Nucleophiles, electrophiles, leaving groups Flashcards
nucleophiles
nucleus-loving (want dat + charge)
examples: anions, molecules with pi bonds, molecules with lone pairs (amides, water, carbonyls)
What factors decrease nucleophilicity?
electronegativity - more electronegative molecules hold their electrons more tightly due to smaller valence shells. and are not as available to participate in bonding.
steric hinderance - decreases molecule’s physical ability to participate in bonding.
acidic (protic) solvents - H+ ions protonate nucleophiles/shield negative portions from reactants
What factors increase nucleophilicity?
negative charge - deprotonation
What are the effects of protic and a-protic and nonpolar solvents on nucleophility?
protic - nucleophilicity increases down a group.
aprotic - nucleophilicity increases up a group.
nonpolar - will not dissolve a polar nucleophile
electrophiles
attracted to electron densities, can accept electrons to make bonds.
stronger electrophiles = more positive
examples: carboncations, carbonyls, haloalkyls, the carbons of alcohols
what makes a good leaving group?
takes electrons
stable after leaving
ex: halides, inorganic esters (phosphate), tosylates, water, alcohols, ethers
poor leaving groups
unstable in free form
strong bases, hydroxides, alkoxides, carbanions, hydride anions
nucleophile must be a stronger base than the leaving group
what is that rate-limiting step in an SN1 nucleophilic substitution?
leaving group leaves forming a carbocation
can be accelerated by a polar protic solvent
SN1
2 steps
leaving group leaves, forming carbocation, then the nucleophile attacks, forming the product.
can use bulkier groups
yields a racemic mixture b/c nucleophile can attack from either side
SN2
1 step
cannot use bulky reactants - steric hinderance makes nucleophilic attack difficult
Yields inverted stereochemistry
