Bonds to Carbon

Bonds to Carbon.—Trimethylarsine forms stable 1 1 addition compounds with boron tribromide and tri-iodide, but the corresponding trimethylantimony derivatives decompose slowly at room temperature to give Me3SbX2 (X = Br or I). The crystalline 1 2 adduct of dimethylchloroarsine and thiourea from singlecrystal data should be formulated as the ionic compound [Me2AsSC(NH2)2] -Cr,SC(NH2)2 the As—S distance is 2.320(2) A. 

A new chloro-arsine, (l-naphthyl)2AsCI, can be prepared by the reaction of 

AS2O3 with the Grignard reagent from 1-naphthyl bromide followed by deeom-position with hydrochlorie aeid. The resolution into optically active isomers of the arsenic(ni) compounds (115) as hydrogen tartrates and the arsonium salts 

The barrier to pyramidal inversion at arsenic has been calculated for a variety of compounds, and where experimental data are available there is satisfactory agreement. The low barrier to inversion in the 6ir-clectron arsoles (117) is noteworthy, pointing to extensive delocalization in the planar transition state and lending support to the contention that cyclic delocalization also occurs in the pyramidal ground state. The six-membered arsabenzene molecule (118 R = H) is 

Substituted arsabenzene molecules with alkoxy- or aldehyde groups in the 4-position (118 R = OMe, OEt, or CHO) have been prepared for the first time. Relatively little is known about the arsenic analogues of nitrenes, but a complex of the four-electron ligand PhAs has been prepared via demetallation of (CO)5CrAsPhLi2. The product, PhAs[Cr(CO)5]2, from an X-ray determination, contains arsenic in trigonal planar co-ordination, and the short (2.38 A) Cr—As bond distance points to 7r-bonding, probably of the (d-p) type. A new cyclic 

Bonds to Carbon.—A novel cyclo-triarsane derivative (88), in which the substituents are constrained into all cis positions, can be obtained by 

A series of monocyclopentadienyl-arsines (91) with fluxional cr-bonded structures can be prepared from halogeno-arsines and either cyclopenta-dienyltrimethylsilane or lithium cyclopentadienide.625 Vibrational spectra of (PhCH2)3MX2, where M = As or Sb and X = F or Cl, are in accord with 

Breitinger and G. P. Arnold, Inorg. Nuclear Chem. Letters, 1974, 10, 517. 

Bonds to Carbon.— An ingenious attempt to prepare five-co-ordinate nitrogen 

Vibrational assignment of the rotational isomers (rotamers) of methyl nitrite has been accomplished by means of a photochemical effect vibrational and electronic spectra were measured for the vapour phase and matrix-isolated samples of H3CONO and DsCONO.  

Heicklen has studied the photolysis of methyl nitrite,which gives methoxyl radicals  

In the lower atmosphere CH3O is an intermediate in the photochemical oxidation of hydrocarbons and may be important in the conversion of NO into NOg. Thus, in an extension of the work of Wiebe et al., CH3ONO was photolysed in the presence of NO, NO plus NO2, and NO plus Og. Nitro-methane has also been photolysed in the gas phase at 313 nm, the major products being CH3ONO = 0.22), HCHO = 0.20), CH3NO = 0.06), and NO = 0.10). When nitromethane decomposes thermally the same basic reaction occurs  

The kinetics of the process in Ar have been followed and a rate constant has been obtained for the first subsequent reaction  

Bonds to Carbon —Phosphorus in) Compounds. A direct route to MezPCl from methyl chloride and red phosphorus has been achieved by modification of the method used for preparation of MePCl2 in which methyl chloride is passed over a mixture of phosphorus and copper powder packed in a glass tube. The inclusion of active carbon in the last part of the tube is the important modification. 

Although the compounds have not been isolated, there is strong evidence from n.m.r. spectrometry for the formation of the mixed cyano-halides P(CN) X3 , where X = Cl, Br, or I, and n = 1 or 2, in reactions between PX3 and silver cyanide in solution in 1-iodopropane/ In addition, a number of ternary mixed cyano-halides were also detected. The mean P—CN bond dissociation energy has been assessed as 84 kcal moP from data on the heats of hydrolysis and formation of crystalline P(CN)3- ° 

Microwave spectra for EtPHj and its deuteriation products suggest a 45 55 mixture of gauche- and trans-rotamers at room temperature. In the 1 2 adducts of trimethylphosphine with PX3 or ASX3 (X = ClorBr), i.r, and Raman spectra showed that trans configurations were adopted for all these monomeric complexes,but that with l,2-bis(dimethylphosphino)ethane the complexes were forced into the cis configuration. 

On account of a possible explosion hazard, only small quantities of the propyne derivatives (CF3 C)3M (M = P, As, or Sb) were synthesized from the required trihalide and LiC CFs in diethyl ether at -78 The bismuth analogue, on the other hand, could not be obtained in sufficiently large amounts to be able to purify it. 

Veltmann, Angew. Chem. Intemat. Edn., 1976, 15, 315. 

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The older t scale places TMS at IOt with most protons bonded to carbon having smaller X values. 

Next, we consider one pair of it electrons and one pair of cj elections. The cj electrons may originate from a CH or from a CC bond. Let us consider the loop enclosed by the three anchors formed when the electron pair comes from a C-H bond. There are only three possible pairing options. The hydrogen-atom originally bonded to carbon atom 1, is shifted in one product to carbon atom 2,... 

In some force fields the interaction sites are not all situated on the atomic nuclei. For example, in the MM2, MM3 and MM4 programs, the van der Waals centres of hydrogen atoms bonded to carbon are placed not at the nuclei but are approximately 10% along the bond towards the attached atom. The rationale for this is that the electron distribution about small atoms such as oxygen, fluorine and particularly hydrogen is distinctly non-spherical. The single electron from the hydrogen is involved in the bond to the adjacent atom and there are no other electrons that can contribute to the van der Waals interactions. Some force fields also require lone pairs to be defined on particular atoms these have their own van der Waals and electrostatic parameters. 

Methyl nitrite has the molecular formula CH3NO2 All hydrogens are bonded to carbon and the order of atomic connections is CONO... 

Many organic compounds have double or triple bonds to carbon Four electrons are involved in a double bond six in a triple bond... 

A vexing puzzle m the early days of valence bond theory concerned the fact that methane is CH4 and that the four bonds to carbon are directed toward the corners of a tetrahedron Valence bond theory is based on the overlap of half filled orbitals of the connected atoms but with an electron configuration of s 2s 2p 2py carbon has only two half filled orbitals (Figure 2 8a) How can it have bonds to four hydrogens ... 

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest Ingold concluded that the carbon-halogen bond breaks m the rate determining step The weaker the carbon-halogen bond the easier it breaks... 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

The stereochemical relationship between the reactant and the product revealed by the isotopic labeling shows that oxygen becomes bonded to carbon on the same side from which H IS lost As you will see m this and the chapters to come determining the three dimensional aspects of a chemical or biochemical transformation can be a subtle yet powerful tool for increasing our understanding of how these reactions occur... 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

Carbon is partially bonded to both the incoming nucleophile and the departing halide at the transition state Progress is made toward the transition state as the nucleophile begins to share a pair of its electrons with carbon and the halide ion leaves taking with it the pair of electrons m its bond to carbon... 

Although this mechanistic picture developed from experiments involving optically active alkyl halides chemists speak even of methyl halides as undergoing nucleophilic substitution with inversion By this they mean that tetrahedral inversion of the bonds to carbon occurs as the reactant proceeds to the product... 

An advantage that sulfonate esters have over alkyl halides is that their prepara tion from alcohols does not involve any of the bonds to carbon The alcohol oxygen becomes the oxygen that connects the alkyl group to the sulfonyl group Thus the configuration of a sulfonate ester is exactly the same as that of the alcohol from which It was prepared If we wish to study the stereochemistry of nucleophilic substitution m an optically active substrate for example we know that a tosylate ester will have the same configuration and the same optical purity as the alcohol from which it was prepared... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

Table 13 1 collects chemical shift information for protons of various types The beginning and major portion of the table concerns protons bonded to carbon Within each type methyl (CH3) protons are more shielded than methylene (CH2) protons and meth ylene protons are more shielded than methme (CH) protons These differences are small as the following two examples illustrate... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

In the reduction R2C=O R2CHOH the hydrogen bonded to carbon comes from BH4 the hydrogen on oxygen comes from an OH group of the solvent (water methanol or ethanol)... 

IS a two step process m which the first step is rate determining In step 1 the nucleophilic hydroxide ion attacks the carbonyl group forming a bond to carbon An alkoxide ion is the product of step 1 This alkoxide ion abstracts a proton from water m step 2 yielding the gemmal diol The second step like all other proton transfers between oxygen that we have seen is fast... 

Examine the electro r static potential map of butanoic acid on t Learning By Modeling and notice how much more in tense the blue color (positive charge) is on the OH hydro gen than on the hydrogens bonded to carbon... 

Nucleophile (Section 4 8) An atom or ion that has an unshared electron pair which can be used to form a bond to carbon Nucleophiles are Lewis bases... 

Xenon Bonded to Carbon. A number of stmcturally well-characterized compounds containing Xe—C bonds are known. In all cases these occur as colorless salts of xenonium cations, R—Xe" where R is a fluorophenyl or alkynyl group. The formation of the pentafluorophenylxenon(II) cation, CgFgXe+ [121850-39-3] (-30W) and CHgC N (0°C) solutions with the anions B(C3F3)3F [121850-40-6], B(CgFg) 2F- [123168-25-2], and... 

The combination of hydroboration and oxidation leads to the overall hydration of an alkene. Notice, however, that water is not a reactant. The hydrogen that becomes bonded to carbon comes from the organoborane, and the hydroxyl group from hydrogen peroxide. 

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