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Organic Reactions

Cracking

In petroleum refining, cracking is a process by which heavy hydrocarbon molecules are broken up into lighter molecules by means of heat and usually pressure and sometimes catalysts. Cracking is the most important process for the commercial production of gasoline.

Example:

C22H44 => C12H20 + C10H24

C17H36 => C9H20 + C8H16




Reforming

Reforming is a processing technique in which the molecular structure of a hydrocarbon is rearranged to alter its properties. The process is frequently applied to low-quality gasoline stocks to improve their combustion characteristics. Opposite to cracking, reforming is used to create larger molecules of hydrocarbons from smaller ones.

Example:

C5H12 + C5H12 => C10H22 + H2




Combustion

Combustion Combustion is a rapid chemical reaction between substances that produces carbon dioxide, water, and energy in a form of heat and/or light.

Example:

2C8H18 + 25O2 => 16CO2 + 18H2O + energy




Addition (Hydrogenation)

Hydrogenation is an addition reaction involving the combination of hydrogen with unsaturated hydrocarbons. Unsaturated organic compounds have at least one pair of carbon atoms bonded by a double or triple bond, alkenes and alkynes. Addition of hydrogens to alkenes and alkynes are called saturation reactions because the reaction causes the carbon atoms to become saturated with the maximum number of attached groups.

Example:

     H H H H              H H H H 
     | | | |              | | | |
   H-C=C-C-C-H + H-H => H-C-C-C-C-H
         | |              | | | |
         H H              H H H H
   
    
  butene   + 2H  =>   butane
   

   
   
   C3H4     + 2H-H => C3H8  
   
propyne  + 2H-H => propane



Aside from alkenes and alkynes, halides also have the ability to add halogens to the carbons of double or triple bonds thus creating organic halides.

Example: 

   
     H H                H H
     | |                | |
   H-C=C-H + Cl-Cl => H-C-C-H
                        | |
                       Cl Cl
   
   
ethene  + chlorine => 1,2-dichlororethane

   
   
   C2H2 + Cl-Cl => C2H2B2
   
ethyne + bromine => 1,2-dibromoethene

Still unsaturated, 1,2-dibromoethene could undergo another addition reaction to produce 1,1,2,2-tetrabromoethane. 

   
    Br Br                              Br Br
     | |                                | |
   H-C=C-H         +     Br-Br   =>   H-C-C-H
                                        | |
                                       Br Br
     
   
1,2-dibromoethene + bromine => 1,1,2,2-tetrabromoethane




Substitution

Substition reaction is a chemical reaction that breaks apart a carbon-hydrogen bond and replaces the hydrogen atom with another atom or atom group.

Example:

     H H H                 H H H
     | | |                 | | |
   H-C-C-C-H + Br-Br  =>  H-C-C-C-H  +  H-Br
     | | |                 | | |
     H H H                Br H H
   
   
propane + bromine => 1-bromopropane + hydrogen bromide




Elimination

Elimination is an chemical reaction in which a pair of atoms or groups of atoms are removed from a molecule by reacting it with an acid or a base. For organic reactions, an alkyl halide reacts with a hydroxide ion to produce an alkene and therefore removing a hydrogen atom and a halide ion from the molecule.

Example:

     H H H                               H H H
     | | |                               | | |
   H-C-C-C-H      +  OH            =>  H-C=C-C-H  +  H-O  +  Br
     | | |                                   |         |
     H BrH                                   H         H
   
   
2-bromopropane + hydroxide ion  =>  propene  +  water + bromide ion

Alcohols could also undergo elimination reactions by reacting with an acid to remove a hydrogen atom and a hydroxyl group thus producing alkenes.

Example:

    
     H H                  H H
     | |                  | |
   H-C-C-H  +  acid =>  H-C=C-H  +  H-O 
     | |                    |         |
     H OH                   H         H
   

   ethanol  +  acid =>  ethene  +  water


Condensation

Condensation is an organic reaction when two molecules combine, usually in the presence of a catalyst, with the elimination of water or some other simple molecule. Catalysts commonly used in condensation reactions include acids and bases. The combination of two identical molecules is known as self-condensation. This process forms larger molecules, many of which are useful in organic synthesis. Aldehydes, ketones, esters, alkynes, and alcohols are among several organic compounds that combine with each other to form larger molecules.

Example:

   CH3OH   +   CH3OH   =>   CH3OCH3   +   HOH
   
methanol  +  methanol  =>  methoxymethane + water


For a carboxylic acid undergoing condensation reaction, it combines with another reactant, forming two products - an organic compound and the byproduct of water. In an event when a carboxylic acid reacts with an alochol to produce an ester and water, this process is called esterification.

Example:

   
             
   CH3COOH    +    HOCH3   =>   CH3COOCH3    +    HOH
   
ethanoic acid  +  methanol  =>  methyl ethanoate  + water




Addition Polymerization

Polymerization is a process in which very small molecules, called monomers, combine chemically with each other to produce a very large chainlike molecule, called a polymer. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. The monomers react to form a polymer without the formation of by-products such as water. The structure has one structural unit, or monomer, that occurs repeatedly. Through polymerization of ethylene (ethene), CH2CH2, the structure of the polymer can therefore be represented by -(CH2CH2)n- Where n can be several thousand. Because of this, polymers have incredible molar masses up to millions of grams per mole.

Example:

   
      H H     H H     H H        H H H H H H 
      | |     | |     | |        | | | | | |
      C=C  +  C=C  +  C=C  =>   :C-C:C-C:C-C:     OR     -(CH2CH2)n-
      | |     | |     | |        | | | | | | 
      H H     H H     H H        H H H H H H
             
         ethylene          part of polyethylene




Condensation Polymerization

In condensation polymerization, two functional groups of two different monomer molecules are joined together which produces a small molecule such as water. The monomers bond at where the hydrogen atoms were taken out to produce water. In order to become a condensed polymer, the monomer molecules must have at least two functional groups. The combination of two identical molecules is known as self-condensation. The reaction between a carboxylic acid and an alcohol creates an ester. If the carboxylic acid and the alcohol were the monomers of the polymer, during polymerization, they would create polyester, and produce water. The polymerization of a carboxylic acid and an amine similarly creates polyamides.

Example:

   COOHC6H6COOH     +     HOCH2CH2OH   =>    COOHC6H6OCH2CH2OH  +  HOH
   
1,4-benzenedioic acid   +   1,2-ethandiol   =>  polyester  + water