Why is 2-bromo-2-methylpropane unable to undergo an reaction? Select an answer and submit. For keyboard navigation, use the up/down arrow keys to select an answer. a bromine is a poor leaving group b the molecule is too congested for nucleophiles to approach

❓ Why Is 2-Bromo-2-Methylpropane Unable to Undergo an SN₂ Reaction?

📘 Question:

Why is 2-bromo-2-methylpropane unable to undergo an SN₂ reaction?

• a) Bromine is a poor leaving group
• b) The molecule is too congested for nucleophiles to approach ✅
• c) A catalyst is required for SN2 reactions to proceed
• d) The molecule is not bulky enough to support SN2 reactions
• e) SN2 reactions are generally slower than SN1 reactions

🔬 Detailed Explanation

2-bromo-2-methylpropane, also known as tert-butyl bromide, is a tertiary alkyl halide widely used in organic chemistry as a reference compound for substitution reactions. It consists of a central carbon bonded to three methyl groups and one bromine atom. Its structural formula can be written as (CH3)3CBr. Understanding why this compound fails to participate in SN₂ reactions requires a closer look at the SN₂ mechanism and the structural features of tert-butyl bromide.

⚛️ What Is the SN₂ Reaction?

The SN₂ reaction is a nucleophilic substitution reaction that occurs in a single, concerted step. The mechanism involves the nucleophile attacking the electrophilic carbon center from the opposite side of the leaving group, displacing it in the same step. Because of this backside attack, the SN₂ reaction causes an inversion of configuration at the carbon center (also known as the Walden inversion).

📌 Key Requirements for an SN₂ Reaction

• Accessibility of the electrophilic carbon
• Strong nucleophile
• Good leaving group (like Br⁻)
• Low steric hindrance

While 2-bromo-2-methylpropane has a good leaving group (Br⁻), it fails the test for accessibility due to its bulky structure.

🚫 Why 2-Bromo-2-Methylpropane Cannot Undergo SN₂

The central carbon in tert-butyl bromide is bonded to three methyl groups, making it a tertiary carbon. This configuration results in extreme steric hindrance, blocking the nucleophile’s pathway to the electrophilic carbon.

💥 Steric Hindrance Explained

Steric hindrance occurs when bulky groups around a reactive center prevent other molecules or atoms from approaching that center. In the case of tert-butyl bromide, the three methyl groups surrounding the central carbon act like a shield, repelling nucleophiles from accessing the backside of the carbon to initiate an SN₂ attack.

🧪 SN₂ Reactivity of Different Halides

The reactivity in SN₂ reactions typically follows this trend:

• Methyl halide > Primary halide > Secondary halide > Tertiary halide

This trend directly reflects the increasing level of steric hindrance as more alkyl groups are attached to the carbon bearing the leaving group.

🔄 Example Comparison:

• Methyl bromide (CH₃Br): Very fast SN₂ reaction
• Ethyl bromide (CH₃CH₂Br): Slower than methyl, but still efficient
• Isopropyl bromide ((CH₃)₂CHBr): Slow SN₂ due to steric hindrance
• Tert-butyl bromide ((CH₃)₃CBr): No SN₂ due to extreme steric hindrance

🔍 Why SN₁ Is More Likely

Unlike SN₂, which requires direct nucleophilic attack, the SN₁ mechanism proceeds via a two-step process:

1. Formation of a carbocation intermediate (slow step)
2. Nucleophilic attack on the carbocation (fast step)

Tert-butyl bromide forms a very stable tertiary carbocation, making it highly favorable for SN₁ reactions, especially in polar protic solvents.

📚 Educational Importance

Understanding why tert-butyl bromide cannot undergo SN₂ is critical in mastering substitution reaction mechanisms in organic chemistry. Students and researchers must learn to evaluate substrate structure and choose the correct pathway (SN₁ vs SN₂) for reaction prediction and planning.

🧠 Additional Factors Affecting SN₂

• Solvent: Polar aprotic solvents (e.g., acetone, DMSO) favor SN₂ reactions.
• Leaving Group: Better leaving groups (like Br⁻, I⁻) facilitate faster reactions.
• Nucleophile Strength: Stronger nucleophiles increase reaction rate.

Even with ideal solvent and nucleophile conditions, the bulky structure of 2-bromo-2-methylpropane prevents SN₂ from proceeding.

🔬 Practical Lab Insight

In laboratory settings, tert-butyl bromide is often used in teaching labs to illustrate SN₁ reaction characteristics. For instance, when reacting with water or ethanol, tert-butyl bromide forms tert-butyl alcohol through a clear two-step SN₁ mechanism.

📈 Real-World Applications

• Pharmaceutical Industry: Understanding substitution mechanisms helps in synthesizing drug molecules with higher yield and purity.
• Organic Synthesis: Predicting SN₂/SN₁ pathways is vital in multi-step reaction design.
• Chemical Education: Mastering these principles lays the foundation for learning advanced reaction mechanisms.

📌 Summary Table: SN₂ Feasibility