Alkyl halides resonance structures

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

Hudson, H.R., Rees, R.G., and Weekes, J.E., Preparation, structure, and nuclear magnetic resonance spectroscopy of triphenyl and trineopentyl phosphite alkyl halide adducts, /. Chem. Soc., Perkin I, 982, 1974. 

The Gabriel synthesis of amines uses potassium phthalimide (prepared from the reaction of phthalimide with potassium hydroxide). The structure and preparation of potassium phthalimide is shown in Figure 13-13. The extensive conjugation (resonance) makes the ion very stable. An example of the Gabriel synthesis is in Figure 13-14. (The N2H4 reactant is hydrazine.) The Gabriel synthesis employs an 8, 2 mechanism, so it works best on primary alkyl halides and less well on secondary alkyl halides. It doesn t work on tertiary alkyl halides or aryl halides. 

The electrophile, an acyl cation, is generated in a manner similar to that outlined in Figure 17.4 for the generation of the carbocation electrophile from an alkyl halide. First the Lewis acid, aluminum trichloride, complexes with the chlorine of the acyl chloride. Then A1C14 leaves, generating an acyl cation. The acyl cation is actually more stable than most other carbocations that we have encountered because it has a resonance structure that has the octet rule satisfied for all of the atoms ... 

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strong base, commonly an organometallic base such as butyllithium or phenyllithium. The hydrogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidic because the carbanion of the conjugate base (the ylide) is stabilized by the inductive effect of the positive phosphorus atom. In addition, a resonance structure with five bonds to phosphorus makes a minor contribution to the structure and provides some additional stabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reaction of triphenylphosphine with the appropriate alkyl halide (see Section 10.9). 

Because of the contribution of structures such as the one on the right to the resonance hybrid, the a-carbon of an enamine is nucleophilic. However, an enamine is a much weaker nucleophile than an enolate anion. For it to react in the SN2 reaction, the alkyl halide electrophile must be very reactive (see Table 8.1). An enamine can also be used as a nucleophile in substitution reactions with acyl chlorides. The reactive electrophiles commonly used in reactions with enamines are ... 

Treatment of alkenes A and B with HBr gives the same alkyl halide C. Draw a mechanism for each reaction, including all reasonable resonance structures for any intermediate. 

The difference between the chemical behaviour of the substitution products of benzene and the corresponding aliphatic derivatives are well known and are reflected in the values of their dipole momcnxs [Table XCVI), In chlorobenzene, in addition to the bond resonance structures, I and //, there are three additional structures ///, IV and V contributing to the molecular resonance and the dipole moment is lowered in comparison with the alkyl halides. 

Katritzky reported that oxime 12, formed by nitrosation of enol 11, reacted with benzyl bromide in the presence of K2CO3 in DMF at room temperature to form oxazole 4 in 43% yield. Since benzyloxime 16 formed by the reaction of diketone 3 and o-benzylhydroxyamine didn t form oxazole 4 in the presence of K2CO3 in DMF, even under forcing conditions (heating at 60 °C), he proposed the following for its formation. The reaction of oxime 12 with benzyl bromide formed nitrone 13, which could also exist as resonance structure 14, followed by cyclization and loss of HO, afforded oxazole 4. Alkyl halides could also used as electrophiles to react with 1,2-diketone nitrones to form oxazoles (17 18). 

Although relatively few alkyl halides react with [M(CO)s] to form straightforward RM(CO)s derivatives, almost all acyl chlorides, RCOCl, react with [M(CO)s] to form the acyl derivatives RCOM(CO)5. Approximately fifteen acyl derivatives of manganese alone have been reported. Acyl derivatives of transition metals such as these manganese compounds appear to be especially stabilized by resonance structures such as (XX), in which the metal-carbon bond has appreciable double bond character, as in the metal carbonyls themselves. The rarity and instability of acyl derivatives of nontransition metals such as tin especially emphasizes the importance of structures analogous to (XX) in stabilizing acyl derivatives of transition metals. 

As mentioned earlier, the allyl cation can be captured at either carbon sharing the positive charge. Table 12.3 gives some average rates of SnI solvolysis for a few common structural types. Primary allylic halides react much faster than primary alkyl halides. Secondary and tertiary allylic halides also ionize faster than their non-allylic counterparts. Resonance stabilization does have an accelerating effect on the ionizations. 

These complexes have the metal significantly (ssO.SA) out of the plane, as is evidenced by the 3D structures of five-coordinate porphyrin complexes determined by X-ray crystallography nd the existence of two o-H and m-H NMR resonances for TPP derivatives and two a-CH2 resonances for OEP, OETPP and meso-alkyl porphyrin complexes. In addition to halides as counterions, a number of other monomeric, five-coordinate, high-spin Fe porphyrins have been reported,... 

The high reactivity of chelated lithium alkyl compounds severely limits structural study of pure compounds, particularly in aromatic solvents. Most of our more recent work on chelated lithium alkyl systems used H and 7Li NMR to observe various metalation reactions like the self-metalation or aging reaction of TMED LiBu in heptane (I, 2). Much of our current insight into the structural features of chelated alkali metal systems comes from careful quantitative study of systems with relatively stable anions like resonance stabilized carbanions (5) and the systems described in this paper. We discuss magnetic resonance experiments on two systems (a) chelated lithium halides Chel LiX, examples of the recently discovered inorganic salt chelates (6), and (b)... 

The thionyl halides range from 1308 cm for thionyl fluoride, through 1233 for thionyl chloride [52] to 1121 cm for the bromide [85]. Nitrogen substitution also raises s=o as there is no resonance. Sulphinamides have been studied by Smith and Wu [86], by Keat et al. [87] and by Steudel [84]. With A -alkyl groups on both sides of the sulphur atom the frequency is close to 1120 cm", but lower values are found for structures such as R—NHSO—R (1060—1037 cm" ) [86]. In these the bands are usually doubled which may indicate dimerisation as in the amides. Two sulphur atoms attached to the S=0 group give frequencies close to 1105 cm-i. 

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