Chem 261 Nov 20, Conjugated = separated by a single bond from a double bond

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1 7 hem 6 Nov 0, 07 REVIEW: onjugated = separated by a single bond from a double bond Examples of onjugated Systems. Double bonds ( or more). Allylic ations. Allylic Anions. Allylic Radicals REVIEW: Molecules below at diastereomers (a class of stereoisomers) 5 E-,-pentadiene 5 Z-,-pentadiene The two molecules below are constitutional (structural) isomers.,-pentadiene is not conjugated. It has double bonds separated by single bonds.,-pentadiene E-,-pentadiene NT conjugated sp,-pentadiene, shown below is not conjugated. It has cumulated double bonds. This molecule is an allene and is a structural isomer with respect to the above,-pentadienes. sp

2 8,-pentadiene the same compound, trans cis, diastereoisomers,-pentadiene,-pentadiene structural or constutional isomers structural or constutional isomers,-pentadiene,-pentadiene Addition Reactions of onjugated Systems % 0% 0 0% 80%,-butadiene -bromo--butene -bromo--butene! These are structural isomers. The product -bromobutene is a,-addition product, whereas -bromo--butene is the product of an,-addition reaction. The numbers (,- or,-) indicate the position where the and added to the,-butadiene. Why does the temperature affect the ratio obtained?

3 9 From the resonance forms (connected by the double-headed arrow) of the allylic cation in the above figure, we see that the positive charge is shared between the and. Note: The allylic cation has two electrons delocalized across the three carbons. It was drawn to have two resonance forms, but it is a single entity, therefore we cannot differentiate and speak of one form being a primary carbocation (therefore less stable) and the other being a secondary carbocation (therefore more stable). Answer to the above question: -bromo--butene has a higher yield at the lower temperature because it is formed faster than -bromo--butene: kinetic control -bromo--butene has lower yield than -bromo--butene at higher temperature due to thermodynamic control. The addition of bromine to the allylic cation is reversible at high temperature. -omo--butene can be converted back to the allylic cation, and then form -bromo--butene which is the thermodynamically more favored product.

4 50 To understand thermodynamic and kinetic control, let s look at the reaction s energy diagram. Transition State (TS) (Progress of a Reaction)! ΔG = Free energy of the reaction this determines ratio of starting material and product Δ = Enthalpy of the reaction (change in bond energies) T = Temperature (in degrees Kelvin) ΔS = Entropy (a measure of disorder) E A = Activation energy (the amount of energy needed for reaction to proceed). This is a factor that determines the rate of a reaction. When E A is small the reaction is fast; when it is large the reaction is slow. Transition State (TS) = Where bonds are partially made and partially broken For an exothermic reaction, there is release of heat and ΔG is always negative (depicted in the energy diagram, the product would be lower on the energy diagram than the starting material).

5 5 Now let s look at the energy diagram of the addition to butadiene reaction TS TS intermediate Kinetic product (-80 o ) Thermodynamic product (0 o )! The activation energy barrier to form the,-product, -bromo--butene is much smaller than the,-product, -bromo--butene (so that it can be formed easier and faster. We call this kinetically favoured). owever, the energy of -bromo--butene is lower than -bromo--butene, so that it is more stable than -bromo--butene (it is thermodynamically favoured). Why is -bromo--butene more stable? The bromine atom is bulky. It likes to stay away from the rest of the molecule to avoid steric clashes. Alkene carbons are somewhat electron deficient. More highly substituted alkenes are more stable due to donation of electron density by the substituents ( vs ),-addition monosubstituted =,-addition disubstituted =

6 5 Practical Applications for onjugated Addition Reaction: Example: Rubber formation Formally by a conjugated addition reaction Rubber is a natural product produced by rubber tree ivea brasiliensis (>500 lbs per acre). The rubber is collected in the form of syrup (latex) and after concentration as raw rubber it is very sticky (like chewing gum). This is not very useful. ther sources of rubber but with very minimal yields:. Sunflower Dandelion (~0 lbs per acre). Lettuce etc. rubber n! isoprene etc polyisoprene Rubber can be viewed as a polymer of -methyl-,-butadiene, which is also known as isoprene. Addition of acid will form a resonance-stabilized cation. Note that nonenzymatic polymerization (e.g. in lab synthesis) would give undesirable trans isomer instead of the cis naturally formed in plants by enzymatic processes. harles Goodyear - Add sulfur and heat to rubber resulting to vulcanized rubber elastic (not plastic (malleable to a new shape) or sticky)

7 5 Another example: Styrene Butadiene Rubber (SBR) synthesis (by conjugated cation) SBR is a copolymer. etc Another representation: Example: Terpenes or Terpenoids or Isoprenoids N an isoprene unit, a 5 unit Nomenclature for multiple 5 units in a terpene: a. ne 5 unit = hemiterpene b. Two 5 units = monoterpene c. Three 5 units = sesquiterpene d. Four 5 units = diterpene Examples of Terpenes st 5 unit nd 5 unit Limonene = a monoterpene with two 5 units

8 5 st 5 unit nd 5 unit α-pinene = a monoterpene with two 5 units st 5 nd 5 rd 5 Geraniol = a sesquiterpene with three 5 units Extra Example: P P -PP P P GPP (Geranyl PP) PP PP PP PP GPP (Geranyl PP) Repeat STERIDS ARTENES