Practically, alkyl fluorides are not used for S N2 reactions because the C-F bond is too strong. This leads to the following reactivity order for alkyl halides Since the bond between the carbon and the leaving group is being broken in the transition state, the weaker this bond is the lower the activation energy and the faster the reaction. It turns out that the two factors lead to the same prediction for halogen leaving group ability: There are two main factors: The strength of the C-X bond, and the stability of the X group after it has left. As you may imagine, however, the nature of the leaving group is an important consideration: if the C-X bond does not break, the new bond between the nucleophile and electrophilic carbon cannot form, regardless of whether the substitution is S N1 or S N2. In our general discussion of nucleophilic substitution reactions, we have until now been designating the leaving group simply as “X”. The electrophile (a) Structure of the alkyl group So in general we want a strong nucleophile. If we have a strong nucleophile, the S N2 reaction will happen faster a weak nucleophile will react more slowly and may not even react. This is because the nucleophile is almost “naked” in aprotic solvents, whereas in polar protic solvents it is surrounded by a cage of solvent molecules. Regarding the solvent, polar aprotic solvents such as DMSO, DMF, acetone or acetonitrile are popular choices for S N2 reactions, because rates are generally faster than with polar protic solvents (water, alcohols, etc.).
Resonance – if the nucleophilic lone pair can be delocalized by resonance, it will make it less nucleophilic.Polar protic solvent, bigger atom is better polar aprotic solvent, smaller atom is better. Within a row – more electronegative atom => weaker nucleophile.Charge – negatively charged => stronger nucleophile.We summarized the main points from 6.5 as follows: Anything which removes electron-density from the nucleophilic atom will make it less nucleophilic. In section 6.5, we learnt what makes a nucleophile strong (reactive) or weak (unreactive). So what makes for a good S N2 reaction? We need to consider what makes a suitable nucleophile, and what makes a suitable electrophile. In practice, the rates of S N2 reactions vary enormously, and for a practicable procedure we need to make sure that the reaction will happen at a reasonable rate. Factors affecting the S N2 reactionĪs we saw in the previous section, in the S N2 reaction the rate of reaction depends on both the electrophile (usually an alkyl halide) and the nucleophile. In this section, we examine what factors will help an S N2 or S N1 reaction be successful. The reactants and conditions we use will depend on what we’re trying to do. When we want to make a chemical in a lab or on a chemical plant, we need to design the reaction so that it works well, and gives a good yield of the product in a reasonable time.
In the same way, the outcome of a reaction (such as nucleophilic substition) depends on many different things – reactants, solvent, etc. So there are many different factors that can affect your grade. As long as you do all of these things then you’re likely to pass (though I can’t give guarantees!).
If you want to do well in this class, there are several things you need to work hard at: Being attentive in class, studying the notes and this textbook (especially before exams), practicing problems, and completing the quizzes and homeworks. Designing a “good” nucleophilic substitution
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