Polarization C-X bonds

C-X bond polarity governs much of the chemistry of haloalkanes. 

The group moment always includes the C—X bond. When the group is attached to an aromatic system, the moment contains the contributions through resonance of those polar structures postulated as arising through charge shifts around the ring. 

I > Br > Cl > F. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the rate-determining step so the ease of C—X bond breaking does not affeet the rate. When this is the ease, the order of reactivity is often F > Cl > Br > I. This order is the result of the polar effeet of the halogen. The stronger bond dipoles assoeiated with the more eleetronegative halogens favor the addition step and thus inerease the overall rate of reaetion. 

Alkyl halides contain a halogen bonded to a saturated, sp3-hybridized carbon atom. The C-X bond is polar, and alkyl halides can therefore behave as electrophiles. 

In the El reaction, C-X bond-breaking occurs first. The substrate dissociates to yield a carbocation in the slow rate-limiting step before losing H+ from an adjacent carbon in a second step. The reaction shows first-order kinetics and no deuterium isotope effect and occurs when a tertiary substrate reacts in polar, nonbasic solution. 

The anions N3 and CN , being strong nucleophiles, form stable C—X bonds (C—N3 very polar C—CN unpolar) which can only be broken with a high energy use. The newly formed neutral compounds are relatively stable when faced with nucleophiles. The nitrate anion fits in between groups three and four and will be discussed in more detail later. 

Effect of Leaving Group. For both SeI and second-order mechanisms, the more polar the C—X bond, the easier it is for the electrofuge to cleave. For... 

The polarized C-X bonds in aliphatic organic halides are electrophilic and can be attacked readily by various nucleophilies. These reactions... 

P-phosphino-NHPs but the reverse reaction of a P-chloro NHP with diphenyl-trimethylsilylphosphine and subsequent reaction with a chloroalkane can be combined to produce high yields of P-alkyl-diphenylphosphines [74], Since the chloro-NHP is recovered in the second step, the overall reaction can be performed by employing this species merely as catalyst (Scheme 13). NMR investigations confirm that the appropriate P-phosphino-NHPs are in fact key intermediates in the resulting catalytic cycle, and it has been pointed out that P-X bond polarization represents a crucial factor for the overall acceleration of the catalyzed P-C... 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

The deshielding of the resonance in lithium carbenoids is explained by the polarization of the elongated C—X bond and shows that the carbocationic structures Ic and 2c have to be taken into account. The postulated high s character of the C—Li bond is also confirmed by the NMR-spectroscopic investigations insofar as enhanced C- Li coupling constants have been observed in lithium carbenoids . Thus, the effect of the metalation that leads to the formation of lithium carbenoids can be summarized as follows (Scheme 3) . 

The products observed in the reactions of Me2Ge with CCI3X (X = Cl, Br), PhCH2X (X = Br, 1), and Ph2CHCl, by proton chemically induced dynamic nuclear polarization ( H CIDNP) are those of net insertion of Me2Ge into the C—X bond and Me2Ge2X2 (X = Cl, Br) (Scheme 14.29). A two-step radical reaction takes place... 

Alkyl halides react mainly by heterolysis of the polar C—X bond. NUCLEOPHIUC DISPLACEMENT... 

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