Lecture 3 Friday January 9 2015 9 14 PM The 1 MO suggests that a bond between C2 C3 has a partial double bond character That is consistent with the fact that the C2 C3 bond is shorter than the regular C C bond Another manifestation of the partial double bond character is the fact that the conjugated dienes can exist in two conformations s trans and s cis and more importantly that there is an activation barrier for the conversion of one to the other The activation barriar is about 6 kcal mol and is associated with the breaking of the bond between C2 C3 Finally it is worth mentioning that the s trans isomers are in general more stable than the s cis isomers Allenes structure In allenes the central carbon is sp hybridized and the terminal ones are sp2 hybridized If you think about the central carbon it has two p orbitals that are perpendicular to each other Those atomic orbitals are used to make bond with the neighboring carbons So the bonds in an allene are perpendicular to each other Also the substituents at the terminal carbons are in the planes that are perpendicular to each other So hydrogen atoms on carbon 1 are in the plane of the paper and hydrogen atoms on carbon 3 are in a plane that is perpendicular to the plane of the board Remember it all starts with the fact that the middle carbon is sp hybridized and as a result the two bonds it forms have to be perpendicular to each other One of the consequences of the fact that the substituents on the terminal carbons are perpendicular to each other is that the compounds 1 and 2 are enantiomers They are mirror images of each other but are not superimposable Interestingly these molecules do not have a chiral center but have a chiral axis Allenes will be chiral as long as each terminal carbon has two different substituents That is sufficient for the existenceres of enantiomers For export Page 1 Lecture 3 Friday January 9 2015 9 31 PM Diene synthesis Dienes are commonly made through elimination reactions Either from alcohols under acidic reaction through E1 Or from halides in the presence of a base through E2 reaction E1 under acidic conditions tends to be reversible and the regioselectivity are we going to remove protons Ha or Hb from the carbocation of the reaction is under thermodynamic control We form a more stable conjugated diene E2 reactions are usually irreversible and are under kinetic control Again under kinetic control we get the conjugated diene as the major products Reactions of conjugated dienes Protonation always to the terminal position as that way we get a more stable allylic cation In this case the reaction of the allylic cation forms only one product For export Page 2 Lecture 3 Friday January 9 2015 9 44 PM However in the following reaction from one allylic cation we can get two products 1 2 addition product and 1 4 addition product It turns out that the product distribution varies with temperature Furthermore if we expose the 1 2 addition product to HBr small amount we get again the mixture of products Reaction diagram reversible reactions thermodynamic control more stable product dominates Irreversible reactions kinetic control faster to form dominates The reaction of the 1 2 addition product with HBr tells us that at 25 C reaction is reversible Product can go back to the cation and then over to another isomer of the product When a reaction is reversible the distribution of the products is determined by the thermodynamic stability of the products thermodynamic control More stable product will be the major product If a reaction is not reversible the product distribution will depend on which product is formed faster which product is easier to make and we say that the reaction is under kinetic control For export Page 3