Unit 10: Radical Reactions
Bonds that split one electron to each atom instead of two to one, and the chain reactions that unpaired electron sets off.
6 topics · 21 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Radical Initiation, Propagation, and Termination
- How it worksHow does light or heat homolytically cleave a weak bond like Br-Br into two bromine radicals to start a chain?
- Worked exampleFind the net bonds broken and formed in one full propagation cycle of methane's radical chlorination.
- Common mistakeWhy doesn't a radical chain reaction stop after just one propagation step even though a product already formed?
Your turn1 of 2
In one full propagation cycle of methane's radical chlorination, a chlorine radical abstracts a hydrogen from methane, then the resulting methyl radical reacts with a new Cl2 molecule. What bonds are broken and formed overall?Halogenation of Alkanes by a Radical Chain
- How it worksHow does a chlorine radical abstract a hydrogen from methane, and how does the resulting radical grab a new chlorine?
- Worked exampleFind every possible monochlorination product of 2-methylbutane by radical chlorination, ignoring selectivity.
- Common mistakeWhy doesn't radical chlorination of a molecule with several different hydrogens give just one clean product?
- Going furtherWhat happens to the chain reaction once two radicals collide and combine, bringing the cycle to an end?
Your turn1 of 2
2-methylbutane, (CH3)2CHCH2CH3, has how many distinct kinds of hydrogen, giving how many possible monochlorination products, ignoring any selectivity between them?Selectivity in Radical Halogenation
- How it worksHow does a more stable radical intermediate, like a tertiary one, form faster and dominate the product mixture?
- Worked exampleFind the major monobromination product of 2-methylbutane, favoring substitution at the most substituted carbon.
- Going furtherWhat happens to a halogenation's selectivity when bromine is swapped out for the far less selective chlorine radical?
Your turn1 of 2
A hydrocarbon has 2 tertiary hydrogen(s) and 7 primary hydrogens. In a radical halogenation, each tertiary hydrogen reacts 1585 times faster than each primary hydrogen. What percent of the monohalogenated product comes from abstraction at the tertiary position?Allylic Bromination
- How it worksHow does resonance spread a radical's unpaired electron over two carbons once it forms right next to a double bond?
- Worked exampleFind the two possible allylic bromination products when cyclohexene reacts with NBS and light.
- Common mistakeWhy doesn't NBS add bromine directly across the double bond the way molecular bromine would?
- Ties togetherHow does an allylic radical's resonance-spread structure connect back to the resonance from the bonding unit?
Your turn1 of 2
Cyclohexene reacts with NBS and light, forming an allylic radical resonance-delocalized between the original allylic carbon and the far carbon of the double bond. Bromine can bond at either resonance position. What is the outcome for cyclohexene specifically?Radical Addition of HBr and Anti-Markovnikov Selectivity
- How it worksHow does a peroxide initiator flip HBr addition into a radical chain that adds bromine first, to the less hindered carbon?
- Worked exampleFind the anti-Markovnikov product when HBr adds to 1-methylcyclohexene in the presence of peroxides.
- Common mistakeWhy doesn't this anti-Markovnikov selectivity show up with HCl or HI addition, only with HBr?
- Going furtherWhat happens to a reaction's regiochemistry when peroxides are simply left out of the same HBr addition?
Your turn1 of 2
HBr adds to 1-methylcyclohexene in the presence of peroxides, favoring the radical mechanism. What is the anti-Markovnikov product?Radical Polymerization
- How it worksHow does a growing polymer radical keep adding one more monomer at a time to extend its chain?
- Worked exampleFind the repeating unit of the polymer formed when styrene undergoes radical polymerization.
- Going furtherWhat happens to a polymer chain's growth when two growing radical chains collide and terminate each other?
Your turn1 of 2
Styrene, CH2=CH-C6H5, undergoes radical polymerization. What is the repeating unit of the resulting polymer?