《有机化学》课程教学课件（Organic Chemistry, Alex Jonathan Roche lecture notes Rutgers The State University NJ, wade 8th）Chapter 07 Structure and Synthesis of Alkenes

Structure and Synthesis of Alkenes Alkenes(olefins)are hydrocarbons which have carbon-carbon double bonds H H =C H- H A double bond is a o bond and a nt bond. Double bond B.D.E. =146 kcal/mol o bond B.D.E. =83 kcal/mol Therefore n B.D.E.must =63 kcal/mol. A n bond is weaker than a o bond. Bonds are more reactive than o bonds,and r bonds are considered to be a functional group. Structure of Alkenes In ethylene(ethene)each carbon is bonded to 3 other atoms,with zero nonbonding electrons=>sp'hybridization. sigma bonding orbitals of ethylene Ch07 Alkenes:Struct +synth (landscape) Page I

Ch07 Alkenes; Struct + synth (landscape) Page 1 Structure and Synthesis of Alkenes Alkenes (olefins) are hydrocarbons which have carbon–carbon double bonds. A double bond is a  bond and a  bond. Double bond B.D.E. = 146 kcal/mol  bond B.D.E. = 83 kcal/mol Therefore  B.D.E. must = 63 kcal/mol. A  bond is weaker than a  bond.  Bonds are more reactive than  bonds, and  bonds are considered to be a functional group. Structure of Alkenes In ethylene (ethene) each carbon is bonded to 3 other atoms, with zero nonbonding electrons => sp2 hybridization

The C-H bonds are formed by overlap of sp'orbitals from the Carbon overlapping with 1s orbital from the Hydrogen. (The C-H bonds in ethane are sp/ls overlapping orbitals.The ethene C-H bonds contain more s character than the ethane C-H bonds and are therefore shorter and stronger). 1.33A H H 1.54A 116.6° 121.7 H ethylene ethane sp'are 1/3 s,whereas sp'are 1/4 s in character.(s orbitals are closer to the nucleus and lower in energy). The carbon-carbon bond in ethene is shorter and stronger than in ethane partly because of the sp'-sp overlap being stronger than sp'-sp',but especially because of the extra it bond in ethene. Ch07 Alkenes:Struct synth (landscape) Page 2

Ch07 Alkenes; Struct + synth (landscape) Page 2 The C-H bonds are formed by overlap of sp2 orbitals from the Carbon overlapping with 1s orbital from the Hydrogen. (The C-H bonds in ethane are sp3 /1s overlapping orbitals. The ethene C-H bonds contain more s character than the ethane C-H bonds and are therefore shorter and stronger). sp 2 are 1/3 s, whereas sp3 are 1/4 s in character. (s orbitals are closer to the nucleus and lower in energy). The carbon-carbon bond in ethene is shorter and stronger than in ethane partly because of the sp 2 -sp 2 overlap being stronger than sp3 -sp 3 , but especially because of the extra  bond in ethene

Pi Bond The unhybridized p orbitals on each carbon align parallel to form a nt bond. The n bond prevents rotation about the C-C bond because it would force the p orbitals into a non overlapping formation (i.e.break the nt bond). CH H CH3 CH CH no overlap with the ends perpendicular ran The nt bond exists with half its electron density above the o bond,and half below. Ch07 Alkenes:Struct +synth (landscape) Page3

Ch07 Alkenes; Struct + synth (landscape) Page 3 Pi Bond The unhybridized p orbitals on each carbon align parallel to form a  bond. The  bond prevents rotation about the C=C bond because it would force the p orbitals into a non overlapping formation (i.e. break the  bond). The  bond exists with half its electron density above the  bond, and half below

Elements of Unsaturation Alkanes are said to be saturated since they have they maximum number of bonds to hydrogen. An alkene is unsaturated. Any t system or a ring system reduces the maximum number of hydrogens a molecule can have. An element of unsaturation relates to 2 missing hydrogens from the saturated formula(C.H2+2). Consider alkane,alkenes,alkynes and cycles: CH,一CH2一CH CH,一CH=CH propane,C.H propene,C.H saturated one element of unsaturation CH2 CH2-CH2 CH,一C=C一H cyclopropane,C.H propyne,CH one element of unsaturation two elements of unsaturation Ch07 Alkenes:Struct synth (landscape) Page4

Ch07 Alkenes; Struct + synth (landscape) Page 4 Elements of Unsaturation Alkanes are said to be saturated since they have they maximum number of bonds to hydrogen. An alkene is unsaturated. Any  system or a ring system reduces the maximum number of hydrogens a molecule can have. An element of unsaturation relates to 2 missing hydrogens from the saturated formula (CnH2n+2). Consider alkane, alkenes, alkynes and cycles:

(Heteroatom complications Heteroatoms are considered anything other than C or H. Halogens These simply substitute for hydrogens in the molecular formula. Therefore just like C2H6 is saturated,so is C2HF2. Oxygen CH3-CH3 is saturated(C2H) CH-O-CH is also saturated(C2HO) An oxygen can be added without requiring any additional hydrogens,so ignore the number of oxygens when calculating elements of Unsaturation. Nitrogen Nitrogen is trivalent,and when it replaces a C in a chain it requires only one hydrogen (-NH-vs.-CH2-),so nitrogens count as half a carbon Thus C.HoN is equivalent to CasHo.(i.e.one element of Unsaturation).) Ch07 Alkenes:Struct synth (landscape) Page 5

Ch07 Alkenes; Struct + synth (landscape) Page 5 (Heteroatom complications Heteroatoms are considered anything other than C or H. Halogens These simply substitute for hydrogens in the molecular formula. Therefore just like C2H6 is saturated, so is C2H4F2. Oxygen CH3-CH3 is saturated (C2H6) CH3-O-CH3 is also saturated (C2H6O) An oxygen can be added without requiring any additional hydrogens, so ignore the number of oxygens when calculating elements of Unsaturation. Nitrogen Nitrogen is trivalent, and when it replaces a C in a chain it requires only one hydrogen (-NH- vs. -CH2- ), so nitrogens count as half a carbon. Thus C4H9N is equivalent to C4.5H9. (i.e. one element of Unsaturation).)

Nomenclature of Alkenes Simple alkenes are named like alkanes (root from the longest carbon chain),but the-ane suffix is replaced by-ene. CH,-CH CH,=CH一CH IUPAC names: ethene propene cyclohexene Common names: ethylene propylene 每03 son Eaurtin ire When the chain is longer than 3 carbons,number the atoms such that the double bond is given the lowest number (i.e.start at the end nearest the double bond). CH2=CH-CH,-CH: CH,-CH-CH,-CH,-CH old IUPAC names: 1-butene 1-pentene new IUPAC names: but-l-ene pent-l-ene 3 CH,一CH=CH一CH3CH3一CH=CH一CH2一CH old IUPAC names: 2-butene 2-pentene new IUPAC names: but-2-ene pent-2-ene Ch07 Alkenes:Struct synth (landscape) Page 6

Ch07 Alkenes; Struct + synth (landscape) Page 6 Nomenclature of Alkenes Simple alkenes are named like alkanes (root from the longest carbon chain), but the –ane suffix is replaced by-ene. When the chain is longer than 3 carbons, number the atoms such that the double bond is given the lowest number (i.e. start at the end nearest the double bond)

Compounds with 2 double bonds are called dienes,3 double bonds are trienes,etc. CH2=CH一CH一CH CH:-CH-CH-CH-CH-CH-CH, old IUPAC names 1.3-butadiene 1.3.5-heptatriene 1.3.5.7-cyclooctatetraene new IUPAC names: buta-1,3-diene hepta-1.3.5-triene cycloocta-1,3.5.7-tetraene For branches,each alkyl group is given a number,but the double bond is still given preference when numbering the chain. 2 CH;-CH-C-CH3 CH;-CH-CH-CH2 CH:-CH-C-CHz-CH2-CH-CH3 CH3 CH3 CH3 CH 2-methyl-2-butene 3-mcthyl-1-butene 3.6-dimethyl-2-heptene 2-methylbut-2-ene 3-methylbut-1-ene 3.6-dimethylhept-2-ene CH CH3 1-methylcyclopentene 2-ethyl-1,3-cyclohexadiene 7-bromo-1.3.5-cycloheptatriene 3-propyl-1-heptene 1-methylcyclopentene 2-ethylcyclohexa-1,3-diene 7-bromocyclohepta-1,3,5-triene 3-propylhept-1-ene 2013 Pearson Education.ind Ch07 Alkenes:Struct synth (landscape) Page 7

Ch07 Alkenes; Struct + synth (landscape) Page 7 Compounds with 2 double bonds are called dienes, 3 double bonds are trienes, etc. For branches, each alkyl group is given a number, but the double bond is still given preference when numbering the chain

When alkenes are substituents,they are termed alkenyl groups,and may be named systematically CH2 多-Hc-&=cH methylene 2-propenyl group group (allyl group) Geometric isomers The rigidity of a it bond gives rise to geometric isomers. When similar groups(not H's)are bound to the same side of the double bond the alkene is said to be cis. When similar groups are bound to opposite side of the double bond it is said to be trans. H H H CH2CH3 CH2CH3 trans-pent-2-ene cis-pent-2-ene Cycloalkenes must have or more carbons before they are large enough to incorporate a trans double bond. cyclohexene trans-cyclodecene Therefore cycloalkenes are deemed to be cis unless specified otherwise. Ch07 Alkenes:Struct synth (landscape) Page 8

Ch07 Alkenes; Struct + synth (landscape) Page 8 When alkenes are substituents, they are termed alkenyl groups, and may be named systematically. Geometric isomers The rigidity of a  bond gives rise to geometric isomers. When similar groups (not H’s) are bound to the same side of the double bond the alkene is said to be cis. When similar groups are bound to opposite side of the double bond it is said to be trans. Cycloalkenes must have 8 or more carbons before they are large enough to incorporate a trans double bond. Therefore cycloalkenes are deemed to be cis unless specified otherwise. CH2 H2C H C CH2 methylene group 2-propenyl group (allyl group) cyclohexene trans-cyclodecene

E-Z Nomenclature The assignment of geometrical isomers as either E or Z follows on from the Cahn-Ingold-Prelog convention Imagine the double bonds being split into two ends,and assign the priority of the substituents Zusammen Entgegen If the two highest priority groups are on opposite sides of the double bond(trans),the isomer is assigned E. If the two highest priority groups are on the same side of the double bond(cis)the isomer is assigned Z. ①'s together Br CH; ①D Br CH C=C becomes Z CI ② ② (Z)-1-bromo-1-chloropropene Ch07 Alkenes:Struct +synth (landscape) Page 9

Ch07 Alkenes; Struct + synth (landscape) Page 9 E-Z Nomenclature The assignment of geometrical isomers as either E or Z follows on from the Cahn-Ingold-Prelog convention. Imagine the double bonds being split into two ends, and assign the priority of the substituents. If the two highest priority groups are on opposite sides of the double bond (trans), the isomer is assigned E. If the two highest priority groups are on the same side of the double bond (cis) the isomer is assigned Z

If there is more than one double bond,then the stereochemistry around each double bond must be specified. Br 3 2 A 6 8 (32,5E)-3-bromoocta-3,5-diene Ch07 Alkenes:Struct synth (landscape) Page 10

Ch07 Alkenes; Struct + synth (landscape) Page 10 If there is more than one double bond, then the stereochemistry around each double bond must be specified