Bromination of cinnamic acids

Bromination of cinnamic acid with NBS in aq. MeOH gives 2-bromo-3-methoxy-3-phenyl propionic acid, PhCH(0Me)CHBrC02H. At moderately high [H+] the reaction is second-order and independent of [H+], whereas at low [H+] the rate increases with decrease in [H+], Succinimide inhibits the reaction182. 

What products are obtained from the bromination of cinnamic acid [(E)-3-phenylpropenoic acid] Draw the structural formulae of the products as Fischer projection formulae. What relationship do the products have to one another ... 

The bromination of cinnamic acid leads to two intermediate enantiomeric cyclic bromonium ions which are subsequently attacked by a nucleophilic bromide ion. Since this attack can occur both at positions 2 and 3, and because the reaction proceeds stereospecifically with inversion, both bromonium ions will give rise to mutually identical pairs of enantiomeric products. This is easier to see from the Fischer projection formulae, which are readily obtained from the zigzag projections by rotation around the C2-C3 bond. 

The bromination of cinnamic acid dissolved in carbon tetrachloride or other inert solvent.offers a convenient system for study. The dibromocinnamic acid produced remains in the carbon tetrachloride solution. The thermal reaction is so slow that it can barely be measured at room temperature and it is entirely negligible in comparison with the photochemical reaction at ordinary intensities. The quantum yield is so large that considerable reaction occurs even if the intensity of light is much reduced by the monochromator or other device for confining the light to a narrow range of frequencies. Furthermore, the reaction is easily and accurately followed by titration with sodium thiosulfate. Potassium iodide is added and the iodine liberated is a measure of the remaining bromine. 

Buckeles, et al. reported [11] that the gas-solid bromination of cinnamic acid and other aryl olefins proceeds in either an absorbed phase or in a film of solution on the surface of thjfe solid. At the end of the reaction with large excess (100 times molar)... 

Formation of bromostyrene. Dissolve 0-2 g. of cinnamic acid (or a cinnamate) in about 5 ml. of NagCOg solution. Add bromine-water drop by drop and note the rapid separation of bromostyrene, CjHjCHiCHBr, as a colourless oil, having a pleasant characteristic odour. 

Method II.—12-5 gms. (1 mol.) of cinnamic acid are dissolved in 65 c.cs. of anhydrous ether, and the solution cooled to 0° in a freezing mixture. 4-3 c.cs. (1 mol.) of bromine are then slowly added from a burette while all but diffused daylight is excluded, as the reaction is very violent in direct sunlight. The ether is removed on a water bath, and the residue recrystallised from dilute alcohol. 

A separate filling was made for each experiment, the whole of the solution being titrated each time. Obviously the concentration of the bromine was changing continuously and the values in the first, third and fourth columns are average values. There was an excess of cinnamic acid, 0.0055 moles per liter or more, in all cases except the first. 

Fig. 32.—Influence of bromine concentration on quantum yield initial concentration of cinnamic acid 0.0110 for points for all other points, 0.0055.

According to another hypothesis frequently suggested, the bromine atoms combine with bromine molecules to give triatomic bromine molecules which are rather unstable and after reaction they regenerate the bromine atoms, ready to repeat the cycle. This chain was applied to the photobromination of cinnamic acid by Purka-yastha and Ghosh.41... 

If these chemical chains involving Br or Br3 were ruled out, one could consider an energy chain, in which the exothermic heat of reaction plus the energy of excitation is sufficient to produce the dibromide of cinnamic acid with a high energy content. The heat of bromination alone is estimated at 16,000 calories. These hot molecules of product then pass their energy over to more bromine atoms which then react to form additional dibromide, as follows,... 

The behavior of a photochemical chain reaction can sometimes be described rather simply in terms of the number of photons absorbed and the chain length. In the photobromination of cinnamic acid it is seen from Fig. 32 that the quantum yield is nearly proportional to the concentration of bromine cB in moles per liter. Then as an approximation... 

Example Photobromination of cinnamic acid to dibromocinnamic acid was carried out in blue light of wavelength 440 nm at 310 K using light intensity of 1.5 10 3 J per second. An exposure of 20 minutes produced a decrease of 0.075 millimole of bromine. The solution absorbed 80 per cent of the light passing through it. Calculate the quantum yield of the reaction. 

Properties of Cinnamic Acid (Section 553).—(a) Test for the double bond in cinnamic acid.—Dissolve about 0.5 gram of the acid in carbon tetrachloride and add, drop by drop, a solution of bromine in the same solvent. If no reaction takes place in 1 minute, warm the solution gently. (Eq.) How does the rate at which this reaction proceeds compare with the rate with which ethylene and bromine react ... 

Phenylpropiolic Acid, CeHs.C C.COOH, may be prepared from cinnamic acid by the reactions which are commonly employed to establish a triple bond in a compound. It is obtained by adding bromine to the ethyl ester of cinnamic acid, and subsequent elimination of hydrogen bromide by alcoholic potassium hydroxide —... 

In this problem, the halogen used for a good leaving group will be bromine. Bzz adds across the doxable bond of cinnamic acid. At this point, base is introduced to abstract the acidic proton so that the carboxylate ion is created. 

What mechanism best explains this observed stereochemistry The starting un-saturated acid and bromine are both achiral, and thus the product, if a chiral molecule, must be racemic. The outer-shell electrons of molecular bromine, however, are highly polarizable and molecular bromine reacts as an electrophile with the nucleophilic tt electrons of the aUcene group of cinnamic acid, in a reaction that is effectively a nucleophilic substitution reaction on a bromine atom. The product is a carbocation, which is, however, stabilized by the ability of fhe bromine atom to donate a lone pair to the carbocation. This interaction results in the formation of a chiral, cyclic bromonium ion intermediate. This intermediate ion is a rigid structure in which the ring section is only open to further attack by a nucleophile (Sn2 attack by the Br ion generated in the first stage) from the side opposite the bromine atom. 

Cocking used ether for extraction of the purified balsamic acids and this solvent is preferable. Further, the B,P, directs the mixed benzoic and cinnamic acids to be titrated but they should be weighed after drying in zacuo over sulphuric acid as they are present in variable proportions the amount of cinnamic acid present may be determined by bromination,... 

An alternative method for the classical Knoevenagel-type preparation of cinnamic acids has been reported this came to light during a study of the bromine-induced decarboxylation of substituted cinnamic acids. Cinnamic acid derivatives can also be prepared by the palladium-catalysed coupling of methyl acrylate with electron-rich aryl iodidessimilarly 5-arylpenta-2,4-dienoic acids (29) can be synthesized from penta-2,4-dienoic acid and bromo-aryls. A Stobbe-type condensation between aromatic aldehydes and methyl propylidenemalonate leads to the , -unsaturated acids (30), probably via a 5-lactone intermediate. ... 

N-chlorosuccinimide, reaction+PhOH, 92 chrysene, bromination of, 405 —, hydrogen exchange with, 258 —, nitration of, 39 cinnamic acid, nitration of, 39 cinnanyl p-totyl ethers, Ciaisen rearrangement of, 472... 

But catalytic reduction of the same phenyl propionic acid gives cis cinnamic acid. Therefore by adding hydrogen under various conditions, one can obtain a desired isomer. The conversion of acetylene into olefinic compounds has been carried out not only for the sake of obtaining the adduct, but Michael studied the various addition reactions for the sake of obtaining a desired product cis or trans. For example, he found that the addition of bromine to acetylene-dicarboxylic acid leads predominantly to the formation of trans isomer. 

Refluxing with zinc in ethanol reduced a-bromocinnamic acid to cinnamic acid in 80% yield [997]. Allylic chlorines in y,y,y-trichlorocrotonic acid were partly or completely hydrogenolyzed by zinc and sodium amalgam [5/9]. Hydrogenolysis of allylic bromine in oc, -unsaturated esters with zinc in acetic acid gave predominantly, y-unsaturated esters in 65-97% yields [998]. 

Method I.—40 gms. (1 mol.) of finely divided cinnamic acid are spread out on a large clock-glass and placed in a desiccator over concentrated sulphuric acid. A dish containing 45 gms. (slightly more than 1 mol.) of dry bromine is supported on a glass tripod above the cinnamic acid, the desiccator is closed, and allowed to stand until all the bromine has evaporated from the dish, and has been absorbed by the acid (about 3 days). The clock-glass is removed, the product exposed to the air for several hours, weighed in order to make sure that the theoretical amount of bromine has been absorbed, and recrystallised from dilute alcohol. 

Although kinetic studies of bromination of methyl ( )-cinnamic acid and methyl (E)-fS-styrylphosphonate suggest analogous mechanisms for both reactions, the stereochemistry of the reactions indicates that product-forming steps of different character must be involved157. 

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