Bearing interchangability

Titanium-cataly2ed ester interchange can be used to prepare polyesters from diester and diols as well as from diacids and diols at considerably higher temperatures. Polymer chains bearing pendant ester and hydroxy functions can be cross-linked with titanates. 

Because of the standardization in envelope dimensions, precision bearings were once regarded as interchangeable among manufacturers. This interchangeability has since been considered a major cause of failures in machinery and the practice should be used with extreme caution. 

This calculation shows that reaction energies and reaction enthalpies are usually about the same, even when reactions Involve gases. For this reason, chemists often use A 5 reaction nd A reaction interchangeably. Because many everyday processes occur at constant pressure, thermodynamic tables usually give values for enthalpy changes. Nevertheless, bear In mind that these are different thermodynamic quantities. For processes with modest AE values and significant volume changes, A " and A H can differ substantially. 

A stereocenter is defined as an atom bearing groups of such nature that an interchange of any two groups will produce a stereoisomer. 

These polymers show lower water uptake than the analogous sulfonated poly(arylene ether sulfone) materials, possibly suggesting some interaction between the aromatic nitrile and sulfonic acid. The phosphine oxide functional moiety could also be used as a compatibilizer with other materials. Sulfonated poly(arylene ether phosphine oxide sulfone) terpoly-mers have been prepared both with sulfonated triphenyl phosphine oxide and with triphenyl phosphine oxide with 3,3 -disulfonate-4,4 —dichlorodiphenyl sulfone as the sulfonic acid bearing monomer. Block copolymers containing phosphine oxide appear to avoid the ether—ether interchange that results when non—phosphine oxide blocks are utilized, and this is being further pursued. ... 

A somewhat different route is used to prepare an analogue that bears additional oxygen. The sequence, in this case, starts by base-catalyzed formylation of the hydro-cinnamic acid derivative (40-1) with ethyl formate. Condensation of the product (40-2) with guanidine in this case leads to a pyrimidone (40-3), with the cyclization involving an ester-amide interchange between guanidine and the ester. Reaction of... 

Structurally characterized heterobimetallic Ln/Al complexes bearing donor-functionalized dianionic diamido ligands of the types [NNN]2" and [NON]2- were obtained from [amide]- [alkyl] (Scheme 52) or [aluminate] [amide] interchange reactions (Scheme 53). 

The metathesis reaction is still occasionally referred to as disproportionation although it bears no resemblance to disproportionation as a chain-growth termination step. Rather, it is analogous to interchange in step-growth polymerization (reaction 10.7 in Section 10.2.2) except that the reactants are broken apart at their double bonds and the fragments reconnected by double bonds. 

Another type of 1,4-diazocine of stereochemical interest is that bearing an N,N -tetramethylene-o-phenylenendiamine moiety, exemplified by structures 86 and 89. A proton-NMR study showed that the heterocyclic ring of an N,N -disubstituted benzo-l,6-diazocine is in boat conformation 148, the boat boat interconversion barrier being more than 25 kcal/mol (76CJC1135). Saunders and Sprake used UV and H-NMR spectroscopy to study the N,lV -diacetyl-5,6,ll,12-tetrahydrodibenzo[h,/]-l,4-diazocine and concluded that there are four possible conformations (Scheme 5) with the interchange 150 151 (or 149 i=f 152) occurring more readily than 149 150... 

More complicated but more realistic is to measure the effect of time and to describe it. Several schemes have been tested with many of them based on interchanges between ill-defined black boxes bearing labels such as "labile or "non labile . These and other schemes were discussed by Barrow [38]. When the rate of reaction is measured using changes in the solution concentration as an indication of the amount of reaction, the results can often be closely described by another modification of the Freundlich equation ... 

---