Nucleophile halide ions

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Due to the presence of a strongly electrophilic carbon centre alkyl halides are susceptible to nucleophilic attack, a nucleophile displaces the halogen as a nucleophilic halide ion (Following fig.). The reaction is called nucleophilic substitution and there are two types of mechanism, i.e. the S I and SN2 mechanisms. Carboxylic acids and carboxylic acid derivatives also undergo nucleophilic substitutions, but the mechanisms are totally different. 

Finally, nucleophilic halide ions can enhance the reactivity by promoting the removal of generated acyl moieties from the metal centers ... 

Ilte reaction of a tertiary alcohol willi HX lakes place by art SnI mech-aniem. Acid protonatea the hydroxyl oxygen Atom, water is expelled to generate a carbocation, and the cation reacts with nucleophilic halide ion to givH the alkyl halide product (Figure 17.7). 

Hydroxide ion and other O-nucleophiles Amines and amide ions S-Nucleophiles Halide ions Carbanions... 

Note that the reaction proceeds with inversion of configuration at the stereogenic center attached to the metal. It is not clear exactly what the initial function of the halogen is in the reaction, however. Scheme 8.7 presents two possible scenarios for the reaction. In path a, the halogen oxidatively adds to the metal to give a cationic complex. The remaining nucleophilic halide ion attacks the complex at the... 

Hydroxide and other 0-nucleophiles Nitrogen nucleophiles Sulfur and related nucleophiles Halide ions Hydride sources Carbanions Azide ions... 

This explains why hydrogen halides with highly nucleophilic halide ions fail to initiate cationic polymerization. 

In the addition of hydrogen halides to alkenes, it is usually found that the nucleophilic halide ion becomes attached to the more-substituted carbon atom. This general observation is called Markovnikov s rule. The basis for this regioselectivity lies in the relative ability of the carbon atoms to accept positive charge. The addition of hydrogen halide is initiated by protonation of the alkene. The new C—H bond is formed from the 7T electrons of the carbon-carbon double bond. It is easy to see that if a carbo-cation is formed as an intermediate, the halide will be added to the more-substituted carbon, since protonation at the less-substituted carbon atom provides the more stable carbocation intermediate. 

When 2-methyl-2-propanol tert-hvXyl alcohol, 65) is treated with concentrated HCl, 2-chloro-2-methylpropane (2-chloro-2-methylpropane ter -butyl chloride, 93) is isolated in 90% yield. Similarly, when 1-butanol (94) is treated with 48% HBr in the presence of sulfuric acid, a 95% yield of 1-bromobutane (96) is obtained. In both reactions, the oxygen of the alcohol reacts as a Br0nsted-Lowry base in the presence of the protonic acids, HCl, or sulfuric acid. The fact that alkyl halides are produced clearly indicates that these are substitution reactions. In previous sections, tertiary halides gave substitution reactions when a nucleophilic halide ion reacted by an Sfjl mechanism that involved ionization to a carbocation prior to reaction with the halide. Primary halides react with a nucleophilic halide ion by an 8 2 mechanism. It is reasonable to assume that tertiary alcohols and primary alcohols will react similarly, i/the OH unit is converted to a leaving group. 

The order of reactivity of hydrogen halides with aUtenes is HI > HBr > HCl, which corresponds to the order of decreasing acidity. The reaction is an electrophilic addition, in which the proton is the electrophile. The alkene, a Lewis base, accepts a proton from the acid and forms a carbocation. Then, the nucleophilic halide ion reacts with the carbocation to form the addition product. 

Primary alcohols react by an Sj 2 mechanism in which water is displaced by the nucleophilic halide ion because the primary carbon atom is sterically accessible to the nucleophile. The alternate mechanism would have a high activation energy because the transition state would resemble a highly unstable primary carbocation. We used a similar argument to explain the direction of electrophilic addition to double bonds. According to the Hammond postulate, the structure of a transition state resembles the species that is most similar to it in energy. In the case of the mechanism for the reaction of an al-... 

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