的 Ra6e8ohgdeYeingt2algroupproecdencnove Anerea2as8gng2ohseR85ubeeto The last"e"and "alkene"is dropped in naming alkenok CH CHCHCH. 11-2 Structure and Bonding in Ethene:The Pi Bond bg8enbondconsistsofsgmandpm CH=CH-CH=CH-CH,- 人人 he pi bond in ethene is relatively weak. aea8ggesgbondngandantbandngand oreote dou 为= E 9 22 Rule 5: Use the IUPAC E,Z system when cis/trans labels are not applicable (3 or 4 different substituents attached to the double-bond carbons). Apply the sequence rules devised for R,S substituent priorities to the two groups on each double-bond carbon. If the two groups of highest priority are on opposite sides of the double bond, the molecule is an E isomer. If they are on the same side of the double bond, the molecule is a Z isomer. Rule 6: Give the hydroxy functional group precedence over the double bond in numbering a chain. Alcohols containing double bonds are named alkenols. The stem incorporating both functions is numbered to give the OH carbon the lowest possible assignment. The last “e” and “alkene” is dropped in naming alkenols. Rule 7: Substituents containing a double bond are named alkenyl. The numbering of a substituent chain containing a double bond begins at the point of attachment to the basic stem. 11-2 Structure and Bonding in Ethene: The Pi Bond The double bond consists of sigma and pi components. Ethene is planar. It contains two trigonal carbon atoms having bond angles close to 120o. The hybridization of the carbon atoms is best described as sp2. The three sp2 orbital on each carbon form σ bonds to two hydrogen atoms and to the other carbon atom. The remaining unhybridized p orbital on each carbon overlap to form a π bond. The electron density of the π bond is equally distributed above and below the plane of the molecule. The pi bond in ethene is relatively weak. The overlap of the two sp2 orbitals to form the σ bond connecting the two carbon atoms is much greater than the overlap of the two p orbitals to form the π bond. As a consequence, the σ bond contributes more to the double bond strength than does the π bond. The relative energies of the bonding and antibonding σ and π orbitals can be summarized: