Carbon-carbon bond formation

• reactions that form carbon-carbon bonds have a central importance to organic chemistry as they provide a means of synthesising new compounds containing more carbon atoms
• formation of nitriles:

  • the nitrile group is the organic functional group -CN
• nitriles from haloalkanes:

  • can be formed by reacting the haloalkanes with sodium cyanide, NaCN, or potassium cyanide KCN in ethanol
  • in this reaction the length of the carbon chain is increased
• CH3CH2CH2Cl + KCN = CH3CH2CH2CN + KCl
• reaction follows a nucleophilic sub mechanism
• nitriles from aldehydes and ketones:

  • aldehydes and ketones will react with hydrogen cyanide in a nucleophilic addition reaction
  • reaction forms c-c bond, thus increasing the number of c atoms in the molecule
  • CH3COCH3 + HCN = CH3C(OH)(CN)CH3 - hydroxynitrile (cyanohydrin)
• • hydrogen cyanide too poisonous to se and increased reaction rate can be obtained in the presence of cyanide ions
  • mixture of sodium cyanide ad sulfuric acid is used - improves safety and increases reaction rate
• nitriles are useful intermediates in the synthesis of other organic compounds such as amines and carboxylic acids
• reduction of nitriles:

  • can be reduced to amines by reacting hydrogen in the presence of a nickel catalyst
• hydrolysis of nitriles:

  • undergo hydrolysis to form carboxylic acids by heating dilute aqueous acid, e.g. HCl(aq)
• forming c-c bonds to benzene rings:

  alkylation:

  • reaction that transfers an alkyl group from a haloalkane to a benzene ring
  • takes place in the presence of a Friedel-Crafts catalyst such as aluminium chloride
• acylation:

  • when benzene reacts with an acyl chloride in the presence of an aluminium chloride catalyst - ketone is formed
  • acylation
  • useful in synthesis as the organic product undergoes the typical reactions of a ketone
• organometallic compounds in c-c bond formation:

  • compounds that contain a carbon-metal bond = organometallic compounds
  • very important for forming c-c bonds in organic compounds to increase the length f a carbon chain
  • containing Mg or Li often used for this purpose
  • an organometallic compound provides a source of a nucleophile called a carbanion - contains c atom with neg charge
  • carbanion reacts with pos charged or delta+ carbon atoms to form a new c-c bond
• Grignard reagents:

  • organometallic compounds called Grignard reagents are made by reacting Mg with an alkyl or aryl haloalkane dissolved in an ether solvent
  • CH3CH2Br + Mg = CH3CH2MgBr
  • this Grignard reagent reacts as the carbanion CH3CH2-
• aldehydes and ketones can be reacted with Grignard reagents, followed by dilute aqueous acid, to form alcohols with a longer carbon chain e.g. butanal: CH3CH2CH2CHO can form a secondary alcohol

  • the mechanism for this reaction can be shown using a carbanion
  • similar to the nucleophilic addition of carbonyl compounds by hydride ions, H- or cyanide ions, CN-
• Hydroxynitriles in nature:

  • species of millipede, Apheloria corrugata stores 2-hydroxy-2-phenylethanenitrile in one of its body segments
  • when attacked the 2-hydroxy-2-phynylethanenitrile is released from a storage chamber where it mixes with an enzyme.
  • the reaction between the enzyme and the hydroxynitrile releases very toxic hydrogen cyanide, together with benzaldehyde
  • the millipede secretes this toxic mixture from glands located in its body segments
  • an adult millipede can produce 0.6mg of HCN - enough to kill ants and deter most other enemies