Ch. 2 - General Chemistry Translated: Finding the Electrons

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 2 - General Chemistry Translated: Finding the Electrons

Problem 80

Chapter 1, Problem 80

The C―H σ bond length in ethane is 1.09 Å. The C―H σ bond in ethene is 1.07 Å. Explain.

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The bond length of a C―H σ bond is influenced by the hybridization of the carbon atom involved in the bond. In ethane, the carbon atoms are sp³ hybridized, while in ethene, the carbon atoms are sp² hybridized.

The sp³ hybridization in ethane results in orbitals with 25% s-character and 75% p-character. In contrast, sp² hybridization in ethene results in orbitals with 33% s-character and 67% p-character.

Orbitals with higher s-character (as in sp² hybridization) are closer to the nucleus, leading to shorter bond lengths. This explains why the C―H bond in ethene (1.07 Å) is shorter than the C―H bond in ethane (1.09 Å).

The increased s-character in sp² hybridized orbitals also results in stronger and more tightly held bonds, contributing to the shorter bond length in ethene compared to ethane.

In summary, the difference in C―H bond lengths between ethane and ethene is due to the difference in hybridization of the carbon atoms, which affects the orbital characteristics and bond strength.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bond Length

Bond length is the distance between the nuclei of two bonded atoms. It is influenced by the type of bond (single, double, or triple) and the atomic radii of the involved elements. In general, shorter bond lengths indicate stronger bonds due to increased overlap of atomic orbitals.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding. In ethane (C2H6), carbon undergoes sp3 hybridization, resulting in single C―H bonds. In contrast, ethene (C2H4) features sp2 hybridization, leading to a double bond between carbons, which affects bond lengths.

Bond Order

Bond order refers to the number of chemical bonds between a pair of atoms. A higher bond order typically results in a shorter bond length and greater bond strength. Ethene has a bond order of 1.5 for the C―H bonds due to the presence of a double bond between the carbon atoms, which contributes to the shorter bond length compared to ethane.

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Looking ahead in Chapter 4, we explain that molecules like CH₃⁺ are Lewis acids or electron pair acceptors. Into which orbital would the new electron pair go?

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There is free rotation around the C―C bond in ethane. There is an extremely high barrier to rotation around the C=C bond in in ethene. Explain.

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In comparison to CH₃⁺ in Assessment 2.82, the related molecule H₃O⁺ is not a Lewis acid at oxygen. Why?

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Given the Lewis structures, indicate the direction of the dipole moment, if there is one.

(e)

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