Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

4. Alkanes and Cycloalkanes

Ring Strain

Organic Chemistry

4. Alkanes and Cycloalkanes

Ring Strain

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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.]

Verified step by step guidance

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.

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.

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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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.