In this tutorial, we’ll explore the hydrohalogenation of alkynes, delving into the intricate mechanism of how alkynes react with hydrogen halides. We’ll highlight common examples that often appear in exams and, importantly, pinpoint frequent student mistakes to ensure you’re well-prepared to tackle this topic with confidence.
Diving into the hydrohalogenation of alkynes, textbooks typically illustrate this with common reactants like propyne and hydrogen bromide.
Here’s how the mechanism usually goes:
Markovnikov’s rule, proposed by the Russian chemist Vladimir Markovnikov in 1870, predicts the outcome of the addition of protic acids (like HCl, HBr, and HI) to unsaturated hydrocarbons, such as alkenes or alkynes.
According to the rule:
When a protic acid is added to an alkene, the acid’s hydrogen (H) will attach to the carbon with the highest number of hydrogen atoms already attached to it.
Conversely, the non-hydrogen part of the acid (e.g., the Cl, Br, or I) will add to the carbon with the fewest hydrogens.
This rule helps us anticipate the major product in many addition reactions. One of the underlying reasons for this pattern is the stability of the carbocation intermediates formed during the reaction; more substituted carbocations are generally more stable than less substituted ones.
When reacting 3-methylbut-1-yne with HBr, the process unfolds as follows:
Now, here’s where things get spicy! You might be scratching your head, wondering, “Why no carbocation rearrangement?”
Especially since a tertiary allylic carbocation is a far more stable option. Well, despite what the textbooks might hint at, not everything follows the beaten path. And believe it or not, sometimes, the mechanisms we’re taught aren’t the whole story. Remember, every quirk in chemistry has its reason, even if it momentarily feels like…well, let’s just say it’s not always as it seems!
Often in sophomore organic chemistry, students learn a mechanism that doesn’t quite align with experimental data. Let’s set the record straight!
Concerted Termolecular Mechanism: The crux of the matter is that recent kinetic studies reveal a termolecular mechanism at play. Imagine an alkyne molecule having a chat with two separate HBr molecules. The result? Our product forms directly, side-stepping any carbocation shenanigans. With no carbocation middleman, there’s zero chance of carbocation rearrangements. If you’ve ever stumbled across a study that proves otherwise or gives a reason (beyond the magic wand effect) for a non-rearranging carbocation, drop a comment below. I’d love a good scholarly surprise!
The Second Step – Just Regular Hydrohalogenation: Moving on, the next phase is textbook hydrohalogenation. Why no carbocation gymnastics here? It’s all thanks to our bromine buddy! The bromine’s electron pairs lend some resonance support, ensuring our intermediate stays stable and predictable. A straight path, with no twists or turns!
Support continues to mount for the termolecular mechanism, and a reaction between but-2-yne and HBr.
Here’s the catch: The midway product of this reaction is overwhelmingly the outcome of an anti-addition. If a carbocation strutted its stuff here, we’d spot both products since carbocations don’t dance to a single stereochemical beat. But guess what? We only see one product. That’s a tell-tale sign!
Two mechanisms stand at a crossroads: the familiar one we share with students (lacking substantial experimental backing) and the more realistic termolecular version. Here’s a golden nugget of advice: Understand your professor’s expectations. A handful of textbooks touch on the termolecular nature. If yours isn’t among them, you’re likely learning the carbocation route. So, before you dive deep, have a chat with your instructor. After all, they hold the grading pen, not me!