Chapter 16: Conjugated Pi Systems and Pericyclic Reactions

3 Classes of Dienes

• Cumulated (adj. = bonds)

• Conjugated

• Isolated

Diene

2 = bonds b/t C atoms

Conjugated dienes can be prepared from allylic halides via an elimination process, Potassium tert-butoxide is used because:

t-BuOK is sterically hindered only acts as a base

Why is the single bond shorter in the conjugated diene? The C2-C3 bond in 1,3 butadiene is formed by the _ of 2 _ hybrid orbitals.

• overlap

• sp²

Since sp² orbitals have more s character than sp³ orbitals, the electron density is _ to the nucleus in an sp² orbital, so the bond is shorter.

closer

Stability of Conjugated Dienes?

tetra sub > tri sub > TRANS di sub > CIS di sub > MONO cub

-126 kJ/mol

-116 kJ/mol

The _ the heat of hydrogenation, the more stable the double bond.

lower

-253 kJ/mol

-225 kJ/mol