SUBSTITUTION BY HALOGEN

The determination of iodine value (IV), AOCS Tg 1-64, is sometimes used to determine the extent of unsaturation. Because the tertiary aUyflc hydrogen ia the compounds is capable of substitution by halogen atoms, this only approximates a value for the degree of unsaturation. 

The nitrites aie most conveniently prepared from the corresponding alcohols by treatment with nitrosyl chloride in pyridine. The crude nitrites can be precipitated by addition of water and recrystallized from appropriate solvents. However nitrites prepared from carbinols in which the adjacent carbon is substituted by halogen, free or esterified hydroxyl or a carbonyl function are very readily hydrolyzed and must be recrystallized with great care. In general the photolysis gives higher yields if purified and dried nitrites are used which do not contain acids or pyridine, although occasionally the addition of small amounts of pyridine is recommended in order to prevent hydrolysis of the nitrite. Traces of acids do in fact catalyze the thermal decomposition of secondary nitrites to equimolar amounts of alcohol and ketone. ... 

Organomercury compounds are weak nucleophiles and react only with very reactive electrophiles. They readily undergo electrophilic substitution by halogens. 

With ketones such as (94), that have alternative groups of a-H atoms to attack, two questions arise (a) which group, the CH2 or the CH3, is attacked preferentially, and (b) when one H has been substituted by halogen, will a second halogen become attached to the same or to the other a-carbon atom. So far as (a) is concerned, it is found that bromination of, for example, MeCH2COCH3, yields 1- and 3-bromobutanones in virtually equal amount (both these bromoketones then undergo very rapid further reaction, cf. p. 296). The inductive effect exerted by a simple alkyl group, R, thus appears to have relatively little effect on the acidity of H, or on the... 

The loosening influence of the carboxyl group on adjacent hydrogen is much smaller. Hence, in carboxylic acids substitution by halogen proceeds with much more difficulty but can be accelerated by illumination and by catalysts (carriers). The point of entry of the halogen is here also always at the carbon atom adjacent to the carboxyl group, the a-carbon atom. 

The hydride surface is quite unreactive. Substitution by halogens, chlorine in the first place, is possible at elevated temperatures. The reaction of carbon blacks with chlorine was first described in a patent by Kloepfer (140). As is shown in Fig. 2, the maximum chlorine uptake occurred between 400 and 500°. It was nearly equivalent to the hydrogen... 

Monobromination of 2-arylfurans para-substituted by halogen or nitro gave high yields (75-84%) of 5-bromo products two molar equivalents of bromine led to 3,5-dibromination (77-96%). Presumably there is better charge delocalization for 3- than for 4-substitution (80CHE334). Ethyl 5-bromo-2-furoate has been made by a bromodesilylation process (91MI1). 

Under high temperature or UV light and in the gas phase, cyclohexene can undergo free radical substitution by halogens. A common reagent for allylic... 

Presumably substitution by halogens in the phosphazene series results in contracted d orbitals and more efficient ir bonding in the ring (see Chapter 16). Unsym-mctrical substitution may allow the normally planar ring to bend. A good example of this is found in l,l-diphenyl-3.3,5.5-lelnifliiorolriphosphatriazcnc ... 

Substitution by halogen may be carried out in three ways (1) by molecular halogenation, in which polarized X2 itself acts as the electrophile (Equation 7.56) (2) by molecular halogenation with a catalyst, in which the role of the catalyst is to polarize the halogen molecule and (3) by positive halogenation in which the halogen is the cation of a salt.132... 

Methyl groups on cyclopolysilanes can also be substituted by halogens without destruction of the ring structure. With SbCls, for instance two or three methyl groups can be removed from (Me2Si)6 depending on the stoichiometric ratio of the reactants. In case of the dichlorodecamethylcyclohexasilanes, the 1,3- and 1,4-isomers are produced exclusively (Scheme 11). Formation of the 1,2 dichloride was never observed. 

Halogenoalkanes Organic compounds in which one or more hydrogen atoms of an alkane have been substituted by halogen atoms such as chlorine. 

Compounds with Reactive Methylene Groups. In this group the coupling components with the greatest industrial importance are the A-acetoacetyl derivatives of aromatic amines (acetoacetarylides) CH3COCH2CONHAr. Anilines substituted by halogen, alkyl, alkoxy, nitro, and acylamino groups are most suitable (e.g., Napthol AS-IRG). 

Trimethylsilyl groups as substituents in metalloles can also be substituted by halogens. Reaction of compounds 89 with PyHBt3 or with ICl/AgBF4 affords 2,5-dibromo- and 2,5-diiodosiloles 90, respectively <19980M5133>. 

It is interesting to compare the reaetion schemes for the titanium derivatives with these proposed for eyclopentadienyliron dicarbonyls. In both cases C5H5 transfer occurs. In the former case the electron-deficient compound is the substrate, in the latter case the substrate is the electron donor. Thus ligand transfer from the titanium is a nucleophilic substitution, whereas the similar reaction for the iron derivatives can be described in terms of electrophilic substitution. The latter conclusion is in agreement with the results of Cramer (S5), w ho has suggested that CO substitution by halogens in the above-mentioned iron eomplexes follows an electrophilic mechanism 218, 219). 

Section IV,B,l,d Phosphoimidazolium compounds have been prepared by the action of an JV-alkylimidazole on a disubstituted acid chloride of tetracoordinated phosphorus in anhydrous conditions. Phosphoryl imidazolides are of value for the conversion of aldoximes into nitriles. Section IV,B,2,e NMR data indicate that, shortly after complexation of imidazole with iodine or iodine monobromide, the CH and NH ring protons are substituted by halogen. Although complex formation also occurs with 1-methylimidazole, there is no substitution. ... 

When the phenyl-5-pyrazoIones are treated with mercuric acetate in alcoholic solution, four acetoxymercuri groups are introduced into the molecule, together with a methoxy group. If, however, the hydrogen in the 4-position be substituted by halogen or methyl groups, mereura-tion only takes place at 160 C. 

Since these alkyl groups have the alkane structure, they should undergo alkane [Reactions, for example, substitution by halogen. But an alkene molecule presents two sites where halogen can attack, the double bond and the alkyl groups. Can we direct the attack to just one of these sites The answer is yes, by our choice of experimental conditions. 

Thus alkenes undergo substitution by halogen in exactly the same way as do alkanes. Furthermore, just as the alkyl groups affect the reactivity of the double bond toward addition, so the double bond affects the reactivity of the alkyl groups toward- substitution. 

Two fundamentally different series of 1,2-dithiolylium salts can be derived from l,2-dithiole-3-thiones. In one series the thione sulfur is conserved in the salt in the other series it is substituted by halogen or hydrogen. 

Compounds that contain one or more benzene rings are called aromatic hydrocarbons. These compounds undergo substitution by halogens and alkyl groups. 

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