Chapter 10: Free Radicals and Organometallics Flashcards
Describe the initiation step of the Peroxide Effect
Light or heat are used to initiate a cleavage of a peroxide to form two R-O radicals
R-O radical attacks the electrophile proton in HBr to forms a hydroxide which is stronger than HBr bond
Br radical is formed
Describe the propagation step of the Peroxide Effect
Bromine radical reacts with nonradical starting alkene material where the radical is formed on most substituted carbon, putting the bromine on the least substituted carbon.
Alkene radical attacks the proton of HBr to form another bromine radical
Describe the termination step of the Peroxide Effect
After all of the alkene is consumed, the bromine radical will react with itself to form Br2 and terminate the peroxide effect.
What is the regioselectivity of the peroxide effect?
The alkene radical formed is a tertiary radical because it is more stable and faster than a primary radical because of the more substituted carbon which has more orbitals for the radical to interact with.
Also due to steric effects in which the Br can more easily attack the less substituted carbon because of the lack of vanderwaal repulsions
What is the hybridization of free-radical alkenes?
sp2 hybridized where radical electron is in the p orbital
What is radical polymerization?
Alkenes react to form polymers in the
presence of free-radical initiators.
What is the propagation step of radical polymerization?
A free radical and a pi-bond electron of an alkene form a bond together where a new radical is formed on the carbon on the other side of the double bond.
What is beta-scission?
Termination of free radical polymerization. Two radicals react with themselves by removing a beta carbon-hydrogen bond to give an alkene and an alkane non radical product
What are the three different types of polymers?
Isotactic: substituents are all on the same side of the polymer chain (most common)
Syndiotactic: substituents alternate regularly on opposite sides
Atactic: substituents are randomly configurated
What is free-radical alkane halogenation?
When an alkane is treated with Cl2 or Br2 in heat to form a mixture of alkyl halide
What are the steps of free radical alkane halogenation?
Initiation: Cl2 or Br2 cleave
Propagation: Br (or Cl) radicals react with hydrogen of CH4 to form a CH3 radical and HBr. CH3 radical then reacts with another Br radical to form CH3Br and another bromine radical
What is the regioselectivity of alkane halogenation?
When more than one type of hydrogen is available on alkane, the product that forms an alkyl halide with the most substituted carbon bonded to the halogen is the most energetically favorable. This is because a free radical tertiary alkane is more stable than a primary radical alkane
Chlorination versus bromination
Chlorination is much less selective than bromination because the light generated chlorine radical is less stable and reaction wants to get rid of it no matter the degree of the carbon radical.
What are organometallic compounds?
Compounds that contain carbon-metal bonds where the stabalized carbonanion can be used as a strong nucleophile or base.
- Grignard Reagent: composed of R-Mg-Br which can be seen as R- and MgBr+
- Organolithium Reagent: composed of R-Li where R- and Li+ are formed.
How are Grignard Reagents and Organolithium Reagents produced?
Both Grignard and organolithium reagents are prepared by adding the
corresponding halide to stirred suspensions of the appropriate metal.
Describe formation of Grignard Reagent
Formed in aprotic diethylether. Ethers are great solvents because they don’t have acidic C-H bonds that react with the carboanion.
R-Br + Mg –> R-Mg-Br (R- MgBr+)
Describe formation of Organolithium Reagent
Formed in hydrocarbon solvents because of its complete lack of acidic protons. Organolithium reagents are MUCH more reactive than other organometallics.
R-Cl + 2Li –> R-Li + LiCl
How are Grignards and Organolithiums used as nucleophiles?
Carboanion reacts instaniously with electrophile and kicks off MgBr as a cation leaving group.
What is a carbene?
CCl2 where carbon has one lone pair and is missing an octet.
The lone pair in the sp2 orbital can act as a nucleophile, and its empty 2p orbital acts as an electrophile
What is the cyclopropane formation from carbenes?
A concerted syn addition where empty 2p orbital acts as an electrophile for pi-bond electrons in alkene, and lone pair in sp2 orbital act as nucleophile and attack the other carbon to form a cyclopropane ring.
Stereochemistry (cis or trans) is conserved
What is the Simmons-Smith reaction?
This reaction prepares cyclopropanes without halogens.
The Simmons-Smith reagent I-CH2-ZnI breaks down to form CH2 with a lone pair on the carbon that reacts with cyclohexene to bond CH2 to both carbons and kick off ZnI2
