To simplify the multitude of possible combinations of reactants, organic reactions fall under specific types.

  • Combustion
  • Substitution
  • Addition
  • Elimination
  • Oxidation/reduction
  • Condensation (dehydration synthesis)
  • Hydrolysis (hydration)
  • Polymerization

Another tool to simplify your task of predicting products is to remember that functional groups will always react the same way.

e.g. all 1o alcohols will react the same way

Some functional groups, because of their specific bonding, do not participate in all reactions.

Combustion

  • Oxygen is always a reactant
  • Complete combustion will produce CO2 and H2O
  • Incomplete combustion will produce a mixture of C, CO, CO2 and H2O depending on the availability
  • Alkanes are the least reactive of the hydrocarbons and alkynes the most reactive

Substitution Reactions

A reaction of alkanes or aromatics with halogens to produce alkyl halides or hydrogen halides.
Hydrogen atom is replaced by another atom or group of atoms.

  • Reaction with F2 is vigorous; Cl2 and Br2 require help

H   H                           heat         H  Br

H—C–C—H       +       Br2 >     H—C—C—H     +       HBr

H   H                                           H  H

If the mole ratio allows, more bromine can react with the bromoethane to produce dibromoethane.

Addition Reactions

  • Limited to alkenes and alkynes
  • Molecule such as hydrogen (H2) or halogen (X2) is added to a double or triple bond

(i)     Halogenation (with Br2 or Cl2) (propyne + Br2)

(ii)    Hydrogenation (with H2) (alkene + H2)

(iii)   Hydrohalogenation (with hydrogen halides) (1-butene +HBr)

(iv)   Hydration (with H2O) (2-butene + H2O)

When the reactants are non-symmetrical, the final product produced in greatest quantities can be predicted. The hydrogen atom will bind to the carbon that has more hydrogen atoms bonded to it. This is known as Markovnikov’s Rule.  The –OH or the –X will add to the more substituted carbon (i.e. has more carbons added to it).

Reactions of Benzene

  • Benzene undergoes a substitution reaction in the presence of a catalyst

Further brominations can produce 1,3-dibromobenzene

  • Reacts with nitric acid to form nitrobenzene. H2SO4 catalyst is required
  • Can also react with alkyl halide in a substitution reaction where the alkyl group displaces a hydrogen from the benzene ring.
  • This usually requires a catalyst (AlCl3)

Elimination Reactions

In this type of reaction, atoms are removed from a molecule to form a double bond.  This type of reaction is the reverse of an addition reaction.   One reactant breaks up to give 2 products.

Common Elimination reactions

  • Alcohols in a strong acid catalyst give an alkene and water
  • Alkyl halides to produce alkenes and a binary acid

Oxidation

  • A reaction in which carbon atoms form more bonds to oxygen (and less to hydrogen)
  • Formation of C=O bond is also classified as an elimination

E.g.   1o alcohol is oxidized to form an aldehyde

O

CH3-OH    +       [O]   =>     H-C-H

2o alcohol is oxidized to form a ketone

OH                                                       O

CH3-CH-CH3 +       [O]   =>     CH3CCH3

Aldehydes are oxidized to form carboxylic acids

Reduction

A reaction in which a carbon atom forms fewer bonds to oxygen or more bonds to hydrogen.

Clues!

  • C=O or C=C bond is reduced to a single bond
    • This also looks like an addition reaction.
  • Presence of strong reducing agents such al LiAlH4, H2/Pt or the symbol [H]

E.g.

  • Carboxylic acids to aldehydes
  • Ketones to 20 alcohols
  • Aldehydes to 10 alcohols

Leave a Reply

Be the First to Comment!

avatar
wpDiscuz