Diagrams 9-5b,c
This generates a cylinder of electron density around the sigma bond.
The C-C bond length for ethyne is 1.20Å which is shorter than ethane (1.54Å) and ethene (1.33Å).
The C-H bond length in ethyne is 1.06Å which is also shorter than in ethane (1.09Å) or ethene (1.08Å).
This is because the C-H bond contains more s character (sp3→sp2→sp) which gives stronger bonds.
Acidity of Alkynes
Terminal alkynes are acidic, the end hydrogen can be removed as a proton by a strong base.
E.g.
Heavy Metal Acetylides
These form more covalent type structures, and the silver and copper acetylides will precipitate from solution. (Test for terminal alkynes).
Synthesis of Alkynes
The carbanions produced are called acetylides, and are strong nucleophiles.
They can react in SN2 displacements of halides or tosylates from primary unhindered substrates to produce internal alkynes.
E.g.
Carbonyl and Epoxide Reactions
Acetylides will also attack other standard electrophiles like carbonyls and epoxides to generate alcohols. (These reactions are very similar to Grignard type reactions).
E.g.
Epoxides also generate longer chain alcohols.
Synthesis of Alkynes via Elimination
In the same way that alkenes are produced by elimination of H-X, alkynes can be produced by elimination of 2 moles of H-X from a geminal or vicinal dihalide.
Strongly basic conditions (KOH or NaNH2, high temps) are required for this transformation, and often the molecule may not be stable to these conditions and may be destroyed or will rearrange.
Base Catalyzed Rearrangements
Often the deprotonated acetylide will rearrange to give a more stable product.
More highly substituted alkynes are more stable.
Sodium amide is such a strong base that during the equilibrating of all triple bond isomers, the terminal alkyne gets deprotonated and exists as the sodium salt, and is therefore prevented from equilibrating further.
Eventually all the alkynes are trapped as the terminal sodium acetylide salts, and on addition of water generates the terminal alkyne.
E.g.
Bond Strengths
Recall that the π bond of an alkene is weaker than its σ bond.
Compound Total Energy Class Approx. Energy
Alkane 83kcal/mol alkane σ 83kcal/mol
Alkene 146kcal/mol alkene π 63kcal/mol
Alkyne 200kcal/mol alkyne 2nd π 54kcal/mol
The sum of the two π bonds in an alkyne are weaker than the sum of two alkene π bonds.
The triple bond is electron rich and acts as a nucleophile (just like a double bond).
Since addition reactions convert π bonds into σ bonds, these reactions are typically exothermic.
Alkynes can add up to two molecules across the triple bond.
Addition of Hydrogen
Hydrogen in the presence of a catalyst will add twice to alkynes to generate alkanes.
This reaction proceeds through a cis alkene intermediate, but cannot be stopped at this stage except with the use of a special catalyst.
Lindlar’s catalyst
This is a partially deactivated (poisoned) catalyst consisting of barium sulfate, palladium and quinoline (the poison).
The hydrogens are delivered simultaneously to the same side of the alkyne, creating syn addition (cis alkenes).
Trans Alkenes
These can be produced by the action of sodium metal in liquid ammonia.
An electron is added to the triple bond, generating a radical anion, which is protonated to generate a vinyl radical.
The radical accepts another electron which produces an anion, which in turn gets protonated, giving the alkene with anti stereochemistry.
The anti stereochemistry arises from the two alkyl groups being as far away from each other as possible at the vinyl radical stage.
Addition of Halogens
Chlorine and bromine add in an analogous fashion as for alkenes.
If only one mole of halogen is added then a dihaloalkene is produced, and a mixture of syn and anti addition is observed.
It is usually hard to control the addition of just one equivalent of halogen and it is more common to add two equivalents to generate tetrahalides.
Addition of Hydrogen Halides
H-X can be added to alkynes just like to alkenes.
For terminal alkynes, the products have orientation as dictated by Markovnikov’s rule.
The vinyl cation is more stable with positive charge on the more substituted carbon.
The second mole of H-X will usually add with the same orientation as the first.
Internal alkynes show little selectivity since both intermediate cations are equally substituted.
The peroxide effect is also observed with the addition of H-Br and peroxides to generate anti-Markovnikov products.
Hydration of Alkynes to Ketones and Aldehydes
There are two different methods:
Mercuric-ion catalyzed hydrations
A mixture of mercuric sulfate and aqueous sulfuric acid is used to perform addition of water across the triple bond.
The orientation is Markovnikov, and the initial products are enols.
Electrophilic addition of the Hg2+ ion generates a vinyl cation, which is attacked by a nucleophilic water molecule.
Deprotonation gives an organomercurial alcohol which, under acidic conditions, has the mercury replaced by a hydrogen, to generate a vinyl alcohol (enol).
Enols tend to be unstable, and isomerise to their more stable ketone form.
Enols are always in equilibrium with their ketone form – this is an example of keto-enol tautomerism.
Hydroboration-Oxidation
Previously we saw hydroboration-oxidation of alkenes gave anti-Markovnikov addition of water across the double bond.
The same occurs for alkynes, except a sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. (Usually di(sec-isoamyl)borane is used).
A vinyl borane is produced with anti-Markovnikov orientation, which then is oxidized using basic hydrogen peroxide to an enol, which tautomerises into the more stable keto form.
Terminal alkynes are converted into aldehydes.
Oxidation of Alkynes
Under mild (cold, neutral) conditions the reaction of potassium permanganate with an alkyne can generate a diketone.
If the mixture becomes too warm or basic, the oxidation proceeds further to generate two carboxylate anions, which on acidification generates two carboxylic acids.
Terminal alkynes are similarly cleaved to give a carboxylic acid and carbon dioxide.
Ozonolysis
Ozonolysis of an alkyne followed by hydrolysis gives similar products to those obtained from permanganate oxidative cleavage.
