Multiple Choice
Which of the following elements is present in all organic compounds?
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2. Molecular Representations
Functional Groups
Multiple Choice
Which of the following functional groups is most likely to show a major IR absorption peak above cm?
A
Alkane
B
Alkyl halide
C
Ether
D
Alkene
Verified step by step guidance1
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.
Recall that alkanes (sp\(\textsuperscript{3}\) C-H) show C-H stretching absorptions just below 3000 cm\(\textsuperscript{-1}\), typically around 2850-2960 cm\(\textsuperscript{-1}\).
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.
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).
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.
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