Electrophiles and leaving groups

An electrophile is a compound that has a relatively low energy empty orbital available for bond-making. An electrophile may be neutral or positively charged. There are three classes of electrophiles Lewis acid electrophiles, rr-bond electrophiles, and cr-bond electrophiles. 

When a nucleophile attacks a 77-bond electrophile, the 77 bond breaks, and the electrons move to the other atom of the 77 bond, which decreases its formal charge by 1. 

Excellent to good leaving groups Pkb Fair to poor leaving groups  

The relief of strain can increase a group s leaving ability. For example, the re-O 

Alkenes that are normally nucleophilic can act as electrophiles toward potent-enough nucleophiles, and alkenes that are normally electrophilic can act as nucleophiles toward potent-enough electrophiles. For example, alkyllithium compounds (RLi) add to the nucleophilic alkene ethylene, and the electrophilic alkene 2-cyclohexen-l-one adds Br2 across its C=C Trbond. These reactions provide additional support for describing alkenes as the chameleons of organic chemistry. 

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

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. 

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

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. 

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

We mentioned before that we need to consider four factors when choosing whether a reaction will go by an SnI or Sn2 mechanism. These four factors are electrophile, nucleophile, leaving group, and solvent. We will go through each factor one at a time, and we will see that the difference between the two mechanisms is the key to understanding each of these four factors. Before we move on, it is very important that you understand the two mechanisms. For practice, try to draw them in the space below without looking back to see them again. 

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 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. 

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