Halogenation preferential

Exercise 17-9 a. Explain why 2-butanone is halogenated preferentially on the ethyl side with an acidic catalyst. (Review of Section 11-3 should be helpful.) b. What product would predominate in the acid-catalyzed bromination of 1-phenyl-2-propanone Give your reasoning. 

When 1-aminopyrazoles were subjected to halogenation, preferential oxidation of the amino group led to 5-halogeno-l, 2,3-triazines (88CPB3838) (see also Part 1, Section B,l,a). 

Dibenzoselenophene is readily alkylated to the selenienyl salts (62), which are powerful methylation and trifluoromethylation agents [98, 99], Halogens preferentially attack a-carbon in selenophenes but the 1,2-dibromo-l-benzoselenophenium bromide (63) was found among the bromination products of 1-benzoselenophene (Fig. 6) [100],... 

Thus the halogens preferentially cause the loss of the radical X through a cleavage of the adjacent bond, whereas the amines preferentially lose a radical through a cleavage of the alpha bond. 

When the imidazole or benzimidazole carries an iV-substituent, reactions which would have involved the conjugate base species either have to take an alternative pathway or may not take place, e.g. diazo coupling and halogenation. Preferential electrophilic attack now occurs at C-5. Exceptions to the usual orientation of substitution are observed in Regel acylations at C-2 (probably via initial A-acylation), and some hydroxymethylations. 

In the latter solvent, cavitation probably helps the formation of the actual reagent, AcO Br+. In the absence of a solvent, side-chain bromination occurs, although in modest yield. Even if a radical mechanism appears probable in this case, the reaction was not examined in sufficient detail for an accurate interpretation. In hexane suspension, solid N-halosuccinimide effects ring halogenation preferentially. The acid catalyst, necessary when the mixture is stirred, can be omitted under sonication. The system consisting of alumina-supported cupric bromide is also efficient to achieve the same reactions.i67... 

Sometimes, the distribution ratio is referred to as the partition coefficient, which is often expressed as the logarithm. Note that a distribution ratio for uranium and neptunium between two inorganic solids (zirconolite and perovskite) has been reported. In solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. In this experiment, the nonpolar halogens preferentially dissolve in the nonpolar mineral oil. 

Substituents with -I,+M effects such as halogen show a similar orientation effect to that of alkyl groups. If the substituent is in the 2-position, substitution occurs at C-5 if it is in position 3, substitution occurs preferentially at C-2 or if steric requirements of the group or electrophile intervene, then substitution occurs at C-5. 

The halogenation of thiophene follows a similar pattern in that (a) chlorination is the most difficult to control and (b) substitution occurs preferentially in the a-positions. With... 

In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as is illustrated by the preparation of 3-bromothiophene... 

Isoxazoles can be halogenated in the 4-position (63AHC(2)365). Ring bromination of oxazoles with bromine or NBS occurs preferentially at the 5-position and, if this is occupied, at the 4-position (74AHCI 17)99). Aminooxazoles are readily halogenated. 

Electrophilic substitution occurs readily in Af-phenyl groups, e.g. 1-phenyI-pyrazoIes, -imidazoles and -pyrazolinones are all nitrated and halogenated at the para position. The aryl group is attacked preferentially when the reactions are carried out in strongly acidic media, where the azole ring is protonated. 

The acid-catalyzed additions of bromide and chloride ion to thiiranes occurs readily, with halide preferentially but not exclusively attacking the most substituted carbon atom of the thiirane. The reaction of 1-substituted thiiranes with acetyl chloride shows a slight preference for halide attack at the less substituted carbon atom (80MI50601). For further discussion of electrophilic catalysis of halide ion attack see Section 5.06.3.3.2. The reaction of halogens with thiiranes involves electrophilic attack on sulfur (Section 5.06.3.3.6) followed by nucleophilic attack of halide ion on carbon. 

Halogenated and halogenoalkyl substituted imines react with diazo alkanes under very mild conditions and preferentially yield aziridines [5, 146 147] Diazonium betaines have been considered as intermediates of these reactions [148,... 

When other halogens are also present, dekalogenation reactions may result m a mixture of products resulting from the elimination of fluorine and mainly of the other halogen [64] Chlorofluoro compounds may be preferentially dechlonnated without the loss of fluorine by using zinc in dimethylformainide solution [65] (equation 33)... 

For the ordinary Diels-Alder reaction the dienophile preferentially is of the electron-poor type electron-withdrawing substituents have a rate enhancing effect. Ethylene and simple alkenes are less reactive. Substituent Z in 2 can be e.g. CHO, COR, COOH, COOR, CN, Ar, NO2, halogen, C=C. Good dienophiles are for example maleic anhydride, acrolein, acrylonitrile, dehydrobenzene, tetracya-noethylene (TCNE), acetylene dicarboxylic esters. The diene preferentially is of the electron-rich type thus it should not bear an electron-withdrawing substituent. 

With respect to the carbonyl substrate, a variety of additional functional groups is tolerated, e.g. ester, ether, halogen. With compounds that contain an ester as well as a keto or aldehyde function, the latter usually reacts preferentially. Due to its mild reaction conditions the Wittig reaction is an important method for the synthesis of sensitive alkenes, as for example highly unsaturated compounds like the carotinoid 17 shown above. 

With bromine monochloride at 0°C in a variety of solvents, 1 was converted into addition products, the product distribution being a function of solvent. A change in halogenating agent also altered the product ratio. (Scheme 4) Nucleophilic displacement reactions between these products and silver fluoride was found to cause preferential bromine substitution (83G149). 

The two isomeric possibilities are 1,2- and 2,1-benzisoxazole. Both are preferentially halogenated by electrophilic halogen in the homocyclic ring, initially in the 5-position, although substituents can modify this behavior [67AHC(8)277]. Nucleophiles attack the heteroring (84MI26). 

Quinones may react with carbon-centered radicals by addition at oxygen or carbon, or by electron transfer (Scheme 5.]6).l74, fi2 195 201 202 The preferred reaction pathway depends both on the attacking radical and the particular quinone (halogenated quinones react preferentially by electron transfer). The radical formed may then scavenge another radical. There is also evidence that certain quinones e.g. chloranil, benzoquinone (38)] may copolymerize under some conditions. ... 

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