Hard vs. Soft Nucleophiles & Electrophiles

Question 1

electrophile define

Answer 1

accept electron pair to form chemical bond in a reaction

electrophiles: molecules w/ FULL or PARTIAL POSITIVE CHARGE or ATOM W/OUT AN OCTET OF ELECTRONS

Question 2

relative electrophilicity

Answer 2

empty orbital > π* orbital > σ* orbital
HOMO = Nu lone pair orbital
LUMO = empty p orbitals on atoms, C=O π’s or C-Hal σ’s

Question 3

relative nucleophilicity

Answer 3

-ve charge > lone pair > π bond > σ bond

LUMO: Empty orbital on E+

HOMO= Nu lone pair orbital:
π & σ bonds are HOMOs
for l.p. and -ve charge HOMO = l.p. HAO sp3 etc.

Question 4

define nucleophile

Answer 4

donates electron pair to electrophile, forming chemical bond

nucleophiles = all molecules with a FREE ELECTRON PAIR or AT LEAST ONE π BOND

Question 5

large nucleophiles…

Answer 5

react v. quickly w/ saturated C but poorly w/ C=O
have high energy lone pairs (high HOMOs)
often uncharged or charge spread diffusely over large orbitals (typically filled 3p or above)
eg. RS-, R3P, bromide, iodide

Question 6

small nucleophiles…

Answer 6

attack C=O rapidly
electrons concentrates close to the often ∂- nucleus (low HOMOs)
usually charged + small (typically not above 2p occupancy) - high charge density
e.g. OH-

Question 7

soft nucleophiles…

Answer 7

large typically and ‘fluffy’
high HOMO
so small HOMO-LUMO gap…
…so dominated by FMO (HOMO-LUMO/ frontier molecular orbital) interactions
low charge density means electrostatics = unimportant

Question 8

hard nucleophiles…

Answer 8

small typically and ‘punchy’
low HOMO
larger HOMO-LUMO gap…
so cannot be dominated by FMO interactions
high charge density ==> electrostatics much more important…
… reaction driven by electrostatics but orbitals still used for reaction to occur