Carbocations are also stabilised by resonance when the electron-poor cationic carbon is adjacent to an electron-rich -system: H H + C C H H + H C H + C H C C H H H Allyl carbocation H + H C H C H H C H H C H + + + Benzyl carbocation Not surprisingly, the order of reactivity of the substrates in SN1 reactions is exactly the same as the overall stability order for carbocations: 3° > 2° ≈ benzyl ≈ allyl > 1° > Methyl (2) The effect of the Nucleophile: The rates of SN1 reactions are independent of the nature of the nucleophile because the nucleophile is not involved in the ratedetermining step. Partial comparison of SN1 and SN2 substitution reactions: Variable Substrate structure: Methyl or 1° SN1 SN2 Does not occur except Common for 1° allyl and 1° benzyl Secondary Mainly allylic and Sometimes benzylic systems Tertiary Stereochemistry Nucleophile Common Racemisation Rate does not depend on the concentration or the nature of the nucleophile. Does not occur Inversion Rate does depend on the concentration and the nature of the nucleophile - SN2 is especially favoured by anionic nucleophiles. Questions for home study and discussion at the Class Tutorial: (1) How does the nature of the leaving-group affect SN2 reactions? (2) How does the nature of the solvent affect SN2 reactions? (3) How does the nature of the leaving-group affect SN1 reactions? (4) How does the nature of the solvent affect SN1 reactions? Alkenes (also known as olefins) - compounds with a carbon-carbon double bond. The simplest alkene - ethene or ethylene: H2C=CH2 CH3CH2 OH Ethanol H2C CHCl Vinyl chloride CH3CHO Acetaldehyde H2C CH2 2.6 x 10 7 tons per year in US HOCH2CH2 OH Ethylene glycol CH3COOH Acetic acid Electronic structure of the carbon-carbon double bond: H H C H H H H C Barrier ca. 3 kcal mol-1 H C H H H Staggered H CH3 C C CH3 H H H H H H H H H H H H H Eclipsed H H H H H H C C C CH3 Staggered H CH3 H Eclipsed Different conformations of the same compound. Interconvert very rapidly at room temperature. Cannot be separated or isolated. H H C H C H C Rotate by 90° around -bond H Barrier 64 kcal mol-1 H H C H CH3 H H C C C C H CH3 trans(Latin: across) H CH3 CH3 cis(Latin: on the same side) The cis- and trans- isomers of alkenes cannot interconvert due to the high barrier to rotation about the double bond - they are different compounds. Nomenclature - the systematic rules for naming alkenes: (1) Find the longest chain of carbon atoms that includes the double bond - then name as for the corresponding saturated hydrocarbon (alkane) but use the termination -ene rather than -ane: CH3 H2C CH CH2CH3 H2C propene H3C butene CH3 HC CH CH3 CH3 C CH C CH3 CH2 butene CH2CH3 hexene (2) Number the carbon atoms of the longest chain, beginning at the end nearest to the double bond: 3 CH3 2 1 3 CH H2 C 1-butene 4 1 C CH 3 2 H H CH3 H3C HC CH H2C 1-propene CH2CH3 2 1 4 CH3 1 2-butene 2 C 3 2-pentene CH2CH3 4 5 (3) Indicate the position of any groups (substituents) attached to the parent alkene chain by the appropriate number: CH3 CH3 C CH3 CH2 1 3 1 C 3 H2C 4 CH2CH3 2 5 6 2 CH3 C CH3 2-methyl-1-propene 3,4-dimethyl-3-hexene 3 3 1 H2C 2 CH2Cl 1 ClHC C CH3 3-chloro-2-methyl-1-propene 2 CH2Cl C CH3 1,3-dichloro-2-methyl-1-propene Some non-systematic names still in common use: H2C CH2 H2C ethylene H2C methylene H2C CH vinyl- CH CH2 allyl- CH2 H2C CH Cl H2C vinyl chloride CH CH2 OH allyl alcohol methylenecyclohexane Reading exercise: Read McMurry (5th Ed.) pp. 192-194 for some further aspects of the rules of alkene nomenclature and work through the problems on pp. 194-195 . The stability of alkenes: Two important generalisations: (1) Trans- alkenes are more stable than cis- alkenes. (2) Alkene stability increases with increasing number of substituents on the alkene carbons. (1) Trans- alkenes are more stable than cis- alkenes. Hoisomerisation = 1 kcal/mol H H C H C CH3 C CH3 CH3 C CH3 cis-2-butene H trans-2-butene Hohydrogenation (kcals/mol) = H2/Pd - 28.6 - 27.6 H2/Pd CH3CH2 CH2CH3 Butane Why is the trans- isomer more stable than the cis- ? H H H C H C H H H C C H H H H C H H C C Reduced steric crowding in the trans- isomer H C H H (2) Alkene stability increases with increasing number of substituents on the alkene carbons. CH3 CH2CH CH2 CH3CH CHCH3 1-butene 2-butene Monosubstituted Disubstituted Hohydrogenation (kcals/mol) = cis- - 28.6 trans- -27.6 H2/Pd H2/Pd - 30.1 CH3CH2 CH2CH3 Butane Why does increased substitution increase the stability of alkenes? Two factors: (A) Bond strength effects. Substituted alkenes may contain three different kinds of C-C bonds: sp2 - sp2 : C C sp2 - sp3 : C C C C sp3 - sp3 : C C C The order of stability of these bonds is sp2 - sp2 > sp3 - sp2 > sp3 - sp3. It is the sp2 - sp3 > sp3 - sp3 difference that is important here. CH3 CH2 CH CH2 CH3 CH CH CH3 1-butene 2-butene 1 sp 3-sp2 bond 1 sp3-sp3 bond 2 sp 3-sp2 bonds 0 sp 3-sp3 bond More stable The more alkyl substituents there are on the alkene carbons the more stable sp3 - sp2 bonds are formed and the more stable is the alkene. (B) Hyperconjugation H C C Filled C-H bonding orbital C Empty * C-C antibonding orbital Donating electrons to: The alkene is stabilised by a sideways-on (i.e. ) overlap of a filled C-H -bonding orbital of an alkyl substituent with the empty * orbital of the carbon-carbon double bond. The more alkyl substituents there are on the alkene carbons the more stabilising interactions of this type can occur and the more the alkene is stabilised. Hyperconjugation results in a net lowering of energy, i.e. the alkene is stabilised.