CHM2123_2023_Hydrazone

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Chemistry

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Oct 30, 2023

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H IGHWAY TO THE HYDRAZONE T HE IDENTIFICATION OF UNKNOWN CARBONYLS S ARA M UDRY , F RANÇOIS M AGNAN While a melting point analysis was an analytical technique of interest in the past for confirming the identity of a reaction’s product, it was inherently limited to products that were already solid at room temperature. As a work-around, chemists would further react liquid products with a derivatizing agent to convert the product into a solid analogue, which could then be analyzed and compared to the reported value. For this experiment, you will perform such an analysis with the help of 2,4-dinitrophenylhydrazine (DNPH), a derivatizing agent for ketones and aldehydes. You are tasked with identifying an unknown carbonyl derivative from a select list of possible reagents. By reacting your unknown carbonyl with DNPH, you will produce the corresponding hydrazone, which you will purify via a quick recrystallization, By measuring the melting point of your purified product and comparing to a list of known values, you will then be able to ascertain the identify of the carbonyl derivatives that was provided to you.
2,4-Dinitrophenylhydrazine (DNPH) is an organic mol- ecule that once enjoyed a certain popularity in the synthetic lab due to its unique properties and reactivity that were often exploited for analytical purposes. The hydrazine moiety (–NH–NH 2 ) is a nucleophile that will readily react with elec- trophilic carbonyls ( e.g. aldehydes and ketones) to form the corresponding hydrazone (Fig. 1). 1. Pedagogical Apply common fundamental laboratory techniques. Perform a synthetic organic transformation. Perform a recrystallization, a useful purification technique for organic solids. Relate the concepts of structure, purity, and melting point. 2. Experimental React a mystery carbonyl derivative with DNPH to convert it into the corresponding hydrazone. Purify the product by recrystallization. Ascertain the identity and purity of the final product via TLC and melting point analysis. OBJECTIVES Also available... KEY TERMS Hydrazine Hydrazone Nucleophilic acyl substitution KEY VIDEOS Reflux assembly Vacuum filtration Recrystallization Thin-Layer Chromatography Melting Point HYDRAZONES AS USEFUL REAGENTS Such a reaction is in-fact fairly unique to aldehydes and ketones, since other carbonyl derivatives ( e.g. amides, car- boxylic acids, esters) are not sufficiently electrophilic due to conjugation effects. For that reason, DNPH was often used in the first half of the 20 th century as a key reagent in analytical schemes. On one hand, DNPH can be useful for the qualitative detection of carbonyl derivatives: in con- tact with reactive carbonyls, DNPH will rapidly form brightly colored hydrazones that readily precipitate. In that context, the formation of a solid upon treating the solution of an unknown molecule with DNPH can be used as a presumptive test for the presence of a ketone or an aldehyde motif in the molecule in question (or the absence of such motifs, if no precipitate is observed). Figure 1 – The reaction of 2,4-dinitrophenylhydrazine (DNPH) with a reactive carbonyl (aldehyde or ketone) results in the formation of a hydrazone (red). H N NH 2 NO 2 O 2 N R R/H O H N N NO 2 O 2 N R/H R + DNPH Hydrazone
3 The identification of unknown carbonyls THE TRANSFORMATION Alternatively, DNPH was used in the identifi- cation of simple liquid aldehydes and ketones. Whereas a melting point analysis is often used to characterize solids, such an analysis can prove to be more problematic when the substance melts below room temperature! In that regard, simple liquid aldehydes and ketones would instead be identified by converting them to their correspond- ing DNPH-derived hydrazone. Following a recrys- tallization, the product would then be analyzed by melting point: by comparing this value to re- ported values, one could potentially identify a given carbonyl derivative – or, at the very least, narrow the list of possibilities. DNPH is an ideal reagent in such a context, as it rapidly reacts with the un- known carbonyl to form solid products that are readily recrystallized and that possess sharp, re- peatable melting points. Whereasyoudonotlearnofhydrazoneformation in CHM2120, you do learn of a similar reaction – the formation of imines . So similar in fact that the mechanisms for the two reactions are effec- tively identical! An imine is made from the reaction of a primary amine 1 (Fig. 2) with a reactive car- bonyl 2 (again, an aldehyde or a ketone): nucle- ophilic attack of the nitrogen onto the carbonyl forms a tetrahedral intermediate 3 . Intermolecular proton transfer steps eventually lead to the for- mation of 4 , which bears an oxonium (–OH 2 + ), a Figure 2 a) The formation of an imine is achieved by reacting a primary amine ( 1 ) with a reactive carbonyl ( 2 ) under acidic catalysis. Following nucleophilic attack and proton transfer steps, the resulting amino oxonium ( 4 ) loses a water-molecule, and the carbon atom is stabilized by the neighboring nitrogen’s lone pair of electrons to form the iminium ( 5 ). Loss of a proton forms the resulting imine and regenerates the acid catalyst. B) Similarly to what was seen in a), a hydrazine ( 7 ) reacts with a reactive carbonyl to eliminate a water molecule and form a C=N double bond, resulting in a hydrazone. great leaving group. Upon its departure, the elec- tron-deficient carbon is stabilized by the neighbor- ing nitrogen, who shares its lone pair of electrons to form a C=N double bond iminium 5 , which sub- sequently loses a hydrogen atom to form the neu- tral imine 6 . Such a reaction is usually done in the presence of an acid catalyst, which protonates the initial carbonyl to make it more electrophilic. The formation of the hydrazone is conceptually identical. The initial primary amine is replaced with the hydrazine derivative 7 , which has two nitrogen atoms. In asymmetrically-substituted hy- drazines, one must consider which of the two ni- trogen atoms will perform a nucleophilic attack, which is dependant on the nature of the functional groups attached. In the case of a hydrazine mono- substituted with an aromatic ring, such as in the case of DNPH, the substituted nitrogen has de- creased nucleophilicity by virtue of resonance de- localization of the heteroatom’s lone pair of elec- trons onto the aromatic ring, which does not occur for the terminal nitrogen. For this reason, the ter- minal nitrogen is the most nucleophilic of the two atoms and will undergo the reaction with the re- active carbonyl. The rest of the mechanism is vir- tually identical to what was described above for the imine: attack of the nitrogen unto the carbonyl yields, after a series of intermolecular proton trans- fers, a C=N double bond bridging the two starting materials ( 11 ). Since the C=N group is function- alized with a second nitrogen atom, the resulting molecule is a hydrazone rather than an imine, since the two vary in properties and reactivities. a) b) R 1 NH 2 R 2 R 3 /H O R 2 R 3 /H OH NH 2 HN R 2 R 3 /H OH 2 NH R 1 R 2 R 3 /H NH R 1 R 2 R 3 / H N R 1 + N H NH 2 Ar R 2 R 3 /H O R 2 R 3 /H OH NH 2 R 1 Cat. H + Ar R 2 R 3 /H OH 2 NH HN Ar R 2 R 3 /H NH HN Ar R 2 R 3 /H N HN A r Cat. H + + ± H + ± H + – H + – H + 1 2 3 4 5 6 7 2 8 9 10 11
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