Alkynes | Organic Chemistry 2

The general formula for alkynes is C_nH_2n-2, where n is the number of carbon atoms. Alkynes are hydrocarbons with at least one triple bond between two carbon atoms, leading to two fewer hydrogen atoms than an alkene, which has the formula C_nH_2n. Alkanes have single bonds only and follow the formula C_nH_2n+2.

Alkynes are named by replacing the -ane suffix in the corresponding alkane with -yne. The key principles of their nomenclature are:

• Find the longest continuous carbon chain containing the triple bond.

• Assign numbers to the carbon atoms in the triple bond, starting from the end that gives the triple bond the lowest number.

• Use the appropriate numerical prefix to indicate the position of the triple bond in the chain.

• Identify and name any substituent groups attached to the carbon chain, and specify their locations using locants.

• Combine the base name of the alkyne, the locants for the triple bond and substituents, and the names of any substituents to give the final systematic name.

The carbons of the triple bond in an alkyne are sp-hybridized, resulting in a linear geometry. The triple bond of an alkyne consists of: 

• one σ bond, formed by the end-on overlap of two sp-hybrid orbitals of carbon atoms
• two π bonds, with each π bond formed by the side-by-side overlap of two 2p orbitals of carbon atoms.

Alkynes are significantly more acidic than other hydrocarbons, such as alkanes and alkenes, due to the sp hybridization of their carbon atoms.  The carbon-hydrogen bond in terminal alkynes is composed of an sp-hybridized carbon and an s-orbital hydrogen, which results in a higher percentage of s-character in the bond. When a hydrogen is removed from an alkyne, the resulting acetylide anion is stabilized by the sp hybridized carbon, which can effectively distribute the negative charge over the p-orbitals of the carbon.

Alkynes exhibit increased reactivity compared to alkenes due to the weaker nature of both π bonds within the triple bond, making them more susceptible to breaking and facilitating addition reactions. Additionally, alkynes are more polarizable than alkenes, signifying that the electrons in their π bonds are loosely held, which contributes to their increased reactivity in various chemical reactions.

Dehydrohalogenation is a chemical reaction that involves the elimination of hydrogen halides (HX) from organic compounds. For example, when we subject a vicinal dihalide to dehydrohalogenation with a strong base, we can form an alkyne. The reaction typically proceeds through the formation of an alkene intermediate, which then undergoes a second dehydrohalogenation to form the alkyne. This is a common method to synthesize alkynes from dihaloalkanes when treated with a strong base such as sodium amide (NaNH₂) in liquid ammonia.

Electrophilic addition reactions to alkynes follow a two-step mechanism where the electron-rich triple bond attacks an electrophile to form a vinylic carbocation intermediate. This reactive intermediate is then attacked by a nucleophile, leading to the addition product. These reactions are regioselective, often following Markovnikov's rule; however, various conditions and catalysts can alter the outcome of the addition.

Alkynes can undergo several types of electrophilic addition reactions such as:

• Halogens: Alkynes react with halogens such as Br₂ or Cl₂ to form dihalides or tetrahalides depending on the molar ratio used.
• Hydrogen halides: Reaction with HX (X = Cl, Br, I) can yield vinyl halides or geminal dihalides, with the latter as the major product in excess of HX.
• Water: In the presence of a strong acid catalyst, alkynes hydrate to form enols which quickly tautomerize to ketones.
• Hydrogen: Alkynes can be hydrogenated to alkenes using a poisoned catalyst (Lindlar's catalyst) or fully to alkanes with a regular catalyst like Pd/C.

Regioselectivity in electrophilic addition to alkynes is influenced by the stability of the intermediate carbocations formed during the reaction. More substituted carbocations are more stable due to greater alkyl group stabilization, known as hyperconjugation, and the inductive effect. For asymmetrical alkynes, electrophilic addition tends to occur in a way that forms the more stable carbocation, leading to the more substituted alkenyl product, following Markovnikov's rule. Additionally, the nature of the electrophile and solvent effects can also play roles in the outcome of the reaction.

The hydrohalogenation of alkynes involves the addition of hydrogen halides (HX) to the alkyne to form first a vinyl halide, and then an dihaloalkane if excess HX is present. Unlike alkenes, where the addition is typically done in one step, alkynes undergo a two-step addition due to the presence of two π bonds. The addition follows Markovnikov's rule, where the hydrogen atom attaches to the carbon with more hydrogen substituents and the halogen to the carbon with more alkyl substituents.

Alkynes react with halogens through a halogen addition reaction where the π bonds of the alkyne are broken to form dihaloalkanes. Initially, the alkyne reacts with one equivalent of the halogen to form a dihalogenated alkene (vinyl halide), and upon further reaction with a second equivalent of halogen, a tetrahaloalkane is formed. This reaction proceeds through a cyclic halonium ion intermediate, and the addition of the halogens is anti, meaning they add on opposite sides of the alkyne plane.

Anti-Markovnikov addition refers to the addition of a protic acid (H-X) to an alkyne in such a way that the hydrogen (H) attaches to the carbon with fewer hydrogens and the halide (X) or other group attaches to the less substituted carbon. This is the opposite of what we observe in Markovnikov's rule, where the hydrogen attaches to the less substituted carbon.

Anti-Markovnikov addition in alkynes is favored when the reacting alkyne is treated with reagents that form radicals. For example, this type of addition is observed in the presence of peroxides in the reaction with hydrogen bromide (HBr), which leads to the formation of radicals. It can also occur in hydroboration-oxidation reactions, where borane (BH₃) adds first in an anti-Markovnikov manner, followed by oxidation which replaces the boron with a hydroxyl group (-OH), giving an enol that tautomerizes to a ketone or aldehyde.

In the hydrogenation of alkynes to alkenes, a poisoned catalyst, such as the Lindlar catalyst (palladium on calcium carbonate partially deactivated), is often used. This selective catalyst allows for the addition of hydrogen to the triple bond to form a cis-alkene without further reduction to an alkane.

For full hydrogenation of alkynes to alkanes, a more active catalyst like palladium, platinum, or nickel is used, which facilitates the addition of hydrogen across the triple bond to generate the corresponding alkane.

The products of alkyne reduction can vary depending on the hydrogenation catalyst used. Using a traditional catalyst like palladium on carbon (Pd/C) will typically reduce an alkyne completely to an alkane. In contrast, using a poisoned catalyst, such as the Lindlar catalyst, allows for the selective reduction of an alkyne to a cis-alkene, stopping at the alkene stage without further reduction to an alkane. Alternatively, sodium in liquid ammonia (the Birch reduction) reduces an alkyne to a trans-alkene by transferring electrons and protons in a stepwise manner.