Mark Onikoff was about to turn in the products he had...

Mark Onikoff was about to turn in the products he had obtained from the reaction of HI with 3,3,3-trifluoropropene when he realized that the labels had fallen off his flasks. He didn’t know which label belonged to which flask. Another student in the next lab told him that because the product obtained by following Markovnikov’s rule was 1,1,1-trifluoro-2-iodopropane, he should put that label on the flask containing the most product and label the flask with the least product 1,1,1-trifluoro-3-iodopropane, the anti-Markovnikov product. Should Mark follow the student’s advice?

Key Concepts

Markovnikov’s Rule

This rule states that in the electrophilic addition of hydrogen halides (HX) to alkenes, the hydrogen atom typically attaches to the carbon atom bearing more hydrogen substituents, while the halide attaches to the more substituted carbon, because this pathway leads to the formation of a more stable carbocation intermediate.

Electrophilic Addition Reaction

This type of reaction involves an electrophile reacting with the electron-rich double bond of an alkene. The process generally proceeds through a carbocation intermediate whose stability influences the selectivity and final product distribution, with regioselectivity often predicted by Markovnikov’s rule.

Carbocation Stability and Substituent Effects

The intermediate carbocation’s stability is a key factor in many addition reactions. Electron-donating groups stabilize positive charges, while strongly electron-withdrawing groups can destabilize them. Alterations in carbocation stability due to adjacent substituents may lead to deviations from the outcomes predicted by traditional rules like Markovnikov’s.

Reaction Mechanism (Ionic vs Radical)

The mechanism by which an addition reaction occurs—whether through an ionic pathway involving carbocations or via a radical pathway—greatly influences the product distribution. Radical mechanisms often result in anti-Markovnikov products, emphasizing the need to consider the specific reaction conditions and the possible mechanisms at play.

Transcript

00:01 So here we're looking at a reaction with a propene, but the 3333 trifluoro propane.

00:12 Okay, so if we were to react this with just the regular markovnikov iodation, iodination.

00:25 What will basically get here, at least in the intermediate first, then the we just, we just, first hydrate or protein is alkene, these two carbocadions.

00:40 Okay, so these two carbocadiones will basically tell us which product is the major versus the minor.

00:51 So we have this carbotidion here, the secondary one, versus this primary one.

00:59 Okay, so we know what the original, the second student, what he or she was mentioning, was, okay, this is a second.

01:08 Secondary carbocadion, secondary carbureans, more stable than primary.

01:15 So then this product, this intermediate would, you know, take presidents.

01:25 But one thing we have to note here is we have this trifluor group.

01:30 So what is the effect of so many fluorides on our carbocary intermediate? okay, so recall, fluorine is the most electronegative atom on the whole periodic table.

01:48 So because of that, it does have some pretty significant electron withdrawing possibilities.

02:03 Yeah, it's basically an electron withdrawing group.

02:06 So what that means is it's polarizing our molecule here.

02:12 Basically to be more negative on this side and be more positive on that side.

02:20 What it's basically doing is donating or allowing for there to be more positive towards our carbocaryone.

02:28 So making a carbocadion essentially more positive.

02:32 Okay.

02:33 Let's just write that down.

02:42 And this is not like a favorable thing.

02:46 This will just allow...