Radical Chemistry 1 Flashcards
What are radicals?
- Radicals are 7-electron species, containing a single, unpaired electron, which is represented by a dot
- Radicals are usually extremely reactive. However, radical reactions proceed under relatively mild, neutral reaction conditions, and are often highly chemo-, regio- and stereoselective
What is a unique property of radicals related to solvents?
Compared with ionic species which form solvent shells, (neutral) radicals are not strongly solvated. This lack of ‘clutter’ makes radicals good for the formation of sterically congested bonds
Radicals have low barriers to sterochemical inversion
Draw the alkyl radical inversion of the following sp³ molecule
- Middle inversion has an sp² arrangment, while the other have sp³
Radicals have a low barrier to sterochemical inversion
Draw the alkenyl radical inversion for the following molecule
Radicals can be generated by…
…homolytic bond cleavage (where a bond between two atoms is broken and the atoms get one electron each)
Draw the curly arrows for the photochemical homolysis of this weak bond
(this is for an nitrite ester but could be more an alkyl nitrite)
Draw the curly arrows for the photochemical homolysis of this weak bond
For a protonated N-chloroamine
Draw the curly arrows for this thermal homolysis of this weak bond
Breaking C-C bonds in the second step isn’t usually favourable but due to the entropy of creating CO₂ gas - disorder
Draw the curly arrows for this thermal homolysis of this weak bond
- Formation of N-N triple bond is a strong thermodynamic driving force of this reaction
The following example is an electron transfer reaction called the Kolbe electrolysis
Draw the curly arrow mechanism and products
- Start by forming the carboxyl radical
- Then carbon dioxide forms as the driver for this reaction
- And the 2R radicals dimerise
The following reaction is an example of dissolving metal reduction
Draw the curly arrow mechanism and the product
- The final product has the ability to undergo further chemistry and dimerise too
For the generation of carbon radicals, it is not ususally covenient to prepare compounds with peroxides or azo functional groups
Instead alkyl halides are common radical precursors
This reaction uses tributylin hydride and AIBN to initate radical chain reaction
Draw the curly arrow mechanism and product
After AIBN is used to initate the reaction, the products then react with tributyltin hydride
Draw the curly arrow mechanism and product
Once the tin-butyl radical is formed, it then goes on to react with an alkyl halide (X-R)
Draw the curly arrow mechanism and product
Then R radical generated is what we want to undertake further chemistry
The by-product is a tin-halide
Once the R radical has reacted, how can we terminate it?
By using tributyltin hydride
(This can also cause early termination if the R radical has not reacted yet)
Dehalogenation is an example of how we can remove a functional group using radical chemistry
Deoxygenation is another example of how we can remove a functional group using radical chemistry
This is the first step, what is the mechanism?
The next step of the deoxygenation is you react it with tributyltin hydride and AIBN
Draw the curly arrow mechanism and the product
Radical Allylation is an example of intermolecular carbon-carbon bond-forming reactions of radicals
What do we regenerate the tributyltin radical from this reaction
