Reaction Mechanisms

• Draw the mechanism for the reaction of a nucleophile (either the cyanide ion or the hydroxide ion) with bromoethane.

  
  

  State what type of mechanism this is.

  Nucleophilic Substitution

  
• How can you tell that the mechanism will be nucleophilic substitution?

  1) The halogenoalkane is primary or secondary

  
  

  and

  
  

  2) The product is not an alkene.

  
  

  and

  
  

  3) If there is a hydroxide ion, it will be cold and in water.
• A species (called Y) has a lone pair of electrons and is being reacted with a halogenoalkane to produce something that is not an alkene. What is Y acting as?


  A nucleophile
• If sodium (or potassium) hydroxide is dissolved in cold water and mixed with a halogenoalkane. What is it acting as?


  A nucleophile
• During nucleophilic substitution, where must the arrows be drawn?

  1) From the middle of the lone pair to the partial positive charge

  
  

  2) From the middle of the C-X bond to the halogen atom

  
  

  3) For ammonia there is an extra step. The arrow must be drawn from the middle of the N-H+ bond to the nitrogen atom.
• Place the C-X bonds in order of how quickly they can undergo nucleophilic substitution (start with the fastest)


  1) C-I (fastest nucleophilic substitution reactions)

  
  

  2) C-Br

  
  

  3) C-Cl (slowest nucleophilic substitution reactions)

  
  

  4) C-F (unreactive so cannot undergo nucleophilic substition)
• What are the different compounds that can be made from a halogenoalkane using nucleophilic substitution?


  1) Primary amine → Secondary amine → Tertiary amine

  
  

  2) Alcohol → aldehyde → carboxylic acid

  
  

  3) Nitrile → Primary amine

  
  

  4) Nitrile → Carboxylic acid
• Draw the mechanism for the reaction of a nucleophile (either with ammonia or water) with bromoethane.

  
  

  State what type of mechanism this is.

  Nucleophilic Substitution

  
• Draw the mechanism for the synthesis of bromoethane from ethene.

  
  

  State what mechanism this is.

  Electrophilic Addition

  
• Draw the mechanism for making ethene from bromoethane

  
  

  State what mechanism this is.

  Elimination

  
• Draw the mechanism for making ethene from ethanol.

  
  

  State what mechanism this is.

  Elimination

  
• Draw the mechanism for making prop-2-ene from propan-2-ol.

  
  

  State what mechanism this is.

  Elimination

  
• During an elimination reaction, where must the arrows go?


  1) From the middle of the lone pair to a hydrogen that is attached to the carbon next to the C-X bond.

  
  

  2) From the middle of the C-X bond to the halogen atom
• What small molecule is always produced from elimination reactions?

  Water
• How can you tell that the mechanism will be elimination?


  1) There is a halogenoalkane and you are removing a hydrogen atom from it to make an alkene.

  
  

  2) The halogenoalkane is tertiary or secondary

  
  

  3) If there is a hydroxide ion, it will be hot and in ethanol.
• A species (called Y) has a lone pair of electrons and is being reacted with a halogenoalkane to produce an alkene. What is Y acting as?

  A base
• If sodium (or potassium) hydroxide is dissolved in hot ethanol and mixed with a halogenoalkane. What is it acting as?

  A base
• Show the mechanism for making CH3Cl using free radical substitution

  
• Define the term 'initiation' in free radical substitution.


  The breaking of the halogen-halogen bond to form two free radicals.
• Define the term 'propagation' in free radical substitution.

  Stage 1: Free radical takes a hydrogen atom from the hydrocarbon to form a stable compound and a new free radical.

  
  

  Stage 2: New free radical reacts with another halogen molecule to form a new stable compound and a new free radical.
• Define the term 'termination' in free radical substitution.

  Free radicals react together to give a compound with no unpaired electrons. This removes free radicals and prevents the chains from growing any more.
• State the three stages that occur in free radical substition

  1. Initiation

  2. Propagation

  3. Termination
• State the conditions and reagents that are needed for free radical substitution.

  1. Alkane

  2. Halogen

  3. UV light

  4. Above room temperature
• State what is meant by the term 'chain reaction' in free radical substitution.

  A self-sustaining series of reactions
• State what is meant by the term 'free radical'.


  A chemical species with an unpaired electron - usually highly reactive
• What does the dot on a free radical symbolise?

  An unpaired electron
• In free radical substitution, the halogen-halogen bond breaks homolytically. What does this mean?


  One electron goes to each of the halogen atoms.
• How do you know if the mechanism is nucleophillic substitution?


  If there is a halogen being added to UV light, and a halogenoalkane is being made
• Give two reasons why chain reactions not very useful.

  1) They produce a mixture of products.

  2) They can occur without light or high temperatures
• Explain why chain reactions are important.


  They remove free radicals from the atmosphere. Free radicals can attack ozone (O3) and lead to ozone depletion.
• What is ozone depletion?


  Ozone thinning rapidly due to chlorofluorocarbons breaking in the presence of UV radiation from the sun.
• What are chlorofluorocarbons?


  Halogenoalkane molecules in which all hydrogen atoms are replaced by chlorine and fluorine atoms.
• Describe a nucleophilic substitution reaction


  A reaction mechanism where a nucleophile attacks a polar molecule and replaces a functional group in that molecule
• Define the term 'nucleophile'.


  An electronic pair donor.
• What type of species act as nucleophiles?

  It is usually a negatively charged ion or has an an atom with a partial negative charge.
• Give three examples of nucleophiles and state which atom their lone pair of electrons is on.

  1) Hydroxide ion (lone pair on the oxygen)

  2) Ammonia (lone pair on the nitrogen)

  3) Cyanide ion (lone pair on the carbon)
• Define the term 'electrophile'

  An electron-pair acceptor
• Describe the mechanism of 'Electrophilic addition'


  A reaction mechanism where a C=C double bond in an alkene opens up and atoms are added to the carbon atoms
• During electrophilic addition reactions with H-X, where must the curly arrows go?


  1) From the middle of the C=C double bond to the partial positive charge on the hydrogen halide.

  
  

  2) From the middle of the C-X bond to the halogen atom

  
  

  3) From the negative charge on the halide ion to the positive charge on the carbocation
• Explain why halogens can act as electrophiles for electrophilic addition reactions.


  Electrons are constantly moving. At any time the halogen molecule could have an instantaneous dipole. The positive end of this dipole is attracted to the electron-rich double bond in an alkene.