use the concept of sp hybridization to account for the formation of carbon-carbon triple bonds, and describe a carbon-carbon triple bond as consisting of one σ bond and two π bonds. list the approximate bond lengths associated with typical carbon-carbon single bonds, double bonds and triple bonds.
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Key Terms
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sp hybrid orbital
Study Notes
The bond angles associated with sp3-, sp2- and sp‑hybridized carbon atoms are approximately 109.5°, 120° and 180°, respectively.
Bonding in acetylene
Finally, the hybrid orbital concept applies well to triple-bonded groups, such as alkynes and nitriles. Consider, for example, the structure of ethyne (another common name is acetylene), the simplest alkyne.
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<p>This molecule is linear: all four atoms lie in a straight line. The carbon-carbon triple bond is only 1.20Å long. In the hybrid orbital picture of acetylene, both carbons are <strong><em>sp</em>-hybridized</strong>. In an <em>sp</em>-hybridized carbon, the 2<em>s</em> orbital combines with the 2<em>p</em>x orbital to form two <em>sp </em>hybrid orbitals that are oriented at an angle of 180°with respect to each other (eg. along the x axis). The 2<em>p</em>y and 2<em>p</em>z orbitals remain non-hybridized, and are oriented perpendicularly along the y and z axes, respectively.</p>
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 and the strongest (965 kJ/mol) of the carbon-carbon bond types. Because each carbon in acetylene has two electron groups, VSEPR predicts a linear geometry and and H-C-C bond angle of 180o.</p>
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<td><strong>Bond Length (pm)</strong></td>
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<td><strong>Ethane, CH3CH3</strong></td>
<td><strong>(<em>sp3</em>) C-C (<em>sp3</em>)</strong></td>
<td><strong>376</strong></td>
<td><strong>154</strong></td>
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<td><strong>Ethylene, H2C=</strong><strong>CH2</strong></td>
<td><strong>(<em>sp2</em>) C=C (<em>sp2</em>)</strong></td>
<td><strong>728</strong></td>
<td><strong>134</strong></td>
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<td><strong>Acetylene, HC≡CH</strong></td>
<td><strong>(<em>sp</em>) C≡C (<em>sp</em>)</strong></td>
<td><strong>965</strong></td>
<td><strong>120</strong></td>
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<p>Notice that as the bond order increases the bond length decreases and the bond strength increases.</p>
<p>The hybrid orbital concept nicely explains another experimental observation: single bonds adjacent to double and triple bonds are progressively shorter and stronger than ‘normal’ single bonds, such as the one in a simple alkane. The carbon-carbon bond in ethane (structure A below) results from the overlap of two sp3 orbitals.</p>
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, however, the carbon-carbon single bond is the result of overlap between an sp2 orbital and an sp3 orbital, while in propyne (C) the carbon-carbon single bond is the result of overlap between an sp orbital and an sp3 orbital. These are all single bonds, but the single bond in molecule C is shorter and stronger than the one in B, which is in turn shorter and stronger than the one in A.</p>
<p>The explanation here is relatively straightforward. An sp orbital is composed of one <em>s</em> orbital and one <em>p</em> orbital, and thus it has 50% <em>s</em> character and 50% <em>p</em> character. sp2 orbitals, by comparison, have 33% <em>s</em> character and 67% <em>p</em> character, while sp3 orbitals have 25% <em>s</em> character and 75% <em>p</em> character. Because of their spherical shape, 2<em>s</em> orbitals are smaller, and hold electrons closer and ‘tighter’ to the nucleus, compared to 2<em>p</em> orbitals. Consequently, bonds involving sp + sp3 overlap (as in alkyne C) are shorter and stronger than bonds involving sp2 + sp3 overlap (as in alkene B). Bonds involving sp3-sp3overlap (as in alkane A) are the longest and weakest of the group, because of the 75% ‘<em>p</em>’ character of the hybrids.</p>
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<h2>Hybridization Summary</h2>
<p> A single bond is a sigma bond. A double bond is made up of a sigma bond and a pi bond. A triple bond is made up of a sigma bond and two pi bonds. Sigma bonds are made by the overlap of two hybrid orbitals or the overlap of a hybrid orbital and a s orbital from hydrogen. Pi bonds are made by the overlap of two unhybridized p orbitals. Lone pair electrons are usually contained in hybrid orbitals.</p>
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The hybrid orbitals used (and hence the hybridization) depends on how many electron groups are around the atom in question. An electron group can mean either a bonded atom or a lone pair. Molecular geometry is also decided by the number of electron groups so it is directly linked to hybridization.
| # of Electron Groups | Hybrid Orbital Used | Example | Basic Geometry | Basic Bond Angle |
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