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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Reactions > SN2

SN2

Nucleophilic Substitution

Previously (Physical Properties of Haloalkanes), we learned that haloalkanes contain a polarized C-X bond, leaving a carbon that is partially positive and a halogen that is partially negative. 

  • An electrophile is an electron poor species that can accept a pair of electrons. A carbon that is connected to a halogen in a haloalkane, for example, is an electrophilic carbon.
  • A nucleophile is an electron rich species that can donate a pair of electrons. You will be exposed to many different kinds of nucleophiles throughout your course of study. Some nucleophiles will be negatively charged species; others will be neutral.
  • Nucleophiles can react with electrophiles. One way in which this occurs is through a process called nucleophilic substitution. In nucleophilic substitution reactions, an electron rich nucleophile bonds with or attacks an electron poor electrophile, resulting in the displacement of a group or atom called the leaving group.
  • Nucleophilic substitution of haloalkanes can be described by two reactions. These two types of reactions are shown in the diagram below. In the first reaction, a negatively charged nucleophile attacks the electrophilic carbon of a haloalkane. Upon attack, the leaving group, which is the halogen of the haloalkane, leaves. The end result is a neutral R-Nu species and an anion. In the second reaction, a neutral nucleophile attacks the electrophilic carbon of a haloalkane. The end result of this attack however, is positively charged product and an anion.

Haloalkanes 02.bmp

Examples of Negatively Charged and Neutral Nucleophiles

6.7 Haloalkanes 06.jpg

Now let's look at two actual examples of these two general equations. In the first reaction shown below, the negative nucleophile, hydroxide, reacts with methyl iodide. Hydroxide takes the place of the leaving group, iodide, forming neutral methanol and an iodide ion. This reaction is the same as the first type of nucleophilic substitution shown above. In the second reaction shown below, the nuetral nucleophile, ammonia, reacts with iodoethane. Ammonia takes the place of the leaving group, iodide, forming the positively charged product, ethylammonium iodide, and an iodide ion. This reaction is the same as the second type of nucleophilic substitution shown above.

Haloalkanes 03.bmp

 sn2.gif           ammonia+hcl.gif

Figure 2. SN2 reaction of methyl chloride and hydroxide ion (left) and ammonia reaction with HCl (right).  

Four Factors to Consider in Determining the Relative Ease at Which SN2 Displacement Occurs

Next section: Using Electron-Pushing Arrows

Contributors

  • Rachael Curtis (UCD)

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Last Modified
12:13, 3 Jul 2014

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