How Organic Molecules Change
The vast array of organic compounds is made possible by a relatively small number of fundamental reaction types that allow for the modification of carbon skeletons and functional groups.
Major Reaction Types
1.Addition Reaction: Two or more molecules combine to form a larger one. This is characteristic of molecules with double or triple bonds (alkenes and alkynes). The pi bond breaks, and new atoms are added to the carbons.
Example: CH₂=CH₂ (ethene) + H₂ → CH₃-CH₃ (ethane).
2.Elimination Reaction: A single reactant splits into two products. This is the reverse of an addition reaction and is often used to form double or triple bonds.
Example: CH₃-CH₂Br → CH₂=CH₂ + HBr.
3.Substitution Reaction: An atom or group of atoms in a molecule is replaced by another atom or group. This is characteristic of saturated compounds like alkanes and alkyl halides.
Example: CH₄ + Cl₂ → CH₃Cl + HCl.
4.Oxidation-Reduction (Redox) Reaction: In organic chemistry, this often involves changing the number of C-O or C-H bonds.
Oxidation: Increases the number of C-O bonds or decreases the number of C-H bonds (e.g., converting an alcohol to a ketone).
Reduction: Decreases the number of C-O bonds or increases the number of C-H bonds (e.g., converting a ketone to an alcohol).
Key Players in Reactions: Nucleophiles and Electrophiles
Nucleophile ('nucleus-loving'): A reactant that is electron-rich. It has a lone pair of electrons or a pi bond and is attracted to positive charges. Nucleophiles are Lewis bases. Examples: OH⁻, NH₃, Cl⁻.
Electrophile ('electron-loving'): A reactant that is electron-poor. It is positively polarized or has an empty orbital and seeks electrons. Electrophiles are Lewis acids. Examples: H⁺, Br⁺.
Many organic reactions can be understood as an attack by a nucleophile on an electrophile.
A Common Mechanism: The SN2 Reaction
The SN2 reaction is a type of nucleophilic substitution.
S = Substitution
N = Nucleophilic
2 = Bimolecular (the rate-determining step involves two molecules: the nucleophile and the substrate).
Mechanism: The nucleophile attacks the carbon atom from the back side (opposite the leaving group) in a single, concerted step. As the new bond forms, the old bond to the leaving group breaks. This process causes an inversion of stereochemistry, like an umbrella flipping inside out in the wind.