Elimiation At Sp3 Centers Flashcards
What is sigma conjugation?
Delocalisation of electrons though sigma bonds
What are the 3 factors that decide whether it will be a substitution or elimination reaction (say the factors that favour elimination)
- Basicity (more basic is elimination)
- Size (larger/bulkier prefers elimination)
- Higher temperature prefers elimination
What is the order of reactivity for E1 and E2 reactions?
Tertiary, secondary, primary
Why is the order for E2 reactions substrates opposite to SN2
For E2, tertiary substrates have greater beta carbons, thus the no. of hydrogens increase and the rate of deprotonation increases. The more substituted alkene is the more stable, making the transition state and final product more stable (less energy required thus formed more rapidly)
What conditions favour E2 reactions
- Increasing concentration of base
- Stronger bases (more acidic more E1 since the protonation of a group can make the leaving group more stable such as H2O)
- No acid catalyst (those favour E1)
Why does the methyl group not go through elimination or SN1 reactions
Carbocation intermediate is too unstable (no sigma donation) for SN1, and no beta carbons to deprotonate for elimination reactions.
Why do elimination reactions favour high temperatures?
Gibbs free energy equation, temperature and entropy (entropy does not change for substitution while it increases for elimination reactions)
For E2 reactions, is the internal or terminal alkene bond more likely to be formed and why?
The more substituted alkene (internal bond) is the more favoured product because factors that stabilize the product (alkene) will also stabilize the transition state and a more substituted product is lower in energy which means lower activation energy required thus more favoured.
What is Hammonds postulate?
If two states, such as a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will result in only a minor reorganisation of molecular structures.
Why are more substituted alkenes more stable?
More substituted means more beta bonds (from neighbouring C-H or C-C) which can overlap favourably with the antibonding pi orbital of the C=C and stabilize it (as it forms a partial MO which is puts the electrons in a lower energy/more stable orbital)
What is the role of each step in determining the product of E1 reactions
Step 1: rate determining step (Ea)
Step 2: product determining step (which part is deprotonated)
what are the conditions required to form the less substituted alkene for E2 reactions?
changing to a bulky base, the steric hindrance prevents access to certain hydrogens (within a ring for eg.) and thus the base would more easily deprotonate a substituent which forms the less substituted product
can E1 and E2 reactions form a less substituted alkene?
E1: no, it always forms the more substituted product (due to the formation of a carbocation)
E2: yes with a bulky base
Which stereoisomer does E1 reactions prefer and why?
E alkene preferred due to less steric hindrance
What is a reaction that has 100% of one product over another called?
specific