Ch07Su08

Ch07Su08 - Structure and Synthesis of Alkenes Alkenes...

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Structure and Synthesis of Alkenes Alkenes (olefins) are hydrocarbons which have carbon–carbon double bonds. A double bond is a σ bond and a π bond. Double bond B.D.E. = 146 kcal/mol σ bond B.D.E. = 83 kcal/mol Therefore π B.D.E. must = 63 kcal/mol. A π bond is weaker than a σ bond. π Bonds are more reactive than σ bonds, and π bonds are considered to be a functional group. Structure of Alkenes In ethylene (ethene) each carbon is bonded to 3 other atoms, with zero nonbonding electrons => sp 2 hybridization. The C-H bonds are formed by overlap of sp 2 orbitals from the Carbon overlapping with 1s orbital from the Hydrogen. eb15eba307c83201bcb825741eb92af9f75ccd8d.doc Page1 C C H H H H
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(The C-H bonds in ethane are sp 3 /1s overlapping orbitals. The ethene C-H bonds contain more s character than the ethane C-H bonds and are therefore shorter and stronger). sp 2 are 1/3 s, whereas sp 3 are 1/4 s in character. (s orbitals are closer to the nucleus and lower in energy). The carbon-carbon bond in ethene is shorter and stronger than in ethane partly because of the sp 2 -sp 2 overlap being stronger than sp 3 -sp 3 , but especially because of the extra π bond in ethene. Double Bond The unhybridized p orbitals on each carbon align parallel to form a π bond. Figure 7-2 The π bond prevents rotation about the C=C bond because it would force the p orbitals into a non overlapping formation (i.e. break the π bond). eb15eba307c83201bcb825741eb92af9f75ccd8d.doc Page2 H H H H 1.33A 1.08A H H H H H H 1.54A 1.09A
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The π bond exists with half its electron density above the σ bond, and half below. Elements of Unsaturation Alkanes are said to be saturated since they have they maximum number of bonds to hydrogen. An alkene is unsaturated . Any π system or a ring system reduces the maximum number of hydrogens a molecule can have. An element of unsaturation relates to 2 missing hydrogens from the saturated formula (C n H 2n+2 ). Consider alkane, alkenes and cycles: Heteroatom complications Heteroatoms are considered anything other than C or H. Halogens These simply substitute for hydrogens in the molecular formula. Therefore just like C 2 H 6 is saturated, so is C 2 H 4 F 2 . eb15eba307c83201bcb825741eb92af9f75ccd8d.doc Page3 H 3 C CH 2 CH 3 propane C 3 H 8 saturated H 2 C CHCH 3 propene C 3 H 6 one element of unsaturation H 2 C CH 2 H 2 C cyclopropane C 3 H 6 one element of unsaturation
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Oxygen CH 3 -CH 3 is saturated (C 2 H 6 ) CH 3 -O-CH 3 is also saturated (C 2 H 6 O) An oxygen can be added without requiring any additional hydrogens, so ignore the number of oxygens when calculating elements of Unsaturation. Nitrogen
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Ch07Su08 - Structure and Synthesis of Alkenes Alkenes...

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