Lab Report on Reductive Amination

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Lab Report on Reductive Amination to synthesize N-cinnamyl-m-nitroaniline (2° amine) by forming the imine from the condensation between cinnamaldehyde and 3-nitroaniline (1° amine), followed by selective reductive amination with sodium boronhydride (NaBH4)

The lab report below was submitted as part of the coursework for CM2121 Organic Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.


Aim:
To synthesize N-cinnamyl-m-nitroaniline (2° amine) by forming the imine from the condensation between cinnamaldehyde and 3-nitroaniline (1° amine), followed by selective reductive amination with sodium boronhydride (NaBH4).
Results and Calculations:
Temperature at which the distillation occurs: 78.0 oC       
Mass of recrystallized product and plastic bag = 3.944 g and of plastic bag = 0.7601g
Mass of recrystallized compound B = 3.944 – 0.7601 = 2.9343g                                            
ncinnamaldehyde = mcinnamaldehydeMcinnamaldehyde = 2.910132.16 = 0.022019 mol
n3-nitroaniline = m3-nitroaniline M3-nitroaniline = 2.755138.12 = 0.019995 mol
Since cinnamaldehyde and 3-nitroaniline reacts in the mole ratio of 1:1, therefore, 3-nitroaniline is the limiting reagent.
Mr of N-cinnamyl-m-nitroaniline = 254.31 g/mol
Theoretical mproduct B = 254.31 × 0.019995 = 5.0849 g (5 s.f)
Percentage yield of product B = 2.93435.0849 × 100% = 57.7 % (3 s.f)
Melting point range: 105.0 °C – 105.3 °C
Discussion:
Mechanism for synthesis of intermediate A and eventual production of compound B
The first part of this experiment was to synthesize the intermediate imine A, N-cinnamylidene-m-nitroaniline. A magnetic stirring bar was placed inside the round bottomed flask to ensure continuous and homogenous mixing of both reactants.
Excess cinnamaldehye was added carefully into the round bottomed flask containing 3-nitroaniline in a drop-wise manner. Heat was only applied after all the cinnamaldehyde had been added, as heating the solution prior to addition may cause the 3-nitroaniline to decompose explosively, producing toxic NO2 gas[1]. The heat applied was also controlled as excess heating could lead to 1,4 conjugate addition instead of the desired 1,2 direct addition.

  1. Formation of intermediate A, N-cinnamylidene-m-nitroaniline

The lone pair on N atom act as a nucleophile to attack the electrophilic C of the carbonyl. After a series of protonations, the water leaves producing the imine intermediate.
Upon collecting 22cm3 of distillate, the mixture was cooled and the remaining residue was dissolved in 25cm3 of 95% ethanol before 0.7838g of NaBH4 dissolved in 25cm3 of 95% ethanol was added in a drop-wise manner.
  1. Formation of compound B, N-cinnamyl-m-nitroaniline
After the mixture was refluxed for 15 minutes, an orange-red solution was obtained. NaBH4, being a mild reducing agent, does not reduce the alkene or nitrobenzene but reduce the iminium group, producing N-cinnamyl-m-nitroaniline, an orange-red compound. The addition of 50cm3 of water to the cooled refluxed mixture ‘destroyed’ the NaBH4 and‘quenched’ the reduction. The mixture was cooled in an ice water bath for further crystal growth and the crude product was obtained via vacuum filtration.
Ethanol was used as the solvent, instead of water. This is because it is only weakly protic. Also, as an organic solvent, it can dissolve the organic reactants and products.
Mechanism for possible side reactions
Care must be taken to prevent the following side reactions, thereby increasing the overall yield.
  1. Hydrolysis of cinnamaldehyde
It was ensured that the apparatus used were dry to avoid the hydrolysis of cinnamaldehyde.

Water is produced from the condensation between 3-nitroaniline and trans-cinnamaldehyde. It may react with the reagent, trans-cinnamaldehyde to form an unwanted diol and lower the overall yield.
  1. 1,4 conjugate addition to cinnamaldehyde
    When the mixture is heated for too long during the condensation of 3-nitroaniline and cinnamaldehyde, 1,4 conjugate addition may occur instead of the desired 1,2 direct addition. This produces the thermodynamic product instead of the kinetic product.
  2. Side reaction of sodium borohydride
     
NaBH4 is a base that is sensitive to water. If water is present, BH4- will deprotonate it to form hydrogen gas and borohydride. The BH3 formed can further react with the hydroxide ion. Thus, NaBH4 is ‘destroyed’.
  1. Side reaction of sodium borohydride and remaining cinnamaldehyde
Since 3-nitroaniline is the limiting reagent, NaBH4 will reduce the remaining cinnamaldehyde to 1o alcohol. This decreases the amount of NaBH4 for reducing imine A.
Recystallization of crude product
The large amount of crude product (7.188g) obtained indicates the presence of side products and impurities.
The pure N-cinnamyl-m-nitroaniline is then obtained via recrystallisation. During crystallization, the crude product was dissolved in sufficient 95% ethanol to ensure that no N-cinnamyl-m-nitroaniline is lost which may affect the yield, and the mixture was heated mildly. A few drops of warm ethanol was added to ensure that all the crude product were dissolved and to dispel the cloudiness. Then, the heating was turn off and the solution was allowed to cool to room temperature.
The saturated solution was allowed to cool slowly with minimum disturbance to provide sufficient time for the newly dissolved crystals to assemble into their optimal crystal pattern. Rapid cooling causes the solid to precipitate out quickly and trap the impurities within the crystals. Hence, the solution was cooled at room temperature before further cooling in an ice bath. The lower temperature of the ice bath will cause the more N-cinnamyl-m-nitroaniline to crystallize hence increasing the yield.
The appearance of the crystals obtained is bright orange red, shiny and granular.
Azeotropic distillation
It was noticed that, upon reaching the temperature of 78.0° C, boiling point of ethanol, the temperature remained constant and distillate was generated. Thus, it can be concluded that the ethanol used as solvent was distilled out.
Imines are unstable and are easily hydrolysed to back to their amine and carbonyl form. Hence, in order to drive the reaction to the right (synthesize more imine), there is a need to remove the water produced. Hence, azeotropic distillation, a process in which a liquid mixture is separated into pure components with the help of an additional substance or solvent, is carried out.[2] In this case, the solvent is ethanol.
In the mixture, both water and ethanol were present. Although the boiling point of water is 100° C, ethanol and water form a minimum-boiling azeotrope (78.0° C) [3], thus water generated during condensation is continuously removed by distilling ethanol-water azeotrope during the course of reaction. [4]
The components in the 22 cm3 distillate are ethanol and water. Water boils at 100 ºC and ethanol boils at 78.0 ºC.  thus the mixture will boil at 78.2 ºC giving a compostion approximately 95.5% by mass of ethanol and 4.5% by mass of water as this is the azeotropic ratio.[2] The ethanol has a higher composition because the boiling point of ethanol (78.0° C), is closer to the 78.2 ºC,  where the vapour and liquid compositions becomes identical.
One of the assumptions is that the composition of ethanol-water is the same as that of the distillate. Another assumption is that the temperature is assumed to be constant at the vapour-liquid interface and vapour free stream are at the bubble point and dew point respectively. The mass and heat transfer processes in the azeotropic distillation are assumed to be vapour-phase controlled and the interaction between diffusion fluxes (water and ethanol) is negligibly small.
Discussion on the yield of N-cinnamyl-m-nitroaniline
The experimental yield tabulated was only 57.7%. There are several factors that attributed to such a yield.
Firstly, the production for imine A is in a dynamic equilibrium. It is not possible for all the reactants to form the imine product. Thus, percentage yield of 100% cannot be achieved.
Other than the equilibration, side reactions may result in either the lost of reactants or the lost of the NaBH4 reducing agent. From the mechanism of side reactions, highlighted in Page 3. Reactions (i), (ii) and (iv), ‘destroyed’ the cinnamaldehyde reactant for the correct condensation with 3-nitroaniline. Hence, with a lower amount of cinnamaldehyde, according to the Le Chatelier’s Principle, the position of equilibrium of the reaction would shift left, resulting in a lower yield in the imine product, which later then affects the yield for N-cinnamyl-m-nitroaniline.
Reaction (iii) highlights the fact that the reducing agent, NaBH4 may also be lost due to reaction with the water presence in the air. However, it is highly unlikely that such side reaction had occurred, as no effervescence of hydrogen gas was observed. In addition, NaBH4 was added in excess, thus, the minute amount loss due to exposure to moisture is minimum. However, a significant amount of NaBH4 could be lost when it react with the remaining cinnamaldehyde, as highlighted in reaction (iv). Thus, this side reaction could contribute to the lower yield of N-cinnamyl-m-nitroaniline.
As the preparation of the amine and the purification of the N-cinnamyl-m-nitroaniline involved many steps of transferring, the small amounts of reagents lost in each step added up to a significant amount eventually. Thus, incomplete transfer of all the products resulted in the loss of some N-cinnamyl-m-nitroaniline.
Lastly, some N-cinnamyl-m-nitroaniline could be lost due to incomplete recrystallization. The low yield may also be attributed to too much ethanol added to dissolve the solid. When excess solvent was used, less solute will crystallize out, as the saturation is lowered.
Discussion on the purity of N-cinnamyl-m-nitroaniline
In order to determine the purity of the N-cinnamyl-m-nitroaniline synthesized, melting point measurement was conducted.Pure crystalline compounds possess characteristic melting points, thus any deviations in the experimental melting points from the theoretical melting point range would indicate the presence of impurities as impurities tend to depress and broaden the melting range. Presence of impurities affects the melting point of the compound due to colligative properties.
The theoretical melting point range of N-cinnamyl-m-nitroaniline is 106.0°C – 107.0°C. [6]
The experimental melting point range of N-cinnamyl-m-nitroaniline is 105.0°C – 105.3°C
The experimental melting point range lies slightly out of the theoretical melting point range as the experiment was not carried under standard conditions.
Also, there are some experimental errors during the measuring of the melting point. When determining the melting range of the crystals, there was difficulty in observing when the first crystals melts as the particles are small. To overcome this, repeated readings should be taken to narrow down the melting range and obtain more accurate readings. Furthermore, the packing of crystals may not be tight enough, allowing air sachets to affect the conduction of heat between the molecules, thus melting point may vary. Dust particles from the surrounding may also inevitably land in the mixtures to lower the purity of the crystals when they are brought to test for their melting points.
Conclusion:
The intermediate imine A is N-Cinnamylidene-m-nitroaniline, while Compound B is N-cinnamyl-m-nitroaniline. Mass and percentage yield of N-cinnamyl-m-nitroaniline obtained is 2.9343g and 57.7% respectively. Its experimentally determined melting-point range is 105.0 °C – 105.3 °C.
References:
[1] 3-nitroanilinehttp://www.analytyka.com.mx/english/MSDS/N/N0038.htm
[2] Azeotropic Distillation

http://www.answers.com/topic/azeotropic-distillation#ixzz1Y1tthHq3
[3] Oxford University. (2003, September 8).
Safety data for Ethanol/water Azeotrope. Retrieved August 28, 2011, from Oxford MSDS: http:// msds.chem.ox.ac.uk/azeotropes/Ethanol-water_azeotrope.html
[5] Positive deviations from Raoult’s Law

http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html
[4] [6] John C. Gilbert, Stephen F. Martin, Experimental Organic Chemistry: A Miniscale and Microscale Approach, Thomson Brooks/Cole, 5th ed, United States (2010) pp572-575
Questions:
Q1 is answered under the section, Discussion, Azeotropic distillation. Q2 is also answered under the same section. Q3 is answered under the section, Discussion, Mechanism

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