Reactions of Alkynes

Reactions of Alkynes

– Many of the reactions of alkynes are similar to the corresponding reactions of alkenes because both involve pi bonds between two carbon atoms.

– Like the pi bond of an alkene, the pi bonds of an alkyne are electron-rich, and they readily undergo addition reactions.

(A) Reactions of Alkynes: Acetylide Chemistry

(1) Formation of acetylide anions (alkynides)

– Acidity of Alkynes is the inportant factor of activity of Alkynes.

– Terminal alkynes are much more acidic than other hydrocarbons.

– Removal of an acetylenic proton forms an acetylide ion, which plays a central role in alkyne chemistry.

– The acidity of an acetylenic hydrogen stems from the nature of the sp hybrid ≡ C-H bond.

– Sodium amide (Na+ –NH2) is frequently used as the base in forming acetylide salts.

Reactions of Alkynes

(2) Alkylation of acetylide ions

– An acetylide ion is a strong base and a powerful nucleophile.

– It can displace a halide ion from a suitable substrate, giving a substituted acetylene.

– If this SN2 reaction is to produce a good yield, the alkyl halide must be an excellent SN2 substrate: It must be methyl or primary, with no bulky substituents or branches close to the reaction center.

Reactions of Alkynes

(3) Reactions with carbonyl groups

– Like other carbanions, acetylide ions are strong nucleophiles and strong bases.

– In addition to displacing halide ions in SN2 reactions, they can add to carbonyl (C=O) groups.

Because oxygen is more electronegative than carbon, the (C=O) double bond is polarized.

– The oxygen atom has a partial negative charge balanced by an equal amount of positive charge on the carbon atom.

– The positively charged carbon is electrophilic; attack by a nucleophile places a negative charge on the electronegative oxygen atom.

The product of this nucleophilic attack is an alkoxide ion, a strong base. (An alkoxide ion is the conjugate base of an alcohol, a weak acid.)

– Addition of water or a dilute acid protonates the alkoxide to give the alcohol

Reactions of Alkynes

(B) Reactions of Alkynes: Addition to trible bond

(1) Reduction to alkanes

– In the presence of a suitable catalyst, hydrogen adds to an alkyne, reducing it to an alkane.

– For example, when either of the butyne isomers reacts with hydrogen and a platinum catalyst, the product is n-butane.

– Platinum, palladium, and nickel catalysts are commonly used in this reduction.

Reactions of Alkynes

(2) Reduction to alkenes

– Hydrogenation of an alkyne can be stopped at the alkene stage by using a “poisoned” (partially deactivated) catalyst made by treating a good catalyst with a compound that makes the catalyst less effective.

– Lindlar’s catalyst is a poisoned palladium catalyst, composed of powdered barium sulfate coated with palladium, poisoned with quinoline. (Equation 1)

– To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry.

– Sodium metal in liquid ammonia reduces alkynes with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes. (Equation 2)

Reactions of Alkynes

Reactions of Alkynes

(3) Addition of halogens (X2 = Cl2 , Br2)

– Bromine and chlorine add to alkynes just as they add to alkenes.

– If (1) mole of halogen adds to (1) mole of an alkyne, the product is a dihaloalkene.

– The stereochemistry of addition may be either syn or anti, and the products are often mixtures of cis and trans isomers.

Reactions of Alkynes

(4) Addition of hydrogen halides (where HX = HCl, HBr, or HI)

– Hydrogen halides add across the triple bond of an alkyne in much the same way they add across the alkene double bond.

– The initial product is a vinyl halide. When a hydrogen halide adds to a terminal alkyne, the product has the orientation predicted by Markovnikov’s rule.

– A second molecule of HX can add, usually with the same orientation as the first.

(5) Addition of water

(a) Catalyzed by HgSO4 / H2SO4

– Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst.

– A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent.

– The hydration of alkynes is similar to the hydration of alkenes, and it also goes with Markovnikov orientation.

– The products are not the alcohols we might expect, however.

Reactions of Alkynes

(b) Hydroboration–Oxidation

– Hydroboration–oxidation adds water across the double bonds of alkenes with anti-Markovnikov orientation.

– A similar reaction takes place with alkynes, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond.

– Di (secondary isoamyl)borane, called “disiamylborane,” adds to the triple bond only once to give a vinylborane. (Amyl is an older common name for pentyl.)

– In a terminal alkyne, the boron atom bonds to the terminal carbon atom.

(C) Reactions of Alkynes: Oxidation of Alkynes

(1) Oxidation to α-diketones

– Under mild conditions, potassium permanganate oxidizes alkenes to glycols, compounds with two -OH groups on adjacent carbon atoms.

– Recall that this oxidation involves adding a hydroxyl group to each end of the double bond (hydroxylation). A similar reaction occurs with alkynes.

– If an alkyne is treated with cold, aqueous potassium permanganate under nearly neutral conditions, an α-diketone results.

– This is conceptually the same as hydroxylating each of the two pi bonds of the alkyne, then losing two molecules of water to give the diketone.

(2) Oxidative cleavage

– Ozonolysis of an alkyne, followed by hydrolysis, cleaves the triple bond and gives two carboxylic acids.

– Either permanganate cleavage or ozonolysis can be used to determine the position of the triple bond in an unknown alkyne.

Reactions of Alkynes

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