Chapter 10 - Radical Reactions Flashcards
What is a radical?
A radical is a species with an unpaired electron.
What are the two types of bond cleavage?
- Heterolytic bond cleavage forms ions
- Homolytic bond cleavage forms radicals
What type of arrows are used to represent radical mechanisms?
Single-barbed arrows, also called fishhook arrows.
How does the geometry of a carbon radical compare to carbocations and carbanions?
A carbon radical is trigonal planar or exhibits a shallow pyramid, while a carbocation is trigonal planar and a carbanion is trigonal pyramidal.
Rank the stability of the following radicals from least to most stable: Methyl, Primary, Secondary, Tertiary.
Methyl < Primary < Secondary < Tertiary
What is hyperconjugation?
Hyperconjugation is the delocalization effect that stabilizes radicals through alkyl groups.
What is the bond dissociation energy (BDE) trend for carbon radicals?
Tertiary radicals have the lowest BDE, making them the most stable.
True or False: Allylic radicals are resonance stabilized.
True
What is the difference between allylic and vinylic positions in terms of radical stability?
Allylic positions are resonance stabilized, while vinylic positions are not.
What are the six common steps in radical mechanisms?
- Homolytic cleavage
- Addition to a π bond
- Hydrogen abstraction
- Halogen abstraction
- Elimination
- Coupling
In radical mechanisms, what is the alpha position?
The position bearing the unpaired electron.
Fill in the blank: A ________ radical is less stable than a primary radical.
Vinylic
What is the significance of bond dissociation energy (BDE) in radical reactions?
BDE indicates the energy required to break a specific bond homolytically, influencing radical stability.
What happens during hydrogen abstraction in radical reactions?
A radical abstracts a hydrogen atom, generating a new radical.
What kind of radical is the triphenylmethyl radical?
A stable radical used as a biosensor.
What is the outcome of coupling in radical reactions?
Two radicals join together to form a bond.
What is the effect of resonance on radical stability?
Resonance stabilizes radicals, making them more stable than non-resonance radicals.
What is the alpha position in an elimination step?
The position bearing the unpaired electron
What forms as a result of an elimination step?
A double bond between the alpha (α) and beta (β) positions
What happens to the single bond at the β position during elimination?
It is cleaved, causing the compound to fragment into two pieces
What is coupling in the context of radical mechanisms?
Two radicals join together and form a bond
List the six common steps in radical mechanisms.
- Homolytic cleavage
- Addition to a π bond
- Hydrogen abstraction
- Halogen abstraction
- Elimination
- Coupling
What is the relationship between homolytic cleavage and coupling?
Homolytic cleavage creates radicals, while coupling destroys them
How are addition to a π bond and elimination related?
Addition to a π bond is the reverse of elimination
What are the two abstraction steps in radical mechanisms?
- Hydrogen abstraction
- Halogen abstraction
How many fishhook arrows are needed for homolytic cleavage?
Two fishhook arrows
How many fishhook arrows are required for addition to a π bond?
Three fishhook arrows
What is the key feature of fishhook arrows in radical mechanisms?
They indicate the movement of electrons
What characterizes the initiation stage of a radical reaction?
Creation of radicals
What occurs during the termination stage of a radical reaction?
Destruction of radicals
What is the most important stage of a radical reaction?
Propagation stage
In the chlorination of methane, what is the net reaction produced?
CH4 + Cl2 → CH3Cl + HCl
What is a chain reaction in the context of radical reactions?
One radical can cause thousands of molecules to react
What is required to initiate a radical chain reaction?
Energy in the form of heat or light
What is the role of radical initiators?
To provide the energy required to start a radical chain reaction
What is the energy change for homolytic bond cleavage of Cl2?
243 kJ/mol
What is the typical temperature required for thermal initiation?
Several hundred degrees Celsius
What is an example of a compound that can act as a radical initiator?
Alkyl peroxides
What is a radical inhibitor?
A compound that prevents a chain process from starting or continuing
Give an example of a radical inhibitor.
Molecular oxygen (O2)
What happens when molecular oxygen couples with radicals?
It destroys them
What is a radical inhibitor?
A compound that prevents a chain process from either getting started or continuing
Radical inhibitors are also called radical scavengers.
How does molecular oxygen (O2) act as a radical inhibitor?
It couples with other radicals, destroying them
Each molecule of oxygen can destroy two radicals.
What is an example of a radical inhibitor besides molecular oxygen?
Hydroquinone
Hydroquinone can undergo hydrogen abstraction to generate a resonance-stabilized radical.
What is the role of ΔG in determining the favorability of a reaction?
A negative ΔG indicates that the reaction favors products over starting materials
If ΔG is positive, starting materials are favored.
What components make up ΔG?
Enthalpy (ΔH) and entropy (–TΔS)
The overall ΔG can be approximated by ΔH when the entropy change is negligible.
What is the significance of bond strength in estimating ΔH for halogenation reactions?
It determines the energy of the bonds broken and formed, influencing the sign of ΔH
Bonds broken require energy input, while bonds formed release energy.
Which halogenation reactions are thermodynamically favorable?
Chlorination and bromination
Iodination is not thermodynamically favorable due to a positive ΔH.
What is the difference in the rate of chlorination versus bromination?
Bromination is generally a much slower process than chlorination
This is due to the endothermic nature of the first propagation step in bromination.
What does the Hammond postulate explain about transition states in chlorination and bromination?
For chlorination, the transition state resembles reactants; for bromination, it resembles intermediates
This affects the radical character and selectivity of the reactions.
What observation is made regarding product distribution in radical bromination of propane?
Bromination occurs at the secondary position more than predicted by statistics
This is due to the stability of the developing radical at the secondary position.
How does the selectivity of bromination compare to chlorination?
Bromination is more selective than chlorination
This is due to the greater difference in energy between transition states for bromination.
What is the significance of the energy diagrams for chlorination and bromination?
They illustrate the differences in transition state stability and energy levels
The rate-determining step’s exothermic nature in chlorination versus endothermic in bromination affects selectivity.
What is the major product expected from radical bromination of 2,2,4-trimethylpentane?
Bromination at the tertiary position
The selectivity for bromination at tertiary positions is significantly higher.
What trend in selectivity is observed among fluorination, chlorination, and bromination?
Bromination shows the highest selectivity for tertiary positions, while fluorination shows very little selectivity
Selectivity values are quantified in Table 10.2.
What is the major product expected from bromination at the tertiary position?
The major product is a monobrominated compound formed from bromination at the tertiary position
Bromination cannot occur at a quaternary site.
What is the significance of avoiding quaternary positions during bromination?
Quaternary positions do not possess a C-H bond, preventing bromination.
What type of mixture is formed when butane undergoes radical chlorination?
A racemic mixture of 2-chlorobutane is obtained.
Why does a racemic mixture form during radical chlorination of butane?
The radical intermediate is trigonal planar, allowing halogen abstraction on either face with equal likelihood.
What happens to the configuration of the starting alkane during bromination at an existing chiral center?
The configuration of the starting alkane is lost, leading to a racemic mixture.
What type of products are formed when radical halogenation occurs at a chiral center that is not involved in the reaction?
Diastereomers are formed instead of enantiomers.
What is the expected outcome when radical bromination generates a new chiral center?
A 1:1 mixture of enantiomers (a racemic mixture) is formed.
What is the purpose of using N-bromosuccinimide (NBS) instead of Br2 in allylic bromination?
NBS provides a controlled source of bromine radicals while minimizing competing reactions with Br2.
What is the result of bromination at an allylic position in cyclohexene?
An allylic bromide is produced, with the allylic radical being resonance-stabilized.
What happens during the initiation step of the CFCs’ reaction with UV light?
CFCs undergo homolytic cleavage to form chlorine radicals.
What is the overall impact of chlorine radicals on ozone?
Chlorine radicals can destroy thousands of ozone molecules through a chain reaction.
What global treaty was established to address ozone depletion caused by CFCs?
The Montreal Protocol was established to reduce chlorofluorocarbon production.
What is the mechanism by which ozone protects against UV radiation?
Ozone absorbs UV light, splitting into O2 and O, converting harmful UV light into heat.
Fill in the blank: The major product of radical bromination of cyclohexene is an _______.
[allylic bromide]
True or False: Allylic bromination can occur without the presence of UV light.
False
What is the relationship between the concentration of CFCs and the ozone layer over Antarctica?
The concentration of ozone has dropped as low as 33% of its 1975 value over Antarctica due to CFCs.
What type of energy transformation occurs as ozone absorbs UV light?
Ordered energy (light) is converted into disordered energy (heat).
What are the two possible outcomes when allylic bromination occurs?
The products can be a racemic mixture or a pair of diastereomers.
What happens during the second propagation step in the mechanism of ozone destruction by CFCs?
A chlorine radical is regenerated, continuing the chain reaction.
What happens to ozone molecules in the stratosphere during the ozone destruction process?
Ozone molecules are destroyed by chlorine radicals formed from CFCs.
What role does entropy play in the process of ozone absorbing UV light?
Entropy drives the conversion of light into heat, increasing disorder in energy.
What is the role of chlorine radicals in ozone depletion?
Chlorine radicals regenerate in the propagation step and can destroy thousands of ozone molecules
Chlorine radicals are produced from chlorofluorocarbons (CFCs) in the atmosphere.
What global treaty was established to address ozone depletion?
Montreal Protocol
It was agreed upon in the late 1980s and has undergone eight revisions.
What has been the trend in CFC concentration in the atmosphere since 1996?
Slowly decreasing
This decrease is a result of the Montreal Protocol regulations.
What is the estimated time for ozone levels in Antarctica to return to 1980 levels?
Five decades
This is due to the long life of chlorine radicals in the upper atmosphere.
What compounds have largely replaced CFCs?
Hydrofluoroalkanes (HFAs)
Also known as hydrofluorocarbons (HFCs).
What are two examples of HFAs?
- HFA-134a (1,1,1,2-Tetrafluoroethane)
- HFA-227 (1,1,1,2,3,3,3-Heptafluoropropane)
HFAs have C−F and C−H bonds but lack C−Cl bonds.
Why are HFAs considered less damaging to the ozone layer than CFCs?
C−F bonds are less susceptible to cleavage by UV radiation than C−Cl bonds
This makes HFAs less likely to produce chlorine radicals.
What are the disadvantages of HFAs?
- Powerful greenhouse gases
- Increasing use by 15% per year
- Higher cost than CFC inhalers
HFAs can contribute to global warming.
What is required to extinguish a fire?
At least one of the three essential ingredients: fuel, oxygen, or heat
Alternatively, stopping the radical chain reaction can also extinguish a fire.
What are halons, and why are they effective as firefighting agents?
Halons are organic compounds containing halogen atoms that speed up termination steps in fire suppression
They were heavily used but contribute to ozone depletion.
What is autooxidation?
A slow oxidation process of organic compounds in the presence of atmospheric oxygen
It typically results in the formation of hydroperoxides.
What initiates the autooxidation process in organic compounds?
Hydrogen abstraction
This step forms a carbon radical.
What role do antioxidants play in food preservation?
They act as radical inhibitors to slow the autooxidation process
Common antioxidants include BHT and BHA.
What is the mechanism by which antioxidants prevent autooxidation?
They react with radicals to generate resonance-stabilized radicals
This process prevents the chain reaction of autooxidation.
What happens when a person overdoses on acetaminophen?
It can lead to a temporary depletion of glutathione, uncontrolled free radicals, and liver damage
Early intervention with N-acetylcysteine can prevent irreversible damage.
What are the natural antioxidants mentioned in the text?
- Vitamin E
- Vitamin C
They help protect cell membranes from oxidation.
How does vitamin E function as an antioxidant?
By undergoing hydrogen abstraction to destroy reactive radicals
Its hydrophobic nature allows it to reach the hydrophobic regions of cell membranes.
What is the structure of a triglyceride?
Contains three long hydrocarbon chains
These chains often include double bonds that are susceptible to autooxidation.
What is the primary function of glutathione in the body?
It acts as a radical scavenger
It is produced in the liver from cysteine, glycine, and glutamic acid.
What is the consequence of uncontrolled free radicals in the body?
They can damage DNA and enzymes, leading to cell death
Controlled radical reactions are essential for various bodily functions.
What are two examples of halons used as firefighting agents?
- Halon 1211 (Freon 12B1)
- Halon 1301 (Freon 13B1)
Halons were effective but are now banned due to their contribution to ozone depletion.
What connection has been suggested between the aging process and natural oxidation processes?
A connection between aging and natural oxidation processes has been suggested, leading to the widespread use of antioxidant products like vitamin E despite a lack of evidence for their effect on aging rates.
Antioxidants are thought to combat oxidative stress, which may contribute to aging.
What is the difference between Markovnikov and anti-Markovnikov addition?
Markovnikov addition installs a halogen at the more substituted position, while anti-Markovnikov addition installs the halogen at the less substituted position.
This distinction arises from the stability of intermediates formed during the reactions.
What role do impure reagents play in the addition of HBr?
Impure reagents can cause the reaction to proceed via anti-Markovnikov addition, placing the halogen at the less substituted position.
This is particularly influenced by the presence of trace amounts of alkyl peroxides.
What is a bromine radical and how is it formed in the radical addition mechanism?
A bromine radical is formed through hydrogen abstraction involving an alkoxy radical and HBr in the initiation step of the radical addition mechanism.
The formation of bromine radicals is crucial for the propagation steps in the anti-Markovnikov addition.
What determines the regiochemistry in radical addition of HBr?
The regiochemistry is determined by the stability of the radical intermediates, favoring the formation of more stable tertiary radicals.
This is similar to how carbocation stability determines regiochemistry in ionic additions.
True or False: Anti-Markovnikov addition can be accomplished with HCl and HI.
False.
Anti-Markovnikov addition can only be accomplished with HBr due to thermodynamic considerations.
What is the significance of ΔG in the context of radical addition reactions?
ΔG must be negative for a reaction to be spontaneous, and it is influenced by both enthalpy and entropy terms.
The enthalpy and entropy contributions can vary based on the specific halogen used in the radical addition.
What is the expected stereochemistry when a new chiral center is formed in radical addition?
The reaction produces a racemic mixture of enantiomers due to the possibility of attack from either face of the alkene.
This occurs because the first propagation step can approach the alkene from either side.
Fill in the blank: The polymerization of ethylene to form polyethylene occurs via a _______ mechanism.
[radical]
Radical polymerization involves a series of initiation, propagation, and termination steps.
What is the effect of chain branching in polymerization?
Chain branching influences the physical properties of the resulting polymer, affecting flexibility and strength.
Different degrees of branching can lead to variations in applications, such as in squeeze bottles versus caps.
What common polymers are formed from substituted ethylenes?
Common polymers include:
* Polyvinyl chloride (PVC)
* Polypropylene
* Polystyrene
* Teflon
Each polymer has distinct applications based on its properties.
What is hydrocracking in the petrochemical industry?
Hydrocracking is a radical process that converts large alkanes into smaller alkanes suitable for gasoline production, performed in the presence of hydrogen gas.
It is a key process in refining and improving fuel quality.
What is the difference in selectivity between radical bromination and chlorination?
Bromination is slower but more selective than chlorination in radical reactions.
This selectivity is important when synthesizing compounds with multiple types of hydrogen.
What are the two thermodynamically favorable processes for halogenation?
Radical chlorination and radical bromination
Both processes are used in synthesis.
Which halogenation method is more selective?
Bromination
Bromination is slower but avoids a mixture of products.
What is the best method to use when different types of hydrogen atoms are present in a compound?
Bromination
It is more selective than chlorination.
What is the main utility of radical bromination in synthesis?
Introducing a functional group into an alkane
This opens the door for a wide variety of reactions.
What are the six different kinds of steps in radical mechanisms?
- Homolytic cleavage
- Addition to a π bond
- Hydrogen abstraction
- Halogen abstraction
- Elimination
- Coupling
What is a radical initiator?
A compound with a weak bond that readily undergoes homolytic bond cleavage
Examples include alkyl peroxides and acyl peroxides.
What is a radical inhibitor also called?
Radical scavenger
It prevents a chain process from starting or continuing.
Which radical reaction has practical use in the laboratory?
Radical chlorination and radical bromination
Bromination is generally a slower process.
What occurs more readily at substituted positions during halogenation?
Halogenation
Bromination is more selective than chlorination.
What happens when a new chiral center is created during a radical halogenation process?
Both possible stereoisomers are obtained
A racemic mixture is formed at chiral centers.
What is allylic bromination?
Bromination that occurs at the allylic position
N-bromo-succinimide (NBS) can be used to avoid competing ionic addition reactions.
What is autooxidation?
The oxidation of organic compounds in the presence of atmospheric oxygen to produce hydroperoxides
It is believed to proceed via a radical mechanism.
What are natural antioxidants that prevent oxidation in cell membranes?
- Vitamin E
- Vitamin C
What is produced when alkenes react with HBr in the presence of peroxides?
A radical addition reaction
What is the result of polymerizing ethylene via a radical process?
Chain branching
Polyvinyl chloride (PVC) is obtained when vinyl chloride is polymerized.
What radical processes are heavily used in the chemical industry?
- Cracking
- Reforming
- Hydrocracking (when performed in the presence of hydrogen)
When can chlorination be used in radical halogenation?
When the starting compound has only one kind of hydrogen atom
What is the first step in drawing resonance structures of radicals?
Look for an unpaired electron next to a π bond
What indicates which bond is the weakest in a compound?
The most stable radical
What are the characteristic patterns to recognize in radical processes?
- Homolytic cleavage
- Addition to a π bond
- Hydrogen abstraction
- Halogen abstraction
- Elimination
- Coupling
What initiates a radical process in halogenation?
Homolytic bond cleavage creates radicals
In predicting the selectivity of radical bromination, where does bromination occur?
At the location of the most stable radical
What happens to the configuration when bromination occurs at a chiral center?
A loss of configuration occurs, yielding both possible stereoisomers
What is the expected regioselectivity when HBr reacts with alkenes?
Br is installed at the less substituted position
