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22 Cards in this Set
- Front
- Back
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dienes |
compounds that have 2 C=C bonds |
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3 classifications of dienes |
1. Cumulated Dienes 2. Conjugated Dienes 3. Isolated Dienes |
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Cumulated Dienes |
(also called allenes) the π bonds are adjacent |
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Conjugated Dienes |
the π bonds are separated by exactly one σ bond |
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Isolated Dienes |
the π bonds are separated by two or more σ bonds |
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Preparation of Conjugated Dienes (1st method) |
-conjugated dienes can be prepared from alkyl halides via an elimination process |
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Preparation of Conjugated Dienes (2nd method) |
Conjugated dienes can also be formed from dihalides via two successive elimination reactions |
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What is used to prevent a competing SN2 reaction from taking place in the preparation of dienes? |
-A sterically hindered base, such as potassium tert-butoxide (t-BuOK), is used to prevent the competing Sn2 reaction from occuring |
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Compare the bond length of the single bond in a conjugated diene vs. the bond length of a typically C-C single bond. |
-the single bond in a conjugated diene is shorter than a typical C-C bond |
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What explains this difference? |
-the different hybridization states that are involved |
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The C-C bond of a conjugated diene is formed from the overlap of what two orbitals? |
-formed from the overlap of two sp2-hybridized orbitals |
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The C-C bond of ethane is formed from the overlap of what two orbitals? |
-formed from the overlap of two sp3-hybridized orbitals |
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How does this impact respective bond length? |
-the electron density of an sp2 hybridized orbital will be closer to the nucleus than the electron density of an sp3 hybridized orbital -as a result, a bond between two sp2 hybridized orbitals will be shorter than a bond between two sp3 hybridized orbitals |
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Compare the stability of 1-butene and 1,3 butanediene. |
-Conjugated double bonds are more stable than isolated double bonds -1,3-butanediene is more stable than 1-butene |
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How has this been experimentally tested? |
-using heats of hydrogenation |
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Compare the heats of hydrogenation for 1-butene and 1,3-butanediene. |
-the heat of hydrogenation of 1,3-butanediene is less than that of 1-butene |
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By how much? |
-15 kj/mol |
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At room temperature, 1,3-butanediene experiences free rotation around the C2-C3 bond, giving rise to how many conformers? |
-two |
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What are they called? |
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What is the activation energy for conversion from the s-cis to the s-trans conformer? |
-approximately 15 kj/mol |
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What is this equivalent to? |
-the stabilization energy associated with conjugated double bonds |
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Thus, the stabilizing effect of conjugation is completely destroyed when...? |
-the C2-C3 bonds are rotated by 90 degrees/1,3-butanediene switches from the s-cis to the s-trans conformer |
