Geminal dihalide alkynes

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 Sodium amide Alkyne Ammonia... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

Double dehydrohalogenation of geminal dihalides (Section 9.7) An E2 elimination reaction of a geminal 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 (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. 

Alkynes react with HC1 and HBr to form haloalkenes or geminal dihalides depending on whether one or two molar equivalents of the hydrogen halide are used. 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

When 2 moles of a hydrogen halide add to an alkyne, the second mole usually adds with the same orientation as the first. This consistent orientation leads to a geminal dihalide. For example, a double Markovnikov addition of HBr to pent-l-yne gives 2,2-dibromopentane. 

Alkynes are prepared by elimination reactions, as discussed in Section 8.10. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne by two... 

After the first elimination of HC1, the reaction may be repeated on the alkene to give the alkyne. If, instead of a geminal dihalide, a vicinal dihalide is used, e.g. RCH2CHC1-CH2C1, then the same... 

Adding a halide acid such as HCl or HBr to an alkyne can create a geminal dihalide via a... 

Addition of a hydrogen halide to an internal alkyne forms two geminal dihalides because the initial addition of the proton can occur with equal ease to either of the sp carbons. 

If, however, the same group is attached to each of the sp carbons of the internal alkyne, only one geminal dihalide is obtained. 

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. 

Geminal dihalides can also be converted to alkynes by dehydrohalogenation. 

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. 

Alkynes react with one molar equivalent of hydrogen chloride or hydrogen bromide to form haloalkenes, and with two molar equivalents to form geminal dihalides. 

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). 

When the starting alkyne is treated with excess HX, two successive addition reactions occur, producing a geminal dihalide ... 

The geminal dibromides can be ruled out, because we only saw one way to make a geminal dihalide—and that was starting from an alkyne. We certainly do not want to start with an alkyne in order to produce the very same alkyne ... 

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