Dehydrohalogenation dihalides

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Double dehydrohalogenation to form terminal alkynes may also be carried out by heating geminal and vicinal dihalides with potassium tert butoxide m dimethyl sulfoxide... 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Double dehydrohalogenation of vicinal dihalides (Section 9 7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of gemmal dihalides... 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Double dehydrohalogenation (Section 9.7) Reaction in which a geminal dihalide or vicinal dihalide, 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. 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

Kraft et al., 1998 Lowen and Van Dyke, 1990 Wei and Tian, 1993], Reactions other than oxidative polymerization are also being studied for producing polyaniline and polythiophene [Kraft et al., 1998 Loewe et al., 2001]. An example is the Pd-catalyzed dehydrohalogenation between aryl dihalides and aryl primary diamines to synthesize polyanilines [Kanbara et al.,... 

LABORATORY METHODS OF PREPARATION 1. Dehydrohalogenation of wc-Dihalides or gem-Dihalides... 

Alkynes are prepared from alkyl dihalides via elimination of atoms or groups from adjacent carbons. Dehydrohalogenation of vicinal- or gemiwaZ-dihahdes is a particularly useful method for the preparation of alkynes (see Section 5.4.5). 

Pyrolysis of bisquaternary ammonium hydroxides 7-12 Cleavage of selenoxides 7-13 Dehydrohalogenation of dihalides or vinylic halides... 

SAMPLE SOLUTION (a) Reasoning backward, we recognize 1,1-dichloroethane as the product of addition of two molecules of hydrogen chloride to acetylene. Thus, the synthesis requires converting ethylene to acetylene as a key feature. As described in Section 9.7, this may be accomplished by conversion of ethylene to a vicinal dihalide, followed by double dehydrohalogenation. A suitable synthesis based on this analysis is as shown ... 

The designated starting material, 1,1-dichloroethane, is a geminal dihalide and can be used to prepare acetylene by a double dehydrohalogenation. 

The 1,2-dihalides are readily trapped by bismethylenecycloalkanes to give products which undergo a double dehydrohalogenation to cyclopropa-aromatics, a reaction discussed in Section IV. 

Telluropheno[2,3-b]quinolines were obtained by dehydrohalogenation of 2,3-dihydro-telluropheno[2,3-A]quinoline 1,1-dihalides with l,5-diazabicyclo[5.4.0]undec-7-ene (DBU) in yields ranging from 12 to 35%. The dihalides were prepared from the 2,3-dihydro tclluropheno[2,3-6]quinolines (p. 374) and /V-bromosuccinimide or iodine in carbon tetrachloride. 

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