (C2) and the adjacent carbon with a hydrogen atom (C3). This can lead to the formation of two products: 3-bromo-1-butene and 1-bromo-2-butene. #Step 3: Explaining the formation of both products#

: In the formation of 3-bromo-1-butene, the proton from HBr adds to the C2 of 3-buten-2-ol which is attached to the hydroxyl group. This results in a secondary carbocation intermediate, as the positive charge is on the second carbon and also involves the hydroxyl group in resonance stabilization. Then, the bromide ion attacks this carbocation, leading to the formation of 3-bromo-1-butene. In the formation of 1-bromo-2-butene, the proton from HBr adds to the C3 of 3-buten-2-ol, resulting in a tertiary carbocation intermediate. The carbon with a positive charge involves the alkyl group in stabilizing the positive charge by hyperconjugation effect. Subsequently, the bromide ion attacks this carbocation, leading to the formation of 1-bromo-2-butene. #Step 4: Predicting the product of the reaction between HBr and 2-buten-1-ol#

: For the reaction between 2-buten-1-ol and HBr, we need to analyze the possible products following a similar electrophilic addition mechanism. In this case, HBr will be added to the double bond of 2-buten-1-ol. Upon adding the proton from HBr to the C1 carbon of 2-buten-1-ol, we have a secondary carbocation. The hydroxyl group can participate in resonance stabilization, similar to what happened in the previous reaction, leading to the formation of 1-bromo-1-butene when the bromide ion attacks this carbocation. It is important to note that the addition of the proton to the C2 carbon is not favored in this case, as it generates a primary carbocation, which is less stable. Therefore, the main product in this reaction will be 1-bromo-1-butene.

Key concepts

Electrophilic Addition Mechanism

Electrophilic addition is a vital reaction type in organic chemistry, especially when explaining how different products can emerge from a single reactant. Let's unfold the steps involved in such mechanisms and apply our understanding to specific reactions. The reaction between 3-buten-2-ol and HBr showcases a typical electrophilic addition with a twist; it demonstrates that multiple products can form, depending on where the electrophile, in this case, the hydrogen ion (H⁺ from HBr), adds to the nucleophile, here the double bond of 3-buten-2-ol.

Initially, the H⁺ targets the electron-rich double bond, which is eager to bond with positive charges. This step is guided by the molecule's structure and the relative stability of potential carbocations that form as intermediates. In simpler terms, think of the double bond as a 'help wanted' sign for electrophiles; it signals where to bind. Following the initial attraction, a carbocation - a carbon atom with a positive charge - is formed when the double bond breaks. The more stable this carbocation is, the more likely it is to form.

Next, the remaining part of HBr, the bromide ion (Br^-), acts as the nucleus seeking to join the positively charged carbon atom. This is the second electrophilic addition step, resulting in the final product. With such a reaction, we see a clear formation path, but predicting the major product can be tricky and hinges on nuances like carbocation stability and molecule's overall structure.

Carbon-Carbon Double Bond Reactivity

In the landscape of organic molecules, carbon-carbon (C=C) double bonds are like social butterflies: they interact readily and form connections easily. But, as with social interactions, the 'personality' of the surrounding group matters. For 3-buten-2-ol, the C=C double bond is flanked by different groups: a hydroxyl (OH) group and a hydrogen (H). The reactivity of such a double bond relies heavily on these neighbors.

Why is the double bond so reactive? It's because the electrons in the double bond are higher in energy and more accessible for reaction than those in single bonds. When an electrophile approaches, these electrons are quick to engage in a chemical embrace. But where the embrace happens affects the eventual outcome of the reaction, leading to different products depending on which carbon of the double bond interacts with the electrophile. Therefore, in 3-buten-2-ol, the double bond's reactivity is a dance of possibility, guided by the stability of the resulting intermediates and the influence of the neighboring groups, driving the reaction toward the most favorable direction.

Carbocation Stability

In the dowry of organic reactions, carbocations are the treasures that electrophiles seek — but not all carbocations are created equal. Their stability is the key that unlocks the reactions' pathways and products. Imagine carbocations as different houses: just as houses are more stable with a good foundation, the stability of carbocations increases with more supporting 'neighbors', known as alkyl groups.

There are three primary types of carbocations, differing in stability:

• Primary: Attached to one carbon group (least stable).
• Secondary: Attached to two carbon groups.
• Tertiary: Attached to three carbon groups (most stable).

Their stability is due to hyperconjugation and resonance. Hyperconjugation is like a neighbor lending you sugar; nearby bonds share electrons with the positive charge, thus dispersing and reducing it. Resonance occurs when the charge can hop between different atoms, like moving furniture to balance a room. In the reactions with 3-buten-2-ol and HBr, the more stable carbocations lead to major products because they’re most likely to form. Therefore, understanding carbocation stability is crucial in predicting the outcome of electrophilic addition reactions.