yazyj
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BedokFunlandJC
There are 3 main types of nucleophilic substitutions :

Nucleophilic aliphatic substitutions, ie. SN1 and SN2 mechanisms.

Nucleophilic acyl substitutions, ie. addition-elimination mechanism, eg. generating esters and amides, hydrolysing esters and amides, etc.

All H2 Chem students need to be familiar with both of the above nucleophilic substitutions in terms of the reactions themselves; while mechanism wise : SN1 and SN2 mechanisms are basic and compulsory for H2 Chem students, while the addition-elimination mechanism is recommended for distinction H2 Chem students to be familiar with.

The 3rd type refers to nucleophilic aromatic substitutions (in contrast the basic H2 Chem syllabus focuses on electrophilic aromatic substitutions, whose mechanism is basic and compulsory for H2 Chem students), for which there are 6 different mechanisms at Uni levels, of which the simpler 3 mechanisms (including the SNAr mechanism) are useful for H3 Chem, Olympiad Chem and distinction H2 Chem students to be familiar with.

Nucleophilic aromatic substitution

Aromatic nucleophilic substitution

A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems:

  • the SNAr (addition-elimination) mechanism
SNAr mechanism
Aromatic SN1mechanism
Substitution via benzyne

The most important of these is the SNAr mechanism, where electron withdrawing groups activate the ring towards nucleophilic attack, for example if there are nitro functional groups positioned ortho or para to the halide leaving group.

https://en.wikipedia.org/wiki/Nucleophilic_aromatic_substitution

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yazyj
380EDF72-7930-4D7B-AB07-0AE93C0593C0.jpeg

Referring to the starred part, why do we have to cool the resulting mixture?  
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BedokFunlandJC

yazyj wrote:
380EDF72-7930-4D7B-AB07-0AE93C0593C0.jpeg

Referring to the starred part, why do we have to cool the resulting mixture?  


Because if a halide ion is present (from nucleophilic aliphatic substitution by OH- ion), you'll want to see a ppt to indicate this. If the solution is too hot, the ppt would dissolve (since Ksp increases with temperature for most ionic ppts), and you would be mislead into a false negative result thinking no halide ion is present.

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