Question

Which of the following type of reaction occurs when I-butyne reacts with HBr? (1) an electrophitic addition (2) Markownikoff's electrophilic addition (3) anti-Markownikoff's radical addition (4) None of the above

Short answer

Option 2: Markownikoff's electrophilic addition.

Step-by-step solution

Identify the Type of Alkyne

I-butyne is a terminal alkyne, which means that the triple bond is at the end of the carbon chain.

Understand the Reaction Type

When a terminal alkyne like I-butyne reacts with HBr, it typically undergoes an electrophilic addition reaction. This is due to the nature of the HBr molecule, where H is electrophilic (positively charged) and Br is nucleophilic (negatively charged).

Apply Markownikoff's Rule

Markownikoff's rule states that in the addition of HX to an alkyne, the hydrogen (H) attaches to the carbon with the greater number of hydrogen atoms, and the halide (Br) attaches to the carbon with fewer hydrogen atoms. This applies to the reaction of I-butyne with HBr.

Conclusion

Given the rules and the type of reaction, I-butyne reacting with HBr follows Markownikoff's electrophilic addition.

Key concepts

Alkyne Reaction

In organic chemistry, alkynes are hydrocarbons with at least one carbon-carbon triple bond. Alkyne reactions are crucial for synthesizing various organic compounds. One common type is the electrophilic addition reaction. This occurs when the triple bond reacts with electrophiles (electron-seeking species). The high electron density at the triple bond makes alkynes highly reactive towards electrophiles. Let's break down what happens in an alkyne reaction with an example:
I-butyne reacting with HBr. Here, HBr is an electrophilic species due to its polar nature (H is slightly positive, and Br is slightly negative). When HBr is added to I-butyne, the initial step involves the formation of a carbocation intermediate. The triple bond electrons of the alkyne interact with the hydrogen of HBr, creating a positively charged carbon (carbocation). After that, Br- (bromide ion) bonds with the carbocation, completing the addition reaction. Alkyne reactions can lead to diverse products, depending on the conditions and the specific reagents used.

Markownikoff's Rule

Understanding Markownikoff's Rule is essential for predicting the outcome of electrophilic addition reactions involving alkynes. This rule states that during the addition of HX (where X is a halogen) to an unsaturated molecule like an alkyne, the hydrogen atom will attach to the carbon atom that already has more hydrogen atoms. Conversely, the halogen will bond to the carbon atom with fewer hydrogen atoms. Applying this to I-butyne and HBr, we get:

• I-butyne has a terminal triple bond (one end of the chain).
• When HBr is added, based on Markownikoff's Rule, the H atom from HBr attaches to the terminal carbon (which has more hydrogen), and the Br atom attaches to the adjacent carbon (which has fewer hydrogen).

This rule ensures the stability of the resulting molecules by favoring the formation of more stable carbocation intermediates. Markownikoff's Rule is a pivotal concept for students to grasp when learning about electrophilic addition reactions in organic chemistry.

Terminal Alkyne

A terminal alkyne is a special type of alkyne where the carbon-carbon triple bond is located at the end of the carbon chain. One notable example is I-butyne. This positioning affects its reactivity and the products formed in reactions. Terminal alkynes are usually more reactive than internal alkynes. This increased reactivity is because terminal alkynes have fewer steric hindrances (less crowded environment around the triple bond) compared to internal alkynes. In terminal alkynes:

• The triple bond involves one carbon atom attached to a hydrogen atom and another attached to a carbon chain.
• This makes terminal alkynes excellent candidates for electrophilic addition reactions where reagents like HBr can easily add across the triple bond.

Understanding terminal alkynes helps in predicting and controlling the outcome of various organic reactions, essential for synthesizing desired organic compounds.