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Chapter 9
Alkynes

09-00-05UN

Title	Terminal and Internal Akynes
Caption
Notes	A triple bond can undergo the same reactions a double bond can undergo. There are two types of alkynes: terminal and internal. Internal alkynes have carbons on both sides of the triple bonded carbons. A terminal alkyne is at the end of a chain and has increased reactivity because the acetylenic hydrogen is acidic.
Keywords	terminal alkyne, internal alkynes

09-00-09UN

Title	Molecular Structure of Acetylene
Caption
Notes	Triple bonded carbons have sp hybrid orbitals. A sigma bond is formed between the carbons by overlap of the sp orbitals. Sigma bonds to the hydrogens are formed by using the second sp orbital. Since the sp orbitals are linear, acetylene will be a linear molecule.
Keywords	acetylene, sp hybrid orbitals

09-00-10UN

Title	Overlap of the p Orbitals of Acetylene
Caption
Notes	Each carbon in acetylene has two unhybridized p orbitals with one non-bonded electron. It is the overlap of the parallel p orbitals that form the triple bond (2 pi orbitals).
Keywords	acetylene

09-00-11UN

Title	Ethane, Ethene, and Ethyne
Caption
Notes	The carbon-carbon bond distance decreases as the s character of the hybrid orbitals increases. An sp3 orbital has a 25% s character, an sp2 has 33% s character, and an sp orbital a 50% s character.
Keywords	s character

09-00-12T01UN

Title	Acidities of Aliphatic Hydrocarbons
Caption
Notes	The acidity of the aliphatic hydrocarbons increases as the s character of the hybrid orbitals increases. An sp3 orbital has a 25% s character, an sp2 has 33% s character, and an sp orbital a 50% s character. The hydrogen of a terminal alkyne can be abstracted by a base but not the hydrogens of an alkene or an alkane.
Keywords	acidity, terminal alkyne

09-00-14UN

Title	Formation of Acetylide Ions
Caption
Notes	Sodium amide is capable of abstracting a hydrogen from a terminal alkyne leaving a negatively charged carbon. The deprotonated alkyne, known as acetylide, is a strong nucleophile that can easily do addition and substitution reactions.
Keywords	sodium amide, addition reaction, substitution reaction

09-00-15UN

Title	Acetylide Ions in SN2 Reactions
Caption	One of the best methods for synthesizing substituted alkynes is a nucleophilic attack by the acetylide ion on an unhindered alkyl halide.
Notes	The deprotonated alkyne, known as acetylide, is a strong nucleophile that can easily do addition and substitution reactions.
Keywords	acetylide

09-00-16UN

Title	Heavy-Metal Acetylides
Caption	Silver(I) and copper(I) salts react with terminal alkynes to form silver and copper acetylides.
Notes	The reaction of terminal alkynes with silver(I) and copper(I) salts form silver and copper acetylides. These acetylides are insoluble so they precipitate in solution. The reaction is used as a qualitative test for the presence of terminal alkynes or as a way to separate terminal alkynes from internal alkynes.
Keywords	terminal alkynes, acetylides, qualitative test

09-00-20UN

Title	Alkylation of Acetylide Ions
Caption
Notes	Acetylide ions are strong nucleophiles that can undergo SN2 reactions. The reaction gives a good yield when the alkyl halide used is a methyl halide or a primary halide.
Keywords	acetylide ion, SN2 reactions, primary halide

09-00-22UN

Title	Acetylide Ions as Strong Bases
Caption	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.
Notes	If the alkyl halide used is hindered, the acetylide ion will act as a base abstracting a proton from the substrate and causing an E2 elimination reaction.
Keywords	acetylide ion, E2 elimination

09-00-23UN

Title	Addition of Acetylide Ion to Carbonyl Groups
Caption
Notes	A nucleophile will add to the carbon of a carbonyl forming the corresponding alkoxide which upon protonation gives an alcohol. The carbon atom of the carbonyl is partial positive and the oxygen has a partial negative charge.
Keywords	acetylide ions

09-01-01UN

Title	Formation of an Acetylenic Alcohol
Caption
Notes	Acetylide ion will attack ketones or aldehydes to form, upon protonation, tertiary and secondary alcohols respectively. The intermediate alkoxide is not isolated, the reaction is worked up under acidic conditions to protonate the alkoxide ions and convert them to alcohols.
Keywords	secondary alcohol, tertiary alcohol, alkoxide

09-01-10UN

Title	Synthesis of Alkynes: Dehydrohalogenation Reaction
Caption
Notes	Vicinal or geminal dihalides can be dehydrohalogenated by strong bases to produce vinyl halides. The vinyl halides can be isolated or can be reacted with a second equivalent of base to produce the corresponding alkyne. The second dehydrohalogenation requires extremely basic conditions and heat.
Keywords	vicinal halide, geminal dihalide, dehydrohalogenation

09-02-08UN

Title	Catalytic Hydrogenation of Alkynes
Caption
Notes	Two molecules of hydrogen can add across the triple bond to form the corresponding alkane. A catalyst such as Pd, Pt, or Ni needs to be used for the reaction to occur. Under these conditions the alkyne will be completely reduced; the alkene intermediate cannot be isolated.
Keywords	hydrogenation, reduction, catalyst

09-02-10UN

Title	Hydrogenation of Alkynes to cis-Alkenes
Caption
Notes	If the catalyst used for the hydrogenation reaction is partially deactivated (poisoned), the reaction can be stopped after the addition of only one mole of hydrogen. The catalyst used is commonly known as Lindlar's catalyst and it is composed of powdered barium sulfate coated with palladium poisoned with quinoline. Nickel boride has also been used succesfully as a catalyst in this reaction. The reaction produces alkenes with cis stereochemistry.
Keywords	catalyst, poisoned catalyst, Lindlar's catalyst, nickel boride

09-03

Title	Mechanism of Hydrogenation if Alkynes Using Lindlar's Catalyst
Caption	Figure 9-3 Catalytic hydrogenation of alkynes using Lindlar's catalyst.
Notes	Both substrates, the hydrogen and the alkyne, have to be adsorbed on the catalyst for the reaction to occur. Once adsorbed, the hydrogens add to the same side of the double bond (syn addition) giving the product a cis stereochemistry.
Keywords	adsorption, syn addition, cis

09-03-02UN

Title	Reduction of Alkynes with Metal Ammonia
Caption	To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.
Notes	A mixture of sodium metal in liquid ammonia can reduce a triple bond to the trans-alkene. The hydrogens will be added to opposite sides of the double bond.
Keywords	trans

09-03-04UN

Title	Mechanism for the Metal-Ammonia Reduction of Alkynes
Caption
Notes	The first step of the mechanism is the addition of transfer of an electron from the sodium to the alkyne forming a radical anion. The radical anion is protonated in the second step by ammonia giving a vinyl radical. This vinyl radical already has the trans stereochemistry that will be observed in the product. A second electron adds to the vinyl radical forming a vinyl anion which is protonated by the solvent to yield the trans alkene.
Keywords	electron transfer, radical anion, vinyl radical, vinyl anion, trans alkene

09-03-05UN

Title	Halogenation of Alkynes
Caption
Notes	Alkynes can add one or two equivalents of halogen across the triple bond. If only one mole of halogen is used the product obtained will be the dihaloalkene. This addition will not be stereoselective and mixtures of cis and trans isomers will be obtained.
Keywords	dihaloalkene

09-03-06UN

Title	Addition of Two Moles of Halogen to an Alkyne
Caption
Notes	Adding two equivalents of halide across the double bond will produce tetrahalides.
Keywords	tetrahalide

09-03-07UN

Title	Hydrogen Halide Addition to Alkynes
Caption
Notes	One or two molecules of hydrogen halides can be added to an alkyne to form vinyl halides or geminal dihalides, respectively. When a terminal alkyne is used, the addition of HX follows Markovnikov's rule.
Keywords	vinyl halides, geminal dihalides

09-03-11UN

Title	Anti-Markovnikov Addition of Hydrogen Bromide to Alkynes
Caption
Notes	By using peroxides, hydrogen bromide can be added to a terminal alkyne anti-Markovnikov. The bromide will attach to the least substituted carbon giving a mixture of cis and trans isomers.
Keywords	peroxides, anti-Markovnikov

09-03-12UN

Title	Mercuric-Ion Catalyzed Hydration of Alkynes
Caption
Notes	Water can be added across the triple bond in a reaction analogous to the oxymercuration-demercuration of alkenes. The hydration is catalyzed by the mercuric ion. In a typical reaction a mixture of mercuric acetate in aqueous sulfuric acid is used. The addition produces an intermediate vinyl alcohol (enol) that quickly tautomerizes to the more stable ketone or aldehyde.
Keywords	tautomerization, vinyl alcohol, enol, mercuric ion

09-03-13UN

Title	Mechanism of Mercuric Ion Catalyzed Hydration
Caption
Notes	The electrophilic addition of mercuric in (Hg+2) creates a vinyl carbocation. A water molecule attacks the carbocation and after deprotonation forms an organomercurial alcohol. Hydrolysis of the alcohol removes the mercury forming a vinyl alcohol, commonly referred to as enol. Enols will isomerize to the corresponding aldehyde or ketone in a process known as keto-enol tautomerization.
Keywords	vinyl carbocation, organomercurial alcohol, enol, tautomerization

09-03-14UN

Title	Keto-Enol Tautomerization
Caption
Notes	Enols are not stable species so the proton from the alcohol shifts to the neighboring carbon and the double bond shifts from the C=C to the C=O position. This process is an equilibrium between the two forms with the keto form being favored, and it is called tautomerization.
Keywords	keto, enol, tautomerization

09-03-15UN

Title	Mechanism of Acid-Catalyzed Keto-Enol Tautomerization
Caption
Notes	In acidic solution a proton adds to the methylene group of the enol. The intermediate is stabilized by resonance. In the second step of the reaction a hydroxyl proton is abstracted by a water molecule forming the keto form.
Keywords	keto, enol, tautomerization

09-03-17UN

Title	Hydroboration of Alkynes
Caption
Notes	Alkynes can be hydrated anti-Markovnikov by using the hydroboration-oxidation reaction. A hindered alkyl borane needs to be used to prevent two molecules of borane to add to the triple bond. A molecule of borane adds to the triple bond to form a vinyl borane. If a terminal alkyne is used, the borane will add to the least substituted carbon.
Keywords	vinyl borane, anti-Markovnikov

09-03-18UN

Title	Oxidation of Vinyl Boranes
Caption
Notes	In the second step of the hydroboration-oxidation, a basic solution of peroxide is added to the vinyl borane to oxidize the boron and replace it with a hydroxyl group (OH). Once the enol is formed it tautomerizes to the more stable aldehyde.
Keywords	peroxide, vinyl borane, tautomerism

09-03-21UN

Title	Permanganate Oxidation of Alkynes to a-Diketones
Caption
Notes	Under neutral conditions, potassium permanganate can oxidize a triple bond into an a-diketone. The reaction used aqueous KMnO4 to form a tetrahydroxy intermediate which loses two water molecules to produce the diketone.
Keywords	potassium permanganate, a-diketone

09-03-23UN

Title	Permanganate Oxidation of Alkynes to Carboxylic Acids
Caption
Notes	If potassium permanganate is used under basic conditions or if the solution is heated too much, an oxidative cleavage will take place and two molecules of carboxylic acids will be produced.
Keywords	oxidative cleavage

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