Answer Problem 37 , parts a-h using 2 -butyne as the...

Answer Problem 37 , parts a-h using 2 -butyne as the starting material instead of propyne. What will be the major product of the reaction of 1 mol of propyne with each of the following reagents? a. HBr (1 mol) b. HBr (2 mol) c. Br $2(1 \mathrm{mol}) / \mathrm{CH} 2 \mathrm{Cl} 2$ d. Br $2(2 \mathrm{mol}) / \mathrm{CH} 2 \mathrm{C} 12$ e. aqueous $\mathrm{H} 2 \mathrm{SO} 4, \mathrm{HgSO}$ f. $\mathrm{R} 2 \mathrm{BH}$ in THF followed by $\mathrm{H} 2 \mathrm{O} 2 / \mathrm{HO}-/ \mathrm{H} 2 \mathrm{O}$ g. excess $\mathrm{H} 2, \mathrm{Pd} / \mathrm{C}$ h. H2/Lindlar catalyst i. sodium amide J. the product of part i followed by 1 -chloropropane

Answer Problem 37 , parts a-h using 2 -butyne as the starting material instead of propyne.
What will be the major product of the reaction of 1 mol of propyne with each of the following reagents?
a. HBr (1 mol)
b. HBr (2 mol)
c. Br $2(1 \mathrm{mol}) / \mathrm{CH} 2 \mathrm{Cl} 2$
d. Br $2(2 \mathrm{mol}) / \mathrm{CH} 2 \mathrm{C} 12$
e. aqueous $\mathrm{H} 2 \mathrm{SO} 4, \mathrm{HgSO}$
f. $\mathrm{R} 2 \mathrm{BH}$ in THF followed by $\mathrm{H} 2 \mathrm{O} 2 / \mathrm{HO}-/ \mathrm{H} 2 \mathrm{O}$
g. excess $\mathrm{H} 2, \mathrm{Pd} / \mathrm{C}$
h. H2/Lindlar catalyst
i. sodium amide
J. the product of part i followed by 1 -chloropropane

Key Concepts

Electrophilic Addition Reactions to Alkynes

Alkynes undergo electrophilic addition reactions in which an electrophile attacks the electron?rich triple bond. These additions can occur in a stepwise mechanism resulting in intermediate carbocations or via a concerted process, depending on the reagent and conditions. This concept underpins many reactions that convert alkynes into alkenes or tetra?substituted products depending on the number of equivalents added and the reaction conditions.

Stereoselectivity in Addition Reactions

Stereoselectivity is a key consideration in many alkyne reactions. Depending on the reagent and mechanism, addition to the alkyne can occur either syn (both groups added to the same face) or anti (groups added to opposite faces). This control over stereochemistry is particularly critical in differentiating between cis and trans addition products from a triple bond.

Regioselectivity in Electrophilic Additions

Regioselectivity refers to the directional preference of adding electrophiles to unsymmetrical alkynes. Although symmetric alkynes do not present regioselectivity issues, understanding this concept is essential for reactions where substitution patterns lead to distinct products. In broader terms, many electrophilic additions to asymmetrical alkynes follow rules such as Markovnikov’s rule.

Hydrohalogenation Reactions

In hydrohalogenation, hydrogen halides add to the carbon–carbon multiple bonds of alkynes. The nature and quantity of the hydrogen halide determine whether the reaction stops at a vinyl halide stage or proceeds to a geminal dihalide formation through sequential additions. This reaction mechanism illustrates how reaction stoichiometry and conditions influence product formation.

Halogenation Reactions

Halogenation of alkynes involves the addition of halogen molecules to the triple bond, typically resulting in dihalogenated products. Often proceeding via an anti addition mechanism, this transformation is useful for synthesizing vicinal or geminal dihalides and reveals the link between electrophilic addition mechanisms and stereochemical outcomes.

Electrophilic Hydration of Alkynes

Electrophilic hydration of alkynes, often conducted in aqueous acid with a mercuric salt catalyst, introduces water across the triple bond. This reaction proceeds through an enol intermediate that typically tautomerizes to a carbonyl compound such as a ketone. This sequence underscores the interplay between electrophilic attack and subsequent rearrangements in forming stable products.

Catalytic Hydrogenation

Catalytic hydrogenation of alkynes allows for selective control over the reduction of multiple bonds. When using a poisoned catalyst like Lindlar’s catalyst, the reaction stops at the cis-alkene stage, whereas a more active catalyst such as palladium on carbon leads to complete reduction to an alkane. This concept illustrates how catalyst choice directly affects the extent of hydrogenation and the stereochemical outcome of the reaction.

Formation and Reactions of Acetylide Ions

Terminal alkynes can be deprotonated by strong bases to form acetylide ions, which are potent nucleophiles. These anions readily participate in nucleophilic substitution reactions, commonly in an SN2-like mechanism, to form new carbon–carbon bonds. This concept is essential when considering coupling reactions that build more complex molecular frameworks.

Hydroboration–Oxidation Reactions

Hydroboration–oxidation is a method used to convert multiple bonds into alcohols or related oxidized products. For alkynes, the initial hydroboration step adds boron regiospecifically and stereospecifically, followed by oxidation to yield enolic intermediates or alkenyl alcohols. This reaction pathway offers a route for anti addition with controlled regiochemistry.

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