For a specific substrate, that may have chance to go through any of the 4 reaction pathways. For this reason it seems rather daunting to suspect the outcome of a certain reaction. We will talk about the methods that have the right to be applied in addressing such problem, and also explain the reasonings behind.

You are watching: Determine whether each of the following reactions occur through an sn1, sn2, e1, or e2 mechanism.


It is an extremely important to know that the structural nature of a substrate (primary, secondary or tertiary) is the most crucial factor to recognize which reaction pathway it goes through. Because that example, primary substrates never ever go v SN1 or E1 because the major carbocations space too unstable. If the substrate might go with a couple of various reaction pathways, climate the reaction conditions, including the basicity/nucleophilicity that the reagent, temperature, solvent etc., pat the important function to determine which pathway is the major one. Our discussions thus will start from the different kind for substrates, then discover the problem effects on the substrate.

Methyl

This is the most basic case. Methyl substrate only go v SN2 reaction, if any kind of reaction occurs. Remove is not possible for methyl substrates, and no SN1 reaction either due to the fact that CH3+ is too unreactive to it is in formed, therefore the only possible method is SN2.

Primary (1°)

Primary (1°) substrates cannot walk with any unimolecular reaction, the is no SN1/E1, because primary carbocations are too stormy to it is in formed. Since primary substrates room very great candidates for SN2 reaction, so SN2 is the predominant pathway when an excellent nucleophile is used. The only exception is the when huge bulky base/nucleophile is used, E2 i do not care the major reaction.

Examples of reactions for primary substrates:


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Figure 8.4a reaction for main substrates

Secondary (2°)

It is most complicated or daunting to predict the reaction the a an additional substrate (2°), because all the pathways are possible. The reaction conditions then come to be very crucial factor. The full four species of reactions deserve to be separated into 3 pathway, the is:

E2: favored through a solid baseSN2: favored through a good nucleophile (relatively weaker base)SN1/E1: it is hard to different SN1 and also E1 completely apart, due to the fact that they both go through carbocation intermediates, and also are favored by negative nucleophile/weak base, because that example, H2O or ROH (solvolysis). Under together neutral condition, SN1 and also E1 usually happen together for an additional substrates, and also increasing the reaction temperature favors E1 over SN1.

It is relatively easy to separate SN2 and E2 pathways from SN1/E1, because both SN2 and also E2 require solid nucleophile or solid base that space usually negatively fee species, while SN1/E1 require neutral conditions.

In bespeak to identify SN2 native E2, we need to have the ability to determine whether a negatively fee anion is a solid nucleophile (for SN2) or a solid base (for E2)? all nucleophiles are potential bases, and all bases room potential nucleophiles, because the reactive part of both nucleophile and base is lone pair electrons. Even if it is an anion is a much better nucleophile or a better base depends on that basicity, size and also polarizability. Generally speaking, the family member stronger bases have actually the stronger propensity to act as base; and also relative weaker base, with little size and an excellent polarizability, have actually the far better tendency to act together nucleophile, view the list provided below.

Strong bases: OH–, RO–(R: small size alkyl group), NH2–

Good nucleophiles (relatively weaker bases): Cl–, Br–, I–, RS–, N3–, CN–, RCO2–, RNH2

Please note that bulky bases, such as t-BuO– and LDA, constantly favor E2 and generate elimination assets that follow Hofmann rule, because they are too large to do back-side strike in SN2.

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Examples of reactions for secondary substrates:

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Figure 8.4b reactions for second substrates

Tertiary (3°)

Tertiary (3°substrates execute not go with SN2 reactions because of steric hinderance. Therefore E2 reaction is the an option when strong base applied, or SN1/E1 pathway v neutral condition (poor nucleophile/weak base). Theoretically speaking, E2 and E1 claimed to provide the exact same elimination product. However, in order come synthesize an alkene from a tertiary substrate, it is a far better choice to usage a strong base the encourage E2 procedure rather go with E1. This is due to the fact that that E1 always combine along with SN1, and it is virtually impossible to avoid the substitution product.

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Figure 8.4c Reaction for tertiary substrates

The over discussions have the right to be summary summarized in the table below, complied with by number of examples. Come predict the reaction outcome, or to architecture synthesis path for a particular case, it is extremely recommend that you do the analysis by complying with the logics mentioned above, instead of simply refer come the table. Also, exercise makes perfect!

Substrate

Preferred Reaction Pathways

Methyl

SN2 reaction

Primary

Predominantly SN2 reaction;

Exception: E2 reaction because that bulky base


SN2 reaction with great nucleophile (e.g., RS–, RCO2–, etc)

E2 reaction with solid base (e.g., OH–, OR–)

SN1/E1 through neutral problem (e.g., H2O, ROH)

TertiaryE2 reaction with solid base (e.g., OH–, OR–)