Question

Why, in the reaction of HBr and HI with unsymmetrical ethers, does the smaller group form the haloalkane?

Short answer

The smaller group forms the haloalkane due to less steric hindrance, allowing easier nucleophilic attack by SN2 mechanism.

Step-by-step solution

Understand the Reaction

When unsymmetrical ethers react with hydrogen halides like HBr and HI, the reaction cleaves the ether into an alcohol and a haloalkane. The reaction proceeds via a nucleophilic substitution mechanism.

Nature of the Reaction

This reaction typically follows an SN1 or SN2 mechanism. The choice between them depends on the substrate and the specific conditions. In this case, SN2 is favored due to steric factors and the strength of the haloacids like HI and HBr.

SN2 Mechanism and Steric Hindrance

In an SN2 reaction, the nucleophile attacks the electrophilic carbon from the opposite side, leading to inversion of configuration. Steric hindrance affects the site of nucleophilic attack - it is easier for the nucleophile to attack the less hindered carbon atom.

Formation of Haloalkane

Because the smaller group in the ether is attached to the less hindered carbon, the nucleophile (I^- or Br^-) attacks this carbon more easily, leading to the formation of the haloalkane from the smaller group. This is due to reduced steric hindrance and more accessible electrophilic carbon.

Summary of Observation

Ultimately, the smaller alkyl group forms the haloalkane in this reaction because it is less sterically hindered, allowing the halide ion to more readily displace the alkoxy group through an SN2 mechanism.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile, which is a molecule or ion with an attraction to positive charges, replaces a leaving group in a molecular compound. In the case of unsymmetrical ethers reacting with hydrogen halides like HBr and HI, the nucleophile is typically a halide ion such as bromide (Br⁻) or iodide (I⁻).

The reaction between hydrogen halides and ethers is a typical example where nucleophilic substitution takes place. The nucleophile attacks the ether, leading to the breakage of the C-O bond in the ether linkage. The result is the formation of an alcohol and a haloalkane, where the haloalkane is formed from the nucleophilic attack on the less hindered carbon of the ether.

• The nucleophile attacks an electrophilic carbon in the ether.
• The leaving group departs, allowing the nucleophile to take its place.
• A haloalkane and an alcohol are the products of the reaction.

SN1 and SN2 Mechanisms

SN1 and SN2 are the two primary types of nucleophilic substitution mechanisms, and choosing between them depends on several factors, including the substrate structure and reaction conditions.

For the reaction of unsymmetrical ethers with HBr and HI, the SN2 mechanism is typically favored. SN2 reactions require a strong nucleophile and are characterized by a single, concerted step where the nucleophile attacks the substrate, displacing the leaving group simultaneously.

• SN1 Mechanism: Involves two steps, forming a carbocation intermediate, often seen with substrates that can stabilize this intermediate, like tertiary carbons.
• SN2 Mechanism: Involves a one-step process where the nucleophile attacks the substrate from the opposite side, resulting in an inversion of configuration at the reactive center.
• SN2 is faster in cases involving primary carbons due to less steric hindrance.

Steric Hindrance

Steric hindrance is an essential factor in determining the path of a nucleophilic substitution reaction. It refers to the restriction of molecular reactions due to the size and spatial arrangement of atoms within a molecule.

In unsymmetrical ethers, one of the carbon atoms is usually less hindered, meaning there is more space for the nucleophile to attack. This difference makes it more accessible for the nucleophile in an SN2 reaction.

• Less hindered carbon atoms are more susceptible to nucleophilic attack.
• Bulky groups near potential reaction sites reduce the rate of reaction.
• In the case of ethers and haloalkane formation, steric hindrance dictates that the smaller group forms the haloalkane.

Haloalkane Formation

Haloalkanes are compounds where a halogen atom is bonded to an alkane carbon. Their formation through the reaction of hydrogen halides with ethers is a classic example of nucleophilic substitution.

When unsymmetrical ethers are treated with strong acids like HBr or HI, the halide ion preferentially attacks the less sterically hindered carbon, forming a haloalkane from the smaller group in the ether. This happens because smaller groups are generally attached to less hindered carbons, allowing the halide ions easier access for displacement.

• Haloalkanes are produced when a halide replaces the alkoxy group in the ether.
• The ease of the nucleophilic attack is determined by steric factors and the strength of the nucleophile.
• This method forms haloalkanes efficiently due to these favorable conditions.