5-1 Bonding, Nomenclature, Properties Bonding, Nomenclature, Properties Structure Hydrogen Deficiency Nomenclature Physical Properties Naturally Occurring Alkenes/Terpenes Alkenes 5 Alkenes

5-2 Unsaturated Hydrocarbons 1. Alkene: contains a carbon-carbon double bond and has the general formula C n H 2n ethene H CC H HH

5-3 Unsaturated Hydrocarbons 2. Alkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2 (Ch 7) ethyne (acetylene)

5-4 Unsaturated Hydrocarbons 3. Arene: benzene and its derivatives (Ch 21-22) or

5-5 Benzene & Phenyl Group Benzene and derivatives studied in Chapters 21 & 22, but some formulas may contain the phenyl group. Benzene Alternative representations for the phenyl group C 6 H 5 -Ph-

5-6 Structure of Alkenes Double Bond: 1  - bond formed by overlap of 2 sp 2 hybrid orbitals 1  - bond formed by overlap of 2 parallel 2 p orbitals  - bonded atoms  - bonded atoms all in a plane  all in a plane  trigonal - bond trigonal - bond angles ~ 120°  - bond perpendicular to the plane H H HHC C

o nm 88 kcals/mol 25 % s tetrahedral 120 o nm 146 kcals/mol 33 % s trigonal 180 o nm 200 kcals/mol 50 % s linear Valence Bond Valence Bond view of “olefins” or double bonds. CC H H H H   H H H H CC   120 o x

5-8 Structure of Alkenes No rotation about a C=C bond - why? Rotation requires breaking  bond ~63 kcal/mol H H HHC C

5-9 Index of Hydrogen Deficiency Index of hydrogen deficiency (IHD): IDH =  (number of rings + number of  bonds) Compare Hs of alkane with Hs in a compound C n H 2n+2 C n H x C 6 H 2(6)+2 IDH = 2 (H reference - H molecule ) = ( ) 2 = 2

5-10 Index of Hydrogen Deficiency Other elements present F, Cl, Br, I add one H - (Group 7) O, S no correction (Group 6) N, P subtract one H (Group 5)

5-11 Index of Hydrogen Deficiency Problem: Problem: isopentyl acetate has a molecular formula of C 7 H 14 O 2. Calculate its IHD reference hydrocarbon C 7 H 16 IHD = (16-14)/2 = 1 Problem: Problem: calculate the IHD for niacin, molecular formula C 6 H 6 N 2 O reference hydrocarbon C 6 H 14 IHD = [14 - (6-2)]/2 = 5 O O Isopentyl acetate

5-12 Nomenclature IUPAC: Root name - longest continuous chain containing the olefin. Number of C’s in C=C chain alkene indicates C=C in chain Number chain - olefin lowest number # hexene 2-ethyl-4-methyl-1-pentene

5-13 Nomenclature IUPAC: Root name - longest continuous chain containing the olefin. Number of C’s in chain with an C=C alkene indicates C=C in chain Number chain - olefin lowest number #- Cyclic #- alkene #- cycloalkene Olefin (functional group) - positions 1,2 Number around the ring to best accommodate substituents x

5-14 IUPAC names? (7S)-7-bromo-4-(2-iodoethyl)-6-methyl-3-octene 7-bromo-4-(2-iodoethyl)-6-methyl-3-octene (S)-6-ethyl-1-fluoro-5,5-dimethyl-1-cyclooctene

5-15 Cis/trans isomerism (recall cycloalkanes - cis/trans) opposite side  -bond trans-1,2-dichloroethene CC Cl H H H C C H cis-1,2-dichloroethene CC H Cl H same side of  -bond trans generally more stable than cis - dipoles and 1,2 interactions

5-16 The Cis,Trans System Configuration is determined by the orientation of atoms of the main chain trans -3-hexene cis -3,4-dimethyl-2-pentene

5-17 Bonding, Nomenclature, Properties Bonding, Nomenclature, Properties Structure Hydrogen Deficiency Nomenclature Physical Properties Naturally Occurring Alkenes/Terpenes Alkenes 5 Alkenes

5-18 Common Names used lab & lecture not on tests

5-19 Cycloalkenes- 3 to 7 “cis” olefins rings not large enough to accommodate trans double bonds C8 limited stability as trans

5-20 E,Z Configuration uses priority rules (Chapter 3) higher priority groups - same side, Z Z higher priority - opposite sides, E E

5-21 E/Z E/Z - priorities of groups on ends of C=C (1) Atom assigned a priority, higher atomic number higher priority. (2) Isotopes - higher atomic mass, higher priority 1 H < 2 H < 3 H [H < D < T] (3) If priority the same, go to the next set of atoms: - CH 2 -H < -CH 2 -O-H < -CH 2 -Cl (4) double (triple) bonds replaced by single bonds. CC C C C C H CCH C H C C C H H C H C H H

5-22 The E,Z System Example: Example: name each alkene and specify its configuration by the E,Z system

5-23 (S)-6-ethyl-1-fluoro-5,5-dimethylcyclooctene IUPAC names? (7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene Stereochemistry? (3Z,7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene ?(trans)-longest chain?

5-24 If same atoms, priority goes to the next point of difference - atomic number 9-bromo-5-(2-methylpropyl)-4-nonene (E)- (4E)- cis-

5-25 cis / trans configuration may be absolute or relative e.g. cis -1,3-dimethylcyclopentane or R,S -1,3-dimethylcyclopentane absolute but trans -1,3-dimethylcyclopentane could be R,R - or S,S- 1,3-dimethylcyclopentane relative E,Z and R,S absolute

5-26 Diene (or diyne, en-yne) longest chain (ring) with both groups: 1,4-cycloheptadiene Nomenclature - more than 1 unsaturated group #,#-alkadiene #-alken-#-yne #,#-alkadiyne number so ene s (enyne, yne s ) have the lowest possible number (if the same ene > yne).

5-27 Dienes, Trienes, and Polyenes n 2 n alkenes with n double bonds, which can be cis,trans isomers, 2 n stereoisomers are possible example 2,4-heptadiene: 2 2 = 4 cis,trans (E,Z)

5-28 Dienes - E or Z per olefin with number 1,6-dichloro-2,4-dimethyl-2,6-decadiene ( 2Z, 4S, 6E)-

5-29 Dienes, Trienes, and Polyenes vitamin A, a biologically important, sight/color four double bonds w/ cis,trans isomerism 2 4 = 16 stereoisomers

5-30 Physical Properties Alkenes are nonpolar compounds attractive forces between molecules are dispersion forces The physical properties of alkenes are similar to those of alkanes to 6.1