Final answer:
The correct Lewis structure for an alkyne isomer of C4H6, with a triple bond situated at the beginning of the molecule, is option d), represented as H2C≡C-CH=CH2, also known as 1-butyne.
Step-by-step explanation:
The question asks to determine the Lewis structure for an alkyne isomer of C4H6. Alkynes are hydrocarbons with a triple bond between two carbon atoms. The general formula for alkynes with one triple bond is CnH2n-2. The structures provided are different isomers of the same molecular formula C4H6.
Among the options given:
a) H−C≡C−CH=CH2 is a molecule with both a triple bond and a double bond, which is not possible for the molecular formula C4H6 because it would result in a molecule with too few hydrogen atoms.
b) H2C=CH−C≡CH contains a double bond before the triple bond, but this is not the correct structure either because the correct Lewis structure for an alkyne should have the triple bond at the end of the molecule, following IUPAC nomenclature rules.
c) H−C≡CH−CH=CH2 has the triple bond between the second and third carbon atoms, which is correct, and a double bond at the end of the molecule.
d) H2C≡C−CH=CH2 is the Lewis structure for 1-butyne, which is an alkyne isomer of C4H6 with the triple bond at the beginning.
Thus, the correct answer is option d) H2C≡C−CH=CH2, as it satisfies the molecular formula and meets the criteria for being an alkyne with the triple bond located at the beginning of the molecule.