ORGANIC CHEMISTRY - CHAPTER 6: PROPERTIES AND REACTIONS OF HALOALKANES

Important Concepts

1. A haloalkane,commonly termed an alkyl halide, consists of an alkyl group and a halogen.

2. The physical properties of the haloalkanes are strongly affected by the polarization of the C – X 

bond and the polarizability of X.

3. Reagents bearing lone electron pairs are called nucleophilic when they attack positively polarized centers (other than protons). The latter are called electrophilic.When such a reaction leads to displacement of a substituent, it is a nucleophilic substitution.The group being displaced by the nucleophile is the leaving group.

4. The kinetics of the reaction of nucleophiles with primary (and most secondary) haloalkanes are second order, indicative of a bimolecular mechanism. This process is called bimolecular nucleophilic substitution (SN2 reaction).It is a concerted reaction, one in which bonds are simultaneously broken and formed. Curved arrows are typically used to depict the flow of electrons as the reaction proceeds.

5. The SN2 reaction is stereo specificand proceeds by backside displacement, thereby producing 

inversion of configuration at the reacting center.

6. An orbital description of the SN2 transition stateincludes an sp2-hybridized carbon center, partial bond making between the nucleophile and the electrophilic carbon, and simultaneous partial bond breaking between that carbon and the leaving group. Both the nucleophile and the leaving group bear partial charges.

7. Leaving-group ability, a measure of the ease of displacement, is roughly proportional to the strength of the conjugate acid. Especially good leaving groups are weak bases such as chloride, bromide, iodide, and the sulfonates.

8. Nucleophilicity increases (a) with negative charge, (b) for elements farther to the left and down the periodic table, and (c) in polar aprotic solvents.

9. Polar aprotic solvents accelerate SN2 reactions because the nucleophiles are well separated from their counterions but are not tightly solvated.

10. Branching at the reacting carbon or at the carbon next to it in the substrate leads to steric hindrance in the SN2 transition state and decreases the rate of bimolecular substitution.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE