Cationic character

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

Blomstrand s formula, which had been largely disregarded, was rediscovered practically simultaneously by Bamberger (1895a) and Hantzsch (1895), who recognized how well it meets the experimental facts. Hantzsch, one of the precursors of physical organic chemistry, also realized the cationic character of these compounds and proposed the present name diazonium ion. 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

Allylic silanes show a pronounced tendency to react at the a-carbon in the presence of bidentate ligands.148 This regiochemistry is attributed to the preferential stabilization of cationic character by the silyl substituent. The bidentate ligands enhance the electrophilic character of the TS, and the cation stabilization of the silyl group becomes the controlling factor. 

Fragmentation can also occur when the a-carbon can support cationic character. 

In this pattern, X acquires partial cationic character and Y partial anionic character while the central atom A is little affected, which is consistent with the resonance diagram of Fig. 3.78. 

HB-5) unusually high cationic character of central atom B and anionic character of terminal atoms A and C, as reflected in natural atomic charges Qa, Qb, Qc), ionic bond orders (6AB(ion) and Aiic<1011)), and electrovalencies (TA(lon), IV1011 . Vc(l0n)). 

The cationic character of the active site, proposed as long ago as 1961,32 has recently been confirmed by the synthesis of a wide series of group 4 metallocene cations able to polymerize ethene and propene without any aluminum cocatalyst.33 36 The finding by Ewen that C2H4(l-Ind)2ZrCl2/MAO... 

In Sn2 reactions, substituents at the central atom that can stabilise cationic character will also stabilise the SN2 transition state leading to products. As well as lowering the energy, such stabilisation moves SN2 saddle points in the direction of cationic species resulting in incipient cationic character in the transition structure and longer bonds to both the nucleophile and the leaving group.162 165... 

Correlation to elogD and fraction ionized Tissue binding of bases and neutrals is primarily driven by lipophilicity and cationic character elogD, pKa, /u in human [32]... 

The two main classes of basic dyes vary in the way in which the basic centre is built into the molecule. The use of pendant quaternary ammonium groups, where the charge is insulated from the chromogen, allows disperse dyes to be given a cationic character. Alternatively, the cationic charge is delocalised within the chromogen. 

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