ch 12 free radical and diene reactions

three steps in a free radical reaction:

1. initiation

2. propagation

3. termination

Free Radical Stability ranking

(least stable) vinylic < methyl < primary < secondary < tertiary < allylic < benzylic (most stable)

what is the rate limiting step in free radical reactions?

initiation

initiation step of free radical reactions

covalent bond breaking to form two radicals

propagation step of free radical reactions

one radical reacting with stable molecule to generate another radical

that carbon radical reacts with another Br2 to form alkyl bromide and another bromine radical

termination step of free radical reactions

two radicals reacting to form a stable molecule (tertiary bromide/chloride)

(CH3)3CH + 1) Br2 2) hv

(CH3)3CBr

difference between Free Radical bromination and chlorination

a racemic mixture can form if a chiral center is created during the reaction

reagents for Free Radical bromination

1) Br2

2) hv

reagents for free radical chlorination

1) Cl2

2) hv

reagents for allylic and benzylic bromination

1) NBS (N-bromosuccinimide)

2) hv

Benzylic bromination cannot form if…?

benzylic carbon is not bonded to a

hydrogen atom

difference between allylic and benzylic bromination

benzylic bromination can only form one product

reagents for Diels-Alder cycloaddition

diene + dienophile —> cyclohexane

diene must be in what conformation for Diels-Alder Cycloaddition to happen?

s-cis

diene + dienophile

cyclohexane

what happens to the three pi bonds in Diels-Alder reaction?

becomes one pi bond and two sigma bonds

is stereochemistry conserved in dienophile ?

yes

When the diene or dienophile is a cyclic molecule, what product is formed?

bicyclic product and endo product will form (substituents on the underside/facing down opposite to the bridgehead carbon of the new six-membered ring)

sustituents will be “dash”

diene:

1) strong acid (HBr)

2) cold

1, 2-addition (less stable product)

monosubstituted alkene (Br is on the middle and double bond on the side)

diene:

1) strong acid (HBr)

2) hot

1, 4-addition (more stable product)

disubstituted alkene (Br is on the side and double bond is in the middle)

How is the activation energy of 1,2-addition vs 1,4-addition?

1,2-addition has lower activation energy because it occurs quicker

How is the gibbs free energy of 1,2-addition vs 1,4-addition?

product of 1,4-addition has lower gibbbs free energy —> thermodynamic product

distinguishing between kinetic and thermodynamic product of 1,2-addition vs 1,4-addition

1,2-addition: kinetic product

1,4-addition: thermodynamic product