Catalysed halogen addition

Higher kinetic orders with respect to halogen (X2) have been discussed in the previous section it is not surprising that other species can take the place of halogen in the kinetic equation. Chloride ion and bromide ion can assist in the addition reactions of bromine and, possibly, iodine. This gives rise to a new kinetic contribution 

The function of the halide ion is not clearly understood its efficiency depends upon the olefin under attack, and as a salt effect of unknown magnitude is simultaneously exerted upon the reaction, the separation of the overall rate into its components is not easy. 

The greater efficiency of the acids HCl and HBr than that expected from their acidities in acetic acid can be explained by postulating attack upon the proton-ated olefin by the trihalide ion. Such acid-catalysed halogenation is observed with both bromine and, to a lesser extent, chlorine. 

In the presence of halogen-carriers , such as iodine or pyridine, further kinetic complexities result. Buckles and Yuk found that the bromination of crotonic acid was a kinetically simple reaction only in the presence of tetra-methylammonium tribromide, the overall kinetic form in ethylene dichloride being 

Iodine-catalysed halogenation is apparently complex the kinetic forms Rate = it[olefin][Cl.][ICl] (4.10) 

---

Three approaches to zinc enolates are commonly adopted the process associated to the classical Reformatsky reaction is based on the insertion of Zn(0) into the carbon—halogen bond of an a-haloester. Two additional routes involve (i) transmetallation of a lithium enolate with a Zn(II) salt (Section V.A) and (ii) the transition-metal-catalysed conjugate addition of diethylzinc to Michael acceptors (Section V.B). 

Such a structure is analogous to those suggested for halogen addition, acid-catalysed hydration, and sulphenyl halide addition, but has. suffered severe criticism. To follow the analogy, such an intermediate would be expected to... 

Bromochloromethane was being prepared in a 400 1 reactor by addition of liquid bromine to dichloromethane in presence of aluminium powder (which would form some aluminium bromide to catalyse the halogen exchange reaction). The reaction was started and run for 1.5 h, stopped for 8 h, then restarted with addition of bromine at double the usual rate for 2.5 h, though the reaction did not appear to be proceeding. Soon afterwards a thermal runaway occurred, shattering the glass components of the reactor. 

Gas-phase acid-catalysed additions of water and methanol to ethanol and its a-halo derivatives have been investigated by computation both reactions are favoured by increasing the electronegativity of the halogen. ... 

Glycals, for example D-glucal (23), being vinyl ethers are susceptible to acid catalysed additions of alcohols which result in the formation of 2-deoxyglycosides, but elimination reactions accompany additions of this type 86) go such products are better prepared by way of halogen adducts or by the alkoxymercuration reaction (see below). On the other hand, the addition procedure can be more suitable than the alcoholysis... 

Tuckermann, M R. Ackermann, C. Golz, H. Lorenzen-Schmidt, T. Senne, J. Stutz, B. Trost, W. Unold, and U. Platt, DOAS-Ob-servation of Halogen Radical-Catalysed Arctic Boundary layer Ozone Destruction during the ARCTOC-Campaigns 1995 and 1996 in Ny-Alesund, Spitsbergen, Tellus, 49B, 533-555 (1997). Turnipseed, A. A., S. B. Barone, and A. R. Ravishankara, Observation of CH, S Addition to 02 in the Gas Phase, J. Phys. Chem.,... 

The activation barrier for the initial hydration depends on the halogen substituents but is 50 kcal moT1 for addition of a single water molecule in the gas phase69. However, it has been shown that this step may be very effectively catalysed by the participation of a second water molecule70 it is thus likely that the carbonyl halide hydrolysis may occur rapidly in contact with a water droplet. 

---