Alkynes are far less reactive to most electrophilic additions than alkenes.

• The main explanation for this discrepancy is the instability of the hybridized vinylic carbocation intermediate created from an alkyne against the sp2-hybridized alkyl carbocation intermediate formed from an alkene.

• Step 1 in the addition of the second mole of HX is the reaction of the remaining p bond's electron pair with HBr to produce a carbocation.

• Because of the delocalization of the positive charge via resonance, the one with the positive charge on the carbon carrying the halogen is favored of the two potential carbocations, as shown in the image attached below.

Hydrochlorination of acetylene was once the primary source of vinyl chloride.

However, when the expense of producing acetylene climbed, makers of vinyl chloride explored alternative ways to use this chemical.

Ethylene was chosen as the beginning ingredient because it can be transformed to vinyl chloride in two steps:

• When ethylene is treated with chlorine, it produces 1,2-dichloroethane, which, when heated in the presence of charcoal or other catalysts, produces 1,2-dichloroethane.

• Enols are in equilibrium with a constitutional isomer created by a hydrogen atom migrating from oxygen to carbon and rearranging the carbon-carbon double bond to form a carbon-oxygen double bond.

The value of K eq shows that 2-butanone (the keto form) is substantially more stable than its enol counterpart.

• In general, keto forms are more stable than enol forms because (1) a C"O p bond is stronger than a C"C p bond, although (2) C-H and O-H s links have equivalent bond strengths.

Tautomers are the keto and enol versions of 2-butanone.

• Tautomers are constitutional isomers that are in equilibrium with one another and only differ in the placement of a hydrogen atom or another atom.

In the presence of intense sulfuric acid and Hg(II) salts as catalysts, alkynes undergo water addition in a process similar to alkene oxymercuration (Section 6.3F).

• HgO, HgSO4, and Hg(OAc)2 are the most often utilized Hg(II) salts for this purpose.

• The addition of water to terminal alkynes follows Markovnikov's rule; hydrogen attaches to the carbon atom of the triple bond holding the hydrogen.

Because the resultant enol is in equilibrium with the more stable keto form, the isolated product is a ketone (an aldehyde in the case of acetylene itself).

An alkyne is reduced in two stages:

• (1) the addition of one mole of H2 to produce an alkene

• (2) the addition of the second mole to the alkene to generate the alkane. In most circumstances, stopping the reaction at the alkene step is impossible.

• However, by carefully selecting the catalyst, the reduction may be stopped after the addition of one mole of hydrogen.

The most often used catalyst for this purpose is finely powdered palladium metal placed over solid calcium carbonate particularly modified with lead salts.

• This is referred to as the Lindlar catalyst.

Stereoselective reduction (hydrogenation) of alkynes over a Lindlar catalyst; syn addition of two hydrogen atoms to the carbon-carbon triple bond.

The net result of hydroboration of an internal alkyne followed by acetic acid treatment is the reduction of the alkyne to a cis-alkene.

• As a result, hydroboration-protonolysis and catalytic reduction over a Lindlar catalyst are two alternate methods for converting an alkyne to a cis-alkene.

• Alkynes can also be converted to alkenes by reacting them with sodium or lithium metal in liquid ammonia or low-molecular-weight primary or secondary amines.

The alkali metal acts as a reducing agent and is oxidized to M1, which dissolves as a metal salt in the reaction solvent.

The stereoselective reduction of an alkyne to an alkene by lithium or sodium in liquid ammonia, NH3(l), includes mostly anti addition of two hydrogen atoms to the triple bond.

Thus, with the correct reagents and reaction conditions, an alkyne can be reduced to either a cis-alkene (through catalytic reduction or hydroboration-protonolysis) or a trans-alkene (by dissolving-metal reduction).

The following process can account for the stereoselectivity of alkali metal reduction of alkynes to alkenes.

• Keep in mind that this process contains two one-electron reductions and two proton-transfer reactions while you analyze it.

• Step 3 determines the stereochemistry of the alkene. The overall equation for the reaction is obtained by adding the four stages and canceling species that occur on both sides of the equation.