Nucleophiles and Electrophiles Leaving Groups

Consider the essential principles of mechanism. Look for nucleophiles, electrophiles, and leaving groups. 

Memorization Task 13.1 (Check your work) Nucleophile, Electrophile and Leaving Group... 

Identify the nucleophile, electrophile, and leaving group in each of the following substitution reactions. Predict whether equilibrium favors the reactants or products ... 

Species such as Y-L can serve first as the addition electrophile and leaving group can serve later as the nucleophile. More addition examples can be found in Chapter 8. 

Note Color is used to indicate the electrophilic (blue), nucleophilic (red), and leaving-group (green) character of the reacting centers. Therefore, a color may switch from one group or atom to another as the reaction proceeds. 

It is important to be able to distinguish when an electron pair donor in a reaction is acting as a base or as a nucleophile. The following reactions are elementary reactions. In each case, determine whether the electron pair donor is acting as a Bronsted-Lowry base or as a nucleophile. Then, identify the acid, electrophile, and leaving group, as appropriate. Finally, use arrows to indicate the movement of electrons. 

This chapter has presented a large number of reactions. Yet most of them fall into one of two classes. A nucleophile or base reacts with a substrate containing a leaving group in either a substitution reaction, in which the nucleophile replaces the leaving group on the electrophilic carbon, or an elimination reaction, in which a double bond is formed between the electrophilic carbon and an adjacent carbon ... 

We shall discuss nucleophilicity and leaving group ability in more detail later. For the moment, the most important aspect is that the rate of an S]sj2 reaction depends on both the nucleophile and the carbon electrophile (and hence the leaving group). Changing the nucleophile or the electrophile changes the value of fc2. 

The imidazo[l,5-a]pyridine ring system has been successfully functionalized with various electrophilic reagents, metalated, and reduced in one case. No nucleophilic displacements of leaving groups have been reported on appropriate derivatives. 

If the mechanism is polar, determine the nucleophilicity, electrophilicity, and acidity of the atoms to which a bonds are made and broken. Under basic conditions, the first step is often deprotonation of an acidic atom, rendering it nucleophilic. Under acidic conditions, the first step may be protonation of a leaving group or a tt bond. 

Substitution —trade a nucleophile for a leaving group Elimination—lose a leaving group and usually a proton to form a pi bond Addition —add a nucleophile or electrophile or both to a pi bond Rearrangement—produce a constitutional isomer... 

The variety of the acids, bases, electrophiles, nucleophiles, and leaving groups may sometimes make an otherwise familiar reaction look unrecognizable. It is very important that you be able to recognize and classify groups into their respective generic classes. 

This section should close with a reminder about charge types. Almost all of the pathways have several charge tyjjes. Nucleophiles can be anionic or neutral electrophiles can be cationic or neutral and leaving groups can depart as anions or neutrals. Reactions catalyzed by a Brpnsted acid can often be similarly catalyzed by a Lewis acid or by a positively charged metal ion. 

Clearly, these data are in opposition to a quasicyclic mechanism (S i—Si, Scheme 24) (1, 2). In such a process, the electrophilic assistance of the counterion M+ determines the apical attack of the nucleophile with the leaving group in an equatorial position, and thus governs the stereochemistry. The use of a... 

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