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The phenylchlorocarbene is formed after α-elimination of hydrogen chloride from benzal chloride when tBuO-.is introduced as base to deprotonate the proton on the benzylic carbon of benzal chloride. The anion formed after the deprotonation of benzal chloride loses a chloride ion spontaneously to form phenylchlorocarbene. A cycloaddition reaction will then occur between the acetylenic carbon of diphenylacetylene and phenylchlorocarbene to form the cyclic 3 member triphenylcyclopropenium chloride. Triphenylcyclopropenium chloride will then lose a chloride anion to form triphenylcyclopropenium cation which then react with nucleophilic tert butoxide to give the triphenylcyclopropenyl tert-butyl ether. The forming of this triphenylcyclopropenyl tert-butyl ether is a result of the Sn1 substitution of the chloride ion of triphenylcyclopropenium chloride with tert butoxide. The ether undergoes a hydrolysis reaction with water whereby the triphenylcyclopropenyl tert-butyl ether is protonated at the oxygen atom to form oxonium ion which will leave as t-butanol to form the bis(triphenylcyclopropenyl) ether. However, both the triphenylcyclopropenyl tert-butyl ether and bis(triphenylcyclopropenyl) ether can undergo hydrolysis with HBr to form 1,2,3 triphenylcyclopropenium bromide.
tBuOK which is a strong base is employed to deprotonate benzal chloride. The reaction should be performed in anhydrous condition otherwise side reaction such as hydrolysis of benzal chloride to benzaldehyde can occur as shown by the equation below:
C6H5CHCl2 + H2O ƒ C6H5CHO + 2 HCl.
tBuOK is highly hygroscopic since it can readily deprotonate water to form tertiary butanol. This will result in less tBuOK available to form both the ether required to give the final product. Therefore, the weighting of the potassium t-butoxide should be done quickly to reduce the exposure to moisture. The moisture collected in the solid potassium t-butoxide can also result in the undesirable reactions such as the hydrolysis of benzal chloride which is stated earlier.
To avoid moisture from the air getting into the reaction setup and causing undesirable side reactions, the setup below was assembled to create an inert condition for the reaction to proceed:
Nonetheless, a small problem still exist in the above reaction setup as N2 gas is flushed from the T shape holder at the top of the condenser. But it will take some time for the whole setup to be completely free from moisture and it is not certain how much time is required to create an inert environment for the reaction to proceed. Thus, another method of setting up the experiment could be to allow the N2 gas to flush in through the RBF with 3 necks. This will enable the setting up of an inert condition to be much faster and reliable.
Other viable ways to remove moisture and O2 from the reaction set up will be to have a Schlenk line where the air is removed from the reaction system with the aid of a vacuum. This method is especially useful if the reagent involved are prone to oxidation, the inert gas involved will usually be required to run through a deoxygenating catalyst in the form of Cu or Mn oxide column before being flushed through the system. This will completely ensure an inert environment whereby even trace amount of moisture and O2 can be removed.
After the deprotonation of benzal chloride, the chloride anion being a good leaving group is spontaneously lost to form the phenylchlorocarbene. The carbene that is formed is highly reactive since it has only 6 valence electrons. This carbene will be a singlet carbene with the lone pair on the benzylic carbon occupying and spin-pairing in the sp2 orbital which is highest occupied molecular orbital(HOMO). This leaves us with the remaining empty p orbital as the lowest unoccupied molecular orbital(LUMO). Hence, the carbene generated here acts as a nucleophile as well as an electrophile. Cycloaddition reaction is able to occur because of the HOMO-LUMO are close in energy and there is favourable interactions between HOMO-LUMO of the carbene and acetylenic carbons as exemplified below:
During the cycloadditon, one π bond is broken and two σ bonds are formed to result in the formation of the triphenylcyclopropenium chloride, a highly reactive intermediate whereby the electronegative chloro anion will leave spontaneously and be substituted by t-butoxide in an Sn1 reaction.
Due to the carbene formed being highly reactive, there are quite a number of side reactions happening as our desired reaction takes place. This provides the reason behind the 40% yield that is expected of the reaction. Several precautions can be taken to produce a higher yield such as slowly injecting benzal chloride into the reaction mixture so that the carbene formed is limited to a small amount to reduce the undesirable side reactions. Several of the side reactions are provided below :
The triphenylcyclopropenyl tert-butyl ether can be formed because of the formation of stable aromatic three member cyclic carbocation as an intermediate. The carbocation intermediate has all the 3 carbon to be trigonal planar in geometry and is sp2 hybridised, hence it is planar and highly conjugated since the 2 π electron is highly delocalized in the 3 member carbocation ring. The 2 π electron in the ring will satisfy the (4n + 2)π electron Huckel rule, this explains the reason behind its aromaticity. The intermediate carbocation is also stabilized by resonance with the 3 phenyl group on each of the 3 sp2 carbon of the 3 member ring. Furthermore, the chloro group is highly electronegative which makes it a good leaving group, this reason coupled with the stability of aromatic carbocation, makes the reaction proceed favourably towards the formation of brown coloured triphenylcyclopropenyl tert-butyl ether.
The O atom of the t-butoxide group is protonated when water is added to the reaction mixture containing triphenylcyclopropenyl tert-butyl. The t-butanol will then leave to form another stable triphenylcyclopropenium cation. Subsequently the carbocation will be attacked by the nucleophilic O atom of triphenylcyclopropenyl tert-butyl ether to form bis(triphenylcyclopropenyl) ether. The overall reaction for this step is an Sn1 substitution of the tert-butoxide group by triphenylcyclopropenyl tert-butyl ether.
The O atom of the bis(triphenylcyclopropenyl) ether is then protonated after HBr is introduced into the reaction mixture to form 1,2,3-triphenylcyclopropenium bromide and 1,2,3-triphenylcyclopropenol. 1,2,3-triphenylcyclopropenol is then once again protonated at the O atom of the hydroxyl group with HBr after which a water molecule leaves to form 1,2,3-triphenylcyclopropenium bromide. A yellow precipitate can be observed at this stage which eventually becomes pinkish purple in colour. This could be due to fact that the final product is stained with a little amount of the triphenylcyclopropenyl tert-butyl ether that have not reacted with hydrogen bromide or the unreacted HBr could be readily oxidized to reddish brown Br2 upon exposure to the air.
In order to minimize the undesirable reaction of phenylchlorocarbene with the phenyl ring of diphenylacetylene, benzal chloride was introduced dropwise over a 4 minute period with constant stirring of the reaction mixture. This precaution was taken so as to keep the amount of carbene low in order to avoid the carbene from reacting with phenyl ring of diphenylacetylene.
At the end of the reflux process, the resultant mixture was allowed to cool and water is added to separate the inorganic layer from the organic layer in a separatory funnel. The technique is employed to separate mixtures that have varying solubilities in the organic and inorganic solvent. Triphenylcyclopropenyl tert-butyl ether is a non polar compound which will result in non polar hydrophobic interaction with organic solvent but not water which is acting as the aqueous layer. Hence, it will be obtained in the organic layer eventually. On the other hand, water was used as the aqueous layer to separate potassium chloride and t-butanol from triphenylcyclopropenyl tert-butyl ether in the reaction mixture. Water being much denser than diethyl ether(density = 0.713gcm-3) will anchor as the bottom layer in the separatory funnel and form the aqueous layer. After water is introduced to the reaction mixture that is refluxed, triphenylcyclopropenyl tert-butyl ether will be hydrolyzed to bis(triphenylcyclopropenyl) ether. Bis(triphenylcyclopropenyl) ether that was left in the aqueous layer is obtained by adding diethyl ether to extract the bis(triphenylcyclopropenyl) ether from the aqueous layer. The extraction process is repeated a number of times using small volume of diethyl ether instead of one time with large volume of diethyl ether so as to extract more bis(triphenylcyclopropenyl) ether. Afterwhich, magnesium sulfate was introduced to the eventual organic layer to get rid of any water remaining in the organic layer and is filtered off thereafter.
The hydrogen bromide was introduced to the organic layer to form the yellow precipitate of 1,2,3-triphenylcyclopropenium bromide that is a result of the acidic cleavage of bis(triphenylcyclopropenyl) ether. Suction filtration was then employed to filter out the yellow precipitate. The mass of product obtained was 0.24 g, this will equate to a lower than expected percentage yield of 18.1% as opposed to the estimated yield of 40%. Reasons for the lower yield could have attributed to the fact that during the separation process in the separatory funnel, not enough shaking and mixing is done to allow the organic layer to mix homogenously with the aqueous layer to obtain the triphenylcyclopropenyl tert-butyl ether and bis(triphenylcyclopropenyl) ether completely. This could be 1 source of error which lead to a lower than expected yield. Another source of error could be during the suction filtration process, some products are flushed into the conical flask rather than being filtered out. This could have lead to a lower than expected yield as well. One solution will be to do the suction filtration of the filtrate again to filter out any residual product still left in the filtrate after the first round of suction filtration.
Next, we move on to discuss a little on triplet carbene. Triplet carbenes are not the carbene involved in this experiment. Normally, triplet carbene have 1 electron in the sp2 orbital and another in the p orbital. Hence, triplet carbenes behave and react in a varying manner from singlet carbenes. Triplet carbene are usually classified as diradical and therefore will undergo radical additions in more than 1 step. Carbenes with triplet nucleophilic electrons will usually form an intermediate with two single electrons whereas carbenes with singlet electron will undergo a one step reaction only. Because of the two varying form of reaction mechanism, reactions of carbenes with singlet electrons will be stereospecific and on the other hand, those of triplet carbene will be non-stereospecific.
Phenyl rings will usually show 3 absorption bands in the UV-vis spectrum because of the π ƒ π* electronic transitions.
The E-band which is the only allowed electronic transition with molar extinction coefficient =47000 is usually seen at 184 nm but this cannot be observed in the UV-vis spectrum obtained as it is out of the regular region of what most UV-vis spectrometer can detect. It is commonly being referred to as the primary band.
The K-band(molar extinction coefficient =7400) which is forbidden is normally seen at 204 nm if substituent on the benzene ring increase the wavelength of the transition into the regular UV-vis region. It is commonly referred to as the secondary primary band.
The B-band(molar extinction coefficient =230) is usally seen at 256 nm and is a forbidden transition. It is commonly known as the secondary band.
The forbidden transitions usually do not mean it does not occur but rather it occurs in a very short period of time only because of change of symmetry induced by the vibrational energy states.
For 1,2,3-triphenylcyclopropenium bromide, the 3 membered aromatic ring of the cation helps to extend the conjugation of the 3 phenyl ring on each of the three carbon of 1,2,3-triphenylcyclopropenium bromide. This in turn results in a bathochromic shift leading to absorption bands of longer wavelength being observed in the UV-vis spectrum. The increased delocalization of electrons reduces the energy gap between the highest occupied molecular orbital and lowest occupied molecular.
The absorption at 301.0 nm will be the π to π* transition that occur in the pi bond of the 3 membered cyclopropenium ring. Normally in simple unsaturated alkene, π to π* transition will happen around 175nm. Because of the 3 membered ring having a phenyl ring at each carbon of the ring in 1,2,3-triphenylcyclopropenium bromide, the energy gap between the π and π* orbital will be smaller, this will result in a shift which ultimately leads to a transition of longer wavelength being seen. This π to π* transition is an allowed transition therefore it has the highest absorbance of 2.6963 amongst the 3 bands being mentioned.
The very weak peaks that are seen are usually due to the background noise hence will not be assigned any stretching mode.
O-H stretch of water
Aromatic C=C stretch of the phenyl ring
C=C stretch of the double bond in cyclopropenium
C-H in-plane bending of aromatic ring
C-H OOP bend (monosubstituted) of aromatic ring
From results and assignment, the peaks obtained is characteristics of the final product obtained with the necessary C=C stretch of the benzene ring and double bond of cyclopropenium being shown in the resultant spectrum. Nonetheless, an anomaly occurs in the form of the OH stretch at 3402.66cm-1. This evidence together with the lack of strong or medium sharp peak in 1000-1300cm-1 region is indicative of the lack of C-O group in the final product. Therefore, it is unlikely to have any presence of ether or alcohol in the final product. Hence, the OH stretch at 3402.66cm-1 should belong to that of water in the final product. The C=C stretch of the cyclopropenium is low perhaps due to the many forms of resonance in the structure which leads to the lowering of the bond order of C=C double bond. This will result in lower bond strength and therefore, a lower force constant, k of the C=C bond in the 3 member cycloprepenium.
There are numerous ways in which water can get into the final product. Firstly, t-BuOK can easily trap water due to it being hygroscopic and during the separation of the organic layer and aqueous layer, it is very difficult to completely remove water from the organic layer since the mixture are all in liquid form. Secondly, inadequate amount of magnesium sulfate may be added resulting in moisture not being completely removed from the final crude product. Furthermore, IR lamp is not utilised in this experiment to dry up the product. Therefore, these reasons sum up to result in the moisture accumulated in the product.
0.24 g of 1,2,3-triphenylcyclopropenium bromide was made with a 18.1% yield. The UV-vis and IR spectrum are indicative of the product synthesized belongs to that of 1,2,3-triphenylcyclopropenium bromide.