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The formation of the phenylchlorocarbene is obtained from an elimination of HCl from benzal chloride. The phenylchlorocarbene undergoes a cycloaddition with diphenylacetylene to form triphenylcyclopropenium chloride. This further undergoes an SN1 nucleophilic substitution with t-BuO- to form triphenylcyclopropenyl tert-butyl ether. The ether can be hydrolyzed with water to bis(triphenylcyclopropenyl) ether, but both ethers can be converted to the ionic bromide salt with HBr.
A strong base such as potassium tert-butoxide is used to abstract a proton from benzal chloride. The reaction must be carried out under a dry environment lest benzal chloride be hydrolyzed into benzaldehyde; C6H5CHCl2 + H2O ïƒ C6H5CHO + 2 HCl. Subsequent oxidation in air may yield benzoic acid.
Potassium tert-butoxide is also hygroscopic and is deactivated by moisture in the air. The transfer of the base during weighing should be fast to minimize the amount of moisture absorbed. The absorbed moisture may also contribute to the hydrolysis of benzal chloride as mentioned earlier.
To prevent moisture from entering the experimental setup, an inert environment was provided with the aid of this setup as shown:
There is however a slight flaw in this experimental setup as the source of nitrogen is from the top and it may not give a clean flush of the air within the round-bottomed flask (RBF). One may argue that it may be inert over time, but how long must it be flushed in this manner to ensure total removal of air? An alternative setup whereby the source of nitrogen enters from a 3-necked RBF and is flushed in one direction would indeed ensure a more inert environment.
Of course, there are better methods to keep moisture and oxygen out of the experimental setup. One way would be to set up a Schlenk line where the evacuation of air via a vacuum and purging with deoxygenated inert gas first passing through a deoxygenation catalyst like Cu (I) of Mn (II) oxide ensures the removal of oxygen below even ppm levels.
After the abstraction of a proton from benzal chloride, a chloride then leaves to generate phenylchlorocarbene. Chloride is a good leaving group and hence leaving behind the carbene molecule which is characterized by the carbon atom only bearing six valence electrons. The carbene generated here is a singlet carbene, further characterized by the lone pair of electrons sitting in an sp2 orbital while a p orbital lies vacant. In this sense, singlet carbenes function as both a nucleophile and an electrophile. The concerted cycloaddition is able to happen owing to favourable HOMO and LUMO interaction between phenylchlorocarbene and diphenylacetylene as shown below:
The product of the cycloaddition whereby 1 π bond is broken and 2 σ bonds are formed is triphenylcyclopropenium chloride, a tertiary alkyl halide.
Owing to the high reactivity of carbenes, there are plenty of side reactions going on concurrently with the main mechanism and hence accounting for the predicted 40% yield as per the lab manual. Good experimental techniques may account for the 62% yield in this work such as the slow introduction of benzal chloride to ensure that the concentration of generated carbene is kept low to prevent side reactions from happening. A few of them are illustrated as shown:
The formation of triphenylcyclopropenyl tert-butyl ether follows SN1 mechanism because of the high stability of the triphenylcyclopropenium ion formed as the intermediate. The 3-membered carbocation ring is aromatic because it is planar and conjugated with 2 π electrons, fulfilling Hückel's Rule of (4n + 2) π electrons. The cation is further resonance stabilized with the 3 phenyl groups. As such, coupling of Cl- being a good leaving group and the aromaticity of the triphenylcyclopropenium ion leads to a good yield of the above mentioned ether, which is light brown in colour, after the tert-butoxide attacks the carbocation.
The addition of water causes the oxygen atom on triphenylcyclopropenyl tert-butyl ether to be protonated. This protonated species then undergoes another SN1 reaction whereby a stable tert-butyl alcohol leaves, generating another stable triphenylcyclopropenyl cation. This is in turn attacked by a molecule of the triphenylcyclopropenyl tert-butyl ether to give bis(triphenylcyclopropenyl) ether.
The oxygen atom of the bis(triphenylcyclopropenyl) ether abstracts a proton upon the addition of HBr to give the 1,2,3-triphenylcyclopropenium bromide and 1,2,3-triphenylcyclopropenol. 1,2,3-triphenylcyclopropenol further reacts with HBr via an SN1 mechanism too to produce the 1,2,3-triphenylcyclopropenium bromide. This is seen as a yellowish precipitate initially but slowly turns pinkish. It could be either attributed to the precipitate trapping some of the earlier triphenylcyclopropenyl tert-butyl ether which has not reacted with HBr yet, or possibly due to the oxidation of HBr to reddish-brown bromine in air since the inert environment has been removed.
In order to prevent localized reaction of benzal chloride with diphenylacetylene, the lab manual called for benzal chloride to be added dropwise over a four-minute duration under stirring. In this experiment however, benzal chloride was added even slower over 10 minutes to keep carbene concentration low.
After reflux, the reaction mixture was cooled and purified by separation using a separatory funnel. Extraction is a technique used to separate compounds based on their different solubility in two immiscible solvents. Triphenylcyclopropenyl tert-butyl ether is relatively non-polar and will preferably form hydrophobic interactions with diethyl ether and stay in the organic layer. Hence, water was added in order to separate potassium chloride and tert-butyl alcohol from the organic layer. The aqueous layer which constitutes water is denser than the organic layer (density of diethyl ether: 0.713g cm-3) and thus, will form the bottom layer in the separatory funnel. Upon addition of water, triphenylcyclopropenyl tert-butyl ether was hydrolyzed to bis(triphenylcyclopropenyl) ether. To extract the bis(triphenylcyclopropenyl) ether that might have entered the aqueous layer, diethyl ether was used to extract the remaining bis(triphenylcyclopropenyl) ether in the aqueous layer to the organic layer. Instead of using a single large volume of solvent for extraction, multiple extractions with small amount of diethyl ether were performed to increase the extraction efficiency. Magnesium sulfate was then added to the combined organic layer to remove water found in it, and then filtered off.
The addition of HBr will yield yellow precipitate of 1,2,3-triphenylcyclopropenium bromide, which forms from the acidic cleavage of bis(triphenylcyclopropenyl) ether. The crude product was then obtained via suction filtration. The mass of 1,2,3-triphenylcyclopropenium bromide produced was 0.82 g, giving a relatively higher percentage yield of 61% as compared to the expected percentage yield of 40%. As the product may not have been totally dried, since the use of an IR lamp was not employed, the presence of water could likely have resulted in a higher percentage yield obtained. This was also marked by the precipitate sticking to the sides of the plastic bag as well as the presence of an O-H stretching band at 3388 cm-1.
The chemistry of a singlet carbene was mentioned earlier and now we can shift the focus to triplet carbenes. Triplet carbenes typically contained one electron in thesp2 orbital and one electron in the p orbital. As such, triplet carbenes react differently from singlet carbenes; different in a way that they can be considered to be diradicals and hence participate in stepwise radical additions instead. Reactions of triplet carbenes go through an intermediate with two unpaired electrons whereas singlet carbenes react in a single concerted manner. Owing to the two different modes of reactivity, reactions of singlet carbenes are stereospecific whereas those of triplet carbenes are non-stereospecific.
The product was also characterized by both UV-vis and IR spectroscopic measurements
Absorbance of bands
Benzene rings usually give three absorption bands due to π ïƒ π* electronic transitions.
The allowed transition (ε = 47000), E-band, observable at 184 nm is not usually observed as it lies out of the routine UV-vis scanning range of most commercial instruments. This is also known as the primary band.
The forbidden transition (ε = 7400), K-band, occurring at 204 nm is often observed if substituent effect shifts it into the routine UV range. This is also known as the secondary primary band.
There is another forbidden transition (ε = 230), B-band, at 256 nm. This is also referred to as the secondary band.
The forbidden transitions are allowed for a small fraction of time owing to disruption of symmetry caused by the vibrational energy states.
In the case of 1,2,3-triphenylcyclopropenium bromide, the additional conjugation brought about by the 1,2,3-triphenylcyclopropenyl cation ring causes a bathochromic shift in the bands to a longer wavelength. The high extent of delocalization of electrons narrows the energy gap between the HOMO and LUMO and hence accounts for the longer wavelengths absorbed.
From the spectrum obtained, we can classify the band at 317.00 nm to be the B-band and the band at 230.50 nm to be the K-band. By Beer-Lambert's law, since A = εcl, absorbance is proportional to the molar extinction coefficient and hence the band with lower absorbance (A = 1.1746) and at higher wavelengths (317.00 nm) is the B-band and the band with higher absorbance (A= 1.4087) and at lower wavelengths (230.50 nm) is the K-Band.
The band occurring at 301.50 nm corresponds to the π to π* transition of the double bond in the cyclopropenium ring. In alkenes, π to π* transition occurred at 175nm. Owing to conjugation with the three benzene rings in 1,2,3-triphenylcyclopropenium bromide, the energy gap decreases and leads to a bathochromic shift to a higher wavelength as observed. This π to π* is an allowed transition hence a higher absorbance value of 1.4500.
The very weak peaks observed can be attributed to the noise and hence are not factored in during the band assignment.
Phenyl C=C stretch
Cyclopropenium C=C stretch
Aromatic phenyl C-H in-plane bends
Aromatic phenyl C-H out-of-plane bending -monosubstituted
Out-of-plane ring bending
From the peaks assigned, we can see that the product obtained was quite pure as all peaks are characteristic of the final product. There is however a stretching peak at 3388 cm-1 and this suggests the presence of an O-H. There is also an absence of strong and sharp of C-O stretching peak(s) between 1300 - 1000cm-1. This further indicates that alcohols and ethers were not present and hence the O-H stretch is attributed to that of water.
There are many sources by which water could have entered the product. Firstly, potassium tert-butoxide is hygroscopic and hence absorbs water readily. Secondly, it can also be attributed to improper extraction of the organic layer from the aqueous layer. Thirdly, there may have been a lack of magnesium sulfate and hence water was not completely absorbed towards the end of the reaction. Lastly, the use of an IR lamp to dry the product was not employed. As such, any moisture present would have clung on to the product and not be evaporated to dryness.
As the phenyl rings were monosubstituted, 4 weak combinations of aromatic overtones should have been observed in the range of 2000 - 1667 cm-1. This was however not seen in the spectrum as the weak peaks were obscured by noise.
The C=C stretch in a cyclopropene ring is expected to be around 1640 cm-1. Conjugation in the 1,2,3-triphenylcyclopropenium ion however increases the single bond characteristic of the C=C bond and hence decreases the bond order. A lower bond order translates to weaker bond strength and hence a smaller force constant, k.
0.82 g of 1,2,3-triphenylcyclopropenium bromide was synthesized with a percentage yield of 62%. The UV-vis and IR measurements obtained confirm that the product obtained was that of 1,2,3-triphenylcyclopropenium bromide.