Triple bonds formation

The observed decrease below the double-bond values may possibly be due to some triple bond formation. 

ADDITION OF THE Os—H BOND OF OsHCI(CO)(P Pr3)2 TO CARBON-CARBON TRIPLE BONDS FORMATION OF ALKENYL DERIVATIVES... 

Addition to carbon carbon triple bonds Formation of benzene derivatives... 

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

Note that there are no stereoisomers around carbon-carbon triple bonds. Each carbon atom involved in triple bond formation can form only one more bond to a chemical group. For example, a butyne (2-butyne) containing a triple bond between the two central carbon atoms looks like this ... 

The problem of directed valence is treated from a group theory point of view. A method is developed by which the possibility of formation of covalent bonds in any spatial arrangement from a given electron configuration can be tested. The same method also determines the possibilities of double and triple bond formation. Previous results in the field of directed valence are extended to cover all possible configurations from two to eight s, p, or d electrons, and the possibilities of double bond formation in each case. A number of examples are discussed. 

The possibilities of double or triple bond formation are most easily discussed in terms of molecular orbitals. The principal type of multiple bond consists of two parts first, a pair of electrons in an orbital symmetrical about the axis of the bond and second, one or more pairs of electrons in orbitals which are not symmetrical about the axis. The first pair of electrons form a bond which differs in no way from the ordinary single, or passed through the axis. They may be regarded as formed by the interaction of two p orbitals, one on each atom, with axes parallel to each other and perpendicular to the axis of the bond. We shall refer to orbitals of this type as rr orbitals. 

Elimination reactions of ( )- and (Z)-benzaldehyde Opivaloyloximes (19a) and (19b) with DBU in MeCN have been found to occur by a nitrile-forming E2 mechanism which is ca 2000-fold faster for the latter isomer in each case.15 The corresponding Hammett substituent constants, activation parameters, and primary deuterium isotope effects, suggest that the anti elimination from (19b) (for which p = 2.4 0.1, H/ D = 2.7 0.3, A/H = 12.5 0.2 kcal mol-1, and A= —31.0 0.6eu) proceeds to (20) via a more symmetrical transition state with a smaller degree of proton transfer, less charge development at the jS-carbon and greater extent of triple bond formation than for syn elimination from (19a) (for which p = 1.4 0.1, kn/kn = 7.8 0.3, AH = 8.8 0.1 kcal mol 1 and A= -23.6 0.4 eu). 

The empirical valence bond model has shown good predicting power if one defines the bond multiplicity to be two for compounds such as CePd or BaPd and three for compounds such as LaRh, Ylr, or RhTh. The limitations of the model could also be shown (40) when applied to assumed quadruple bond formation, as in RuV or ThRu or to double bond formation as in CePt or ThPt. The case of ThRu (41) shows that quadruple bonding is approached where suitable valence states are available, but not fully achieved, apparently because of the directional requirements of the bonding in a diatomic molecule. The examples of CePt and ThPt (40) show the calculated values that have been based on an assumed double bond to be too low. Platinum has no suitable valence state for triple bond formation, but apparently forms triple bonds with other... 

Cases of Ei eliminations are common in pyrolysis. Most of these reactions occur with double or triple bond formation. Several examples are given below. 

The double (or triple) bond formation does not take place exclusively between carbon atoms. It may occur between carbon and nitrogen or carbon and oxygen. An example is the pyrolysis of p-hydroxy olefins ... 

Table II shows bond parameters of various molecules, which are averaged if equivalent bonds exist. Well potentials are not adopted in DV-Xa calculatins here. The bond index(C-C) of CjH, or is 1, 2 or 3 respectively. The bond index (C-C) of is 1.4 and the bond index (B-F) of BFj is 1.2, both of which are sums of a (7 -bond and smaller n -bond contributions. The bond index(C-O) of CO near to 3 and its partitioned energy as large as that of C-C of C H suggest a triple bond formation. The bond indices (S-F) of SF, (B-B) of B H and (Fe-C) of FeCCjHj) (Fig.2) are smaller than 1. But the partitioned energies for those bonds are negative and explain their stabilities. On the other hand, the atomic orbital bond population for (Fe-C) of FelCjHj) is negative and can t explain its stability.

Perchlorotolane reacts with oleum to give cyclic a,a -dihydroxystilbene sulphate [100]. Presumably, the reaction is initiated by an addition of disul-phuric acid to the triple bond (formation of enol disulphate), followed by... 

Figure 2.19 n-Overiap between parallel orbitals is responsible for double and triple bond formation. Various changes in geometry and overlap account for stronger or weaker tr-bonds. 

Triple bond formation at nitrogen and oxygen The stereoelectronic control of reactivity at multiple bonds is not limited to all carbon species. Due to the presence of a stereoelectronically defined lone pair, sp -hybrid-ized nitrogen compounds provide a number of illustrative examples of slereochemically directed reactivity (Figure 7.34). 

Intramolecular Addition to CsC Triple Bonds Formation of Unusually Stable Vinyl Cations... 

A nitrogen atom can fill its octet by sharing three electrons with another nitrogen atom, forming three covalent bonds, a so-called triple bond. The triple bond formation of nitrogen is shown in Figure 7-3. 

A Yes, a possible explanation for the value for difluorine being anomalously low is that the other halogens have available d orbitals and that a p to d promotion would allow triple bond formation. 

---