[ayush]

see this question

CH3CH2CH2CH2OH + CONC. H2SO4 --->CH3CH2CH=CH2
                                    OR
                                                  --->CH3CH=CHCH3

In this question i don't think the rearrangement of carbocation is possible becoz for rearrangement 2⁰ or 3⁰ carbons are necessary

but here it's only primary so what will be the answer now

[azmanam]

first, I don't think the initial elimination goes through a carbocation at all (formation of a primary carbocation is highly unlikely).  Second, you didn't draw the arrows correctly.  The reaction is reversible in every step. 

given that the reaction is reversible, can you see a pathway to get to the internal olefin from the primary alcohol?

[ayush]

ok here i am posting that what we are taught about acid dehydration reaction

1) FORMATION OF PROTONATED ALCHOL
CH3CH2CH2CH2-O-H + H⁺ CH3CH2CH2CH2-O(H⁺)-H

2) FORMATION OF CARBOCATION
CH3CH2CH2CH2-O(H⁺)-H CH3CH2CH2C(⁺)H2


NOW TELL ME WHETHER AFTER THIS STEP REARRANGEMENT WILL OCCUR OR NOT SO AS TO GET MAJOR PRODUCT OF THIS REACTION

[azmanam]

to the extent that the carbocation you have drawn forms, yes it will rearrange immediately.  What I am saying  is I don't think the terminal alkene is formed through an E1 mechanism.   

The primary carbocation is soooo unstable that I don't propose the primary carbocation forms at all.  I think the terminal alkene forms through a different pathway. I don't propose the internal alkene forms through a carbocation rearrangement mechanism.

[ayush]

oh thanks for the suggestions

[orgopete]

to the extent that the carbocation you have drawn forms, yes it will rearrange immediately.  What I am saying  is I don't think the terminal alkene is formed through an E1 mechanism.   

The primary carbocation is soooo unstable that I don't propose the primary carbocation forms at all.  I think the terminal alkene forms through a different pathway. I don't propose the internal alkene forms through a carbocation rearrangement mechanism.

I am not proposing a primary carbocation, but I am not opposed to a term of "primary carbocation like". This might be like a Friedel Crafts alkylation with butyl bromide which gives extensive rearrangement.

It appears that you know the data from this experiment (I don't). Perhaps you could indicate the ratio of internal to terminal alkene in this reaction. Do you have label results as well? I am surmising that you are implying a hydride transfer from C3, right?

[azmanam]

No.  I'm suggesting E2 (after protonation) to give the primary alkene, then hydration to the secondary alcohol then elimination to the internal alkene.

[orgopete]

How could you distinguish between a simple hydride transfer to a secondary carbocation and elimination? Is there something about these conditions that will not rearrange while the F.C. reaction does? We know F.C. alkylation gives extensive rearrangement. I think that comes directly from the halide, no?

[azmanam]

I was going to talk about antarafacial sigmatropic rearrangments, but that's not what's going on here.

deuterium labelling study'll probably sort things out?  start with 2,2-dideutero-1-butanol.  if deuterium migrates to the primary carbon atom, it's carbocation rearrangement.  if it's elimination first, deuterium will either not be in the product or still on the internal carbon atom.  With a large enough excess of water, it's unlikely that the terminal alkene will be 'protonated' with HDO to give a false reading of deuterium at the primary position.

It's not that I think the carbocation won't rearrange... it's that I think the primary carbocation won't form to any appreciable extent.  I don't think there will be a carbocation to even engage in rearrangement.

[orgopete]

1-Bromobutane will give a large proportion of 1-methylpropylbenzene in a F.C. alkylation. Bromide is not a good leaving group in this reaction so a Lewis acid catalyst is added to pull electrons away from the primary carbon. I don't see how the bromine-Lewis acid complex is different from the protonated OH in pulling electrons away from the primary carbon. I am not convinced that one must be an E2-elimination simply because primary carbocations are unstable. I am not arguing that they are stable, just that because they are unstable, they can have many avenues of reactivity. A primary carbocation rearrangement could be common to both reactions for the formation of products derived from a secondary carbocation. As a lumper rather than a divider, I prefer common properties over special properties.

Without looking, I presume that neopentyl bromide has been solvolyzed and rearranges to the tertiary carbocation which leads to rearranged products. In terms of chemical education, I would use a primary carbocation as a presumed high energy and highly reactive intermediate.

[azmanam]

Huh.  after reading further, it does appear to be a 1,2 sigmatropic rearrangement.  They should be thermally suprafacial, and are predicted to be spontaneous.

Anyway,  I looked into it some more.  Results in image below.  The soph organic text we use here is Brown/Foote/Iverson.  They note two pathways for the dehydration of butene.  E2 or E1 (after a hydride shift).  But they don't propose a primary carbocation.  If that's the real pathway, then D should be incorporated at the primary position

MSU e-text discusses the solvation of neopentyl bromide and also invokes a 1,2-methyl shift... this time only after a primary carbocation has been formed (it's also where the suprafacial sigmatropic rearrangement is discussed). http://www.cem.msu.edu/~reusch/VirtTxtJml/rearrang.htm#top1

It would also make sense that elimination/rehydration wouldn't be the dominant pathway to the internal alkene.  That would seem to indicate that controlling the temperature/time of the reaction should favor more terminal alkene, which I don't think anyone's proposing.  I think the internal alkene will be the dominant dehydration product in this reaction at all temperatures, especially cool(er) temperatures.