Answer

Infrared (IR) Active Vibrations Analysis

A molecular vibration is IR active if it involves a change in the dipole moment of the molecule. Below is an explanation of which vibrations qualify based on this criterion:

• trans-3-hexene, the C=C stretch — Although symmetric, the C=C bond in trans-alkenes generally involves a change in dipole due to differences in surrounding groups. IR active.
• CH₃CH₂CH₂C≡CH, the C≡C stretch — This terminal alkyne has an asymmetric environment around the triple bond, producing a dipole moment change. IR active.
• CH₃CH₂CH₂C= CCH₂CH₂CH₃, the C=C stretch — This internal alkene is asymmetric due to differing substituents, so its stretching vibration changes the dipole moment. IR active.
• (CH₃)₂C=O, the C=O stretch — The carbonyl (C=O) group has a large dipole and always causes a significant dipole moment change during vibration. IR active.
• (CH₃CH₂)₃C–Cl, the C–Cl stretch — The C–Cl bond is polar and stretching it alters the dipole moment. IR active.

Conclusion

All five vibrations involve asymmetric electron distributions or dipole-active functional groups. Thus, all are IR active and would show characteristic peaks in an IR spectrum.