[bobbym46]

Hello, I've recently been studying the various methods of alkene synthesis in class and cant seem to get these questions right on mastering chemistry.... I did the mechanisms out by hand and came up with these products no matter how many different times I try. Any help is appreciated.

Link to picture of problems http://imgur.com/a/WQw3u

[bobbym46]

I solved the rxn with 2-butanol heated with sulphuric acid by listing the major/minor zaitsev products with the non zaitsev one but still cant solve the other two and I feel like Im very close with this one but cant come up with the right answer ? how else would this even react
http://imgur.com/FxJr5V6

[orthoformate]

I solved the rxn with 2-butanol heated with sulphuric acid by listing the major/minor zaitsev products with the non zaitsev one but still cant solve the other two and I feel like Im very close with this one but cant come up with the right answer ? how else would this even react
http://imgur.com/FxJr5V6

have you discussed hydride shifting/alkyl shifting in lecture?

[bobbym46]

yes but for the one in the picture 2-bromo-2,3-dimethylbutane the carbons in the middle are both tertiary as well as the 2 on the end primary so no shift would occur right ? now im confused wouldnt the intermediate look like this and if the sodium hydroxide is heated wouldnt it favor elimination, which makes double bonds ?

[lb2]

No carbocation will form under strongly basic conditions such as in your second reaction. With a strong base, look for E2. With no strong base and a secondary or tertiary electrophile such as in your two other reactions, look for E1 (which means you would have to look into C+ rearrangements, but only if they can occur.)

[bobbym46]

how would that change the product for the second reaction ? its still would have a double bond in the middle ? and there are no cis/trans isomers so im just stuck on how its wrong.. Thank you for your advice i got the third one now It has 3 products, which took me awhile to visualize for some reason

[lb2]

It depends what the question is. If you are only looking for the major product in each reaction, then your original answers were all correct. If you are instead looking for all the possible products, then you need to look at (1) regiochemistry and (2) stereochemistry. In addition, if you can form carbocations, the regiochemistry can be influenced by C+ rearrangements.

[orgopete]

yes but for the one in the picture 2-bromo-2,3-dimethylbutane the carbons in the middle are both tertiary as well as the 2 on the end primary so no shift would occur right ? now im confused wouldn't the intermediate look like this http://imgur.com/525EiSW.png and if the sodium hydroxide is heated wouldn't it favor elimination, which makes double bonds ?

In lab, we did an experiment in which we added a t-butyl halide to a solution of NaOH/water/acetone with an indicator. We measured the time for which the HX neutralized the NaOH. Logically, you might think this is an E2 elimination reaction, but the kinetics are entirely E1. This simply means the solvolysis reaction is faster than the E2 elimination.

I'm presuming the products are 2,3-dimethyl-2-butene and some 2,3-dimethyl-1-butene. This is one of those sticky problems. What is the mechanism? This can be hard to know. However, I have argued that listing NaOH as a reagent MEANS than NaOH is a kinetically relevant reagent. That is, the rate depends upon the NaOH concentration. In that case, it should be a second order reaction. I guessing an additional factor can the the number of hydrogens that can be pulled. One for the main product and six for the minor product.

The way that I rationalize a reaction such as this one is that formation of a Zaitsev product means that even though the reaction may be second order, the polar solvent stretches the C-Br bond which gives it some E1-character. It is the E1-character that results in the more substituted product. In contrast, the reactions with KOtBu are in less polar or non-polar solvents. The reactions then behave more like an E2 reaction in which the most acidic hydrogen determines the product.

PS, I am not a fan of posting images in other websites as they may delete the images. See Additional Options to attach images.

[bobbym46]

I just cant figure out what product I am missing at all.... so frustrating

[orgopete]

Okay, everything I said is wrong. If it is an E1, then I'm guessing 2,3-dimethyl-2-butanol.

[AWK]

Okay, everything I said is wrong. If it is an E1, then I'm guessing 2,3-dimethyl-2-butanol.

Where is elimination?

[bobbym46]

If its a strong base I thought it was an E2, Im getting very confused with this question and only have 2 attempts left to answer it.. my answers look right, can anyone else suggest all of the organic products ? also doesnt heat favor elmination

[lb2]

@bobbym46,

This is an intro organic chemistry course and I'm pretty sure your instructor wants you to learn the "textbook" reactivity. In that sense, you are indeed right that with a strong base and a tertiary electrophile you should expect E2 products. So the two products that you drew are indeed the two and only "textbook" products that would be expected. I am an organic chemistry professor who has been teaching this material at university for over ten years to undergraduates: if my question were for students to list all the elimination products in the reaction that you showed, I would expect them to show the two products you found.

Problem is I don't know what the complete question in your problem is, and what your instructor wants you to write. As orgopete nicely describes, this kind of reaction is textbook E2, but it is well known (through fun experiments such as the one he describes) that this picture is a crude over-simplification and that a many reactions with tertiary electrophiles have much more E1 character (to the point of even forming the carbocation) than what can be discussed in an intro course.

So what am I saying? I don't know what your prof wants you to write but you've covered all E2. If that is not a complete enough answer for your instructor, then you can look into substitution products (through SN1?) or carbocation rearrangement products (the initially formed C+ in your problem shouldn't rearrange as it can't form a more stable one, but maybe that's still what your instructor is looking for when asking for ALL the possible products?)

Hope this helps.



@orgopete

That is a cool experiment. Way back when I was an undergraduate, we had a half-semester of looking into classic physical organic chemistry concepts. I don't remember exactly what the experiment was, but we had something similar and I just remember that it completely confused me at that time as I just couldn't appreciate that the complexity is what made it fascinating.

That whole half-semester was a lot of fun as the professor's intent was mostly to debunk some of those classic "myths". I remember we looked into why it makes no sense to propose the occurrence of real free carbocations in solution, why most SN1 reactions are only first-order because the second step is at saturation and how you can make certain SN1 reactions completely second-order, how emphasizing the quantitative treatment of pKa values is pretty terrible when trying to predict the reactivity or organic compounds outside of (1) aqueous solutions at (2) very low concentrations, etc. It was a lot of fun! Nothing I would do in my intro classes with my undergrads though.

As an aside, I do agree that the product distribution can, minimally, depend on the tiny differences of acidity when using hindered based like KOt-Bu, but would argue that the steric effects are by far the dominant factor.

Cheers!

[bobbym46]

Got the answer stupid software was bugged... Thank you for everyones help