Free Radical Halogenations and Diels alder

Light and heat reactions with alkenes

Arrow rules (again)

• Arrows point away from electrons, NEVER EVER EVER toward electrons.

• Atoms that are attacked by arrows (or electrons) are called electrophiles

• Atoms that are do the attacking are called nucleophiles.

Radical Reactions

Reactions that involve only one electron.

Adding X2 to Alkanes

Alkanes are generally very unreactive compounds. Since they only have sigma bonds. However, the do react with Cl2 and Br2, when heat (hv) (Δ) or ROOR (peroxide)

• this is a radical reaction

Alkene Halogenation reaction mechanism

3 steps

Initation

• Halogen splits into two Halide radicals

Propagation

• The Halogen forms a bond with a hydrogen and forms a Carbon hydrogen radical. This radical occurs on the most substituted carbon.

• Then the radical will bond with a halogen molecule to create an Alkyl Halide and a halogen radical. [this continues the propagation steps]

Termination

• Ends the reaction by combining different radicals into non-radical byproduct.

Br vs Cl (Free radical Halogenation)

Br ALWAYS ends up on the most substituted Carbon or on a resonance stabilized carbon

Cl is less selective and can give mixtures of products

Initiation steps key

• An initiation step does not have radicals on the left

• Needs radicals on the right.

Propagation steps key

A step with one or more radicals on both sides of the equations.

Termination steps key

Radicals on the left side of equation,

NO radicals on the right.

Do radical intermediates undergo rearrangments?

No

NBS

NBS adds a single bromine to the carbon that’s one position away from the double bond.

NBS radical intermediate

NBS in benzyl rings

Diels alder Cycloaddition reaction mechanism

Door hinge reaction my beloved :3

There is a reaction between a conjugated diene and a dienophile.

Diels alder with a triple bond.

Diene and dienophile reactions.

Diels alder with Cyclic dienes

You shift the cyclic diene up where it does not have the alkenes.

If it points right, what is it called? Where does it point?

It is the endo, it points down

If it points left , what is it called? Where does it point?

Exo, up

What cannot undergo diels alder?

Things that are stuck and cannot shift into a pacman positions. IE, this one.

Radical Stability trend

Other differences between Endo and Exo

Have different activation energy

• Endo is typically more favored in comparison to exo