Introduction
Reduction reactions are very important in chemistry as they are effective processes for synthesizing new compounds from existing ones. In organic chemistry, a compound is said to be reduced if it acquires new hydrogen. There are diverse reagents for carrying our reduction reactions. Two of these are sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) - the latter is a more powerful reducing agent compared to the former.
In this experiment, NaBH4 was used to reduce p-Nitrobenzaldehyde to form p-Nitrobenzyl alcohol. NaBH4was used over LiAlH4 in this experiment, because LiAlH4 would also reduce the nitro group of the aldehyde.
Experimental:
The synthesis of p-Nitrobenzyl alcohol from p-Nitrobenzaldehyde was done using the experiment procedures in CHM 243 Lab Manual.
Results:
It should be noted that during the reaction of p-Nitrobenzaldehyde with NaBH4, there was no observed changes in the color or temperature of the reaction mixture. Figure 1 shows the spots in the TLC plate of the pure aldehyde, pure alcohol, and the reaction mixture at 15 and 30 minutes reaction times. The corresponding distance travelled of each spot is shown in table 1. The computed retardation factors (Rf) are also shown in the same table.
Figure 1: TLC results
Sample Computations:
Rf = distance traveled by the solvent / distance traveled by the solute
Rf (Pure p-Nitrobenzaldehyde) = 3.50cm/4.50cm = 0.78
Rf (Pure p-Nitrobenzyl alcohol) = 1.00cm/4.5cm = 0.22
Rf (Reaction Mixture at 15mins) = 1.00cm/4.5cm = 0.22
Rf (Reaction Mixture at 30mins) = 1.50cm/4.5cm = 0.33
Discussion:
The reaction between p-Nitrobenzaldehyde and NaBH4 is well known in chemistry. Accordingly, the reduction reaction is facilitated by the nucleophilic attack of a hydride ion to the carbonyl group of the aldehyde. The overall reaction is shown below:
Figure 2: Chemical reaction between p-Nitrobenzaldehyde and NaBH4 (Wright State University 3).
The reaction in figure 2 can be represented using the following chemical reaction equation:
C7H5NO3 + NaBH4 C7H7NO3 + NaBH3
Overall, the reaction is nucleophilic addition (Hunt n.p.). The nucleophilic attack of the hydride ion is selective to the carbonyl group because the carbonyl group is partially polarized with the carbon atom having a partial positive charge and the oxygen atom has a partial negative charge as brought about by their differences in electronegativity – oxygen being more electronegative than carbon. The hydride ion on the other hand bonds preferentially with the partial positive carbon over boron. The reaction mechanism for the above reaction is shown below. The reaction between p-Nitrobenzaldehyde and NaBH4 is chemoselective, because the hydride ion preferentially attacks the carbonyl carbon over the nitro group substituent at the para position in the benzene ring.
Figure 3: Complete reaction mechanism of p-Nitrobenzaldehyde and NaBH4 reaction.
Based from the chemical structure of the product and the starting material, it can observed that the product is capable of hydrogen bonding through its alcohol (--OH) substituent. It should be noted that in TLC, there are two phases used: the mobile phase and the stationary phase.
The stationary phase is usually made from silica gel that is a supported in an inert material such as a piece of glass. The silica gel is functionalized with (--OH) substituent during its preparation unto the inert material. This means that the surface of the stationary phase is polar and is capable of hydrogen bonding.
The mobile phase, on the other hand, is usually an inert liquid or gas – or both. Dichloromethane is an example of a compound usually used as the mobile phase in TLC. Dichloromethane is non-polar and can exist in both liquid and gas phases at room temperature. It is important in TLC that the surrounding air is made up mostly with an inert compound, favorably the vapor of the mobile liquid phase.
The separation of compounds using TLC lies in the difference in the polarity between the stationary phase and the mobile phase, and the polarity of the compounds being separated. Accordingly, a non-polar compound will become dissolved to the mobile solvent, while a polar compound would adsorb strongly to the stationary phase. When a compound gets strongly adsorbed to the stationary phase, it will have a very low Rf value. On the one hand, when a compound gets dissolved to the mobile phase, it will have a high Rf value. Hence, the Rf values in TLC could help identify and separate different compounds contain in a given mixture (Clark n.p.).
Since the product of the reaction between p-Nitrobenzaldehyde and NaBH4 is capable of hydrogen bonding with the stationary phase, it can be expected that it will have a very low Rf value. The starting material, p-Nitrobenzaldehyde, however, is not capable of hydrogen bonding; hence, it can be expected that it will have high Rf value.
Using the principle employed in TLC, the Rf values of the compounds and mixtures in table 1 can be identified and the speed of the reaction between p-Nitrobenzaldehyde and NaBH4 to form p-Nitrobenzyl alcohol can be properly estimated. Accordingly, the Rf value of the pure aldehyde is the highest (0.78). This value proves that p-Nitrobenzaldehyde is non-polar as it got dissolved by the non-polar mobile phase. The Rf value of the pure alcohol is the lowest (1.00) which proves that it is polar. Hence, if the formation of p-Nitrobenzyl alcohol has completed within the 30mins reaction time, the spot of the reaction mixture would be very close or equal to that of the pure alcohol.
Interestingly, the spot at 15mins interval has the same Rf value as the pure alcohol, which indicates that the reaction has proceeded to completion to form p-Nitrobenzyl alcohol. This inference is verified further by the increased in the Rf value of the reaction mixture spot at 30mins reaction time interval. The increased Rf value at 30mins could mean that the there have been less polar side products formed after 15mins. Hence, it can be inferred that the reaction between p-Nitrobenzaldehyde and NaBH4 proceeds to completion within 15 mins reaction time.
Conclusion:
The selection of NaBH4 as the reducing agent for p-Nitrobenzaldehyde is a good choice since it allowed the maintenance of the integrity of the nitro group. If the stronger reducing agent LiAlH4 was used, then there will be an array of side products with both the carbonyl and nitro groups being reduced, in which case the use of TLC to monitor the progress of the reaction may not be a good choice as well.
With regards to the results of this experiment, it can be concluded that the formation of p-Nitrobenzyl alcohol has proceeded to completion within the 15 minutes reaction time. It can also be concluded that there have been side products formed after the 15minutes reaction time, and these side products are less polar than p-Nitrobenzyl alcohol.
Works Cited:
Clark, Jim. Thin Layer Chromatography. 2007. ChemGuide. 2 Aug 2016. <http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html>.
Hunt, Ian. Nucleophilic Addition. N.d. Web. 2 Aug 2016. <http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch17/ch17-3-0.html>.
Wright State University. Reduction of Carbonyl Compounds. 2016. Web. 2 Aug 2016. <http://www.chm.wright.edu/feld/chm216/CHM%20216%20Ex%207%20Reduction.pdf>.
