Chapter_3_notes

Chapter_3_notes - Chem 2261 Notes Alkenes(Bruice Chapters 3 General Molecular Formulas Alkene general formula = CnH2n(same as cycloalkanes What is

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Chem 2261 Notes Alkenes (Bruice, Chapters 3) 1 General Molecular Formulas Alkene general formula = C n H 2n (same as cycloalkanes) What is the general formula for a cyclic alkene (see, for example, cyclohexene, below)? Nomenclature of Alkenes and Cycloalkenes The IUPAC name of an alkene is obtained by replacing the "ane" ending of the alkane with "ene." Some of the smaller alkenes frequently referred to by their common names. common name IUPAC name ethylene ethene propylene propene cyclopentene cyclohexene isobutylene 2-methylpropene CH 3 H 2 CC H 2 CH 3 CH 3 –– Rules for Naming Alkenes (see p. 126-129) 1. Determine the parent name by selecting the longest chain that contains both carbon atoms of the double bond and changing the ending of the name of the alkane of identical length from -ane to -ene (e.g., pentane becomes pentene). 2. Number the chain so as to include both carbon atoms of the double bond and begin numbering at the end nearer the double bond. 3. Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached. CH CH 2 CH 2 H 3 C CH CH H 3 C CH 3 H 3 CCH 2 CH 2 CH 2 C CH 2 CH 2 CH 3 CH 2 Other rules are analogous to those for substituted alkanes (see Chapter 2). 4. Substituents are cited in alphabetical order (the prefixes sec , tert , di, tri, and tetra are ignored in determining alphabetical order, but the prefixes iso, neo, and cyclo are not ignored). CH 2 CH CH CH CH CH 3 Cl C H 3 C CH 3 CH 3 H 2 CH 2 CH CH 2 CH 2 CH CH 3 CH 3 CH CH 3 CH 3 H 3 C 5. If the same number for the alkene functional group suffix is obtained in both directions, the correct name is the one that contains the lowest substituent number. H 3 CH C CH 2 CH 3 CH 3 Br 3-bromo-5-methyl-3-hexene or 4-bromo-2-methyl-3-hexene which is correct? 4- tert -butyl-5-chloro-2-heptene 4-isopropyl-7-methyl-1-octene
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Chem 2261 Notes Alkenes (Bruice, Chapters 3) 2 6. Number cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions and that also gives the substituent the lower numbers at the first point of difference. Because the double bond is always between carbons 1 and 2, a number is not needed to denote its position. 7. Name compounds containing both a double bond and an alcohol as alkenols (or cycloalkenols) and give the alcohol carbon the lower number. OH OH 8. Two frequently encountered groups are vinyl and allyl . Br Cl common name IUPAC name vinyl bromide bromoethene allyl chloride 3-chloropropene 3,5-dimethylcyclohexene 1-methylcyclopentene 2-methyl-2-cyclohexenol 4-methyl-3-penten-2-ol
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Chem 2261 Notes Alkenes (Bruice, Chapters 3) 3 Cis-Trans Isomerism Alkene carbons are held together by both sigma ( σ ) and pi ( π ) bonds (double bond = one σ + one π bond). As we noted earlier (see p.14 of Chapter 1 notes handout), π bonding is strongest when the two p orbitals contributing to the bond are exactly parallel and drops to zero when the two p orbitals are rotated 90° from this parallel arrangement. Consequently, the two alkene carbons and all four
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This note was uploaded on 04/21/2008 for the course CHEM 2261 taught by Professor Crowe during the Fall '08 term at LSU.

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Chapter_3_notes - Chem 2261 Notes Alkenes(Bruice Chapters 3 General Molecular Formulas Alkene general formula = CnH2n(same as cycloalkanes What is

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