Substituents halogenation

The halogen substituents are the most important members of this group and the most important of the anti-HoIleman groups. The facts concerning their influences in nitration (table 9.5) are ... 

The halogen substituents ( — 7 +M) owe their o p-orientating effect, achieved in spite of the deactivation, to polarisability by the conjugative process. The strength of the inductive deactivation is seen in the sequence of the two ratios quoted. 

This is also confirmed by the observation that in the reaction of 2.4- or 2,5-dihalogenothiazoles with anionic nucleophiles, the halogen in position 2 reacts first (8, 9, 35). This halogen is strongly activated by the aza substituent and by the other halogen substituent. Only when the halogen in position 2 is substituted more than 90% can the substitution proceed in position 4 or 5,... 

Hydroxyl groups take precedence over ( outrank ) alkyl groups and halogen substituents m determining the direction m which a carbon chain is numbered The OH group is assumed to be attached to C 1 of a cyclic alcohol and is not numbered... 

Halogen substituents are slightly deactivating but are ortho para directing... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

Like hydroxyl groups and ammo groups however halogen substituents possess unshared electron pairs that can be donated to a positively charged carbon This electron donation into the TT system stabilizes the intermediates derived from ortho and from para attack... 

A halogen substituent Irke hydrogen rs monovalent and when present rn a molec ular formula rs treated as rf rt were hydrogen for counting purposes... 

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... 

Aryl halides are compounds m which a halogen substituent is attached directly to an aro matic ring Representative aryl halides include... 

Alkyl halide (Section 4 1) Compound of the type RX in which X IS a halogen substituent (F Cl Br I)... 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

Geminal dihalide (Section 9 7) A dihahde of the form R2CX2 in which the two halogen substituents are located on the same carbon... 

Hydroxyall l Hydroperoxides. These compounds, represented by (1, X = OH, R = H), may be isolated as discreet compounds only with certain stmctural restrictions, eg, that one or both of R and R are hydrogen, ie, they are derived from aldehydes, or that R or R contain electron-withdrawing substituents, ie, they are derived from ketones bearing a-halogen substituents. Other hydroxyalkyl hydroperoxides may exist in equihbrium mixtures of ketone and hydrogen peroxide. 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

Halogen Substituents. Halogen functional groups are readily replaced by nucleophiles, eg, hydroxide ion, especially when they ate attached at the a- or y-position of the pyridine ting. This reaction has been exploited in the synthesis of the insecticide chlorpyrifos [2921-88-2J (43) (42), and the insecticide tiiclopyi [55335-06-3] (44) (14,43). 2,3,5,6-Tetiachloiopyiidine [2402-79-1] reacts with caustic to form the hydioxylated material [6515-38-4], which then can be used to form (44) and (43). 

Fiber-Reactive Dyes. These dyes can enter iato chemical reaction with the fiber and form a covalent bond to become an iategral part of the fiber polymer. They therefore have exceptional wetfastness. Thein main use is on ceUulosic fibers where they are appHed neutral and then chemical reaction is initiated by the addition of alkaH. Reaction with the ceUulose can be by either nucleophilic substitution, using, for example, dyes containing activated halogen substituents, or by addition to the double bond in, for example, vinyl sulfone, —S02CH=CH2, groups. 

Reduction of the halogen substituent has been carried out by different procedures such as catalytic hydrogenation using palladium-carbon or Raney nickel, red phosphorus and hydroiodic acid, and zinc and sulfuric acid (66AHQ6)347). 3-Deuteropyrazole has been... 

The Bergmann variation of the Balz Schiemann reaction is a two-step process featuring copper- or copper halide-catalyzed decomposition of aqueous or acetone solutions of arenediazonium fluoroborates containing alkyl or halogen substituents [30] A recent modification is a one-step technique featuring simultaneous diazoti-zation and decomposition by addition of aqueous sodium nitrite at 25 °C to a mixture of fluorobonc acid, copper powder, and 2 isopropyl 6 methylaniline to give 2-isopropyl-6 methylfluorobenzene in 73% yield [37]... 

Selectivity is not an issue in the conversion of alcohols to alkyl halides. Except for certain limitations to be discussed in Section 8.15, the location of the halogen substituent in the product conesponds to that of the hydroxyl group in the starting alcohol. 

When we discussed elimination reactions in Chapter 5, we learned that a Lewis base can react with an alkyl halide to form an alkene. In the present chapter, you will find that the same kinds of reactants can also undergo a different reaction, one in which the Lewis base acts as a nucleophile to substitute for the halogen substituent on carbon. 

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