Explain the following observation. When 3-methyl-2-butanol is treated with HBr, a single alkyl b

Explain the following observation. When 3-methyl-2-butanol...

Key Concepts

Carbocation Stability

The stability of carbocations plays a crucial role in determining the pathway of reactions involving alcohols and hydrogen halides. More substituted carbocations (such as tertiary or stabilized secondary ones) have lower energy and are more stable due to hyperconjugation and inductive effects. This stability can drive processes like rearrangement where a less stable carbocation converts into a more stable one before nucleophilic attack occurs.

1,2-Shift (Rearrangement) Mechanism

A 1,2-shift is a rearrangement process in which an alkyl or hydride group moves from an adjacent carbon to a carbocation center to yield a more stable carbocation. This rearrangement occurs spontaneously when it leads to a significantly more stable intermediate, as seen in reactions where the formation of a tertiary carbocation is favored over a secondary one.

Reaction Mechanism: SN1 versus SN2

The mechanism by which substitution occurs often depends on the nature of the substrate. In the case of secondary or tertiary alcohols, protonation of the hydroxyl group leads to the formation of a carbocation intermediate, typically following an SN1 reaction pathway, which can rearrange before nucleophilic attack. For primary alcohols, the direct displacement via an SN2 mechanism is more common because the formation of a primary carbocation is energetically unfavorable, and rearrangements are less likely.

Transcript

00:01 Hello, today we are doing problem 9 .56, and this problem asks us to explain the following observation.

00:06 So when we react to this first molecule here being 3 -metol -2 -butanol, and when we treat it with hbr, a strong acid, a single -alchol bromide is isolated.

00:17 Specifically, this is due to the 1 -2 carbocatine rearrangement and hydride shift.

00:24 However, when we react 2 -methyl, 1 -proinol, and we treat it with the same reagent, we don't give you.

00:30 Get any rearranging, you don't get any carboketan rearrangements, and we still form this alcohol bromide product.

00:38 But it's asking us, why do we observe a carbocatin rearrangements or hydride shift in the first example, but not the second example? so let's go over the mechanism to see why that is.

00:52 So obviously we have a strong acid, so that dissociates to h plus and br minus.

00:56 We have to identify a nucleophile, that's being our alcohol of oxygen, that can pick up this proton here.

01:06 So now we have this protonated water, which is not stable, not favorable.

01:11 Oxygen likes to have negative or positive or neutral charge.

01:15 So because it has a positive charge, water will spontaneously fall off.

01:22 Now we have a secondary carbocato.

01:26 So adjacent to the secondary carbocatone, we have this beta proton, which can do something called a hydride shift to form a more stable intermediate.

01:38 And that leaves us with now a tertiary.

01:39 Carbocadion, in which point this bromine ion, which is being the nucleophile at this point, can attack with 50 % equal probability from the top as from the bottom, giving us a proscemic mixture of products.

01:54 So as you see here, the top example goes through an s in one mechanism in which we form a carbokadone intermediate.

02:04 And to represent the two products that this bromine ion can represent, either being a top or bottom addition substitution reaction, you can just draw a squiggly line showing that it's both a wedge and a dashed lines, meaning coming out and into the page respectively...

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