Reactions

Classification of Organic Reactions

There are several ways in which organic reactions can be characterized or organized.  While there are easily millions of different chemical reactions, we can organize them into a few groups with common attributes.  To be successful in organic chemistry you must be organized.

Classification by electron movement

We can divide all organic reactions into three main groups.

1. Polar reactions - electrons move or reorganize in pairs with two electrons per orbital.
   Most of the reactions you will encounter in organic chemistry are this type.  Acid-base reactions are a simple example of these.  We use curved arrows with a double arrowhead to show two electrons moving in polar reactions
   
   
2. Radical Reactions - single electron reorganization with some orbitals partially filled.
   You will examine a few radical reactions in this course.  Radical reactions proceed through three distinct types of reaction steps 1) radical Initiation, 2) radical propagation, and 3) radical termination steps.  Curved arrows for radical reactions have a single arrowhead to represent one electron moving.
   
   
3. Pericyclic Reactions - concerted reactions that proceed through a cyclic transition state.
   You will encounter a few pericyclic reactions in Organic Chemistry I and II.  The Diels-Alder Reaction is a type of concerted pericyclic reaction.  Concerted means that all bonds are formed and broken in a single step at the same time.  (Like a concert where all musicians play at the same time in concert).

Classification by Reaction Type

Reactions can be characterized by their reaction type or net outcome.

1. Addition Reactions - Two molecules come together to form one product.  This is an example of the electrophilic addition to alkenes.
   

   

2. Elimination Reactions - These are the reverse of addition reactions.  One molecule splits into two pieces.
   
   

   
   
   

3. Substitution Reactions - One group of atoms is exchanged or substituted for another.
   
   

   

4. Rearrangement Reactions - A single molecule rearranges.
   
   

Classification as Oxidation or Reduction

1. Oxidation reactions
   In general chemistry, you learn that when something is oxidized it loses electrons.  In organic chemistry oxidation is the loss (or decrease) in electron density.
   
   In an Oxidation reaction the number C-O, C-X or C-N bonds increases, while the number of C-H bonds decreases.
   
   
   
   Note:  The carbonyl carbon had 2 C-O bonds (i.e. C=O), now it has 3.  By replacing the H with a more electronegative O, the electron density decreases on this carbonyl carbon.
   
   
2. Reduction Reactions
   In general chemistry, you're taught that when something is reduced there is a gain of electrons.  In organic chemistry, a reduction is an increase in electron density
   
   In a reduction reaction the number C-O, C-X or C-N bonds decreases, while the number of C-H bonds increases.
   
   
   
   Note: The number of C-H bonds increased.  Since H is less electronegative than C the electron density on the C atoms increases.

Classification by Common Functional Group Type

Functional Class	General Structure	Characteristic Reactions
alkanes		Radical halogen substitution
alkenes		Electrophilic addition to C=C Oxidation and reduction
alkynes		Electrophilic addition to C≡C Oxidation and reduction
arenes (aromatics)		Electrophilic aromatic substitution (EAS) Nucleophilic aromatic substitution (NAS) reduction
alkyl halides		nucleophilic substitution (SN1/SN2) elimination (E1/E2)
alkenyl halides		Electrophilic addition to C=C Oxidation and reduction
aryl halides		Electrophilic aromatic substitution Reduction Organometallic reactions
alcohols		Nucleophilic substitution Elimination Acid/Base
ethers		Cleavage by HX
epoxides		Ring-opening (nucleophilic and acidic)
aldehydes/ketones		Nucleophilic addition Enols and enolate chemistry
carboxylic acid derivatives		Nucleophilic acyl substitution
amines		Acts as nucleophile or base
nitriles		Nucleophilic addition
nitro		Reduction