The allylic alkene gives two products- the 1,2 product, and the 1,4 product.
However, the benzylic alkene only gives the one product (analogous to the 1,2 product), instead of multiple products (like the 1,4 product, 1,6 product, and 1,8 product). Why is this the case?
The allylic carbocation has two resonance forms, and so two positions where a nucleophile can attack, yielding two different products (1,2 and 1,4).
The benzylic carbocation has four resonance forms, and so in theory, four positions where a nucleophile can attack. But all but one of these resonance forms yield a product that is not aromatic, and so do not form.
Aromatic compounds are very stable; aromatic starting materials tend to only form products that are also aromatic.
MendelSet practice problem # 581 submitted by Matt on July 9, 2011.
Write out the mechnanism for the formation of the 1,2 and 1,4 products of the reaction below.
This is an SN1 reaction, but the carbocation is special because it's allylic.
If the Br- nucleophile attacks the 3º resonance form, the 1,2 product is formed. (This is the product that would have formed if resonance was not involved.)
If the Br- nucleophile attacks the 2º resonance form, the 1,4 product is formed.
MendelSet practice problem # 572 submitted by Matt on July 8, 2011.
Write the structure of the major organic product of each reaction.
a) is a free-radical bromination. The radical will be formed at the most stable position, which will be the allylic position due to resonance (see problem 569). There are two allylic positions in this molecule, so the more substituted one will be the more stable; the bromine will add to the 3º allylic carbon.
b) is an E2 reaction. There are two types of beta protons and so two possible alkene products. One is an isolated diene and the other is a conjugated diene. Conjugated dienes are more stable, and so that will be the major product.
MendelSet practice problem # 571 submitted by Matt on July 8, 2011.
For the anion and cation species, used curved arrows. For the radical species, use hooks.
Notice that arrows always go from high electron density to low electron density. So for negatively charged species, the arrow starts at the lone pair. For positively charged species, the arrow starts at the double bond and goes towards the positive charge.
(Radical resonance uses hooks instead of arrows and so is a little different.)
MendelSet practice problem # 569 submitted by Matt on July 8, 2011.