Mechanisms of chemical reactions Flashcards
What do reaction mechanisms involve?
- Breaking of bonds in reactants.
- Sometimes formation of intermediates.
- Formation of bonds in products (or rearrangement of electrons).
- Occur in small steps involving either 1 (unimolecular) or 2 (bimolecular) species.
What are nucleophiles?
‘Nucleus lovers’. Electron rich species which often possess a lone pair of electrons or full negative charge. Can possess a C=C or benzene ring.
What are electrophiles?
‘Electron lovers’. Polar species which can possess a full positve charge, a partial positive charge or can be C atoms bonded to electronegative atoms such as the halogens.
What is a substitution reaction?
Replacing one single bonded atom/group with another. Number of electrons in the bonds remain unchanged.
What is an elimination reaction?
Removing 2 atoms or groups to form a C=C bonding a bond between the 2 removed atoms/groups. Product will lose 2 electrons. Called a dehydration when water is eliminated.
What is an addition reaction?
Adding 2 atoms/groups (electrophiles) to transform a C=C bond (nucleophile) into a C-C bond. Product has 2 more electrons. Called a hydration reaction when water is added.
What is a hydrolysis reaction?
Water is added to break a molecule in 2. Opposite is condensation.
What is a redox reaction?
The ‘energy reactions’ that make life possible. Reduction and oxidation.
When is curly arrow notation used?
To show rearrangement of electron pairs. Starts from a nucleophiles lone pair or midpoint of C=C bond and ends in space (forming a new bond to an electrophile) or on an atom (forming an anion). Cannot create or destroy electrons or charges.
When is fish hook arrow notation used?
In radical reactions to show rearrangement of single electrons. Can also point toward each other to show 2 radicals combining to form a bond.
Describe the SN1 nucleophilic substitution reaction mechanism.
Step 1 - Starting molecules is tetrahedral shape. Slow step where the C-Halogen bond is broken down to form an intermediate (carbocation). OH- not involved so the rate equation only involves the substrate. Carbocation is in trigonal planar shape.
Step 2 - The fast step where the C-OH bond forms on the opposite side to form a tetrahedral product.
Describe the SN2 nucleophilic substation reaction mechanism.
One slow step. Starting molecule is tetrahedral. C-Halogen bond is broken at the same time as the C-OH bond is formed. In the middle of the step a 5-coordinate bipyramidal transition state is formed. Since OH- is involved in the slow step, the rate depends on both the OH- and the substrate.
What happens when a chiral C carbon undergoes SN2?
The product is chiral but inverted, the ‘blown-out umbrella’ mechanism.
What happens when a chiral C carbon undergoes SN1?
The product is racemic (50/50 mix). The nucleophile can attack the carbocation from either way, giving a mix of R and S enantiomers.
How do you know whether a particular reaction will go via Sn1 or Sn2?
Halomethane and primary haloalkanes will go via the SN2 because the nucleophile has plenty of room fro the backside attack and the H atoms would not be very good at stabilising the carbocation that would occur in SN1. Tertiary haloalkanes will go via the SN1 pathway because the carbocation is stabilised by the partial electron donation from the 3 attached carbon atoms. The incoming nucleophile is blocked for the SN2 pathway.
