Control thermodynamics

Adadurov, G.A. and Gol danskii, V.I., Transformations of Condensed Substances Under Shock-Wave Compression in Controlled Thermodynamic Conditions, Russian Chem. Rev. 50 (10), 848-957 (1981). 

If one of the kinetic factors approaches zero first, the reaction rate is said to be kinetically controlled if the thermodynamic factor falls first, the control is thermodynamic. The aerobic consumption of organic species and highly reduced compounds such as H2(aq) and H2S(aq) invariably show kinetic controls, because the thermodynamic drive for the oxidation of these compounds is quite large. Since methanogens and sulfate reducers operate under a considerably smaller thermodynamic drive, in contrast, it is not uncommon for their reaction rates to be controlled thermodynamically. 

The selectivity of reactions can be controlled thermodynamically or kinetically. In the first case, the pathways leading from a starting material or a common intermediate to the products are reversible. Here, the relative portion of the products is determined by their different heats of formation. This constitutes the minor... 

THERMODYNAMIC CONTROL KINETIC CONTROL THERMODYNAMIC CYCLE Thermodynamic equation,... 

As the data in Figure 15 show, a plot of SO2 adsorbed vs. temperature yields a curve with the classical shape of an adsorption isotherm divided, as indicated on the figure, into three distinct regimes. From the literature (46), we can speculate about the chemistry. Regime 1 is associated with the strong chemisorption of SO2 as sulfite and is controlled thermodynamically... 

If a stereogenic double bond is established by this Elcb elimination, one usually observes a trans- or an / -sclcctivity. This experimental finding could have two origins (1) product development control (Section 4.1.3), if the stereoselectivity occurs under kinetic control, or (2) thermodynamic control. Thermodynamic control comes into play if the cis, trans- or E,Z-isomeric condensation products can be interconverted via a reversible 1,4-addition of NaOH or KOH. In the trans- or isomer of an ce,/l-un saturated carbonyl compound the formyl or acyl group may lie unimpeded in the plane of the C=C double bond. This geometry allows one to take full advantage of the resonance stabilization C=C—0=0 <-> C—C=C—0 . ... 

In printing, a film of ink is formed by wetting the surface with the compression force of the rollers. This force spreads the ink over the surface and into any capillaries that may be present. Spreading and penetration are controlled thermodynamically and kinetically. Measurement of the contact angle can be used to determine the thermodynamics of wetting. This angle can be used also to determine the contribution that polarity and dispersive forces of the liquid make to the wetting of the surface. 

A real example comes in the acylation (Chapter 28) of the enolate from the keto-acetal above and alongside. The molecule is folded downwards and the enolate is essentially planar. Addition presumably occurs entirely from the outside, though the final stereochemistry of the product is controlled thermodynamically because of reversible cnolization of the product whatever the explanation, the black ester group prefers the outside. 

Generally speaking, PEVD is a modified form of chemical vapor deposition (CVD). A comparison between PEVD and conventional CVD is schematically shown in Figure 1 for a product (D) formed from reactants (A) and (B). In a CVD process, both reactants (A) and (B) are supplied through a vapor phase at the same side of a solid substrate (E). They react chemically at the surface of the solid substrate (E), aided by some type of catalytic effect, to form a desired product (D). In a PEVD process, one reactant (A) is transported from one side (source) of a solid substrate (E) to the other side (sink) under well-controlled thermodynamic and kinetic conditions. At the sink side, reaction with (B) occurs to form (D). Further growth of (D) into a continuous thin film with the desired thickness in a PEVD process also relies on (A) transported in the solid state now through (E) and (D) to react with (B). 

See also steric-approach control thermodynamic CONTROL. 

For exchange of tertiary alcohols direct fluorination without activation can be achieved by use of aHF or other sources of acidic fluoride (Scheme 2.61). This type of reaction in an acidic medium proceeds via a stabilized carbocation by an Sjjl mechanism. Fluoride addition is often reversible, and the stereochemistry of the reaction is controlled thermodynamically only by the relative free enthalpies of the possible product isomers. 

Bain and Whitesides concluded that competitive adsorption is controlled thermodynamically rather than kinetically. They argue that it is difficult to imagine the adsorption mechanism which would kinetically favour preferential adsorption of longer-chain alkane-thiols152. This is consistent with the recent observation that while thermodynamically controlled competitive adsorption leads to the preferential adsorption of long-chain thiols, short-chain alkanethiols actually adsorb faster128,129,160. 

The reaction of triethylborane with a ketone in the presence of a catalytic amount of diethylboryl piva-late provides another direct route to the corresponding boryl enolate (11 Scheme 8).1121 This is probably the first time that the alkenyloxyborane was isolated and fully characterized.21 Unfortunately this method requires rather vigorous reaction conditions and has not been used frequently. The stereochemistry of the resulting boryl enolates is probably controlled thermodynamically. 4-Dialkylboryloxy-2-isopropyl-6-methylpyrimidine (12) may be used specifically for the preparation of boryl enolates from aldehydes.22... 

There is no final consensus on whether procyanidin biosynthesis is controlled thermodynamically or enzymatically. In either case proanthocyanidins are synthesized through sequential addition of flavan-3,4-diol units (in their reactive forms as carbocations or quinone methides) to a flavan-3-ol monomer [218]. Based on the latest findings there is some evidence that different condensation enzymes might exist which are specific for each type of flavan-3,4-diol [64] and that polymer synthesis would be subject to a very complex regulatory mechanism [63]. But so far, no enzyme synthetase systems have been isolated and enzymatic conversion of flavanols to proanthocyanidins could not be demonstrated in vitro [219]. If biosynthesis was thermodynamically controlled, the variation in proanthocyanidin composition could be explained by synthesis at different times or in different compartments [64], The hypothesis of a thermodynamically controlled biosynthesis is based on the fact that naturally and chemically synthesized procyanidin dimers occur as a mixture of 4—>8 and 4—>6 linked isomers in approximate ratios of 3-4 1 [220]. Porter [164] found analogous ratios of 4—>8 and 4—>6 linkages in proanthocyanidin polymers. 

A reaction is said to be under thermodynamic control when there is sufficient energy to allow it to be reversible. When a reaction is under thermodynamic control, the relative amounts of the products depend on their stabilities. Because a reaction must be reversible to be under thermodynamic control, thermodynamic control is also called equilibrium control. 

Theoretical calculations, which allow an approximation to the intimate mechanism, usually start from a point (1) where the unsaturated species is already attached to the metal and cis to the M-X bond. In the reverse reaction, /l-elimination of X, the metal has to provide a cis coordination vacant, which is represented by an empty square in (2) in Eq. 6.1 and throughout the chapter. In practice there are steps previous to the insertion or the /I-elimination process which only very rarely are specified as separate from the insertion itself. It is important to be aware that these previous steps can have profound influence on the reaction rate even in cases when they are not rate determining, because they can control thermodynamically the concentration of the active species 1 or 2. In the literature, the word insertion is often applied to the whole process of transformation of some M-X entity and free A=B into 2. The specific insertion step can be then referred to as the X-nfigrafion step. 

From the resultant M+(CH4) complex the reaction proceeds via the C-H bond activation transition state (TS) to give the hydrido-metal-methyl cation complex, HMCHJ. In this step the C-H o-bond is broken and M-H and M-CH3 bonds are formed. Also, the oxidation number of the M-center increases by two. In order to analyze the reactivity of TMCs toward C-H (as well as H-H and C-C) bond, one has to elucidate the factors controlling thermodynamics and kinetics of the reaction M+(CH4) HMCHJ. 

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