Organic chemistry Assistance Lecturer Amjad Ahmed Jumaa  Nucleophilic aliphatic substitution.  Substitution nucleophilic bimolecular (SN 2 ).  The (SN 2 ) mechanism of nucleophilic substitution. 1

 Nucleophilic aliphatic substitutions:  Alkyl halides:  We shall consider as alkyl halides all compounds of the general formula R─X, where R is any simple alkyl or substituted alkyl group. For example.  Nucleophilic aliphatic substitutions:  Alkyl halides:  We shall consider as alkyl halides all compounds of the general formula R─X, where R is any simple alkyl or substituted alkyl group. For example.

 Alkyl halides are classified into primary, secondary, tertiary according to the degree of substitution of the carbon that bears the halogen atom(X).  RCH 2 X (1º) primary alkyl halide, example, ethyl bromide CH 3 CH 2 Br.  R 2 CHX (2º) secondary alkyl halide, example, 2-chloropropane, CH 3 CHBrCH 3.  R 3 CX (3º) tertiary alkyl halide, example, 2-chloro-2- methylpropane.  Alkyl halides are classified into primary, secondary, tertiary according to the degree of substitution of the carbon that bears the halogen atom(X).  RCH 2 X (1º) primary alkyl halide, example, ethyl bromide CH 3 CH 2 Br.  R 2 CHX (2º) secondary alkyl halide, example, 2-chloropropane, CH 3 CHBrCH 3.  R 3 CX (3º) tertiary alkyl halide, example, 2-chloro-2- methylpropane.

 Substitution nucleophilic bimolecular (SN 2 )  The term bimolecular is used here since the rate-determining step involves collision of two particles. For example: The reaction between methyl bromide and hydroxide ion to yield methanol.  Substitution nucleophilic bimolecular (SN 2 )  The term bimolecular is used here since the rate-determining step involves collision of two particles. For example: The reaction between methyl bromide and hydroxide ion to yield methanol.  When hydroxide ion collides with a methyl bromide molecule at the face most remote from the bromine, and when such a collision has sufficient energy, a C─OH bond forms and the C─Br bond breaks, liberating bromide ion. The (SN 2 ) reactions the order of reactivity of R─X is CH 3 X > 1º > 2º > 3º.

 The (SN 2 ) mechanism of nucleophilic substitution:  The reaction proceeds in one (single) step, the hydroxide ion acts as a nucleophile. While the C─Br bond is breaking, the C─O bond is forming. The reaction involves forming the transition state. Look at the following steps.  The hydroxide ion (OH - ) attacks the methyl bromide from the opposite side to form the transition state and the product methanol in one step.

The transition state: hydroxide ion (OH - ) attacks from the opposite the C─Br bond. The transition state in the following figure:

Problem: expect the product when 2-bromooctane treated with the sodium hydroxide in the presence of (ethanol-water) as solvent. Explain the transition state for the product. Solution : The transition state for the product:

Follow problem: write an equation for the reaction of 2- bromopropane with sodium hydroxide in the presence of the (ethanol-water) as a solvent, explain the transition state for the product.