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In 2-butene, the isomer with both methyl groups on the same side of the C=C double bond is called the:
Atrans-isomer
Bmeso-isomer
Ccis-isomer
Dracemic isomer
Answer & Solution
Correct answer: C. cis-isomer
1. Geometric isomers of alkenes are named according to whether similar groups sit on the same side or opposite sides of the double bond.
2. The text states: "The 2-butene isomer in which the two methyl groups are on the same side is called a cis-isomer; the one in which the two methyl groups are on opposite sides is called a trans-isomer."
3. "Meso" and "racemic" describe unrelated concepts (internal symmetry and mixtures of enantiomers), not cis/trans alkene geometry.
4. So the same-side arrangement is specifically the cis-isomer.
_Source: OpenStax Chemistry Ch 20 "Organic Chemistry", p.1105 §20.1 Hydrocarbons_
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