Dienes

Question 1

Name 3 classes of Dienes, and explain their connectivity and p-orbitals.

Answer 1

1. Isolated. separated by one or more sp3 hybridized carbons. pi bonds are independent.
2. Conjugated. connected by a single C-C bond. pi bonds overlapping and therefore ‘extended’
3. Cumulated. one carbon is present in 2 C=C bonds.

Question 2

Which diene is most stable?

Answer 2

Conjugated dienes are most stable because of the delocalized energy/stabilization energy created by the overlapping p-orbitals. Cumulated are lest stable. Lower heats of hydration are more stable than high heats of hydration.

Question 3

Describe an Allene.

Answer 3

linear, non planar arrangement of cumulated diene. has a chirality axis.

Question 4

Name 2 ways to prep a diene. What is the preferred conformation?

Answer 4

1. dehydration of alcohol (w/ existing alkene). — acid, heat
2. dehydrohalogenation of alkyl halide (w/ existing alkene). — base, heat
   the preferred conformation is conjugated diene.

Question 5

Difference between Kinetic Product and Thermodynamic product?

Answer 5

kinetic – formed the fastest, most stable intermediate.

thermodynamic – most stable product.

Question 6

What is conjugate addition? direct addition?

Answer 6

1,2-addition is direct. 1,4-addition is conjugated. Occurs with Hydrogen Halides.

Question 7

Name a few important notes about Diels-Alder Reaction.

Answer 7

• prepares a cyclohexene
• concerted
• cyclic intermediate
• involved orbital overlap
• syn addition
• cis/trans relationship of subs. is retained
• requires heat
• diene in s-cis conformation
• dieneophile has an EWG directly attached to alkene
• intermolecular reaction is possible
• 1,6 rule in synthesis

Question 8

Explain Endo and Exo adduct.

Answer 8

Endo : kinetic product. SAME side. more steric hinderance.

Exo : thermodynamic product. OTHER side. does not have stabilizing effect of partial overlap (like in endo)

Question 9

Steric Hinderance may prevent a Diels-Alder Reaction. [true or false]

Answer 9

True. consider configuration of both dieneophile and diene.