Alkyl halide polarizability

Alkyl halides and related alkylating agents react with pyridines to form N-alkylpyridinium salts (Scheme 2.7). These compounds are much more stable than their 7V-acylpyridinium equivalents and can often be isolated as crystalline solids, particularly if the halide ion is exchanged for perchlorate, tetrafluoroborate or another less polarizable counter anion. 

The acidity difference of hydrogen atoms adjacent to divalent sulfur compared to oxygen stems from the greater polarizability of sulfur and the longer C-S-bond length d-orbitals are not involved. In most cases treatment of dithianes with w-BuLi at temperatures of -30 °C is sufficient for the preparation of the lithio-derivatives. With pKA values of approximately 30, lithiated dithianes can react with aldehydes or ketones, epoxides and acid derivatives, but also with alkyl halides without competing elimination reactions. 

Because sulfur is larger and more polarizable than oxygen, thiolate ions are even better nucleophiles than alkoxide ions. Thiolates are such effective nucleophiles that secondary alkyl halides often react to give good yields of Sn2 products. 

Sulfur compounds are more nucleophilic than the corresponding oxygen compounds, because sulfur is larger and more polarizable and its electrons are less tightly held in orbitals that are farther from the nucleus. Although ethers are weak nucleophiles, sulfides are relatively strong nucleophiles. Sulfides attack unhindered alkyl halides to give sulfonium salts. 

However, this is not a complete picture of the factors that contribute to the reactivity of functional groups. For example, the electronegativity difference between carbon and iodine is relatively small. In the much larger iodine atom the bonding orbitals are further from the nucleus than in chlorine and are more polarizable during the course of a reaction. These differences affect both the a- and ii-bonds. Thus many of the reactions of the alkyl halides and of carbonyl compounds may be rationalized in terms of the polarization of the bonds and the polarizability of the component atoms. 

This is the order of reactivity of alkyl halides in displacement reaction which is familiar to all organic chemists. Although it might not have been predicted, the polarizability factor almost always appears to outweigh the permanent polarization factor in reactions of saturated molecules. 

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 deprotonations proceed with extreme ease, even at temperatures in the region of — 100°C.Most of the reactions reported are couplings with carbonyl compounds. The species have a low thermostability, and one may wonder whether their basicity (and polarizability) is sufficiently high to warrant good yields with electrophiles that are considerably less reactive than carbonyl compounds, e.g. alkyl halides. 

Both van der Waals forces and dipole-dipole interactions must be overcome in order for an alkyl halide to boil. As the halogen atom increases in size, the size of its electron cloud increases. As a result, both the van der Waals contact area and the polarizability of the electron cloud increase. 

The sulfur analogs of ethers are called sulfides or thioethers. Sulfur is an excellent nucleophile because its electron cloud is polarizable (Section 10.3). Sulfides, therefore, react readily with alkyl halides to form sulfonium salts—a reaction that an ether cannot undergo because oxygen is not as nucleophilic and cannot accommodate a charge as easily. 

Recall from Table 9.7 that thiols are moderate nucleophiles and thiolates are good nucleophiles. As such, they rank as better nucleophiles in both their neutral and anionic states as compared to alcohols and alkoxides, respectively. The reason thiols and thiolates are better nucleophiles is that the sulfur atom is more polarizable than oxygen, and increased polarizability enhances nucleophilicity. Also, because the values for thiols are generally less than 11, thiolate anions react with secondary alkyl halides primarily through an Sj 2 substitution mechanism rather than an E2 elimination. The reverse is true for the significantly more basic alkoxides. 

Among alkyl halides, the boiling point increases with increasing size of the halogen alkyl fluorides have the lowest boiling points, alkyl iodides the highest. Dispersion forces are mainly responsible. Induced-dipole/induced-dipole attractions are favored when the electron cloud around an atom is easily distorted. This property of an atom is its polarizability and is more pronounced when the electrons are farther from the nucleus (iodine) than when they... 

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