Electrophilic Aromatic Substitution Essay
Electrophilic Aromatic Substitution
lectrophilic Aromatic Substitution(1) Nitration of Methyl Benzoate(2) Synthesis of 1,4-Di-t-butyl-2,5-dimethoxybenzene byFriedel-Crafts Alkylation of 1,4-DimethoxybenzenePurpose1)To carry out the nitration of methyl benzoate, and then identify the major product formed (position at which nitro-group substitution takes place) by thin-layer chromatography (TLC), the percent yield and the melting point range.
2)To synthesize 1,4-Di-t-butyl-2,5-dimethoxybenzene by Friedel-Crafts Alkylation of 1,4-Dimethoxybenzene, and then determine the percent yield and melting point range.
Procedure*Please refer to the lab handout 6 and Macroscale and Microscale Organic Experiments (Williamson, 2003).
* Part II of the experiment (Synthesis of 1,4-Di-t-butyl-2,5-dimethoxybenzene by Friedel-Crafts Alkylation of 1,4-Dimethoxybenzene) was carried out by Ashley and me. Part I (nitration of methyl benzoate) was carried out by Jenny.
Physical Quantity TableType of substanceMolecular FormulaMolecular Weight (g/mol)Density(g/cm3)M.P.(oC)B.P.(oC)Methyl benzoateC8H8O2136.161.094-15198-200Methyl 2-nitrobenzoateC8H7NO4181.141.289-13104Methyl 3-nitrobenzoateC8H7NO4181.14-78-80289Methyl 3,5-dinitrobenzoateC8H6N2O6226.14544-106-109-Methyl 4-nitrobenzoateC8H7NO4181.15-94-96-1,4-DimethoxybenzeneC8H10O2138.161.0555-582131,4-Di-t-butyl-2,5-dimethoxybenzeneC16H26O2250.37-104-105-2-methyl-2-propanolC4H10O74.12-25.482.4Hazard Concentrated sulfuric acid and nitric acid are highly corrosive.
ObservationPart II Friedel-Crafts AlkylationThe concentrated sulfuric acid used was yellow. 1,4-Dimethoxybenzene was in white crystal form. The t-butyl alcohol solidified in room temperature, so it took a while to heat it up and return to liquid form. After concentrated sulfuric acid was added to the t-butyl alcohol, acetic acid and 1,4-Dimethoxybenzene mixture, the solution became light brown in color. After warming for a while, white precipitate could be observed at the bottom of the tube. After water was added, the white precipitate dissolved and the solution turned milky. The crystals obtained after recrystallization are in form of large slides. They are grinded into powdery or smaller granules for melting point test.
DataPart IMass of methyl benzoate = 0.30gMass of recrystallized product = 0.28436gMelting Point Range of nitration product = 75 oC – 83 oCChemical CompoundsDistance from B to spots (cm)Distance from B to SF(cm)Distance from B to spots (cm)/Distance from B to SF (cm)Retention FactorRfMethyl benzoate1.684.501.68/4.500.37(ortho) Methyl 2-nitrobenzoate1.304.521.30/4.520.29(meta) Methyl 3-nitrobenzoate1.804.501.80/4.500.40Methyl 3,5-dinitrobenzoate1.304.401.30/4.400.30(para)Methyl 4-nitrobenzoate1.804.301.80/4.300.42
Unknown Product1.804.301.80/4.300.42Table one showing the Retention factor of unknown nitration product with comparison to standard valuesB = baselineSF = solvent frontDrawing of TLC PlatesPlate #1 : Plate #2:Key1 = Methyl benzoate2 = Methyl 2-nitrobenzoate3 = Methyl 3-nitrobenzoate5 = Methyl 3,5-dinitrobenzoate4 = Methyl 4-nitrobenzoateP = Unknown nitration ProductPart IIMass of 1,4-Dimethoxybenzene = 0.1220gMass of recrystallized product (1,4-Di-t-butyl-2,5-dimethoxybenzene)= 0.085gMelting Point Range of alkylation product = 97oC -102 oCResult &CalculationPart IBased on data from both melting point test and TLC, the nitration product is likely to be methyl 3-nitrobenzoate (details in discussion part).
Theoretical Mass1 mole of methyl benzoate produced 1 mole of methyl 3-nitrobenzoate:Mole of methyl benzoate = 0.3g/136.16 g mol-1 = 2.20 x 10-3 molðMole of methyl 3-nitrobenzoate = 2.20 x 10-3 molMass of methyl 3-nitrobenzoate = (2.20 x 10-3 mol) x (181.15g mol-1) = 0.3991gPercent YieldPercent yield = 100% (actual mass/theoretical mass) = 100% (0.28436g/0.3991g)= 71.2%Part IITheoretical Mass1 mole of 1,4-Dimethoxybenzene produced 1 mole of 1,4-Di-t-butyl-2,5-dimethoxybenzene:Mole of 1,4-Dimethoxybenzene = 0.1220g/138.16 g mol-1 = 8.83 x 10-4 molðMole of 1,4-Di-t-butyl-2,5-dimethoxybenzene = 8.83 x 10-4 molMass of 1,4-Di-t-butyl-2,5-dimethoxybenzene= (8.83 x 10-4 mol) x (250.37 g mol-1) = 0.2211gPercent YieldPercent yield = 100% (actual mass/theoretical mass) = 100% (0.085g/ 0.2211g)= 38.4%
DiscussionIn this experiment, two types of electrophilic aromatic substitutions were studied—nitration and Friedel-Crafts alkylation. Aromatic substitution involves the substitution of one (or more) aromatic hydrogens with electrophiles. Monosubstitution is possible only if the monosubstitution product is less reactive than the original reactant. If the reactivity of the monosubstitution product equals or exceeds that of the original reactant, the monosubstitution product(s) will proceed on to polysubstitution products. There are two reasons why the monosubstitution products might be less reactive:a)Electronic reasons: if the E group that added is electron withdrawing, it will make the product aromatic ring less electron rich and subsequently less reactive toward subsequent electrophile addition.
b)Steric reasons: Replacement of a small H with a large E group will make the monosubstitution product more crowded, which may interfere with the subsequent addition of additional electrophiles.
The existing substituent attached to benzene, which could either be ortho-para directing or meta directing, determines the position of the new substituent that will attach to the benzene ring. Electron donating (activating) groups such as alkyl and carbonyl groups are ortho-para directors while electron withdrawing (deactivating) groups except Br and Cl, are meta directors.
In part one, the hydrogen atom on a substituted benzene (methyl benzoate) was substituted with a nitro group (-NO2) to see the directing effect of the ester group. The product was then analyzed by both thin layer chromatography (TLC) and melting point test.
The melting point range determined was 75 oC – 83 oC. Comparing this value to the standard melting points in the physical quantity table, it was found that the product was most likely to be methyl 3-nitrobenzoate, with the accepted melting points of 78 oC 80oC. Using TLC, the retention factor calculated for the product was 0.42. By comparison, the closet retention factors of the standards were 0.42 (for methyl 4-nitrobenzoate) and 0.40 (for methyl 3-nitrobenzoate). Since the product spot was a bit large, this suggested the possibility of overlapping and the product could be methyl 4-nitrobenzoate and/or methyl 3-nitrobenzoate. Combining the two results (melting point and retention factor) together, it was concluded that the major product was methyl 3-nitrobenzoate. This made sense because the -COOCH3 substituent on the benzene ring of methyl benzoate has two effects on the reaction that would be explained below.
First, because of the partial positive charge on the carbonyl carbon, -COOCH3 is a ring deactivator, meaning that nitration of methyl benzoate occurs slower than nitration of unsubstituted benzene. This ring deactivation is due to an electron-withdrawing effect by the substituent. Since the electrophile NO2+ is seeking a concentration of electron density to react with (the pi cloud of the benzene ring), electron-withdrawing substituents that remove some electron density from the ring make the ring less electron-rich and, therefore, less reactive toward the NO2+ ion.
The deactivation of methyl benzoate by the -COOCH3 group is not a major problem because the NO2+ ion is such a powerful electrophile that the reaction occurred at ice-bath temperatures anyway. In fact, it was important to make sure the reaction mixture did not get too warm or the nitrated methyl benzoate would react with a second NO2+ to form dinitro-methyl benzoate. However, the dinitro-methyl benzoate could be removed by recrystallization.
The second effect that the -COOCH3 substituent has on the reaction is that of directing the incoming NO2+ ion to replace specific hydrogens on the benzene ring. Specifically, the -COOCH3 group is a meta-director, which means that the H’s meta to it are the most reactive and the most likely to be substituted. Meta-directing, like ring deactivation, is caused by the electron-withdrawing effect of the -COOCH3 substituent. The simplest explanation is as follows: The oxygens bonded to the carbonyl carbon of the -COOCH3 substituent are electronegative and thus induce a partial positive charge on that carbon. Resonance delocalization of the pi electrons of the ring distributes the partial positive charge to other carbon atoms in the molecule. Since the NO2+ is searching for an electron-rich carbon, not one with a partial positive charge, the carbons at meta positions are the most ones for the NO2+ to react. This leads to the expected meta substitution (mechanism attached).
The percent yield was determined to be 71.2%, which was reasonably high. The TLC results indicated that there could be some ortho-para isomers present in the product as impurities but there was no dinitro-methylbenzoate. The product yield would go down after recrystallization and due to mechanical loss.
In part two, 1,4-Di-t-butyl-2,5-dimethoxybenzene was synthesized by reacting 1,4-dimethoxybenzene with tertiary butyl alcohol in the presence of sulfuric acid as a Lewis acid catalyst. The reaction occurred via the Friedel-Crafts alkylation mechanism, and involved the attack of the aryl group at the electrophilic trimethylcarbocation. The resulting product was recrystallized using methanol and characterized by testing the melting point.
The -OCH3 is ortho-para directing and activating, so it put the t-butyl cation in the proper location during the reaction. This was because the positively charged electrophile (t-butyl cation) was more likely to react at a center that is more negatively charged and substitution occurs at the ortho and para positions. The accepted melting point for 1,4-Di-t-butyl-2,5-dimethoxybenzene is 104-105oC, and our determined melting point was 97 oC -102 oC. The slight depression in melting point could due to impurities present in the product.
The percent yield was determined to be 38.4%. The percent yield and melting point of the final product achieved in the experiment showed that the product was the desired 1,4-Di-t-butyl-2,5-dimethoxybenzene. The melting point was off by about three degrees. This slight depression in melting point could have been the result of a small amount of water in the crystals or an inaccurate thermometer reading due to heating the crystals a bit too fast. The latter was likely to be true because the melting point range obtained was too wide. The percent yield was lowered due to mechanical loss during handling (leaving crystals on the surfaces or not fully collected from the flasks as they got stuck).
To improve the experiment, a second recrystallization could be performed to improve the purity of the crystalline product (though the percent yield might be compensated). More careful transferring of the recrystallized product was necessary to minimize product loss and more careful monitoring of power level was important in melting point test so that the melting point range was not widened due to rapid temperature rise. Moreover, the TLC could be performed again with product spotting on both plates (instead of only one) to eliminate differential migration effect of each plate on the overall retention factor.
Answer to the questionsP. 3711.Methyl benzoate dissolves in concentrated acid to increase the rate of the reaction by increasing the concentration of the electrophile, the nitronium ion (NO2+), as shown in the equation below:2.I would expect the structure of the dinitro ester to be methyl 3,5-dinitrobenzoate because of meta-directing effects of the ester and the first nitro group on the addition of the second nitro group (both ester and nitro group are electron withdrawing and therefore lead to ring deactivation).
3.The positive charge is delocalized to the carbon with the NO2 attached; this places it adjacent to the positive charge on the nitrogen, which is very destabilizing. Therefore the electrophile (NO2+) prefers to attach the meta-position. (resonance structures attached)4.Peaks at 3101cm-1: C=H stretch (of benzene ring)Peaks at 1709cm-1: C=O stretch (of ester group)Peaks at 1390cm-1: N-O stretch (representing the nitro group)P. 3854.Mechanism attached.
5.1,4 isomer is the major product in alkylation of dimethoxybenzene and none of the other isomers listed were very unlikely to form because the methoxy groups at 1,4 positions are electron- donating and therefore are ortho-para-directing. The electrophiles (electron-seeking) thus would attack at the positions near the methoxy groups. Also, more resonance structures could be written for the arenium ions resulting from ortho and para attack than from meta attack, which suggests that the ortho- and para- substituted arenium ions should be more stable. The transition states leading to the ortho- and para-substituted arenium ions occur at unusually low free energies. As a result, electrophiles react at the ortho and para positions very rapidly.
6.The carbocation can be generated from various starting materials such as an alkene, alcohols or alkyl halides. In place of t-butyl alcohol, one could use t-butyl chloride. However, this reaction is not convenient to do in the lab (HCl would also be generated). 2-methylpropene could also be used. Both of these yield the same carbocations as the reaction that was performed in the experiment, and thus yield the same product when reacted with 1,4-dimethoxybenzene.
Williamson, Macroscale and Microscale Organic Experiments, 4th Edition, 2003, Chapter 28 & 29, P. 367 – 3852)Solomons and Fryhle, Organic Chemistry, 8th edition, 2004, P. 669 – 680