For which of the following reactions would you expect eliminat...

Question

For which of the following reactions would you expect elimination to be more favored than substitution?

(c)

Accepted Answer

All right. Hi, everyone. So this question is asking us which reaction condition is more favorable to elimination over substitution. Here we have two different reactions which we'll discuss in greater detail in a moment. But option A says reaction one is more favorable. Option B says reaction two is more favorable. Option C says both reactions favor elimination over substitution. And option D says neither of the two reactions favors elimination but substitution. So a few notes to consider when it comes to elimination and substitution reactions recall that for elimination, we have E one and E two reactions. Both of these feature the formation of an all keen, but the mechanism is slightly different, right? When it comes to an E one mechanism, it is a multi step process that's characterized by the formation of a carbo cadion. So a carpal cion has to form in an E one reaction before the akin is produced. Now, on the other hand, an E two mechanism is concerted. In other words, everything is happening in one step. As for substitution, we have S and one and Essen too. Now SN one and E one mechanisms are similar in the sense that both are characterized by a multi step mechanism in which a chromo cion is formed and similar to E two, an sn two mechanism is concerted, meaning everything is happening in one step. Now, there are quite a few factors that give us information as to what reaction is favored in a given circumstance. We consider STAIC hindrance, the nature of the solvent, the nature of the substrate and the strength of the base or the nucleo file. Let's start off by examining the nature of the substrate for reactions. One and two, we have two ocho halides in which the halogen or bromine is attached to a secondary carpet. Now, a secondary Ocu Halide can favor either elimination or substitution. If the starting material was a primary alkyl Halide, then sen two would be favorable because there is less STAIC hindrance. On the other hand, a tertiary cul Halide would favor either acid one or E one due to the stability of a tertiary carbo cion. Now, on the other hand, we can also consider the strength of a nucleo file or a base. Both reactions in this case involve the use of sodium azide. Now, the presence of sodium indicates that this is going to be an ionic compound in which sodium has a positive charge and azide has a negative charge. So generally speaking, when the nucleo file or a base is considered negative or has a negative charge, it's considered to be quote unquote strong and generally tends to favor the bi molecular reactions. So that's E two or SN two. Now, depending on whether or not a certain compound is more effective as a base or nucleo file than elimination or substitution can be favored in that scenario. And we can also consider the effect of a solvent. In this case, both reactions involve dmso or dimethyl sulfoxide as are solvent. DMSO is a commonly used polar a product solvent which therefore once again favors either E two or SN two. So to break this tie, we have to introduce a bit more nuance into our conversation about the substrate. Earlier, I mentioned how steri hindrance can shift or rather favor one reaction towards another. So I'm going to go ahead and revisit that topic because the substrate of reaction one has an o substituent next to the secondary carbon that contains are bromy an additional o substituent. Whereas reaction to substrate does not, that is going to go ahead and allow us to discuss the concept of STAIC hindrance a little bit more, right? Or in more detail. And what I mean by that is recall first and foremost that an essence two reaction is favored by substrates that are less hysterically hindered. Because the first step of the mechanism involves the approach of the Nile towards the carbon that contains the leaving group. The easier it is for the nucleo to reach that particular carbon, the easier the S and two reaction becomes meaning it's more favorable. Now, even though the conditions discussed for reaction, one would generally favor SN two over E two, the presence of the additional alke group next to our leaving group introduces an added element of steri hindrance to the equation right to the mechanism itself. So what that means is that it would be more difficult for a Nile to approach the substrate of reaction. One because of that extra Ocu group blocking or making its approach more difficult. This makes the SN two pathway less favorable and therefore make E two a little bit more favorable, especially because aide is coincidentally a strong base. Now, on the other hand, the substrate of reaction two does not have this issue, right? It is not hysterically hindered because there are no additional all kill groups on adjacent carbons. So this means that A I in the case of reaction two can more easily approach the carbon that contains our leaving group which makes sad two more favorable in that case. So what is the bottom line here? The bottom line is that the substrate in reaction one is more hysterically hindered and therefore favors elimination as opposed to substitution. So there you have it, our answer is going to be option a in the multiple choice because reaction one is more favorable to elimination over substitution. And the reason for this is because the potential nucleo file would experience more STAIC hindrance as it approaches the substrate of reaction one as opposed to reaction two. So with that being said, thank you very much for watching and I hope you found this helpful.

For which of the following reactions would you expect elimination to be more favored than substitution?
(c)

Key Concepts

Elimination vs. Substitution Reactions

Zaitsev's Rule

Sterics and Nucleophilicity

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