Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

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

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Mullins 1st Edition

Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

Problema 3

Capitolo 13, Problema 3

Rank C―H σ the following bonds in terms of their bond length. Explain your ranking ( 1= longest ; 3 = shortest).

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Step 1: Understand the relationship between bond length and hybridization. Bond length is influenced by the hybridization of the carbon atom bonded to hydrogen. The more s-character in the hybrid orbital, the shorter the bond length. For example, sp hybridized carbons have the shortest bond length, followed by sp2, and then sp3 hybridized carbons.

Step 2: Analyze the structures provided. The first structure is a benzene ring with an sp2 hybridized carbon bonded to hydrogen. The second structure is a terminal alkyne with an sp hybridized carbon bonded to hydrogen. The third structure is a cyclopentane ring with an sp3 hybridized carbon bonded to hydrogen.

Step 3: Compare the hybridization of the carbon atoms. The sp hybridized carbon in the alkyne has the highest s-character (50%), resulting in the shortest bond length. The sp2 hybridized carbon in the benzene ring has intermediate s-character (33%), resulting in a bond length longer than sp but shorter than sp3. The sp3 hybridized carbon in cyclopentane has the lowest s-character (25%), resulting in the longest bond length.

Step 4: Rank the bond lengths based on the hybridization. The bond length ranking is as follows: (1) Cyclopentane (sp3, longest bond length), (2) Benzene (sp2, intermediate bond length), (3) Terminal alkyne (sp, shortest bond length).

Step 5: Summarize the reasoning. The bond length decreases as the s-character of the hybrid orbital increases. This is because orbitals with higher s-character are closer to the nucleus, pulling the bonded atoms closer together and shortening the bond length.

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Bond Length

Bond length is the distance between the nuclei of two bonded atoms. It is influenced by the size of the atoms involved and the type of bond (single, double, or triple). Generally, as the number of shared electron pairs increases, the bond length decreases due to stronger attraction between the nuclei and the shared electrons.

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Single bonds, double bonds, and triple bonds.

σ Bonds

σ (sigma) bonds are the strongest type of covalent bond formed by the head-on overlap of atomic orbitals. They are characterized by a cylindrical symmetry around the bond axis and are typically formed between s and p orbitals. The presence of σ bonds in a molecule significantly affects its geometry and stability, influencing the overall bond length.

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Single bonds, double bonds, and triple bonds.

Factors Affecting Bond Length

Several factors influence bond length, including atomic size, electronegativity, and hybridization. Larger atoms tend to form longer bonds due to increased distance between nuclei. Additionally, differences in electronegativity can lead to variations in bond character, while hybridization can affect the spatial orientation of orbitals, further impacting bond lengths.

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