4. Substitution Flashcards
What is nucleophilicity?
How much atom/ion/molec wants to give extra e- to bond w/ nucleus (ie. They’re basically anions and theoretically Lewis base)
Protic vs aprotic solvents
free floating p+, reactive p+ (ie. water, alcohols); when H = connected to electroneg atom, good for Sn1 vs no free floating p+, nonreactive p+ (ie. diethyl ether); when H = not connected to electroneg atom, good for Sn2
Trend of nucleophilicity in protic vs aprotic solvent
I- > Br- > Cl- > F- vs opposite trend
Which type of carbocation are most reactive in Sn1?
tertiary carbocation b/c it is most stabilized with pos charge when alkyl groups leave. secondary carbocation can react too but it won’t be as stable as tert carbocation
Will a primary carbocation or methyl halide ever react in Sn1?
No b/c it’s not stable when alkyl groups leave
Steps of Sn1 mechanism
1) lose a leaving group => carbocation intermediate (that’s sp2 hybridized trig planar) + anion, this is the rate limiting step, 2) nucleophile attacks electrophilic carbocation, this is a fast step 3) racemic product b/c nuc can attack either side and not just backside attack. Watch out for acid/base mechanisms and enantiomer products
How to do solvolysis?
Do Sn1 mechanism but keep using same nuc to rearrange the charge of previous nuc
Steps of Sn2 mechanism
1) nuc does backside attack on reactant and leaving group leaves at same time => concerted rxn, 2) pentomeric transition state (aka C has 5 bonds during transition state), 3) product = inversion of configuration (ie. R reactant will become S product and vice versa). You need a strong nuc for Sn2 to occur
Sn1 summary
Reactivity: tert > sec > pri > methyl (tert first b/c it can stabilize cation)
Works better for polar protic solvents b/c they stabilize cation and anion
Rate = k[R-L] —> first order b/c rate limiting step involves only 1 molec
Sn2 summary
Reactivity: Methyl > pri > sec > tert (tert last b/c too much steric hindrance)
Works better with polar aprotic solvents b/c they help make strong nuc
Rate = k[R-L][nuc] —> second order b/c rate limiting step involves 2 molec
In what situations would secondary substrate would undergo Sn1 or Sn2?
Sn1: weak nuc and polar protic solvents
Sn2: strong nuc and polar aprotic solvents
If a nuc has 2 oxygens, which one is more nucleophilic?
Carbonyl O
4 major factors affecting nucleophilicity. Nucleophilicity trend on periodic table
Charge: more neg —> more nucleophilic b/c of inc e- density –> donate more e-
Size: the smaller, the better b/c less sterically hindered; BUT if nuc = neutral, then bigger size –> better nuc b/c you have more e- to donate
Electronegativity: less electronegativity —> better nucleophile b/c high electronegativity means that atoms = less likely to share e- density
Solvent: worse in protic solvents b/c the solvent can protonate or H bond w/ the nuc —> defeats the purpose of the nuc
Inc down and left
What’s a concerted rxn vs stereospecific rxn?
When a rxn consists of 1 step like Sn2 vs when configuration of reactant determined configuration of product like in Sn2 mechanisms b/c nuc does backside attack —> product is basically the inverted reactant
What makes a good leaving group? LG trend on periodic table?
lower pKa/weak bases (weak bases are most stable, low energy; it must accept e-), have to be stable once they leave molec; bigger = better b/c basicity dec as radius inc, more electroneg = better b/c LG will take e- and leave, forming resonance structures after leaving = better b/c resonance dec basicity. Inc down and right
