Alkyl and aryl halides

In principle, a halogen (F, Cl, Br, and I) can replace any hydrogen in a hydrocarbon. The organic halides thus produced constitute one of the largest groups of organic compounds, their number increased because different halogens may be attached to the same carbon  

Similar nomenclature obtains for fluoro-, bromo-, and iodo-alkanes. 

The names of the alkyl halides follow the lUPAC system (Chapter 3 and Appendix II) with the halide being given the lowest number consistent with its position. K there is more than one kind of halogen present, that is, a mixed halide, the halogens are listed in alphabetical order, which also takes precedent over their position. Finally, as indicated above, if there is more than one halogen of the same kind (i.e., two bromines, three fluorines), the preflxes di- (for two), tri- (for three), tetra- (for four), and so on, are affixed, but they are not used when considering the alphabetical ordering. 

In the event there is another substituent present, such as an alkyl group, the halogen is considered equivalent to the alkyl group and receives the number consistent with its position and alphabetical order and is thus also cited as a prefix (i.e., the halogens, as a group, are regarded as substituents). 

Some typical alkyl, alkenyl, and aryl halides, and their names follow. 

In that absence of other Lewis acids alkyl-halides undergo direct electron addition at the electrode surface with subsequent stimulated electron transfer to (1) a second substrate and coupling (/ —/ ) or (2) a Br0nsted acid (HA H20) to replace the C X bond with a C—H bond. For example 

In both cases the overall process is an irreversible two-electron reduction via either (1) an EE path or (2) an ECEC path the first electron transfer is the most difficult and depends on the substrates electrophilicity. In the presence of hydronium ion the primary electron transfer will be to the most electrophilic center, for instance 

the reduction of n-BuI is the equivalent of the addition of two hydrogen atoms [H ] (generated via the electrochemical reduction of the two hydronium ions). 

the PhCl6 exhibits six irreversible two-electron reductions (each product species less electrophilic than its precursor) to yield at —2.8 V versus SCE benzene (PhH) an overall 12-electron process  

Although such electrolyses are done in aprotic solvents (e.g., DMF, DMSO, MeCN), even the most rigorously dried solvent contains 3-20 mM H20 (50-350 ppm). If the solvent has a degree of Br0nsted aciditity (e.g., alcohols and ketones), it can serve as a source of hydrogen atoms. An especially lucid 

C-F Stretch (strong) at 1400-1000 cm . Monofluoroalkanes absorb at the lower-frequency end of this range, while polyfluoroalkanes give multiple strong bands in the range 1350-1100 cm . Aryl fluorides absorb between 1250 and 1100 cm.  

C-Cl Stretch (strong) in aliphatic chlorides occurs in the range 785-540 cm . Primary chlorides absorb at the upper end of this range, while tertiary chlorides absorb near the lower end. Two or more bands may be observed, due to the different conformations which are possible. 

C-Br Stretch (strong) in aliphatic bromides occurs at 650-510 cm, out of the range of routine spectroscopy using NaCl plates or cells. The trends indicated for aliphatic chlorides hold for bromides. Aryl bromides absorb between 1075 and 1030 cm .  

FIGURE 2.72 The infrared spectrum of carbon tetrachloride (neat liquid, KBr plates). 

Infrared spectral data for halogen-containing compounds are covered in this section. Except for fluorine-containing samples, it is difficnlt to determine the presence or the absence of a halide in a compound via infrared spectroscopy. There are several reasons for this problem. First, the C-X absorption occurs at very low frequencies, to the exbeme right of the spectrum, where a number of other bands appear (fingerprint). Second, the sodium chloride plates or cells that are often used obscure the region where halogens absorb (these plates are transparent only above 650 cm ). 

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The cationic nickel(I) macrocycle, [Ni(tmc)], reacts with 1,4-dihaloalkanes to form ethylene with no evidence for the formation of cyclobutane. The mechanism is thought to proceed via an initial electron transfer to give the monoalkyl radical and free halide. The radical species then reacts with the nickel cation to give an organonickel complex, which undergoes a further electron transfer reaction with another [Ni(tmc)] to provide the products as in equations (66)-(68). 

Evidence for the organonickel intermediate is provided by the hydrolysis products obtained. The [Ni(tmc)] complex reacts with 1,5-dihaloalkane by a similar 

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The products from a mixture of alkyl and aryl halides may be represented by the following scheme ... 

Na2S04 Ketones, acids, alkyl and aryl halides 12 1.25 150... 

Cadmium alkyl and aryl halides, RCdX, as weU as cadmium allyls have been prepared by Grignard reactions but, as yet, have not realized any commercially important uses despite reactivity toward a number of organic and inorganic materials. 

Next to the formation of Grignard reagents, the most important application of this reaction is the conversion of alkyl and aryl halides to organolithium compounds, but it has also been carried out with many other metals, (e.g., Na, Be, Zn, Hg, As, Sb, and Sn). With sodium, the Wurtz reaction (10-93) is an important side reaction. In some cases, where the reaction between a halide and a metal is too slow, an alloy of the metal with potassium or sodium can be used instead. The most important example is the preparation of tetraethyl lead from ethyl bromide and a Pb—Na alloy. 

The addition of Grignard reagents to isocyanates gives, after hydrolysis, N-substituted amides. This is a very good reaction and can be used to prepare derivatives of alkyl and aryl halides. The reaction has also been performed with... 

Scheme 28 Domino iron catalysis for cross-coupling of alkyl and aryl halides [90]...

The high reactivity of the cadmium metal powder is clearly demonstrated by the ready oxidative addition of a variety of alkyl and aryl halides. 

Many alkyl and aryl halides have very low solubilities in water, but they are miscible with each other and with other relatively nonpolar solvents. 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

As in the case with the other members of group IVA, the mixed alkyl and aryl halides of lead are also known, and their reactions with water, alcohols, amines, and other organic compounds can be used to prepare a large number of other derivatives. 

Being a versatile reducing agent, lithium aluminium hydride reduces both alkyl and aryl halides to hydrocarbons. 

An extension of the facile formation of the o-allylcobalt carbonyl compounds provides the basis for the conversion of alkyl and aryl halides into ketones and... 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

This method is of quite general applicability and the carbonyl compound may be an aldehyde, a ketone, or an ester. Similarly, the halide may be chloride, bromide, or iodide although yields are generally lower with iodides. Alkyl and aryl halides react with equal facility and the alkyl halide may be primary, secondary, or tertiary. A few examples of the yields obtained with a variety of reagents are given in Table I (the yields quoted are obtained by g.l.c. analysis of the reaction mixture using an internal standard ). 

Finally, polymer 594 has been used as an arene-catalyst to activate nickel from nickel(II) chloride and lithium, in order to perform hydrogenation of different organic substrates such as afkenes, afkynes, carbonyl compounds and their imines, alkyl and aryl halides (chlorides, bromides and iodides), aromatic and heteroaromatic compounds as well as nitrogen-containing systems such as hydrazines, azoxy compounds or Af-amino oxides, giving comparable results to those obtained in the corresponding reaction in solution . 

Alkylation and arylation with alkyl and aryl halides... 

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