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Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 3 - Alkanes and Cycloalkanes: Properties and Conformational AnalysisProblem 67
Chapter 2, Problem 67

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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Using bond sites to predict hybridization

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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Understanding steric effects.

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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Related Practice
Textbook Question

Calculate the energy difference between each pair of conformations shown by drawing and comparing Newman projections down the indicated bonds in each.

(b)

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Textbook Question

In Chapter 5, we introduce reaction coordinate diagrams as a plot of potential energy versus the progress of a reaction. Consider the reaction coordinate diagram drawn for the 'reaction' of conformation A becoming conformation B. Which structure is present at the top of the hill?

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Textbook Question

Correct the following incorrect names using standard IUPAC nomenclature. [Draw a compound that corresponds to the incorrect name, and then rename it.]

(a) 4-methylhexane

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Textbook Question

In contrast to ethane and other alkanes studied in this chapter, there is no free rotation around any bonds in cyclopentane (shown below). Why?

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Textbook Question

Looking ahead In Chapter 5, we explain that the equilibrium constant (Keq) for a reaction can be calculated based on the difference in energy between reactants and products, according to the following equation:

Keq=e−ΔERTK_{eq}=e^{-\(\frac{\Delta E}{RT}\)}

Using this equation, calculate the equilibrium constant for the 'reaction' shown. [For the rest of the book, if not otherwise specified, assume room temperature (298K).]

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Textbook Question

Correct the following incorrect names using standard IUPAC nomenclature. [Draw a compound that corresponds to the incorrect name, and then rename it.]

(b) 1,5-dimethylcyclohexane

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