Question

The correct decreasing order towords reaction with HBr for the given all kones is given as: C=C(C)C C=C(C)OC 1 2 C=C(C)NC 3 4 (a) \(3>1>2>4\) (b) \(3>2>1>4\) (c) \(2>3>1>4\) (d) \(4>2>1>3\)

Short answer

The correct order is (c) 2 > 3 > 1 > 4.

Step-by-step solution

Understand the Problem

The task is to determine the correct order of reactivity of the given alkenes towards HBr addition. Alkenes undergo electrophilic addition reactions, and their reactivity is influenced by factors such as the stability of the carbocation intermediate, which follows Markovnikov's rule.

Analyze Each Structure's Reactivity

We have four alkenes to consider: tert-butene (C=C(C)C), 2-methoxypropene (C=C(C)OC), allylamine (C=C(C)NC), and 1-butene (simply represented as "1" for structural priority, but not shown directly in the smiles notation). The priority in reactions usually depends on the stability of the carbocation that would form during the Markovnikov process. Substitution and resonance effects enhance this stability.

Assign Expected Stability Based on Structure

- For C=C(C)OC (2-methoxypropene), the electron-donating effect from the methoxy group stabilizes the carbocation. - For C=C(C)NC (allylamine), the amine group also stabilizes the carbocation but less effectively than the methoxy group due to its basicity. - For C=C(C)C, the tertiary carbocation formed is quite stable. - 1-butene forms a primary carbocation, which is the least stable.

Compare Relative Orders of Reactivity

When comparing the structures: 1. 2-methoxypropene has the most stable intermediate carbocation due to resonance and inductive effects from the methoxy group. 2. Allylamine stabilizes the carbocation to a lesser extent. 3. Tert-butene forms a relatively stable tertiary carbocation. 4. 1-butene forms a less stable primary carbocation.

Sequence the Order of Reactivity

Based on the analysis, the order of reactivity should be 2-methoxypropene > allylamine > tert-butene > 1-butene. Matching this with the options provided, the correct order is presented in option (c): 2 > 3 > 1 > 4.

Key concepts

Markovnikov's Rule

Markovnikov's Rule is a fundamental principle in organic chemistry for understanding electrophilic addition reactions like those involving HBr and alkenes. The rule states that when a hydrogen halide (like HBr) adds to an alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already bonded, while the halogen atom attaches to the carbon with fewer hydrogen atoms. This pattern is due to the formation of a more stable carbocation intermediate.

When an alkene reacts with HBr, the double bond breaks, and the hydrogen atom from the HBr attaches to one of the carbon atoms. According to Markovnikov's Rule, it attaches to the carbon that results in the more stable carbocation on the adjacent carbon. This intermediary plays a key role in determining the final structure of the product.

In our example, when comparing different alkenes reacting with HBr, the one forming the most stable carbocation following Markovnikov's Rule shows the highest reactivity. This is critical because it helps predict and explain the outcome of chemical reactions and the order of reactivity among various alkenes.

Carbocation Stability

Carbocation stability is crucial for predicting the reactivity in electrophilic addition reactions like the ones discussed with HBr and alkenes. The stability of a carbocation, which is a positively charged carbon atom, greatly influences the rate of reaction and the product formation.

Several factors determine the stability of carbocations:

• **Degree of Substitution**: Tertiary carbocations (attached to three other carbon atoms) are more stable than secondary ones, which in turn are more stable than primary carbocations.
• **Resonance Stabilization**: When the positive charge can be delocalized across multiple atoms through resonance, the carbocation is more stable. An example is the methoxy group in 2-methoxypropene, which provides resonance stabilization.
• **Inductive Effect**: Electron-donating groups can stabilize carbocations through the inductive effect, where electrons are shared with the positively charged carbon.

In the exercise, 2-methoxypropene created the most stable carbocation due to the powerful resonance and electron-donating effects of the methoxy group. Tert-butene, despite its tertiary carbocation, is next stable. Allylamine benefits from the nitrogen's lone pair but is less effective than methoxy in resonance donation. 1-butene, forming a primary carbocation, is the least stable.

Reactivity of Alkenes

The reactivity of alkenes in electrophilic addition reactions depends significantly on the stability of the carbocation formed as an intermediate. Different substituents and the degree of alkyl substitution affect this reactivity. More stable carbocations lead to a higher reactivity of the corresponding alkene.

When alkenes react with hydrogen halides like HBr, alkenes with substituents that can stabilize the intermediary carbocation will react faster. That's why 2-methoxypropene, with its methoxy group providing resonance stabilization, exhibits the highest reactivity. The presence of such groups can enhance the electron density around the carbocation, stabilizing it through resonance or inductive effects.

In contrast, alkenes like 1-butene, which form primary carbocations, react much slower due to the lack of such stabilizing groups. The general order of reactivity, therefore, follows the ability of substituents to stabilize the carbocation, emphasizing the crucial role of carbocation stability in determining the behavior of alkenes during electrophilic addition reactions.