Bond distances alkyl halides

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m methanol The bond angles at carbon are approximately tetrahedral as is the C—O—H angle A similar orbital hybridization model applies to alkyl halides with the halogen connected to sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

The carbon-halogen bond distances in acyl halides increase in the direction F < Cl < Br < I, and are similar, but slightly larger than, those of the alkyl halides (Table 7). Nuclear quadrupole resonance frequencies of halogen compounds suggest that the charge density on the chlorine atom of an acyl chloride is greater than that on an alkyl chloride (Table 8). 

General Characteristics. In accordance with the high tendency for formation of Co(CO)4 (2 ) and the acidity of (3), the former componnd behaves in many respects as a psendohalide see Pseudohalide). Relative rates of electrophilic snbstitntion with Mel and MeOTs classify (2 ) as a hard and relatively weak nncleophile see Hard Soft Acids and Bases). Compounds of the type RCo(CO)4 are prepared by two different methods reaction of an electrophihc halide, snlfate, or tosylate (RX) with (2 ), or reaction of HCo(CO)4 with alkenes. Simple alkyl derivatives have low stability nevertheless, many studies have been conducted in which species have been identified as RCo(CO)4 intermediates. Apart from alkyl and aryl derivatives, for example MeCo(CO)4 and PhCo(CO)4, (2 ) forms derivatives with main group residues R3E and R2E and with the main group metals Zn, Cd, and Hg as well as with transition metals, as shown by examples in Scheme 4. Co E bonding in these compounds appears to be predominantly covalent. The bond distances can be long, and consequently, the coordination number of Co can be high. 

The tertiary halide is in parentheses because it usually does not react by an Sn2 mechanism. The primary factor in this order of reactivity is steric hindrance, i.e., the ease with which the nucleophile can come within bonding distance of the alkyl halide. 2,2-Dimethyl-1-bromopropane... 

Nncleophilicity is also related to polarizability, or the ease of distortion of the electron clond surronnding the nucleophile. The partial bond between the nncleophile and the alkyl halide that characterizes the Sn2 transition state is more fnlly developed at a longer distance when the nncleophile is very polarizable than when it is not. An increased degree of bonding to the nucleophile lowers the energy of the transition state and... 

The three alkyl groups of a tertiary alkyl halide make it impossible for the nucleophile to come within bonding distance of the tertiary carbon, so tertiary alkyl halides are unable to undergo Sn2 reactions. 

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