The Halides

The di- and tri-iodides of thorium are well characterized, but the analogous compounds with the other halogens are still somewhat uncertain. So the iodides are treated first. 

This is dimorphic (111). The a modification is obtained by heating Thii with thorium metal in a sealed tantalum tube at 957 K. (The hot, lower halides of thorium corrode glass or silica rather quickly above about 773 K.) This form is pseudoorthorhombic with a = 787.0, b = 2699, and c = 3149.2 pm. It belongs to the space group B2/b or Bb, both monoclinic groups, but y 90° (ill). 

These chains are cross-linked by thorium-thorium pairs. The thorium-thorium distance, chain to pair, is 380.0 pm, and the pair separation is 346.0 pm, which is less than in thorium metal (360.0 pm). Clearly there is metal-metal bonding in these units. Along the chains, the distance between thorium atoms is short enough to yield a conduction band, and this variety of This can be regarded as a one-dimensional metal, with the thorium bonded pairs normal to this dimension nil). 

The thorium-iodine distances (330, 322, and 325 pm) are similar to those in Thl4 (313-329 pm) and Thl2-(see later). 

The compound reacts rapidly with aqueous solutions, yielding hydrogen. 

Since the chloride ion is much larger than the fluoride ion, and its nuclear charge is more thoroughly screened by negative charges, chlorine has not been observed to form an SiCle configuration. A 

17 Stock, The Hydrides of Boron and Silicon, pp. 32-3 (Cornell University Press, 1933). 

The ease with which the tetrahalides dissociate thermally increases rapidly in the series from tetrafluoride to tetraiodide. While the chlorine-silicon bond ordinarily is not considered mobile, it undergoes an interesting redistribution reaction with the isocyanate bond in the preparation of chloroisocyanates of silicon.18 

Zinc may be used in place of aluminum in a similar reaction. The method also may be used for the vapor-phase hydrogenation of organo-silicon halides such as methyltrichlorosilane  

The reaction may involve the intermediate formation of a metal hydride which undergoes metathesis with the silicon halide, but no evidence for such a mechanism has been accumulated. 

The dihalides of Si and Ge are polymeric solids that are relatively unimportant compared to those of Sn and Pb. The latter elements are metallic in character and have well-defined +2 oxidation states. Physical data for the divalent halides are shown in Table 11.1. The compounds of Si(II) are relatively unstable because the reaction 

Because of the Lewis acidity of the SnX2 compounds, they will react with additional halide ions to form complexes as illustrated by the equation 

These complexes have the pyramidal structure shown as 

Salts containing these ions can be isolated as solids with large cations such as R4P+. Because they have an unshared pair of electrons on the Sn atom, these ions are Lewis bases that form adducts with Lewis acids such as boron halides  

As a result of their being able to react as Lewis acids, compounds having the formula SnX2 also form many complexes with molecules containing O and N as the electron donor atoms (H20, pyridine, amines, aniline, etc.). 

Metals with large ions of low charge form saline halides with three-dimensional ionic lattices. The salts have high m.p. and b.p. and are good conductors when fused. Most of the halides of the first three A sub-groups belong to this class. 

Non-metals and many B sub-group metals form volatile, non-conducting halides which usually have molecular lattices. 

Consideration of the chlorides of the first three elements in Gps.I-IV reveals sharp changes in volatility in passing along the periods (Table 91). 

as BeClg shows, a rigid division into saline and volatile halides is not possible. Many metals which form ions such as or give 

Halides frequently form hydrates which differ in properties from the anhydrous materials. Many M fluorides e.g, AIF3) are quite insoluble when made by dry methods but the hydrates (e.g. AlFg-S.bHgO) produced from solutions dissolve readily in water. The co-ordination of water molecules round cations greatly reduces the lattice energies. 

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Iron(III) chloride forms numerous addition compounds, especially with organic molecules which contain donor atoms, for example ethers, alcohols, aldehydes, ketones and amines. Anhydrous iron(III) chloride is soluble in, for example, ether, and can be extracted into this solvent from water the extraction is more effective in presence of chloride ion. Of other iron(III) halides, iron(III) bromide and iron(III) iodide decompose rather readily into the +2 halide and halogen. 

The result in Equation [1] is explained by a carbocation rearrangement involving a 1,2-hydride shift the less stable 2° carbocation (formed from the 2° halide) rearranges to a more stable 3° carbocation, as illustrated in Mechanism 18.8. 

The +2 halides are more stable for tin and lead, S11X2 and PbX2. 

The 2° halide can react by an E2 or 3 2 reaction with a negatively charged nucleophile or base. 3ince I is a weak base, substitution by an 3 2 mechanism is favored. 

Table A2.3.2 Halide-water, alkali metal cation-water and water-water potential parameters (SPC/E model). In the SPC/E model for water, the charges on H are at 1.000 A from the Lennard-Jones centre at O. The negative charge is at the O site and the HOH angle is 109.47°.

Boron and aluminium halides show many similarities but also surprising differences. Table 7.2 gives the melting and boiling points of the MX3 halides. 

The alkyl halides are also of great importance in synthetic operations (e.g.) using Grigard reagents (p. 280). acetoacetic ester (p. 269) and malonic ester (p. 2- S)-... 

The Fittig Reaction, in which sodium reacts with a mixture of an aryl and an alkyl halide, forming the sodium halide and the corresponding hydrocarbon ... 

To determine which halogen is present, take 1-2 ml. of the filtrate from the sodium fusion, and add dilute sulphuric acid until just acid to litmus. Add about 1 ml. of benzene and then about 1 ml. of chlorine water and shake. A yellowish-brown colour in the benzene indicates bromine, and a violet colour iodine. If neither colour appears, the halogen is chlorine. The result may be confirmed by testing the solubility of the silver halide (free from cyanide) in dilute ammonia solution silver chloride is readily soluble, whereas the bromide dissolves with difficulty, and the iodide not at all. 

Place a mixture of 0-5 g. of finely powdered thiourea, 0-5 g. of the alkyl halide and 5 ml. of alcohol in a test-tube or small flask equipped with a reflux condenser. Reflux the mixture for a j)eriod depending upon the nature of the halide primary alkyl bromides and iodides, 10-20 minutes (according to the molecular weight) secondary alkyl bromides or iodides, 2-3 hours alkyl chlorides, 3-5 hours polymethy lene dibromides or di-iodides, 20-50 minutes. Then add 0 5 g. of picric acid, boil until a clear solution is obtained, and cool. If no precipitate is obtained, add a few drops of water. RecrystaUise the resulting S-alkyl-iso-thiuronium picrate from alcohol. 

Where R and R are identical, the dialkylmalonic ester may be prepared in one operation by treating 1 mol of ethyl malonate with 2 mots each of sodium ethoxide and the alkyl halide (usually bromide or iodide). 

Aryl or alkenyl halides attack the central carbon of the allene system in the 2,3-butadien-l-ol 120 to form the 7r-allyl intermediate 121, which undergoes elimination reaction to afford the o,/3-unsaturated ketone 122 or aldehyde. The reaction proceeds smoothly in DMSO using dppe as a ligandflOl]. 

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

Pyrrole derivatives are prepared by the coupling and annulation of o-iodoa-nilines with internal alkynes[291]. The 4-amino-5-iodopyrimidine 428 reacts with the TMS-substituted propargyl alcohol 429 to form the heterocondensed pyrrole 430, and the TMS is removed[292]. Similarly, the tryptophane 434 is obtained by the reaction of o-iodoaniline (431) with the internal alkyne 432 and deprotection of the coupled product 433(293]. As an alternative method, the 2,3-disubstituted indole 436 is obtained directly by the coupling of the o-alky-nyltrifluoroacetanilide 435 with aryl and alkenyl halides or triflates(294]. 

The 2-substituted 3-acylindoles 579 are prepared by carbonylative cycliza-tion of the 2-alkynyltrifluoroacetanilides 576 with aryl halides or alkenyl tri-flates. The reaction can be understood by the aminopalladation of the alkyne with the acylpalladium intermediate as shown by 577 to generate 578, followed by reductive elimination to give 579[425]. 

The Li compound 588 formed by the ort/io-lithiation of A. A -dimethylaniline reacts with vinyl bromide to give the styrene derivative 589(433]. The 2-phe-nylindole 591 is formed by the coupling of l-methyl-2-indolylmagnesium formed in situ from the indolyllithium 590 and MgBr2, with iodobenzene using dppb[434]. 2-Furyl- and 2-thienyllithium in the presence of MgBr2 react with alkenyl halides[435]. The arylallenes 592 and 1,2,4-alkatrienes are prepared by the coupling reaction of the allenyllithium with aryl or alkenyl halides[436]. 

The intramolecular insertion of an internal alkyne into an aryl or alkenyl halide 727 generates aryl- or alkenylpalladium as an intermediate, which is trapped with an organozinc or organostannane to give 728. Overall cis addition to the alkyne takes place[595,596]. The reaction of the alkenylstannane 730 with the 2-bromomethylfuran 729 is used for the introduction of a prenyl group[597]. 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

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