Predict the products from reaction of 5 -decyne with the...

Predict the products from reaction of 5 -decyne with the following reagents: (a) $\mathrm{H}_{2},$ Lindlar catalyst $\quad$ (b) $\mathrm{Li}$ in $\mathrm{NH}_{3}$ (c) 1 equiv $\mathrm{Br}_{2}$ (d) $\mathrm{BH}_{3}$ in THF, then $\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{OH}^{-}$ (e) $\mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{HgSO}_{4}$ (f) Excess $\mathrm{H}_{2},$ Pd/C catalyst

Predict the products from reaction of 5 -decyne with the following reagents:
(a) $\mathrm{H}_{2},$ Lindlar catalyst $\quad$ (b) $\mathrm{Li}$ in $\mathrm{NH}_{3}$
(c) 1 equiv $\mathrm{Br}_{2}$
(d) $\mathrm{BH}_{3}$ in THF, then $\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{OH}^{-}$
(e) $\mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{HgSO}_{4}$
(f) Excess $\mathrm{H}_{2},$ Pd/C catalyst

Key Concepts

Full Catalytic Hydrogenation

Full catalytic hydrogenation uses catalysts like palladium on carbon (Pd/C) under an excess of hydrogen gas to completely reduce an alkyne to an alkane. Unlike selective hydrogenation with poisoned catalysts which stops at the alkene stage, here the hydrogenation process proceeds to the fully saturated alkane. This method is fundamental in synthetic chemistry for achieving complete reduction of multiple bonds.

Hydroboration-Oxidation of Alkynes

Hydroboration-oxidation is a two-step reaction that first adds a borane reagent (such as BH3) to an alkyne in a syn fashion, forming a boron-containing intermediate. Subsequent oxidation with hydrogen peroxide in basic conditions converts this intermediate into an enol, which then tautomerizes to form a carbonyl compound. This sequence represents an anti-Markovnikov hydration, providing a route to synthesize ketones or aldehydes from alkynes with controlled regiochemistry.

Mercury-Catalyzed Hydration (Oxymercuration) of Alkynes

Mercury-catalyzed hydration involves the addition of water to an alkyne in the presence of acid and mercury(II) salts, such as HgSO4. This process proceeds via the formation of a mercurinium ion intermediate that facilitates the addition of water and leads to an enol. The enol then rapidly tautomerizes to a more stable carbonyl compound (typically a ketone). This method enables Markovnikov addition of water to alkynes under relatively mild conditions.

Dissolving Metal Reduction

Dissolving metal reduction involves the use of alkali metals (such as Li or Na) in liquid ammonia to provide single electrons that convert an alkyne into a trans-alkene. This reaction proceeds through a radical anion intermediate and is stereoselective, yielding E-alkenes. It is a prototypical example of reduction via electron transfer, offering an alternative to catalytic hydrogenation with different stereochemical outcomes.

Electrophilic Halogenation of Alkynes

Electrophilic halogenation of alkynes is the addition of a halogen molecule (like Br2) across the triple bond. When used in stoichiometric amounts, the reaction generally stops at the formation of a dihalogenated alkene. The mechanism typically involves the formation of a cyclic halonium ion followed by nucleophilic attack, resulting in anti stereochemistry in the addition product. This reaction is key for introducing halogen substituents into alkenes.

Partial Hydrogenation with Lindlar Catalyst

This concept involves reducing an alkyne to a cis-alkene using a poisoned catalyst such as Lindlar's catalyst. The catalyst is designed to stop the hydrogenation at the alkene stage by selectively deactivating further reduction, leading to syn addition of hydrogen atoms. It is an important method in organic synthesis when stereoselectivity (specifically, the formation of Z-alkenes) is required without over?reducing to alkanes.

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