Chapter 3 The Nature of Organic Reactions-Alkenes

Chapter 3 The Nature of Organic Reactions-Alkenes - Chapter...

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Chapter 3 The Nature of Organic Reactions: Alkenes Alkenes are hydrocarbons that contain carbon = carbon double bond. 3.1 Naming Alkenes 1. Alkenes are called unsaturated cause it have fewer hydrogens per carbon than related alkanes due to their double bonds. Example ethylene has formula C 2 H 4 whereas ethane has formula C 2 H 6 2. Their molecular formula is defined as C n H 2n 3. Three steps to naming alkenes a. Name the parent hydrocarbon i. Find the longest carbon chain that contains double bond and name the compound using the suffix –ene ii. THE PARENT HYDROCARBON MUST CONTAIN ALL THE DOUBLE BONDS IN THE MOLECULE b. Number the carbon atoms in the chain i. Begin numbering at the end nearer the double bond ii. If the double bond equidistant from the two ends, begin numbering at the end nearer to the first branch point.
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c. Write full name i. Number the substituents according to their position in the chain and list them alphabetically ii. Indicate the position of double bond by giving the number of the first alkene carbon and placing the number directly before –ene suffix iii. More than one double bond, indicate the position of each and placing the number before any of the suffixes –diene, triene, and so on. d. Cycloalkenes i. Numbering begins such that double bond is situated between C1 and C2 ii. First substituent has the lowest number possible 4. See page 74 and 75 for examples 3.2 Electronic Structure of Alkene 1. carbons in double bond have three equivalent sp 2 hybrid orbitals which lie in a plane at angles of 120 o . The fourth carbon orbital is an unhybridized p orbital
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perpendicular to the sp 2 plane . when two such carbons approach each other, they formed σ bond by head on overlap of sp 2 orbitals and π bond by sideways overlap of unhybridized p orbitals. 2. The doubly bonded carbon and the attached atoms lie in a plen 3. For rotation to take place around a double bond, the part of the bond ( bond) would have to be broken temporarily. Thus, the energy barrier to rotation around a double bond must be at least as great as the strength of the bond itself 3.3 Cis-Trans Isomers 1. Lack of rotations around carbon-carbon double bond results in cis-trans isomers in disubstituted (or trisubstituted or tetrasubstituted) alkene 2. Disbustituded: two substituent other than hydrogen are bonded to double bond carbon trisubstituted: three substituent other than hydrogen are bonded to double bond carbon tetrasubstituted: four substituent other than hydrogen are bonded to double bond carbon 3. when the two substituents are on the opposide side of double bond, it’s called cis. When the two substituents are on the same side of double bond, it’s called trans. Example, cis-But-2-ene and trans-But-2-ene 4. since bond rotations cant occur at room temperature, the two conformations (isomers) cant spontaneously interconvert and are different chemical compounds 5. such isomers are called cis-trans isomer
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6. cis-trans isomerism occurs whenever both double bond carbons are attached to
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This note was uploaded on 07/16/2011 for the course CHE 391 taught by Professor Mckague,b during the Spring '08 term at University of Toronto- Toronto.

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Chapter 3 The Nature of Organic Reactions-Alkenes - Chapter...

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