Dihedral Angle | Bond Angle | |
|---|---|---|
Definition | A dihedral angle is obtained when two planes pass through three atoms and two bonds, of which one bond is common to two planes. The separation between the two planes, or the angle of intersection, is expressed as the dihedral angle (ϴ) and denoted in degrees (°). | A bond angle is a geometrical angle between two bonds originating from the same atom in a covalently bonded molecule, expressed in degrees (o). |
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Describes | It reveals the angle by which the two adjacent carbons’ substituents eclipse or depart when one of the two adjacent carbons undergoes free rotation. | It describes the angle by which the two covalent bonds, each containing three atoms, are spread apart so that their interelectronic bond repulsions are minimized. |
Uses | It is used to describe the rotational property of single bonds. Such a study is important for understanding the various conformations (eclipsed, staggered, or skewed) that molecules generally adopt while undergoing free rotations and their associated energetics. Conformations form part of stereochemistry, which studies the impact of rotations or their absence on a molecule’s stability, physical and biological properties, and chemical reactions. | Bond angles are responsible for the three-dimensional shapes of organic molecules, inferred from the VSEPR theory. Some common molecular shapes include linear, trigonal planar, tetrahedral, and others, although the bond angles are affected by the presence of lone pairs, which in turn influence the molecular shape. The geometry or shape of a molecule can affect its polarity, physical properties, stability, and chemical behavior. |
Examples | The carbon (2) and carbon (3) of butane can generate an eclipsed conformation at 0 °, 120 °, and 240 °, and a staggered conformation at 60 °, 180 °, and 360 °, thereby raising and lowering the energetics of the molecule. Molecules tend to adopt conformations that minimize their energy. Therefore, at low temperatures, the stable form that butane would seek is the staggered conformation, that too, the one in which the two methyl (-CH3) groups would be anti to each other. | The bond angle between H-C-H in a methane molecule is 109.5o. Four separate bonds due to this angle give a methane molecule a tetrahedral shape. However, the presence of lone pair(s) in ammonia and water molecules sharply reduces the bond angle, and the shape deviates from tetrahedral to trigonal planar and bent.
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Representation | Organic chemistry utilizes Newman and Sawhorse projections to visualize the dihedral angles in the various conformations that organic molecules adopt. | Organic chemistry uses dashed and wedged bonds to show in three dimensions the bond angles of organic molecules. |
Measurement | NMR (J-coupling), X-ray crystallography, and molecular modeling. | X-ray Diffraction, Electron diffraction, and Microwave spectroscopy. |
Related -
Newman Projections - Introduction and Importance (Chapter - Drawing Organic Structures)
Sawhorse Projections (Chapter - Drawing Organic Structures)
Conformational Isomerism (Chapter - Stereochemistry)
