Elimination Reactions Flashcards
What do all elimination reactions include?
They all include loss of elements of the reactant and an introduction of an additional pi-bond in the product.
What is the removal of an H and a halogen called?
Dehydrohalegonation, obviously. Also, the halogen always gains some electrons and the hydrogen donates its electrons to form the pi bond.
What are alpha and beta carbons?
An alpha carbon is attached to the leaving group (the halogen typically) and a beta carbon is attached to the alpha carbon.
What are mono- di- tri- and tetrasubstituted alkenes?
They are alkenes with the number of R groups proportional to the prefix, so one R group is monosubstituted, two R groups is disubstituted, etc.
What does the number of R groups bonded to an alkene increase?
The stability of the molecule, because the increase in the number of sp3 electron-donating groups increases.
What favors the speed of an E2 reaction?
A very strong base such as -OH or -OR, a better leaving group (such as I instead of F), and polar aprotic solvents. This is basically identical to the SN2 mechanism. Also, the more sterically hindered the E2 reaction is the faster it will proceed (opposite to SN2 here), this is because it forms a more stable alkene (disub better than monosub)
What determines which E2 product will be present in higher quantities?
The one that is more stable (more substituted).
What does regioselective mean?
It means that one constitutional isomer is selected more than other when more than one is possible. Similar to stereoselective.
Which conformation is most common for an E2 reaction to take place?
The main molecule will be anti periplanar more often than syn periplanar when reacting. In both cases, however, all reacting atoms are aligned in a plane.
What are requirements of an E2 reaction for cyclohexanes?
Both the halogen and the hydrogen must be axial so that they are in the same plane. This means the reaction only occurs when the cyclohexanes are in the less stable state.
What type of solvent increases the rate of E2 reactions?
Polar aprotic solvents, because they do not hide the strong base required for this mechanism.
What solvents should you use when you only want elimination and not substitution?
DBU and DBN, because they are very hindered strong bases, and this causes SN2 not to happen at all.
What is the Zaitsev rule and what are its exceptions?
The Zaitsev rule states that when there are multiple beta-hydrogens, the most stable alkene will form in the highest quantity, ie the most substituted alkene.
How must the beta-hydrogen and the leaving group be oriented in order to undergo E2 elimination?
They must be anti periplanar to each other, and for rings they must both be diaxial.
