Which would have the lowest frequency the C−H bond in ethane, a C−H bond in ethene, or a C−H bond in ethyne?
The frequency of a bond in a molecular vibrational spectrum relates to the strength of the bond and the mass of the atoms involved. Stronger bonds typically vibrate at higher frequencies, and bonds with heavier atoms vibrate at lower frequencies.
In the context of carbon-hydrogen (C−H) bonds, ethane (C2H6), ethene (C2H4, also known as ethylene), and ethyne (C2H2, also known as acetylene) are progressively unsaturated hydrocarbons, with ethane being an alkane with single bonds, ethene being an alkene with one double bond between the carbon atoms, and ethyne being an alkyne with a triple bond between the carbon atoms.
The strength of the C−H bond is different in these molecules due to the differing hybridization states of the carbon atoms to which the hydrogen atoms are bonded:
1. In ethane, the carbon atoms are sp3-hybridized, leading to a relatively lower C−H bond strength and therefore a lower vibrational frequency compared to C−H bonds in more highly hybridized environments.
2. In ethene, the carbon atoms are sp2-hybridized. The sp2 hybridization results in a slightly stronger C−H bond because the s-character is higher in sp2 (33%) than in sp3 (25%) hybrid orbitals. A higher s-character correlates with stronger bonds. Consequently, the C−H bonds in ethene have a higher vibrational frequency compared to those in ethane.
3. In ethyne, the carbon atoms are sp-hybridized, having the most s-character (50%). This results in the strongest C−H bonds, as these bonds are closer to the carbon nucleus and more strongly held. Therefore, the C−H bonds in ethyne possess the highest vibrational frequency among the three.
In summary, the C−H bond in ethane would have the lowest frequency, in ethene it would have a higher frequency, and in ethyne it would have the highest frequency among the three.