Synthesis and Purification of Nitrophenols

Synthesis and Purification of NitrophenolsAbstractOrtho and mirror symmetry-nitrophenol was synthesized using an electrophilic remindful substitution of phenol and dilute nitric acid. Isolation of the crude crossway used a dichloromethane followed by a short vortex and sodium sulfate for water removal. Separation of the ortho and para products was completed using pillar chromatography to collect the eluent in ten vials vials 1-5 collected o- and vials 6-10 collected p-nitrophenol. Thin layer chromatography confirmed synthesis of o-nitrophenol collected in vial 3, 4 and 5 and p-nitrophenol in vial 7.1H proton magnetic resonance showed o-nitrophenol being the spectrum with more than jackets, due to the asymmetric structural difference creating more nuclear environments for the proton to posticipate in.Introduction oxybenzenes, due to their rich electron density, atomic number 18 highly susceptible to undergo electrophilic substitution reactions. The hydroxyl group on the redole nt ring of the phenol promotes charge delocalization therefore, allowing for stabilization through resonance. i such electrophilic substitution reaction is that of nitration. First, an electrophilic attack of the phenol takes place, resulting in a carbocation intermediate stabilized by resonance1. Next, the nitronium ion nitrates the phenol ring, producing p-nitrophenol and o-nitrophenol (Figure 1). The hydroxyl group of the phenol is an ortho para director therefore, the meta isomer is not produced. However, by products such as 2,4-dinitrophenol and 2,4,6,-trinitrophenol may be present in excess amounts of nitric acid. Once nitration is complete, the crude product slew be purified through column chromatography and monitored through tender loving care.Thin layer chromatography (TLC) is a chromatographic technique used to separate the comp onenessnts of a mixture using a shorten stationary phase. TLC functions on the same principle as all chromatography a compound will have diffe rent affinities for the mobile and stationary phases and this affects the speed at which migrates2.After a separation is complete, individual compounds appear as spots separated vertically. Each spot has a retention constituent (Rf) which is equal to the distance migrated over the total distance covered by the solvent. The Rf pissula is2In this experiment the difference in Rf set will allow for identification between o- and p-nitrophenol. When comparing two different compounds under the same conditions, the compound with the larger Rf think of is less opposite because it does not stick to the stationary phase as long as the arctic compound, which would have a lower Rf value2.Column chromatography is a profitable analytical technique for small-scale separation and purification using similar principles as TLC3. The charged, stationary phase remains either silica gel or alumina and the mobile phase can be dichloromethane (DCM)/hexane or DCM/ethyl acetate depending on the polar ity of the sample. Therefore, the more polar isomers will adsorb to the silica gel and take longer to wash than the less polar isomers3. In the above reaction, the ortho product should elute first as it is less polar than the para product.ResultsTotal percent yield using mass values Table 1Table 1 Mass of fractions 1-10Vial Number vacate Clean Vial (g)Dry Vial Weight (g)Product exclusively (g)113.349713.46630.1166213.335713.3370.0013313.160513.16080.0003413.081913.35430.2724513.205413.31470.1093613.283813.67430.3905713.200713.51760.3169813.046413.09770.0513913.315713.46820.12251013.581813.83760.2558Table 2. 1H NMR spectrum of o-nitrophenolAtomAtom is part of a groupPeak multiplicityPeak observed (ppm)Peak compute (ppm)AHydroxylSinglet10.710.84BAreneDoublet7.157.07CAreneTriplet7.06.59DAreneDoublet8.28.00EAreneTriplet7.67.22Table 3 1H NMR spectrum of p-nitrophenolAtomAtom is part of a groupPeak multiplicityPeak observed (ppm)Peak calculated (ppm)AAreneDoublet8.158.24BAreneDoublet6. 87.0CHydroxylSinglet5.456.0Table 4 IR spectrum of o-nitrophenolFunctional GroupMolecular MotionObserved Wavenumber (cm-1)Literature protect Range2-4 (cm-1)Peak IntensityPeak ShapeAromatic alcoholO-H continue3240.313550-3500WeakBroadAromatic C=CC=C Stretch1613.371600-1430Medium calculativeAromatic nitroNO2 Asymmetric Stretch1530.131540-1500Medium lancinate Aromatic nitroNO2 symmetric Stretch1471.311370-1330MediumSharpTable 5 IR spectrum of p-nitrophenolFunctional GroupMolecular MotionObserved Wavenumber (cm-1)Literature Value Range2-4 (cm-1)Peak IntensityPeak ShapeAromatic alcoholO-H Stretch2999.353550-3500WeakBroadAromatic C-HIn planeC-H bending1259.931275-1000MediumSharpAromatic nitroNO2 Asymmetric Stretch1517.921540-1500MediumSharpAromatic nitroNO2 Symmetric Stretch1326.381370-1330StrongSharpAromatic C=CC=C Stretch16001600-1430MediumSharpFigure 2 TLC plate A Figure 3 TLC plate BTable 6 Rf valuesCompoundRetention Factor (Rf)Relative Polarityo-nitrophenol0.93Less polarp-nitropheno l0.07More polarDiscussionIn this experiment a nitrophenol synthesis was carried out. The total percent yield is 42.7% as evident in Equation 2. Equations 2 and 3 show o-nitrophenol yield being 54.66% and p-nitrophenol being 45.34%. It could be assumed that not all of the organic matter was collected during the crude isolation phase.Two TLC analyses were performed to further determine the identity of o- and p- nitrophenols. The analysis on plate A determined that the fractions collected correspond to o-nitrophenol. This was concluded based on the distance the spots traveled up the plate. The o-nitrophenol complex is less polar than both the silica gel on the TLC plate and the p-nitrophenol complex. Therefore, it was expected to travel further up the plate. The fractions collected on TLC plate B correspond to p-nitrophenol this complex is polar and adheres to the polar silica gel of the plate. The Rf value (retention factor) obtained for o-nitrophenol is 0.93. The Rf value obtained fo r p-nitrophenol is 0.07. Compounds with larger retention factors are less polar as they do not stick to the polar solvent. The fractions collected on plate A are all pure as only one spot is observed per lane. Lanes 1 and 2 do not show any spots because the fractions were collected too early and no product exists. The only pure fraction collected on plate B is the one in lane 7. Lanes 8, 9, and 10 each have multiple spots suggesting that by-products are present. Lane 6 does not have any spots meaning that only solvent, not product exists.To confirm the identity of the product, 1 H NMR spectroscopy were used. The 1 H NMR spectrum of p-nitrophenol it is easily distinguishable because it contains only 3 observed primes- A, B and C at 8.15 ppm, 6.8 ppm and 5.45 ppm accordingly. Peak A is a doublet and belongs to the protons adjacent to the deshielding nitro group. The proton pair adjacent to the hydroxyl group show a doublet signal at 6.8 ppm on the spectrum. The singlet showing lack o f splitting must(prenominal) belong to the hydroxyl group, but it is far below expected values of around 10 ppm4. This is due to the intermolecular hydrogen hold fast in this compound. The spectrum for o-nitrophenol has louvre observed peaks. The hydroxyl group is just above 10.5 ppm, which is in normal melt down. Peak D which is a doublet belongs to the proton closest to the nitro group at 8.2 ppm. The triplet at present across the nitro group peak E has a values of 7.6 ppm. This value generally would be expected at 7.0 ppm, but the ortho and para positions are more deshielded due to the resonance structure observed in Figure 4 and 5.Comparing resonance structures of p-nitrophenol and phenol explains why pnitrophenol is more acidic (Figure 4, Figure 5). Phenol can donate an electron pair to the aromatic system from the hydroxide group. P-nitrophenol has a ring deactivating nitro group that withdraws electron density from the aromatic system. This allows the hydroxyl proton to be removed because of the partial positive charge on that side of the system. The conjugate base is then stabilized by the nitro group taking away an electron pair from the banly charged oxygen to form a double bond with the ring system. The stable conjugate base means that it cant form a new bond with the free proton, thus making p-nitrophenol more acidic than phenol. However with phenol, there is no electron withdrawing group, allowing oxygen to retain its negative charge. The conjugate base formed is very unstable and will immediately bond with any available proton. Also, o-nitrophenol has the nitro group in close proximity to the hydroxyl, thus allowing for intramolecular hydrogen bonding to occur. This slightly lowers the acidity of o-nitrophenol compared to pnitrophenol because the hydroxyl proton is made unavailable by the negative oxygen on the nitro substituent. Whereas in p-nitrophenol, intermolecular bonding occurs between other p-nitrophenols contributing to the overall stability of the compound.The IR spectrum of o-nitrophenol was given however, the IR spectrum of p-nitrophenol was obtained experimentally. The IR spectrum for o-nitrophenol shows the following stretch forthes O-H stretch C=C stretch aromatic NO2 asymmetric stretch and an aromatic NO2 symmetric stretch. The O-H stretch is caused by the hydroxyl group on the phenol ring. The observed value is 3240.31 cm-1 this corresponds to the literary works value range of 3550-3500 cm-1. The peak was broad and exhibited strong intensity. The C=C stretch is caused by the aromatic ring of the phenol. The observed value is 1613.37 cm-1 this corresponds to the literature value range of 1370-13130 cm-1. The peak was sharp and exhibited moderate intensity. The aromatic NO2 asymmetric stretch is caused by a nitro group. The observed value is 1530.13 cm-1 this corresponds to the literature value range of 1540-1500 cm-1. The peak was sharp and exhibited smedium intensity. The aromatic NO2 symmetric str etch is likewise caused by the nitro group.The p-nitrophenol IR spectrum exhibited many of the same peaks. The observed peaks are as follows O-H stretch C-H bending aromatic NO2 asymmetric stretch aromatic NO2 symmetric stretch and C=C stretch. The O-H stretch is caused by the hydroxyl group on the phenol ring. The observed value is between 3726.38 and 2999.35 cm-1 this corresponds to the literature value range of 3550-3500 cm-1. The peak was broad and exhibited weak intensity. The C-H in plane bend is caused by the aromatic ring of the phenol. The observed value is 1259.93 cm-1 this corresponds to the literature value range of 1275-1000 cm-1. The peak was sharp and exhibited medium intensity. The aromatic NO2 asymmetric stretch is caused by a nitro group. The observed value is 1517.92 cm-1 this corresponds to the literature value range of 1540-1500 cm-1. The peak was sharp and exhibited strong intensity. The aromatic NO2 symmetric stretch is also caused by the nitro group. The obs erved value is 1326.38 cm-1 this corresponds to the literature value range of 1540-1500 cm-1. The peak was sharp and exhibited medium intensity.ConclusionThe synthesis of o- and p-nitrophenol was performed using an electrophilic aromatic substitution of a nitro group in dilute acidic conditions. This was followed by column chromatography to separate the o- and p forms and TLC to confirm that the synthesis and purification was successful. The capture of o-nitrophenol and of p-nitrophenol was successful due to having product in vials 3,4,5 and 7 as seen on the TLC plates (Figure 2 nand 3). IR spectra of o- and p-nitrophenol also confirm a successful synthesis due to the differences in the aromatic OH streches (Table 4, Table 5). The experiment may be considered a success because of the differences between the IR spectra confirming the synthesis of o- and p-nitrophenol. The IR spectra may be improved by more homogenous packing of the column. Also, waiting to collect a darker yellow elu te may have increased yield of o-nitrophenol due to not capturing only solvent in vials 3-4.ReferencesStawikowski, M. Experiment 5 Synthesis and Purification of Nitrophenols BlackBoard.Touchstone, Joseph C. Practice of thin layer chromatography. 2nd ed. New York Wiley, 1983.PrintSmiley RA Ullmanns Encyclopedia of Industrial Chemistry. John Wiley and Sons. Richards, S. A., and Hollerton, J. C.. Essential Practical NMR for Organic Chemistry (1). Hoboken, GB Wiley, 2010, 2.