Cinnamic acid bromination

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. 

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. 

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. 

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. 

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. 

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. 

Examples of the solvent-dependent competition between nucleophilic substitution and / -elimination reactions [i.e. SnI versus Ei and Sn2 versus E2) have already been given in Section 5.3.1 [cf. Table 5-7). A nice example of a dichotomic y9-elimination reaction, which can proceed via an Ei or E2 mechanism depending on the solvent used, is shown in Eq. (5-140a) cf. also Eqs. (5-20) and (5-21) in Section 5.3.1. The thermolysis of the potassium salt of racemic 2,3-dibromo-l-phenylpropanoic acid (A), prepared by bromine addition to ( )-cinnamic acid, yields, in polar solvents [e.g. water), apart from carbon dioxide and potassium bromide, the ( )-isomer of l-bromo-2-phenylethene, while in solvents with low or intermediate polarity e.g. butanone) it yields the (Z)-isomer [851]. 

Phenylpropynoic acid (13) can be prepared from cinnamic acid esters by the addition of bromine to the double bond followed by dehy-drobromination with alcoholic potassium hydroxide (Scheme 5.14). 

The electrophilic character of chlorine and bromine in their attack on olefins is confirmed by recent studies of substituent effects in styrenes and cinnamic acids [ 102]. The slope of the Hammett plot for chlorination = — 4.01) is consistent with a classical carbonium ion intermediate of type (i) (p. 91), but the smaller value [q — — 2.23) for bromination indicates less delocalisation of charge into the aromatic ring, and is consistent with the bromonium ion intermediate. 

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