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Nucleophilic aromatic substitution occurs when an electron rich nucleophile attacks an electron deficient species on an aromatic ring. The substituent on the aromatic ring is replaced with another substituent given that the former is a good leaving group. And a good leaving group would be a substituent which can accommodate more negative charge or is more stable due to distribution of charges or resonance. The substituent on the benzene ring has an effect on the reactivity of the aromatic ring. Effects such as inductive effect wherein the electronegativity of the substituent attached is greater which therefore pulls the electron density towards itself. For example, Nitrogen, Oxygen and halogens are more electronegative than the Carbon on the ring therefore, it pulls the electron density towards itself and therefore deactivating the ring. Deactivation of the ring implies that the reaction would be slower and activating the aromatic ring on the other hand means that the reaction would be faster. Another thing to consider in nucleophilic aromatic substitution is the site of where nucleophilic attack takes place. There are different orientation of reaction depending how substituted the ring is. It could be ortho, meta or para to the substituent (if it's monosubstituted). So when these two factors that are taken into consideration are combined, it will then tell us the reactivity and where the nucleophile will attack/or direct the new substituent and also tell us which of the product will form in majority.
To carry out this experiment a mixture of the following was obtained in a 125 mL Erlenmeyer flask : 1.010g of 2,4-dinitrochlorobenzene; 0.6800g of m-aminobenzoic acid; and lastly 25 mL Dimethylformamide (DMF). Upon dissolving the solids to the liquid DMF, it became a clear yellowish brown homogenous liquid solution. The solution was heated in a water bath for one hour with occasional swirling of the flask. The solution 100mL of deionized water was added, and the solution became an opaque yellow mustard colored homogenous liquid solution. The solution was run through a vacuum filter to obtain the yellowish brown pasty precipitate. The precipitate was then removed from the filter paper and was mixed initially with 25mL 95% ethanol and heated in an Erlenmeyer flask. The yellow brown precipitate then suspends in the liquid ethanol and more ethanol was added until all the precipitate was dissolved and the solution turned into a clear yellowish brown liquid. The solution was removed from the hot plate and deionized water was added until the solution became a mustard yellow cloudy opaque solution. The solution was cooled to room temperature and was run through vacuum filtration. The dark mustard yellow brown precipitate formed was then obtained in a pre-weighed watch glass and air dried.
Data and results:
Mass of 2,4-dinitrochlorobenzene (DNCB) = Required # of moles x M.W. of (DNCB)
=0.005mol x 202.6g/mol = 1.013g DNCB
Mass of m-aminobenzoic acid (MABA) = Required # of moles x M.W. of (MABA)
=0.005mol x 137.1g/mol = 0.6855g MABA
Theoretical Yield =
Percentage Error =
Table 1. Properties of Reactants and Products
Dark yellowish brown crystals
Light brown powder
Clear, colorless liquid
Clear, colorless liquid
m-(2,4-dinitroanilino) benzoic acid
Clear, colorless liquid
Table 2. Data and Results
Physical properties after reaction
Mixed and heated:
Mass of DNCB: 1.010g
Dark Yellowish Brown Liquid solution
Mass of MABA: 0.680g
Light brown Powder
Volume of DMF:25 mL
Clear, colorless liquid
Added Deionized Water
Volume used: 100mL
Dark Yellowish Brown semi-transparent Liquid
Opaque Mustard Yellow liquid solution
Precipitate obtained: Yellowish Brown powder
Added 95% ethanol
Initially added 25 mL
Then added more till precipitate disappeared and solution is clear
Yellowish brown clear liquid solution
Added Deionized Water
Volume used: added until solution became cloudy
Mustard cloudy yellow liquid solution
Precipitate: Mustard yellowish Brown powder
Weight of Watchglass
Final mass of product
Nucleophilic aromatic substitution of 2,4-dinitrochlorbenzene(DNCB) by m-aminobenzoic acid (MABA) was done to produce m-(2,4-dinitroanilino)benzoic acid. 2,4-dinitrochlorbenzene(DNCB) which appeared to be granulated dark yellowish brown solid is a benzene derivative. It has a Chlorine on position one of substituted halobenzenes that also carry two nitro groups (electron withdrawing substituents)inÂ ortho and para position, which in this experiment, was actually substituted by a nucleophilic attack of a good nucleophile, MABA, which has electron donating substituents. (see fig. 1 p.1)(refer to table 1 for properties of reactants).
Fig.2 Electrostatic potential surface ofÂ chloro-2,4-dinitrobenzene
"Comparison of the electrostatic potential surfaces illustrates the considerably stronger chlorine-carrying carbon's positive polarization by the electron-withdrawing nitro groups, which enables a nucleophilic attack on this carbon (red means a more negative potential, blue means a more positive potential)". Dr. Oliver Kamp http://www.chemgapedia.de
Initially, DNCB which appeared to be granulated dark yellowish brown solid was combined with MABA which can be described as a light brown powder in a polar aprotic dimethylformamide (DMF) solvent which is a clear colorless liquid. DMF stabilizes the transition state which enhances the addition-elimination reaction also, it does not solvate the nucleophile. As the solids dissolves in the solvent. MABA becomes deprotonated and attacks DNCB in DMF solution therefore making it a good nucleophile. The solution turned into a dark yellowish brown opaque solution. To facilitate reaction, the solution was heated in a water bath for an hour. The solution turned into a semitransparent yellowish brown solution lighter than before.
The reaction proceeds easily according to the number of the substituents in the aromatic ring that withdraw electrons from the ring itself. However, substituents in orthoÂ andÂ paraÂ position, above all, promote a nucleophilic attack, while substituents inÂ metaÂ position almost never do. Â± These observations may be explained by a mechanism.
Figure 3. MECHANISM: Nucleophilic aromatic substitution of 2,4-dinitrochlorobenze by m-aminobenzoic acid.
This nucleophilic aromatic substitution is an addition-elimination mechanism. The aromatic system is broken by the attack of the nuclephile, MABA- and not the other way around. An intermediate cabanion is formed and delocalized. It is then distributed among the formerly aromatic,Â sp2-hybridized ring carbons. The intermediate carbanion's negative charge is stabilized by the resonance and (-)inductive effects the substituents create. Stabilization of NO2 in a para positionis the best because it is in the center which has the largest negative charge density. Ortho position has a lesser degree than the para position stabilization but still contribute. Since Chloride atom is a better leaving group and because it can accommodate more of the negative charge than the incoming nucleophile, it then splits off.
To precipitate the product and remove DMF which is soluble in water, deionized water was added. The solution turned mustard yellow opaque solution. The dark yellow powder precipitate was obtained by vacuum filtration of the solution. 95% of ethanol was then added and the solution was heated to boil to dissolve impurities and product. Once the solution turns yellow- brown and clear, it was then removed from heat and deionized water was added to precipitate product. A dark yellow brownish powder product m-(2,4-dinitroanilino) benzoic acid is not soluble in water therefore it precipitates from the solution. The melting point determination was omitted because it is greater than 240 Â°C. The final product was weighed and a mass of 1.421g was obtained.
In this experiment, the nucleophilic aromatic substitution of 2,4-dinitrochlorobenzene by m-aminobenzoic acid was successfully performed. Better understanding on how benzene substituent's contributes to the reactivity and orientation of the nucleophilic attack which therefore predicts the structure of the final product. The final mass of the final product m-(2,4-dinitroanilino)benzoic acid was 1.421g. The theoretical yield of 1.516g was not fully obtained. A pecent error of 6% might be due to experimental errors such as failure to have exact or accurate measurements of reactants, loss of product during transferral steps. Also, some of the product was loss due to the failure of the vacuum filtration system because of deficient aspirator in the laboratory. Having a better vacuum filtration setup in a highly controlled laboratory setup will probably contribute to a higher product yield.
Answers to Questions.
Why is dimethylformamide rather than water used as the solvent in this reaction?
DMF is a polar aprotic solvent which does not have a proton to donate in the reaction. It also does not solvate the nucleophile therefore not affecting the product of reaction. It also stabilizes the transition state therefore enhancing the addition elimination reaction in the experiment.
What product may be formed when chlorobenzene is reacted with sodium hydroxide using water as a solvent.
Since Chloride is a better leaving group than a hydroxide, a benzene with a hydroxide in the 2nd carbon will be produced in this reaction.
From the FT- IR spectrum (figure. ) assign the major preaks for m-(2,4-dinitroanilino) benzoic acid. Assign the protons in the NMR spectrum (Figure ) for 2,4-dinitrochlorobenzene.