What alkenes would you expect to be obtained from the acid-catalyzed dehydration of 1-hexanol?

Name: Problem 42, Chapter 12: Organic Chemistry
Uploaded: 2020-07-29T20:48:57-08:00
Duration: 3 min 31 s
Channel: Tom Rutherford
Description: What alkenes would you expect to be obtained from the acid-catalyzed dehydration of 1-hexanol?

What alkenes would you expect to be obtained from the...

Key Concepts

Zaitsev's Rule

Zaitsev's rule predicts that in elimination reactions, the most substituted alkene is generally favored because it is thermodynamically more stable. In the context of alcohol dehydration, when multiple alkenes can be formed, the elimination process will favor the formation of the alkene with the greater number of alkyl substituents attached to the double bond, leading to the major product being the more substituted, stable alkene.

Carbocation Rearrangement

When a carbocation intermediate is formed, it may undergo rearrangements, such as hydride shifts, in order to transform into a more stable carbocation. This rearrangement plays a critical role in determining the final products of acid?catalyzed dehydration, as a more stable carbocation can lead to a different alkene than what would be produced by direct elimination, altering the regiochemistry of the product.

Acid?Catalyzed Dehydration

Acid?catalyzed dehydration is a reaction in which an alcohol loses a water molecule in the presence of an acid to form an alkene. This process typically involves the protonation of the hydroxyl group, turning it into a better leaving group, and then the elimination of water, which can occur via either an E1 or an E2 mechanism depending on the substrate and reaction conditions.

Elimination Mechanisms

Elimination reactions involve the removal of substituents from adjacent atoms, leading to the formation of a double bond. In acid?catalyzed dehydration, the reaction often proceeds through an E1 mechanism, which involves the formation of a carbocation intermediate followed by deprotonation, although under some conditions an E2 mechanism might compete. The mechanism chosen can influence the distribution and stereochemistry of the resulting alkenes.

Transcript

00:01 Okay, this problem is asking us to react this primary alcohol with h2s -o -4 in heat in order to get an alken.

00:06 And it's asking us which alkenes are we going to get.

00:09 So let's go ahead and draw this mechanism.

00:10 So first up, i noticed that i have an alcohol that is considered to be basic in relation to my h -2.

00:16 So i'm going to have the protonation of my alcohol.

00:18 Okay, and that's going to form this product.

00:21 Okay, where i have my protonated alcohol, which has oh2.

00:25 And that has a positive charge on the oxygen.

00:28 Okay, so in an alken formation, we essentially have a protein formation, we essentially have a protein, a leaving group leave and then a base deprotonate an acidic hydrogen in order to make an alkyene.

00:37 So let's go ahead and do that.

00:39 First up, i'm going to have my alcohol slash water leave.

00:43 But if i realize that if i make that water leave, i'm going to end up with a primary carbocyan.

00:48 And i can't have a primary carbonyatine.

00:50 That's just too unfavorable.

00:51 So at the same time that i have my water leave, i'm going to have another molecule of water, come in and deprotonate this hydrogen.

00:59 Okay, now avoid the formation of the primary carbocadine.

01:04 So this is almost behaving in kind of like an sn2 reaction, in which i have two simultaneous steps...