Which of the following functional groups is most likely to show a major IR absorption peak above $3000$ cm${-}^1$?

• A. Alkane
• B. Alkyl halide
• C. Ether
• D. Alkene

Answer: (D)

Verified step by step guidance

1. Understand that IR absorption peaks above 3000 cm\textsuperscript{-1} typically correspond to X-H stretching vibrations where X is carbon or another atom, and the exact position depends on the hybridization of the carbon atom bonded to hydrogen.
2. Recall that alkanes (sp\textsuperscript{3} C-H) show C-H stretching absorptions just below 3000 cm\textsuperscript{-1}, typically around 2850-2960 cm\textsuperscript{-1}.
3. Recognize that alkenes (sp\textsuperscript{2} C-H) have C-H stretching absorptions just above 3000 cm\textsuperscript{-1}, usually in the range of 3020-3100 cm\textsuperscript{-1}, due to the higher s-character of the sp\textsuperscript{2} carbon-hydrogen bond.
4. Note that alkyl halides and ethers do not have C-H stretches significantly above 3000 cm\textsuperscript{-1}; their characteristic absorptions are usually in other regions (e.g., C-X stretches or C-O stretches).
5. Therefore, to identify the functional group with a major IR absorption peak above 3000 cm\textsuperscript{-1}, focus on the presence of sp\textsuperscript{2} C-H bonds, which are characteristic of alkenes.