14.2 Organic Reactions

Reactions that organic compounds undergo

• Organic reactions are classified using two ways:
  1. By the type of reagent used:
     • Nucleophilic
     • Electrophilic
  2. By what happens during the reaction:
     • Addition
     • Substitution
     • Elimination
• Some common reactions:
  -Free-radical substitution
  -Electrophilic addition
  -Electrophilic substitution
  -Nucleophilic addition
  -Nucleophilic substitution
  -Redox (oxidation and reduction)
  -Hydrolysis

[ Note : You do not need to be familiar with these names at this moment. ]

Homolytic and heterolytic bond fission

• Organic reactions involve the breaking of covalent bonds. There are two ways in which a covalent bond can be broken, by homolytic or heterolytic fission

• Homolytic fission is the breaking of a covalent bond in such a way that one electron goes to each atom, forming free-radicals.
• A free-radical is an atom or a group of atoms with an unpaired electrons formed from the homolytic fission of a covalent bond and is very reactive

• Heterolytic fission is the breaking of a covalent bond in such a way that both electrons go to the same atom, forming cation and anion

Curly arrows

• Curly arrows are used in organic chemistry to represent the movement of electron(s). It is often used in the presentation of an organic mechanism

• The tail of the arrow shows where the electron(s) originates from while the head shows the place where electron(s) is/are moved The electrons can either originate from a lone pair or from a bond.
• Half arrows show the movement of one electron while a full arrow shows the movement of two In the diagram above, the first is a half arrow while the second is a full arrow.
• Curly arrows can only be used to represent the movement of electron(s), they may not be used for other purposes to avoid  ambiguity

Nucleophiles and electrophiles

• A nucleophile is a species which contains a lone pair of electrons and isattracted to regions of positive charge or electron-deficient sites. They are often negatively-charged or carry a partial negative charge.
• Some examples of nucleophiles are NH3, CN⁻, OH⁻, Cl⁻, Br⁻ and H2O.
• An electrophile is an electron-deficient species which can accept electrons and is attracted to regions of negative charge or electron-rich sites. They are often positively-charged or carry a partial positive charge
• Some examples of electrophiles are H⁺, Br⁺, Cl⁺ and NO2⁺.