Aromatic Hydrocarbons

Exer 1

INTRODUCTION

        Aromatic hydrocarbons are reactive towards electrophiles due to their high electron density, that is why it usually occurs in electrophilic mechanism. But aryl halides, having halogens that are electron-withdrawing, allow them to be also reactive with nucleophiles and undergo the nucleophilic aromatic substitution (NAS). One type of this reaction is addition-elimination, SnAr. This takes place when an aromatic ring is activated by the strong electron-withdrawing group/s attached to it. The ring activation is more effective when electron-withdrawing group, like nitro group, is ortho and para to the good leaving group, usually halides. This is because in ortho and para positions, there is delocalization of charge. (rxn 1 mcmurry 552)

But when the electron-withdrawing group is in meta position relative to the ipso carbon (carbon bearing the leaving group), the negative charge is not delocalized since the positive nitrogen is not adjacent to the carbanion. (rxn 2 mcmurry 552)

        In this experiment, 2,4-dinitrophenylhydrazine is to be synthesized from 1-chloro-2,4-dinitrobenzene and hydrazine. Hydrazine will be generated from hydrazine sulfate, that is why it is important to dissolve first the hydrazine sulfate powder in water rather than mixing it to 1-chloro-2,4-dinitrobenzene immediately. Hydrazine should be generated because it is the nucleophile that will attack the nucleophilic site. Hydrazine sulfate alone cannot be the nucleophile since it has no lone pairs for NAS to occur, unlike the hydrazine. The overall reaction for this synthesis is (rxn 5 old kor)

        To synthesize 2,4-dinitophenylhydrazine, the reaction starts with the attack of the nucleophile which is hydrazine to the ipso carbon which bears the chloride, the leaving group. This step is the addition part in addition-elimination reaction, which forms a Meisenheimer complex. (rxn 3 old val)

The elimination part can either be the ejection of the nucleophile which will regenerate the starting material, 1-chloro-2,4-dinitrobenzene, which will be the side product; or the ejection of the chloride, the leaving group, which will generate the desired product, 2,4-dinitrophenylhyrdazine. The expulsion of one of the substituents attached to the ipso carbon is brought by the excess electron density and regeneration of the aromatic ring. (rxn 4 old val & magmech for side product)

In the synthesis proper, the refluxed mixture containing the product should be filtered and then eventually washed with cold ethanol. It is necessary that the ethanol is cold so that the residue filtered form the mixture will not redissolve and go straight to the filtrate.

In order to minimize the occurrence of the side reaction which is the regeneration of the starting  material, _____

        The product, 2,4-dinitrophenylhyrdazine, is very useful as a major reagent in many characterization tests. This 2,4-dinitrophenylhydrazine can be used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. A positive test is signalled by a yellow or red precipitate (known as a dinitrophenylhydrazone.) If the carbonyl compound is aromatic, then the precipitate will be red; if aliphatic, then the precipitate will have a yellow color. In this experiment, acetone and benzaldehyde are the carbonyls to be used in characterizing the synthesized product whether it is 2,4-dinitrophenylhydrazine or not. The following are the reactions of 2,4-dinitrophenylhydrazine with acetone and benzaldehyde. (rxn 6 exer1 word)