Topic 7: Elimination and the Formations of Alkenes Flashcards
Can we tune the reactivity of an electron-rich compound to favour its selective function as a Brønsted base?
Yes, use steric crowding around the electron-rich centre; this tends to attenuate (reduce) nucleophilicity without having a significant impact on Brønsted basicity
Non - nucleophilic Brønsted bases
Allow elimination to occur not nucleophilic substitution
Electron rich
Commonly used Brønsted bases
Alkoxides and hydroxides, MOR / MOH [pKa (ROH) ~ 16–18, pKa (H2 O) ~ 14]
M = usually an alkali metal, Li, Na, K
NaOt Bu and KOt Bu are particularly widely used bases as t-butyl substituent ensure these alkoxides are very poor nucleophiles
In the case of 3° alkyl halides why does elimination often predominate
easier on Steric grounds for a molecule to function as a Brønsted base and abstract a proton from a b-carbon than it is for the
molecule to function as a nucleophile and attack the cationic carbon
Why is the alignment of orbitals so important in elimination
It ensures a smooth redistribution of electron density from the s C–H bonding molecular orbital (MO) to the empty p AO as the b-carbon undergoes rehybridization, from sp3 to sp2 , to form the new p-bond and this minimises the activation energy of this step
E1 reaction meaning
E signifies an elimination reaction
1 tells us that the reaction has a unimolecular rate-determining step
What’s special about an antiperiplanar arrangement in elimination
Adopting an antiperiplanar arrangement between the C–H and C–LG ensures a smooth redistribution of electron density as the reaction progresses along the reaction coordinate from the starting material to the alkene product. The energy of the T.S. is minimised
Movement of electron in E2
2 electrons in the sigma C–H bond being broken, are used to form the pi bond in the products
sp3 to sp2 hybridisation
Which mechanisms can primary alkyl halides undergo
SN2
E2
Which mechanisms can secondary alkyl halides undergo
SN1
SN2
E1
E2
Which mechanisms can tertiary alkyl halides undergo
SN1
E1
E2
In E1 mechanisms, which alkene is usually favoured?
Why?
more substituted
thermodynamic stability
3 bonds in alkynes
C-C sigma bond formed by overlap of 2 sp hybrid orbitals
2 C-C orthogonal pi bonds formed by side on overlap of p-oritals
2 C-H bonds formed by overlap of C sp orbital and H s orbital
Valence bond theory
Bonds are formed from the sharing of two electrons between atoms, and that the electronic structure of the two atoms
forming the bond is largely preserved. The bonds are formed by the overlap of two orbitals, facilitating the sharing of two
electrons between the atoms.
Which alkene isomer is usually more stable?
E
due to steric intractions
