Halogenation geminal 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... 

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

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 geminal dihalides. 

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. 

CHEC-II(1996) <1996CHEC-II(8)345> covered a range of reactions of substituents, including hydrolyses of esters to carboxylic acids and geminal dihalides to aldehydes, reduction of aldehydes and halogens directly attached to the ring, O-methylation with diazomethane, and reduction of nitroso compounds. A selection of reactions which have... 

A geminal dihalide (Latin, geminus, twin ) has the two halogen atoms bonded to the same carbon atom. A vicinal dihalide (Latin, vicinus, neighboring ) has the two halogens bonded to adjacent carbon atoms. 

Geminal dihalide (Section 8.10) A compound that has two halogen atoms on the same carbon atom. 

This mechanism occurs particularly with aryl halides, phosphates, and ammonium salts, and with geminal dihalides. The reactions can be stimulated by light, or inhibited by p-dinitrobcnzcnc, and aryl halides can react exclusively by the SrnI mechanism or by Sn2 on halogen, depending on the structures of R3S11 and of ArX, and on the presence or... 

Consistent with this generalization, addition of hydrogen halides to terminal allenes initially gives the vinyl halide if the second double bond reacts, a geminal dihalide is formed. The regioselectivity of the second step is consistent with Markovnikov s rule because a halogen atom can stabilize a carbocation by resonance (see Section 3.4.1). 

If the two halogens are on the same carbon (geminal dihalides) or on adjacent carbons (vicinal dihalides), the two consecutive E2 dehydrohalogenations can result in the formation of a triple bond. This is how alkynes are commonly synthesized. 

A geminal dihalide (abbreviated gem-diAiaAiAf has two halogen atoms bonded to the same carbon geminus, Latin twins). Ketones can be converted to m-dichlorides by reaction with phosphorus pentachloride, and the jf wj-dichlorides can be used to synthesize alkynes. 

The addition of HX to alkynes follows a similar mechanism as that for addition to al-kenes. The major difference is the fact that a vinyl carbenium ion is formed. The stability of these cations is lower than trigonal sp carbenium ions, and thus the addition of HX is slower than with alkenes. The regiochemistry of addition is the same as with alkenes, in that the halogen attaches to the more substituted carbon. Since halogens can stabilize adjacent carbenium ions via resonance, the addition of a second equivalenf of HX places the carbenium ion on the substituted carbon, and therefore geminal dihalides are the major products formed from the addition of two HX molecules (Eq. 10.13). 

GC-MS (Sect. 15.14) A is chromatograph-mass spectrometer used for the analysis of a mixture that contains several compounds, geminal (Sect. 10.4) Two groups connected to the same carbon atom. For example, a geminal dihalide is a compound with two halogens connected to the same carbon atom. 

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