SOCl 2В and PBr 3В for Conversion of Alcohols to Alkyl Halides.

Mesylates and Tosylates as Good Leaving Groups.

Mechanism and Stereochemistry of SN2 Reactions with Practice Problems.

The SN2 Mechanism: Kinetcis, Thermodynamics, Curved Arrows and Stereochemistry with Practice Problems.

All You Need to Know About the S N2 Reaction Mechanism.

Take Now Check Also inВ Nucleophilic Substitution Reactions: In addition to this, there is also a multiple-choice quiz on substitution and elimination reactions: The answers are available to registered members at Is it SN1 SN2 E1 or E2 Mechanism With the Largest Collection of Practice Problems Check these practice problems-there are plenty. There is always an exception, if not more than one.

DonвЂ™t take words like вЂњneverвЂќ вЂњalwaysвЂќ literally. Here we are only talking about general traits and major products. Note: In real life, a mixture of compounds is obtained in most cases. The Role of the Solvent in SN1, SN2, E1, and E2 Reactions The effect of the solvent on nucleophilicity and basicity is another factor (not the main though) to be considered. If it is a non-bulky base, look further into the substrate вЂ“ primary substrates do S N2, secondary and tertiary do E2 as the major mechanism. In summary, follow these steps to identify if the mechanism is S N1, S N2, E1, or E2:ДЎ) Determine if the base/Nu is strong or weakДў) If it is a strong, bulky base вЂ“ E2 only. Now, remember that non bulky indicates that both S N2 and E2 are possible, and hereВ you need to look at the substrate: if it is primary, you will get S N2, if it is secondary or tertiary, you get E2as the major product: Notice also that a non bulky strong base such as NaOEt would follow the ZaitsevвЂ™s rule in an E2 reaction. Other common bulky bases are DBU, DIPEA, and LDA. If it is a sterically hindered base, then it can only be E2 вЂ“ done: The basic nucleophiles are carbon, oxygen, or nitrogen-containing (bearing the lone pairs or the negative charge) species. If you react any of the non-basic nucleophiles with a substrate, you will only get an S N2 product, unless (there are always exceptions, sorry) it is a tertiary substrate, then S N2 is impossible, only S N1 (check this post on choosing between S N1 and S N2).

So, memorize this table with strong bases and nucleophiles: That is why if we classify the strong bases as bulky (sterically hindered) and non-sterically hindered, which helps us determine whether it will react as a base or a nucleophile. To understand this, remember that steric hindrance favors elimination over substitution for any mechanism. The difference is that the butoxide can only (take it as mostlyвЂ¦ I know) act as a base, but the ethoxide reacts as a base and a nucleophile depending on the substrate. Non-nucleophilic means most often it reacts as a base while nucleophilic base means it can react both as a base and a nucleophile and this where the problems start.ДЇor example, ethoxide and butoxide ions are (relatively) strong bases. Bases can be nucleophilic and non-nucleophilic. Strong bases take us to the right side (E2 or S N2). Notice now that we are already done with the left side of the flowchart and the only riddle is to choose between S N2 and E2. The reason why heat favors elimination over substitution is covered in this post вЂ“ scroll to the вЂњ Why does Heat Favor Elimination?вЂќ section. If heat is mentioned, then it is a hint to you that E1 elimination is the main mechanism in that reaction: And choosing between E1 and S N1 is easy вЂ“ the main factor is the heat. The weak reactants are mainly going to be the water and alcohols. That is if you know the most common strong and weak bases/nucleophiles (see below). Keeping this in mind, what we see is that essentially you will never have to choose between S N1 vs E2 and S N2 or E1 vs E2 and S N2. LetвЂ™s summarize this again: if strong вЂ“ S N2 or E2, if weak вЂ“ S N1 or E1.

If the base/nucleophile is weak, then the mechanism is unimolecular вЂ“ E1 or S N1. This only happens when the base (if the mechanism is E2) or the nucleophile (if S N2) is strong вЂ“ very reactive. Well, you first need to remember that S N2 and E2 are bimolecular reactions, meaning that the substrate (alkyl halide) and the base/nucleophile both participate in the rate-determining step. So, letвЂ™s get started and break this into choosing between two mechanisms rather than four. But remember, this is mastered only by doing lots of practice problems. If you are reading this post, welcome to the world of S N1 S N2 E1 E2 business! The good news is that you are not alone and that also, we are going to make it as easy as it can get.

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