Textbook Question
The heat of combustion of cis-1,2-dimethylcyclopropane is larger than that of the trans isomer. Which isomer is more stable? Use drawings to explain this difference in stability.
4. Alkanes and Cycloalkanes
Ring Strain
Problem 67
Textbook Question
The normal C(sp3)–C(sp3) bond length is 1.54 Å. The normal bond angle for an sp3-hybridized carbon is 109.5°. The following molecule experiences large deviations from these normal values. Explain these deviations. [Molecular models would be helpful here.]
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The molecule shown contains a central carbon atom bonded to three bulky tert-butyl (t-Bu) groups and one hydrogen atom. The tert-butyl groups are large and sterically demanding, which leads to steric hindrance around the central carbon atom.
Steric hindrance occurs when bulky groups are in close proximity, causing repulsion between their electron clouds. This repulsion forces the bonds to adjust in length and angle to minimize strain.
The C(sp3)–C(sp3) bond length is greater than the normal value of 1.54 Å because the bulky tert-butyl groups push each other apart, elongating the bonds to reduce steric repulsion.
The bond angles around the central sp3-hybridized carbon are greater than the normal tetrahedral angle of 109.5° because the tert-butyl groups need more space to accommodate their size. This results in an expansion of the bond angles.
These deviations from normal bond lengths and angles are a direct consequence of steric strain caused by the large size of the tert-butyl groups. Molecular models can help visualize how the bulky groups interact and distort the geometry of the molecule.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hybridization
Hybridization is the concept that describes the mixing of atomic orbitals to form new hybrid orbitals, which can explain the geometry of molecular structures. In the case of sp³ hybridization, one s and three p orbitals combine to create four equivalent orbitals that are oriented tetrahedrally, leading to a bond angle of 109.5°. Deviations from this ideal angle can occur due to steric effects or the presence of lone pairs.
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Steric Hindrance
Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together, which can affect bond lengths and angles. In molecules with bulky groups or multiple substituents, the spatial arrangement can lead to increased strain, causing bond lengths to deviate from the norm. This is particularly relevant in larger organic molecules where substituents can influence the overall geometry.
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Lone Pair Repulsion
Lone pair repulsion is a key factor in molecular geometry, as lone pairs of electrons occupy more space than bonding pairs. This increased electron density can push bonded atoms closer together, altering the expected bond angles. In sp³ hybridized carbons, the presence of lone pairs can lead to significant deviations from the ideal bond angle of 109.5°, resulting in distorted molecular shapes.
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