Well, this question is both simple (being a 50/50 choice) and complicated (as they are expecting you to offer an explanation). I am going to try to guide you in thinking through the explanation without indicating which choice is correct -- we'll see if this is successful.... It's always better to give the student the tools to generate the answer, rather than simply answering the question.
This question is clearly from the section of Organic dealing with SN1/SN2/E1/E2 reactions. Its aim to to make sure you know which reaction conditions favor each of those four reaction mechanisms, so you can know which mechanism(s) is/are most likely to occur. You should visit the section of your textbook and/or lection notes which covers this material, so you know what you are expected to know, as different people emphasize different things, so my answer won't necessarily match exactly what your professor wants you to know. As such, I'm going for a more expansive explanation, to cover all the bases -- your professor may not need you to think this completely about this problem.
First of all, inspection of the desired product indicates clearly that the reaction must be some sort of substitution reaction (SN1 or SN2). If an elimination reaction were to have occurred, the product would have a double bond. As such, you want to pick the option which is MOST likely to undergo a substitution under the given reaction conditions (SN1 or SN2 -- you have yet to think through which of those two is more likely), and LEAST likely to undergo either E1 or E2 under these conditions. One of the two possibilities will give (at best) a mixture of substitution and elimination products, while the other option will give EXCLUSIVELY substitution, with NO POSSIBILITY of any elimination products forming.
There are many different conditions which can "tip the scales" in favor of one mechanism over the others -- some of which are not specifically indicated in the problem. I'll mention some of those first, for completeness sake, even though they won't help you decide here.
First, temperature. Although all reactions (both substitution and elimination reactions) occur more rapidly at higher temperatures, elimination reactions are favored more because they have a larger increase in entropy than substitution reactions do (not delving any deeper into that part right now). As a result, eliminations (E1 and E2) are favored at higher temperatures. This is usually not the deciding factor for E2 reactions -- other reaction conditions will usually dictate an E2 reaction. But it is often the ONLY reaction condition which favors E1 over SN1. All other reaction conditions which facilitate E1 also facilitate SN1. As a result, they often occur together, and temperature is the only condition that shifts the balance from the SN1 product (favored at lower temperatures -- when the other conditions favor SN1/E1) and the E1 product (favored at higher temperatures).
Second, solvent. Protic solvents -- usually water or alcohols like methanol or ethanol, though any molecule containing an O-H or N-H bond is considered protic -- accelerate both SN1 and E1 reactions, and slow down SN2 reactions a ton. If the solvent is A-protic (aprotic means "not protic", so the solvent does NOT have an O-H or N-H bond) -- so, THF, acetone, DMSO, acetonitrile, etc. -- the rates of SN1 and E1 are slower, while the rate of SN2 is MUCH faster. (The effect of solvent on E2 reactions is not zero, but much smaller than the effect on the other three mechanisms.)
Unfortunately, neither temperature nor solvent are indicated above.... But in a problem which DOES specify either solvent or temperature, you could use those factors to help you decide which mechanism is more likely for a given reaction.
The two factors which (very clearly) indicate which mechanism(s) would be favored for each option are 1) the substitution of the substrate, and 2) the nature of the nucleophile/base. The "substrate" is the molecule with the leaving group (in both cases here, iodide), and the "nucleophile" or "base" is, in these two cases, the negatively charged alkoxide -- methoxide in the first option, and, uh, 1-phenylethoxide in the second (deprotonated 1-phenylethanol).
The substitution of the substrate is probably the most dominant factor in determining which mechanism is favored. SN2 reactions go most rapidly when steric hindrance around the leaving group is minimized, so they are STRONGLY favored for methyl halides, quite fast for primary, much slower for secondary halides, and almost never occur for tertiary halides. This is because any substituents on the same carbon as the leaving group sterically hinder the "backside attack" required for SN2. Fewer substituents means faster SN2. The halide in the first option is secondary, which is not that great for SN2, while the halide in the second option is methyl, which means the SN2 reaction would be VERY fast.
SN1 and E1 reactions both proceed through a carbocation intermediate, so the effect of substitution is largely predicated by stability of the cation intermediate. Since more substituted cations are more stable, SN1 and E1 are fastest for tertiary halides, slow for secondary, and essentially never occur for primary or methyl halides. The only exception is when the cation is ALSO stabilized by resonance -- even a primary halide can undergo SN1 or E1 reaction if the cation is stabilized by resonance. The second option would go through an unstabilized methyl carbocation, which is EXTREMELY unfavorable, so it will NOT undergo any reaction by either SN1 or E1. The first option COULD undergo either SN1 or E1, since although secondary is typically not that great, it does have resonance stabilization in the cation formed.
For E2, the structure of the substrate is not as critical -- E2 reactions can happen for primary, secondary, or tertiary substrates. They tend to be less favorable than SN2 for most primary substrates, but MORE favorable than SN2 for most secondary substrates, and ALL tertiary substrates (since SN2 will not occur at tertiary). But obviously, in order for an elimination to occur, the substrate must have at least two carbons -- to form the new C=C pi bond! As such, a methyl halide CANNOT undergo E2 -- or E1 for that matter. So the second option CANNOT do E1 or E2 -- only SN1 or SN2. (You should begin to see the answer.)
Lastly, the nature of the base/nucleophile is important. Both SN2 and E2 require a strong nucleophile or base in order to occur. A poor nucleophile will undergo SN2 only very slowly, and a poor base will undergo E2 only very slowly. In contrast, the strength of the base/nucleophile has NO EFFECT on the rate of SN1 or E1, since the rate determining step does NOT INVOLVE the nucleophile or base. As such, both E1 and SN1 occur equally well in the ABSENCE of any strong nucleophile or base. In fact, I teach that SN1/E1 mechanisms are typically only the primary mechanism when there is no strong base or good nucleophile present. Since bases and nucleophiles are often very similar in structure, E2 and SN2 can often be in competition with each other. Some very sterically hindered bases will generally prefer E2 over SN2, and some polarizable, weak bases can do SN2 well without competition from E2. But in these two cases, the methoxide and 1-phenylethoxide are both strong bases AND good nucleophiles, so SN2 and E2 can compete. Of course, since the second option can’t do E2 (see above), that option would ONLY do SN2. But since there IS a strong base/good nucleophile present in each case, I would not expect SN1 or E1 to be the fastest mechanism.
Well, tying everything together, I guess here’s the answer – if you’ve read this far, you’re a champion…. Since a strong base is present, it is very likely to be SN2 or E2, much more than SN1 or E1. And the second option CANNOT do E2, since the substrate has only one carbon. As such, the second option is the clear winner. The first option COULD do either SN1 or E1, since it would go through a resonance stabilized cation. But that’s not likely to be the fastest mechanism, since the methoxide would likely cause SN2 or E2 to be faster. And since it is a secondary substrate, I would think SN2 would be pretty slow for steric reasons. As a result, the first option is most likely to undergo an E2 reaction, which would form a different product. This may occur as a competition with SN2, which WOULD form the desired product, but since the halide in secondary, that is not likely to be the major product, if it is formed at all. The second option can ONLY do SN2 – both E1 and E2 are impossible, since the substrate can’t even form a pi bond, and SN1 cannot occur, since the cation intermediate would be extremely unstable. And the SN2 mechanism WILL form the desired product.
Pamella P.
I see. Thank you very much!10/10/23