OCAASL S1: Chapter 9 - Substitution Reactions

Draw S_N2 process

What are the two functions of the LG?

1) Withdraws e^- density from C atom it’s attached to → makes C electrophilic

2) stabilises -ve charge after being expelled

Why is S_N2 called S_N2?

RoR is dependent on concentration of both Nu^- and E⁺ i.e. 2nd order rxn

Draw mechanism for S_N1.

Why does S_N1 reaction result in both enantiomers?

Occurs via trigonal planar intermediate - Nu can attack from both sides (unlike S_N2).

Why is S_N1 called S_N1?

Rate only dependent on the E⁺

What is referred to as the substrate in substitution and elimination reactions?

The ELECTROPHILE!

Which substrates work better for S_N2?

1º - less steric hindrance for the Nu^- to attack at electrophilic centre

What’s most important in S_N1 reactions?

Stability of CC (formed in rds)

Which substrates work best (and which don’t) for S_N1?

3º - alkyl groups (EDGs) stabilise the CC

1º substrates are usually unreactive.

Which rxn will occur with 1º or 2º substrate?

Most likely S_N2

Which rxn will occur with 3º substrate?

Most likely S_N1

Other than alkyl groups, how else can CCs be stabilised?

Resonance!

Look out for LGs in benzylic or allylic positions → will generate resonance-stabilised CCs after LG leaves.

Which type of Nu^- favours S_N2?

Strong Nu^- favours S_N2.

(Weak one disfavours it.)

Which type of Nu^- favours S_N1?

None. S_N1 is only dependent on substrate conc - not Nu^-.

How does Nu^- type affect competition between S_N1 and S_N2?

Strong Nu^- favours S_N2.

Weak one disfavours S_N2 → allows S_N1 to compete successfully!

Which factors affect Nu^- strength?

1) Charge

2) Polarisability (more important)

Why are I^- and HS^- particularly strong Nu^-s?

I and S are highly polarisable atoms (large + lots of e^-s) → stronger

Does LG identity affect S_N1 and S_N2?

Yes! If LG is bad, neither can operate.

S_N1 is more sensitive to LG identity.

Why is S_N1 more sensitive to LG identity?

RDS is the loss of LG to form a CC and LG.

RDS is sensitive to stability of CC and LG → LG must be highly stabilised.

What’s a good LG like?

Conjugate base of strong acid (i.e. weak base since they’re highly stabilised e.g. I^-)

What’s a bad LG like?

Stabilised base

How can you convert ^-OH into a good LG?

Protonate it w/ a strong acid!

State + draw the 6 most common LGs

Halides: I^-, Br^-, Cl^-

Sulfonate ions: tosylate (^-OTs), mesylate and triflate

Which solvent greatly affects competition between S_N1 and S_N2?

Polar aprotic solvents!

Why do S_N reactions need polar solvents?

‘Like dissolves like’

S_N1 needs it to stabilise CC (needs polar more than S_N1)

S_N2 needs it to dissolve the Nu^-

Will rarely see a substitution rxn in a non-polar solvent.

What’s a polar protic solvent?

Solvent that has proton connected to an EN atom (e.g. H₂O or EtOH)

Protic since solvent can serve as source of protons.

What’s a polar aprotic solvent? Show 4 common e.g.s

Solvent doesn’t have proton on an EN atom

(Solvent can still have H atoms, but they won’t be connected to EN atoms.)

e.g. acetone, DMSO, DME and DMF

Why do polar aprotic solvents favour S_N2?

Polar aprotic solvents aren’t good at forming solvent shells around Nu^-.

→ Frees up Nu^- to react (doesn’t have to shed solvent shell)

Which of S_N1 and S_N2 are susceptible to CC rearrangements, and why?

S_N1!

S_N1 involves CC intermediate — S_N2 doesn’t.

Which two factors should you consider in each problem?

Steric and electronic.

Electronic effects (e.g. Nu^-. LG and solvent effects) are usually more complicated.