PM1B- Stuff Hard To Memorise Flashcards
Discuss the stereochemical features of the E1 reaction and explain whether it is a stereospecific or stereoselective reaction.
As the carbocation has a finite lifetime, rotation about single bonds can occur and this may lead to mixtures of configurational isomers.
Can have either E or Z isomer.
It is generally true that E1 reactions make mostly E-alkenes as the transition state leading to its lower energy.
However some of the Z-alkene will also result and the reaction is stereoselective.
Compare and contrast E2 and SN2 mechanisms.
Rates of the E2 reactions are affected by the strength of the base employed which is similar to SN2 reactions where ROR is affected by the nature and concentration of the nucleophile.
Discuss the stereochemical features of the E2 reaction and explain whether it is a stereospecific or stereoselective reaction.
Elimination occurring from the lower energy staggered conformer rather than from the higher energy eclipsed conformer.
The attacking base and departing leaving group being as far apart as possible in the transition state for ANTI-PERIPLANAR ELIMINATION.
The relative configuration of the substrate controls the geometry of the alkene produced and the reaction is said to be stereospecific.
Difference between stereospecific reaction and stereoselective reaction?
A stereospecific reaction is one that leads to the production of a single isomer as a direct result of the mechanism of the reaction and the stereochemistry of the starting material.
The relative configuration of the substrate controls the geometry of the alkene product and the reaction is said to be stereospecific.
Whereas stereoselective reaction gives rise to one predominant product because the reaction pathway has a choice.
In E2 reactions, for acylic molecules, elimination takes place from either two limiting conformations.
What are the two limiting conformations?
which is generally favoured and why?
Anti-periplanar and syn- periplanar.
Anti-periplanar elimination is generally favoured due to:
Elimination occurring from the lower energy staggered conformer rather than from the Higher energy eclipsed conformer.
The attacking base and the departing leaving group being as far apart as possible in the transition state.
What are the factors that promote E1 mechanism as opposed to E2 mechanisms?
Similar to those the promote SN1 rather than SN2:
- Nature of the carbocation: tertiary > secondary> primary. This is due to hyperconjugation, i.e the donation of the electron density through the sigma bond framework. In E1 tertiary carbocations form more like than primary .
- Nature of the solvent: a good ionising, ion solvating medium will favour E1 but have little effect on E2 reactions.
- Use of a strong base/ high conc of base will enhance E2 reactions.
- Steric effects of the substrate have less effect on elimination reactions as the base attacks H not C.
What are the stereochemical consequences of SN1 reactions
The carbocation is planar.
The bonding electrons are in SP2 orbitals and there is an empty p orbital.
The nucleophile can attack from either face of the carbocation (front or back).
So when x,y and Z are different the product will be a mixture of 2 enatiomers.
As there is an equal probability of attack on either face, a racemic mixture results and loss of stereochemistry.
Are SN2 reactions stereospecific or stereoselctive?
Stereospecific
Structure and function topic:
What is the order of intensity for important interactions?
Which interactions are important for initial attraction?
1.Electrostatic
2.ion-dipole
3.Dipole-dipole
4.van der waals
Electrostatic and ion- dipole important for initial attraction as these are the ones that have an effect over the longest distance and it is therefore these that provide the initial attractive force.
Dipole-dipole, H bonding and van der waals hold molecule once in position.
Why do elimination and substitution reactions often compete ?
Since the reactive part of a nucleophile or of a base is an unshared electron pair, all nucleophiles are potential bases and all bases are potential nucleophiles.
Does tertiary structure of protein have intermolecular or intramolecular bonding and why?
Interactions between side chains therefore INTRAmolecular bonding
Does quaternary structure have intramolecular or intermolecular bonding ?
INTERmolecular bonding as quaternary is made up of several subunits of proteins.
What is the difference between a reversible and irreversible inhibitor and which is generally preferred? (4 marks)
A reversible inhibitor competes with the natural substrate, binding to the active site of enzyme more strongly to inhibit the enzyme. (1 mark). This is called competitive binding- the inhibitor has a similar shape to the substrate and competes with it.
Irreversible inhibitors form a permanent covalent bond to a key amino acid in the active site and permanently blocks the active site (1 mark)
Reversible inhibitors are preferred as adding more of the natural substrate will reduce/remove the inhibition effect (1 mark) whereas irreversible inhibitors are permanent, which, if they bind to non-target proteins or sites they can lead to greater side-effects (1 mark).
What is the difference between uncompetitive and non-competitive inhibitors?
Uncompetitive inhibitor bind reversibly to an enzyme when the substrate is already bound to the active site, they bind to the enzyme-substrate complex.
Increasing substrate conc has no effect on inhibiton.
Level of uncompetitive inhibition is dependent on sufficient levels of substrate to form the enzyme substrate complex.
Non-competitive inhibitors bind to allosteric sites to inhibit enzyme activity but DO NOT inhibit substrate binding.
What are receptors ?
Are protein molecules embedded within the cell membrane with part of their structure on outside of cell
