Double Markovnikov addition of HBr to terminal alkyne 1-butyne

Steps:

• Identify Sibrene as 1-butyne (CH₃CH₂C≡CH), a terminal alkyne with formula C₄H₆.
• First HBr adds Markovnikov: H to terminal C, Br to internal C, forming 2-bromobut-1-ene (CH₃CH₂CBr=CH₂).
• Second HBr adds Markovnikov to the alkene: H to =CH₂, Br to CBr, yielding 2,2-dibromobutane (CH₃CH₂CBr₂CH₃).
• Skeletal formula is a butane chain with geminal Br's on carbon 2.

Why D is correct:

• D shows the geminal dibromide on carbon 2, matching the regiochemistry of Markovnikov's rule for HX addition to terminal alkynes (H adds to less substituted C).

Why the others are wrong:

• A: Depicts 1,1-dibromobutane, wrong regiochemistry (Br's on terminal carbon).
• B: Shows monoaddition product (2-bromobut-1-ene), ignores excess HBr requiring double addition.
• C: Represents 1,2-dibromobutane, incorrect as it implies vicinal rather than geminal Br's.

Final answer: D