Synthesis Topic 6 Flashcards
Why SN2 is not possible for aromatic rings?
Because nucleophile neegs to attack leavingt group at 180 degrees. Nucleophile has high electron density and it will be approaching aromatic ring that is also high in electron density. As a result they will repel each other.
Why SN1 is unfavourable for aromatic rings?
As ring will need to form cation which will have sp2 orbital orthogonal to the π system. As a result positive charge cannot be stabilised with resonance delocalysation.
How nucleophilic aromatic substitution occurs?
By a two step addition elimination reaction with electron poor aromatic compounds with -M substituents (e.g. NO2, CN, COR, CO2R)
In nucleophilic aromatic substitution by addition-elimination what is rate determining step?
Nucleophilic addition
How this intermediate is called?
Meisenheimer complex
What happens in nucleophilic aromatic substitution by addition-elimination?
Nu attacks C near LG, electrons from double bond moves near electron withdrawing group.
Then negative charge from electron withdrawing group forms double bong and LG leaves.
Which halogen is the best LG and which is the worst?
F>Cl>Br>I
Why fluorine is the best leaving group out off all halogens?
Loss of leaving group is a fast process. However it accelerates the first slow step as a result of F being very small minumising steric clash with a nucleophile. Also F is inductively very strongly electron withdrawing which makes ring more electron defficient and more likely to react with nucleophile.
Why diazonum salt has a perfect leaving group?
Because LG would be N2 gas that is very stable duo to the strong N-N triple bond.
How diazonium salts are formed?
They are redily available from anilines
What is mechanism of diazotisation?
Draw a reaction mexhanism of bromobenzene with NaNH2 and liquid NH3?
What is this molecule called?
Benzyne, it is electron-deficient and therefore highly electrophilic.
