Chemistry Mechanisms

Nucleophilic Substitution
1. During this type of reaction, the halogen from a haloalkane is replaced by a nucleophile.
  Anmerkungen:
  - A nucleophile is an electron pair donator,
  1. 1. The electrons of the nucleophile attack the slightly positive Carbon in the Carbon-Halogen bond.
    Anmerkungen:
    - Since the Halogen is more electroneqative than the Carbon, the electrons in the covalent bond tend towards the Halogen making the Carbon slightly positive.
    1. 2. The bond between the Carbon and Halogen break and a bond forms between the same Carbon and the nucleophile.
2. The conditions required for this type of reaction to take place is that it needs to be done under aqueous conditions and under reflux, if the nucleophile is a cyanide ion.
  1. If the nucleophile is a Hydroxide ion, then, it has to be done under distillation,
3. Examples of Nucleophiles are OH-, CN- or NH3.
  1. When it is an OH-, an alcohol is produced, and a halogen ion.
  2. If it is a CN-, a nitrile compound is produced.
  3. If it is a NH3, an amine is produced.
    1. Primary amines have one alkyl group, secondary amines have 2 alkyl groups and tertiary amines have 3.
      1. Ethylamine, Methylamine, Ethyldecylamine.
Elimkination reaction
1. 1. During this, a nucleophile attacks a Hydrogen from a Haloalkane.
  Anmerkungen:
  - The conditions are different.
  1. 2. The electrons move from the covalent bond between the C-H to the Carbon-Halogen bond.This is due to the high density of e-'s on the Hydrogen.
    1. 3. As a result, the electron in the C-Halogen bond obviously tends towards the Halogen and the bond breaks.
      1. 4. Double bond forms between the Carbons that have lost a Hydrogen and a Halogen.
        The products are an alkene, water and a halogen ion.
2. The conditions:
  1. The reaction has to be ethanolic and done under reflux.
Electrophilic addition
1. 1. During this, an electrophile attacks the double bond of an alkene.
  Anmerkungen:
  - An electrophile is an electron acceptor e.g. H+
  - A double bond is an area of high electron density.
  1. 2. An intermediate forms with a Carbocation in it.
    Anmerkungen:
    - A carbocation is a positive Carbon ion.
    1. 3. The rest of the electrophile is attracted the positive carbocation, and bonds form between these.
2. For example, when ethene reacts with a Hydrogen halide.
  1. The Hydrogen halide is an electrophile and is attracted to the double bond, breaking it.
    1. A single bond form between the Hydrogen of the Hydrogen halide and a Carbon. The second carbon forms a carbocation. This is an intermediate.
      1. Finally, the Halogen is attracted ato the carbocation so forms a single bond with it to form a halogenoalkane e.g. chlorethane.
Electrophilic substitution
1. This usually occurs between an electrophile and a benzene.
2. 1. Firstly, since the benzene ring is an area of high e- density, it attracts an electrophile e.g. A nitrate ion (NO2-).
  1. 2. The electrophile uses the delocalized e-'s in the ring to form a single bond with one of the Carbons. This makes the ring unstable so it becomes a positive cation.
    1. 3. To make the benzene stable again, electrons from one of the C-H bonds moves towards the ring, and the bond breaks.
      1. 4. The benzene is stable again but this time, the electrophile has substituted a Hydrogen.
        5. A hydrogen ion has been released and benzylic product forms e.g. nitrobenzene.

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