Acids a-halogenation

When halogenated phenols or phenolic ethers are nitrated with nitric acid a halogen o- or p- to hydroxyl or alkoxyl group can also be replaced. The ease of replacement appears to be in the order Cl< Br[Pg.128]

They usually require a co-catalyst, such as water, an organic acid, a halogen acid, or an organic halide, which serves as the proton or cation donor as it associates with the Lewis acid (Eq. 22.10-22.12). 

DICHLOROACETYLENE (7572-29-4) Unstable explosive when exposed to heat, air, or shock. Violent reaction with oxidizers and acids. A halogenated acetylene compound. 

Saturated aliphatic acids a-halogen-substituted aliphatic acids aj5-unsaturated acids Aryl acids... 

Cobalt is a bluish silvery metal, exhibits ferromagnetism, and can exist in more than one crystal form it is used in alloys for special purposes. Chemically it is somewhat similar to iron when heated in air it gives the oxides C03O4 and CoO, but it is less readily attacked by dilute acids. With halogens, the cobalt(II) halides are formed, except that with fluorine the (III) fluoride, C0F3, is obtained. 

Nitrogen and sulphur absent, (i) If only one halogen is present, acidify with dilute nitric acid and add excess of silver nitrate solution. A precipitate indicates the presence of a halogen. Decant the mother liquor and treat the precipitate with dilute aqueous ammonia solution If the precipitate is white and readily soluble in the ammonia solution, chlorine is present if it is pale yellow and difficultly soluble, bromine is present if it is yellow and insoluble, then iodine is indicated. Iodine and bromine should be confirmed by the tests given below. 

Thus a second method was envisaged, the reaction of a nitrile, hydrogen selenide, and an a-halogenated ketone in the presence of a condensation catalyst, which can be POCl, or POCI3 with a Lewis acid such as PCI3 or anhydrous ZnCl. The use of fresh AICI3 leads to the formation of tarry side-products. 

The O-S exchange method in presence of a-halogenated carbonyl compound is a very good one for thiazole compounds. The thioamide is prepared in situ by the action of amide upon phosphorus pentasulphide with solvent. The a-halogenated aldehyde reacts directly. But the O-Se exchange cannot be performed with a-halogenated carbonyl compounds because of the apparition of phosphoric acid. (Scheme 3), The C-Se bond is very sensitive to add pH. 

The a-halogenated acids or their esters (105) also react with thiourea to give 2-amino-4-hydroxythiazoles (106a) or their 2-amino-4-thiazolone (106b) (1, 247, 254, 530) or 2-imino-4-oxathia2olidine (106c) tautomers (Scheme 47). 

Unlike Its acid catalyzed counterpart a halogenation m base cannot normally be limited to monohalogenation Methyl ketones for example undergo a novel polyhalo genation and cleavage on treatment with a halogen m aqueous base... 

The mechanism of the haloform reaction begins with a halogenation via the eno late The electron attracting effect of an a halogen increases the acidity of the protons on the carbon to which it is bonded making each subsequent halogenation at that car bon faster than the preceding one... 

An electronegative substituent particularly if it is attached to the a carbon increases the acidity of a carboxylic acid As the data m Table 19 2 show all the mono haloacetic acids are about 100 times more acidic than acetic acid Multiple halogen sub stitution increases the acidity even more trichloroacetic acid is 7000 times more acidic than acetic acid ... 

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

Hell-Volhard-Zelmsky reaction (Section 19 16) The phos phorus tnhahde catalyzed a halogenation of a carboxylic acid... 

Organosulfur Halides. When sulfur is directly linked only to an organic radical and to a halogen atom, the radical name is attached to the word sulfur and the name(s) and number of the halide(s) are stated as a separate word. Alternatively, the name can be formed from R—SOH, a sulfenic acid whose radical prefix is sulfenyl-. For example, CH3CH2—S — Br would be named either ethylsulfur monobromide or ethanesulfenyl bromide. When another principal group is present, a composite prefix is formed from the number and substitutive name(s) of the halogen atoms in front of the syllable thio. For example, BrS—COOH is (bromothio)formic acid. 

Stannous fluoride probably was first prepared by Scheele in 1771 and was described by Gay-Lussac and Thenard in 1809. Commercial production of stannous fluoride is by the reaction of stannous oxide and aqueous hydrofluoric acid, or metallic tin and anhydrous hydrogen fluoride (5,6). Snp2 is also produced by the reaction of tin metal, HP, and a halogen in the presence of a nitrile (7). 

The limitations of this reagent are several. It caimot be used to replace a single unactivated halogen atom with the exception of the chloromethyl ether (eq. 5) to form difluoromethyl fluoromethyl ether [461 -63-2]. It also caimot be used to replace a halogen attached to a carbon—carbon double bond. Fluorination of functional group compounds, eg, esters, sulfides, ketones, acids, and aldehydes, produces decomposition products caused by scission of the carbon chains. 

Chemical Properties. A combination of excellent chemical and mechanical properties at elevated temperatures result in high performance service in the chemical processing industry. Teflon PEA resins have been exposed to a variety of organic and inorganic compounds commonly encountered in chemical service (26). They are not attacked by inorganic acids, bases, halogens, metal salt solutions, organic acids, and anhydrides. Aromatic and ahphatic hydrocarbons, alcohols, aldehydes, ketones, ethers, amines, esters, chlorinated compounds, and other polymer solvents have Httle effect. However, like other perfluorinated polymers,they react with alkah metals and elemental fluorine. 

Iodine monochloride [7790-99-0] ICl, mol wt 162.38, 78.16% I, is a black crystalline soHd or a reddish brown Hquid. SoHd ICl exists ia two crystalline modifications the a-form, as stable mby-red needles, d = 3.86 g/mL and mp 27.3°C and as metastable brownish red platelets, d = 3.66 g/mL, mp 13.9°C and bp 100°C (dec). Iodine monochloride is used as a halogenation catalyst and as an analytical reagent (Wij s solution) to determine iodine values of fats and oils (see Fats and fatty oils). ICl is prepared by direct reaction of iodine and Hquid chlorine. Aqueous solutions ate obtained by treating a suspension of iodine ia moderately strong hydrochloric acid with chlorine gas or iodic acid (118,119). 

Beside being acidic, a significant industrial chemical property of phenol is the extremely high reactivity of its ring toward electrophilic substitution. If steric conditions permit, the substitution leads first to the formation of the 2- or 4-mono derivative, then to the 2,4- or 2,6-diderivative, and finally to the 2,4,6-triderivative. The halogenation of phenol produces mono-, di-, and tribal ophenols. 

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