[rickk443]

I just recently took an exam for O-Chem I and the question asked to draw the mechanism and major product for:
1-ethyl-2-iodocylcohexane (substrate) and MeOH w/heat (reagent).

I drew the E1 mechanism with the respective product. My professor marked it wrong and said a SN1 mechanism would dominate.

There is a problem in Organic Chemistry by: D. Klein that is almost identical; except EtOH w/heat is the reagent and they have the major product listed as E1.

Who is correct? Thanks in advance. 

[orgopete]

Ask your professor for a literature reference to the products in this reaction. Bring in your copy of Klein. Here is another reference.

"Since the SN1 and E1 reactions proceed via the same carbocation intermediate, the product ratios are difficult to control and both substitution and elimination usually take place."
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx3.htm#hal8

When I had problems that could be ambiguous, I asked students to draw the substitution or elimination product to avoid this issue.

In this case, as a secondary iodide, it will slow somewhat. So, I see heat is indicated. Heat increases elimination. If it were me, I'd argue this point.

[fledarmus]

Methanol is actually quite a bit better as a nucleophile than ethanol - I wouldn't be surprised if you got elimination for ethanol and substitution for methanol.

That being said, I would not like to have to predict which one I would get for either reaction - I would look it up or run it in the lab. I know, neither option is particularly useful on the test.

[rickk443]

Thanks for the reply, orgopete. Yeah, it's one of those questions where a more accurate answer would be to list both E1 and SN1 products, so I don't understand why he would specifically asked for one major product. I even took the MIT OpenCourseWare Exam#3 (Spring 2005) and they never asked a question such as this. If the reaction could produce multiple products, they just asked for the reaction mechanism for each product.

I don't think my professor really "gets it." I mean aren't we supposed to step back and look at the bigger picture (the reasons why we are learning all this). From what I understand, the more products (and increased difficulty in controlling the outcome of a reaction) makes a given reaction pathway more inefficient. Therefore, you need to learn how to spot them, so you can avoid them and only use them as a last resort.

Also, he said that EtO- (reagent) in EtOH (solvent) and a primary alkyl halide (substrate) will give a major product that follows an E1 (instead of SN2) mechanism. When we asked him to explain why, he said that EtO- is a strong base and not a good nucleophile (I thought it was both) and that the polar protic solvent makes it act more like a base. These reasons do make sense; however, I could not find any literature on this either and my textbook says only a strong bulky base (i.e. t-BuOK) follows this rule. Has anyone else heard of this?

[rickk443]

I see what your saying fledarmus. However, I think most of agree he should not be that technical when the ratio of products are so close.

This actually brings up another question (about nucleophilic strength). I actually have both the Klein and McMurry Organic Chemistry textbooks. Klein says that nucleophilic strength increases DOWN a group in a polar protic solvent and increases UP a group in a polar aprotic solvent. So, Klein is essentially saying that a smaller nucleophile is a stronger nucleophile. McMurry doesn't mention this, but says that, in general, nucleophilicity increases DOWN a group. Now, both of these make sense [1. (Klein) a smaller nucleophile is more unstable; therefore, more reactive and 2. (McMurry) a larger nucleophile is more electron-rich and is more reactive because its valence electrons are more loosely held]. But, don't both of these statements contradict each other?