tailieunhanh - Lecture Organic chemistry - Chapter 11: Alkenes

This chapter presents the following content: structure of the double bond, the sigma bond, the pi bond, orbital energies, how weak is the pi bond? Bond strengths in alkenes, alkenyl hydrogens are relatively acidic, degree of unsaturation, heteroatoms and degree of unsaturation,. | Chapter 11: Alkenes C C Double bond Names: ending ane ene 1. Find longest chain wit both Csp2 carbons in it. Rules: An octene 1 2 3 4 5 6 7 8 4 For example: ethene, propene, butene, etc. 3. Name and # substituents, in alphabetical order 4-Ethyl-3-methyl-3-octene 2. Number the chain with C C close to terminus 1 2 3 4 5 6 7 8 A 3-octene (only the first of the two Csp2 carbons is named by a #) 4 4. Cycloalkenes 1 2 3 CH3 C C 1 2 3-Methylcyclohexene by definition and start such as to give substituents lowest possible numbering 5. Stereoisomers: R R R R cis trans Cis/trans used for 1,2-disubstituted ethenes. 6. For tri- and tetrasubstituted alkenes: E, Z naming. Use R, S priority rules at each sp2-carbon separately, to find yhe higher priority group at each end. E-4-Ethyl-3-methyl-3-octene 1 2 3 4 5 6 7 8 4 Opposite sides: E Same side: Z Colloquial: double bond position R R Internal Terminal R R CH2 8. Substituents: Alkenyl CH2 CH Ethenyl (vinyl) CH2 CH CH2 2-Propenyl (allyl) 9. Exocyclic | Chapter 11: Alkenes C C Double bond Names: ending ane ene 1. Find longest chain wit both Csp2 carbons in it. Rules: An octene 1 2 3 4 5 6 7 8 4 For example: ethene, propene, butene, etc. 3. Name and # substituents, in alphabetical order 4-Ethyl-3-methyl-3-octene 2. Number the chain with C C close to terminus 1 2 3 4 5 6 7 8 A 3-octene (only the first of the two Csp2 carbons is named by a #) 4 4. Cycloalkenes 1 2 3 CH3 C C 1 2 3-Methylcyclohexene by definition and start such as to give substituents lowest possible numbering 5. Stereoisomers: R R R R cis trans Cis/trans used for 1,2-disubstituted ethenes. 6. For tri- and tetrasubstituted alkenes: E, Z naming. Use R, S priority rules at each sp2-carbon separately, to find yhe higher priority group at each end. E-4-Ethyl-3-methyl-3-octene 1 2 3 4 5 6 7 8 4 Opposite sides: E Same side: Z Colloquial: double bond position R R Internal Terminal R R CH2 8. Substituents: Alkenyl CH2 CH Ethenyl (vinyl) CH2 CH CH2 2-Propenyl (allyl) 9. Exocyclic alkenes: Alkylidenecycloalkanes Methylidenecyclohexane (Methylenecyclohexane) OH ( SH) > ene 1 2 3 OH 2-Propen-1-ol 7. Structure Of The Double Bond Electron rich Two components: the -bond and the -bond The Sigma Bond Ethene The Pi Bond Orbital Energies Bond is relatively weak How Weak Is The Pi Bond? Ea = 65 kcal mol-1 Typical C-C bond strength ~90 kcal mol-1 (Table 3-2) Bond Strengths In Alkenes Unusually strong because C uses sp2 hybrids C C 33% s character. In contrast: sp3 has 25% s character Net effect: relatively e-withdrawing H : sp2 Has Alkenyl Hydrogens Are Relatively Acidic C C H pKa ~ 44 Compare CH3CH2-H pKa ~ 50 + CH3Li RCH C H Li CH4 + C C H H H R Therefore, in principle: Problems: regio-, stereoselectivity. Better via alkenyl organometallics: CH2 C H Br + Li CH2 C H Li C C Br H H R Mg + C C MgBr H H R Useful: React with carbonyls Degree Of Unsaturation Molecular formula tells us how many rings and/or bonds are present in a molecule. Reference is a saturated acyclic

• Hoá học
• Organic chemistry
• Lecture Organic chemistry
• Hóa học hữu cơ
• Bài giảng Hóa học hữu cơ
• Structure of the double bond
• The sigma bond