AH Chemistry

• Organic synthesis is used to create molecules by design
• Synthesis in chemistry allows professionals to produce molecules that are needed as drugs or materials
• Synthesis can also improve the efficiency of established chemicals by testing and redesigning
• To propose a synthesis


  • Be familiar with named reactions taught in the higher course
  • Know what the reactants are
  • Know what the products are
  • Know how they are accomplished (reagents, mechanisms etc)
  • Know the limitations (conditions, multiple products, isomeric products, solvents etc)
• A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product
• Types of reactions


  • Addition
  • Condensation
  • Hydrolysis
  • Oxidation
  • Reduction
  • Substitution
  • Elimination
  • Acid/Base
• Reaction mechanisms of more complex substances can be clearly outlined and understood using the simple idea of functional groups and intermolecular bonding studied in the higher course
• Functional groups and simple chemical reactions learnt in higher chemistry in the context of simple families can be applied to more complex molecules
• Homolytic fission


  Bond breaking that produces free radicals, where each atom keeps the electrons it was contributing to the covalent bond
• Heterolytic fission


  Bond breaking where one of the atoms takes both the electrons being shared in the covalent bond, occurs when a polar bond is broken
• Alkanes are not very reactive due to the non-polar nature of their bonds, but they do react with halogens in the presence of sunlight or UV exposure
• Chlorination of methane (chain reaction)

  1. Initiation: Cl2 -> Cl• + Cl•

  2. Propagation: CH4 + Cl• -> CH3• + HCl, CH3• + Cl2 -> CH3Cl + Cl•

  3. Termination
• Polarity of carbon bonds influences how organic molecules will react
• Chlorination of methane is an example of a chain reaction
• Chain reaction


  1. Initiation

  2. Propagation

  3. Termination
• Substitution reaction


  The reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms
• The initiation step in the reaction between methane and chlorine is slow and is used to determine the rate of the reaction
• Alkenes


  • Contain at least one carbon to carbon double bond
  • Undergo sp2 hybridisation
• Synthesis of Alkenes


  1. Dehydration of alcohols

  2. Base induced elimination of hydrogen halides from monohalogenoalkanes
• Dehydration of alcohols


  OH group is removed along with a hydrogen atom on an adjacent carbon
• Phosphoric acid is preferred over sulfuric acid for the elimination reaction due to extra side reactions with sulfuric acid
• Base induced elimination of hydrogen halides to form monohalogenoalkanes


  Heating the monohalogenoalkane with ethanolic potassium or sodium hydroxide
• If there are more than three carbon atoms in the reactant which is undergoing elimination, alkenes may be formed
• Alkenes can react with hydrogen, halogens, hydrogen halides and water
• Catalytic addition of hydrogen (hydrogenation)

  Catalysed by nickel or platinum
• Addition of halogens to form dihaloalkanes

  Also known as halogenation, can be used to detect unsaturation
• Addition of hydrogen halides to produce monohalogenoalkanes

  Step 1: Electrophilic attack forming carbocation, Step 2: Nucleophilic attack
• Markovnikov's Rule


  The H atom of H-X adds to the carbon atom which already has more hydrogen atoms
• Addition of water (hydration) forming alcohols

  Acid catalysed, follows carbocation reaction mechanism, Markovnikov's Rule applies
• Hydrohalogenation and hydration are examples of electrophilic addition with a carbocation intermediate
• Halogenoalkanes


  Also known as alkyl halides, important in organic synthesis, contain a positively charged carbon vulnerable to nucleophilic attack
• Types of monohalogenoalkanes


  • Primary: carbon bonded to halogen is bonded to 1 other carbon
  • Secondary: carbon bonded to halogen is bonded to 2 other carbons
  • Tertiary: carbon bonded to halogen is bonded to 3 other carbons
• Elimination reactions of monohalogenoalkanes

  Reaction with alkali (dissolved in ethanol)
• Substitution reactions of monohalogenoalkanes

  Nucleophilic attack on the polar carbon-halogen bond
• Reaction with aqueous alkalis to form alcohols


  1. Br + NaOH-(aq) → R-OH + Na+Br-
• Reaction with alkoxides (dissolved in alcohol) to form ethers


  C2H5O-Na+(alc.) + C3H7Br → C2H5OC3H7 + Na+Br-
• Reaction with sodium cyanide to form nitriles


  Nitriles can then be hydrolysed to carboxylic acids
• Nucleophilic substitution reaction to form amines

  Reaction of monohalogenoalkane with ammonia
• It appears in the above mechanism that there are two different reagents but in reality it is the same reagent and therefore allows production of the ether
• Reaction with sodium cyanide to form nitriles

  1. Monohaloalkane reacted with sodium cyanide

  2. Production of the nucleophilic substitution reaction is a nitrile