OC307-9-11-Ch7 - Organic Chemistry Alkenes H D Roth...

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Organic Chemistry Alkenes H. D. Roth 1 Chemistry 307 Chapter 7 Structure and Synthesis of Alkenes Alkenes are hydrocarbons with (C=C) double bonds. Many alkenes occur in interesting natural products and others are of great industrial importance: ethylene is the largest volume industrial organic compound. 1. IUPAC nomenclature Rule 1: Find the longest chain containing the alkene function Rule 2: Number the ene function low Rule 3: Substituents are named as "prefixes" in alphabetical order 4-ethyl-1-methylcyclohexene Rule 4: Geometric isomers with like substituents are differentiated by the terms cis - trans - (Latin) trans -hex-3-ene cis -hex-3-ene cyclopentene trans -cyclodecene Rule 5: Geometric isomers with different substituents are designated by the terms Z- (zusammen) E- (entgegen) (German) h l h l l h h l Z- E- The pairs of geminal substituents are assigned priorities according to the Sequence Rule. Rule 6: the OH function has priority over the ene function H 2 C 2-propyl-6-hepten-1-ol OH 3-propyl-1-octene
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Organic Chemistry Alkenes H. D. Roth 2 Rule 7: small alkene side chains are alkenyl- trans -1-butenylcyclopropane Please, review priorities (atomic number of directly bonded atoms, work to first point of difference, count double bonds twice, triple bonds three times, use mass number for isotopes). Here is your chance to practice ! H H 2 N C CH 3 H HO O D H CH 3 2. Structure and Bonding in Alkenes Alkenes contain two sp2 hybridized carbon atoms connected by one ! bond and one " bond. We can “construct” the bonds of ethylene by combining two sp2 hybridized carbon atoms. H H H H ! bond " bond C H C H H H Alkenes are planar with bond angles of ~120°; the " bond prevents free rotation about the C=C bond. This is the reason for cis - trans isomerism (geometric isomerism). The C=C bond is shorter and stronger than the C–C bond. C H C H H H 1.33 Å 1.08 Å 116.6° 121.7° The " bond is weaker (more reactive) than the ! bond. Thermal isomerization is possible; the energy required (E a = 65 kcal mol 1 ; 272 kJ mol 1 ) identifies the strength of the " bond. As the overall strength of the
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Organic Chemistry Alkenes H. D. Roth 3 C=C bond is 173 kcal mol 1 (725 kJ mol 1 ), we can assign 108 kcal mol 1 (453 kJ mol 1 ) to the strength of the single bond. H H H H H H H H H H H H 3. Physical properties of alkenes i) boiling points are similar to those of alkanes; ii) melting points reflect the different packing of cis- and trans- alkenes: cis-isomers have lower melting points; iii) bonds between alkyl groups and sp2 hybridized carbons are
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