Organic Chemistry

Grignard reagents

45.1.3 Example

Predict whether the following reactions will undergo SN2 or SN1 and tell why.

1:

Figure 128

2:

Figure 129

3:

Figure 130

Answers:

1. 
   

   SN2. Good nucleophile, polar solvent.

   
   


2. 
   

   SN1. Tertiary carbon, polar solvent. Very slow reaction rate.

   
   


3. 
   

   SN2. Primary carbon, good nucleophile, nonpolar solvent.

   
   

45.2 Grignard reagents

Grignard reagents are created by reacting magnesium metal with a haloalkane. The mag-
nesium atom gets between the alkyl group and the halogen atom with the general reaction:

R-Br + Mg→R-Mg-Br

Gringard reagents are very reactive and thus provide a means of organic synthesis from
haloalkanes. For example, adding water gives the alcohol R-OH. Basic: R-X + Mg →
R-Mg-X For example (X=Cl and R=CH3): CH3-Cl + Mg → CH3MgCl (methylmagne-
siumchloride)

45.3 Elimination reactions

With alcoholic potassium hydroxide, haloalkanes lose H-X and form the corresponding
alkene. Very strong bases such as KNH 2 /NH 3 convert vic-dihalides (haloalkanes with two
halogen atoms on adjacent carbons) into alkynes.