AS/ A-level Chemistry: Nucleophilic Substitution

In this chapter of AS/A-Level Chemistry, we will learn about Nucleophilic Substitution (S_N).

Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens

• The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

In AS/A-Level Chemistry, halogenoalkanes can be hydrolysed by OH^- ions by nucleophilic substitution. This is where a nucleophile (e.g. :OH^-) attacks a molecule and is swapped/substituted for one of the attached groups (e.g. Br ^δ-). In this case the Carbon (C^δ+) to Halogen (X^δ-) bond is POLAR as halogens are much more electronegative than the carbon so they draw in electrons making the Carbon slightly/delta positive. The bond looks like this:

C^δ+ — X^δ-

Thus, the carbon can be easily attacked by a nucleophile who likes positive areas. This mechanism occurs:

*C-Br bond breaks heterolytically (unevenly)

• Primary – react by SN2 where 2 molecules/ions are involved in the rate determining step
• Secondary – react by SN1 and SN2
• Tertiary – react by SN1 where 1 molecule/ion is involved in the rate determining step

In AS/A-Level Chemistry, You can see by the rate equation if there are 1 or 2 molecules in the rate determining step, which in turn, tells you if the mechanism is SN1 or SN2.

EXAMPLE:

Rate = k[X][Y] = 2 molecules in rate determining step = SN2 = primary/secondary halogenoalkane

OR

Rate = k[X] = 1 molecule in rate determining step = SN1 = tertiary/secondary halogenoalkane

• The nucleophile in this reaction is the cyanide, CN^- ion
• Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
• The product is a nitrile
  • For example, bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile

• The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain
• When it comes to AS/A-Level Chemistry, This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material

• Refer to AS/A-Level Chemistry, It is very important that the ammonia is in excess as the product of the nucleophilic substitution reaction, the ethylamine, can act as a nucleophile and attack another bromoethane to form the secondary amine, diethylamine

That's it! You got it!!

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