A Fourth Alkene Addition Pattern - Free Radical Addition (2024)

Free Radical Addition Of HBr To Alkenes With ROOR (Peroxides)

We’ve seen that there are three major alkene reactivity patterns [carbocation, three membered ring, and concerted], but there are two minor pathways as well. This post discusses one of them: free-radical addition of HBr to alkenes, which shows the opposite regioselectivity (anti-Markovnikov) than “normal” addition of HBr to alkenes (Markovnikov) which follows the “carbocation” pathway.

Table of Contents

  1. Free Radical Addition Of HBr To Alkenes Leads To “Anti-Markovnikov” Products
  2. An Outline Of The Free Radical Mechanism For Addition Of HBr To Alkenes In The Presence Of Peroxides
  3. Initiation Of The Free-Radical Process Through hom*olytic Cleavage Of Peroxides By Heat Or Light
  4. Formation Of The Bromine Radical From The Alkoxide Radical And HBr
  5. Propagation Step #1 : Addition Of Bromine Radical To The Alkene Occurs So As To Give The Most Stable Carbon Radical
  6. Propagation Step #2: The Resulting Carbon Radical Removes A Hydrogen Atom From H–Br, Regenerating The Bromine Radical
  7. The Termination Step
  8. Summary: Free-Radical Addition Of HBr To Alkenes
  9. Notes
  10. (Advanced) References and Further Reading

1. Free Radical Addition Of HBr To Alkenes Leads To “Anti-Markovnikov” Products

As discussed previously, alkenes normally react with HBr to give products of “Markovnikov” addition; the bromine ends up on the most substituted carbon of the alkene, and the hydrogen ends up on the least substituted carbon. However, something interesting happens when the same reaction is performed in the presence of peroxides and heat / light: the pattern of addition changes!

Instead of Br ending up on themostsubstituted carbon of the alkene, it ends up on the least. [The stereochemistry of the reaction, however, is unchanged: it still gives a mixture of “syn” and “anti” products.]

A Fourth Alkene Addition Pattern - Free Radical Addition (1)

This so-called “anti-Markovnikov” addition is intriguing. What difference could the presence of peroxides, and furthermore heat (or light) make to this reaction?

2. An Outline Of The Free Radical Mechanism For Addition Of HBr To Alkenes In The Presence Of ROOR (Peroxides)

This reaction occurs through a free-radical process. (For a primer on free radical chemistry, you might want to check out this introductory article on Free Radical Reactions). Here is an outline of the mechanism:

  • Peroxides contain a weak oxygen-oxygen bond [approximately 35 kcal/mol; compare to C-H at approx 100 kcal/mol]
  • Heating leads tohom*olytic fragmentation of this bond – that is, the bond breaks such as to leave one unpaired electron on each atom. Strong sources of light [e.g. a floodlight or other source of light radiation which reaches into the near UV] can also serve to sever this bond.
  • The resulting highly reactive alkoxy radical can then abstract a hydrogen from H-Br, giving a bromine radical. The bromine radical is the species that adds to the alkene.
  • Addition to the alkene will preferably occur in such a way that themost stablefree radical is formed [in the case above, the tertiary radical]. That’s why bromine ends up on the least substituted carbon of the alkene. (See: 3 Factors Which Stabilize Free Radicals)
  • This tertiary radical then removes hydrogen from H-Br, liberating a bromine radical, and the cycle continues.

3. Initiation Of The Free-Radical Process Through hom*olytic Cleavage Of ROOR (Peroxides) By Heat Or Light

Only a trace [catalytic] amount of peroxide is required to get the reaction started, although of course at least one molar equivalent of HBr is required to result in full addition of HBr to the alkene.

In the first step, addition of energy (in the form of heat or light) leads to hom*olytic fragmentation of the weak O–O bond to generate two new free radicals. “hom*olytic” means that the bond is broken such that each atom receives the same (“hom*os” = Greek for “same”) number of electrons.

(Most of the bond breakage we see in organic chemistry is heterolytic, where the bond breaks unequally. )

A Fourth Alkene Addition Pattern - Free Radical Addition (2)

The singly barbed arrows depict the movement of single electrons; two alkoxy radicals are formed. Since there is a net increase in the number of radicals (0 →2) this is an initiation step.

Common “peroxides” for this purpose are t-butyl peroxide or benzoyl peroxide. [Note 1]. Alternatively other free-radical “initiators” such as AIBN can also be used.

Only a catalytic amount of peroxides are used to initate this reaction (typically 10-20 mole %, although more can be used, especially when added batchwise).

4. Formation Of The Bromine Radical From The Alkoxide Radical And HBr

In the next step, one of the oxygen radicals from step 1 removes a hydrogen from H–Br in another hom*olytic process.

A Fourth Alkene Addition Pattern - Free Radical Addition (3)

Here, we’re forming an H–O bond (bond dissociation energy of 102 kcal/mol for H–O in CH3OH) and breaking an H–Br bond (bond dissociation energy of 87 kcal/mol) , so a difference in energy of about 15 kcal/mol makes this process essentially irreversible.

(Note: since this process does not change the number of free radicals, it is technically a propagation step)

5. Propagation Step #1 : Addition Of Bromine Radical To The Alkene Occurs So As To Give The Most Stable Carbon Radical

Once formed, the bromine radical can then add to the alkene.

In a relatively “flat” alkene such as 1-methylcyclohexene, addition of the radical will occur with equal probability from either face.

The question is,whichatom of the double bond does the free radical attack? The bond could break two different ways, after all.

  • Attack of the bromine radical on the more substituted carbon would result in a new free radical on a secondary carbon.
  • Attack of the bromine radical on the less substituted carbon would result in a new free radical on atertiarycarbon.

Free radicals are electron-deficient species and are stabilized by adjacent electron donors. The more stable free radical intermediate is thetertiaryfree radical, and that is why addition occurs predominantly at the less substituted carbon (i.e. the carbon attached to the fewest number of carbons).

This explains the “anti-Markovnikov” selectivity of the reaction.

A Fourth Alkene Addition Pattern - Free Radical Addition (4)

6. Propagation Step #2: The Resulting Carbon Radical Removes A Hydrogen Atom From H–Br, Regenerating The Bromine Radical

In a second propagation step in the main sequence, the resulting carbon radical removes a hydrogen from another equivalent of H–Br, giving the final addition product.

Alkyl free radicals are sp2-hybridized, and are shallow pyramids that invert easily.

H–Br, therefore, can react on either face of the free radical [Note 2]. If it attacks on the same face as the Br, then we obtain a “syn” product. If it attacks on the opposite face of the Br, then the product is“anti“.

A mixture of both will be obtained. The reaction is not stereoselective.

A Fourth Alkene Addition Pattern - Free Radical Addition (5)

A bromine radical is generated by this process, which can then add to another equivalent of alkene (propagation step #1).

7. The Termination Step

When the concentration of HBr and alkene become low relative to the concentration of free radical, termination can occur (See post: Initiation, Propagation, Termination). This could occur through a variety of specific pathways (not shown) involving recombination of two free radicals to generate a new bond.

8. Summary: Free-Radical Addition Of HBr To Alkenes

This reaction pathway is most commonly observed (in Org 1 and Org 2, anyway) for addition of HBr, although a rich chemistry of radical addition reactions to alkenes exists (particularly for organostannanes).

NEXT POST: Ozonolysis of Alkenes

Notes

Related Articles

  • Alkene Reactions: Ozonolysis
  • Initiation, Propagation, Termination
  • Free Radical Initiation: Why Is “Light” Or “Heat” Required?
  • Alkene Addition Pattern #2: The “Three-Membered Ring” Pathway
  • Addition Pattern #1: The “Carbocation Pathway”
  • Alkene Addition Pattern #3: The “Concerted” Pathway
  • 3 Factors That Stabilize Free Radicals
  • Alkene Hydrohalogenation Mechanism And How It Explains Markovnikov’s Rule
  • Free Radicals Practice Quizzes
  • Alkene Reactions Practice Problems (MOC Membership)
  • Alkene Addition Reactions: “Regioselectivity” and “Stereoselectivity” (Syn/Anti)
  • Stereoselective and Stereospecific Reactions

Note 1. Benzoyl peroxide enjoys a common household use as an acne cleanser, and even makes an appearance in this classic ad for Oxy skin care (“Oxycute ‘Em”.)

Note 2. The geometry of free radical carbons is that of a shallow pyramid with a low barrier for inversion, allowing for reactivity on either face. The exception is in weird cases where inversion would be highly disfavored, such as on a bridgehead (See Bicyclic Molecules and How To Name Them).

(Advanced) References and Further Reading

Bond Dissociation Energies From Lowry & Richardson, “Mechanism and Theory In Organic Chemistry“, Harper & Row, 1987 pp 161-162

  1. The Peroxide Effect in the Addition of Reagents to Unsaturated Compounds and in Rearrangement Reactions.
    Frank R. Mayo and Cheves Walling
    Chemical Reviews 1940, 27 (2), 351-412
    DOI:
    10.1021/cr60087a003
    F. R. Mayo was a student of the prominent chemist M. S. Kharasch, and together they first described the “peroxide effect” in the anti-Markovnikov addition of HBr to alkenes, ascribing it to a free-radical mechanism.
  2. ADDITION OF HYDROGEN BROMIDE TO 4,4-DIMETHYLPENTENE-1
    M. S. Kharasch, Chester Hannum, and M. Gladstone
    Journal of the American Chemical Society 1934, 56 (1), 244-244
    DOI: 10.1021/ja01316a504
    This communication describes both products that are obtained from HBr addition to the title olefin via the electrophilic and radical mechanisms.
  3. THE PHOTO-ADDITION OF HYDROGEN BROMIDE TO OLEFINIC BONDS
    WILLIAM E. VAUGHAN, FREDERICK F. RUST, and THEODORE W. EVANS
    The Journal of Organic Chemistry 1942, 07 (6), 477-490
    DOI: 10.1021/jo01200a005
    The radical addition of HBr can be initiated not just by peroxides, but also by light, as this paper describes.
  4. The Peroxide Effect in the Addition of Reagents to Unsaturated Substances. XXII. The Addition of Hydrogen Bromide to Trimethylethylene, Styrene, Crotonic Acid, and Ethyl Crotonate
    Cheves Walling, M. S. Kharasch, and F. R. Mayo
    Journal of the American Chemical Society 1939, 61 (10), 2693-2696
    DOI: 10.1021/ja01265a034
    Kharasch and co-workers reported the hydrobromination of styrene in dilute pentane solution with dibenzoyl peroxide to give an 80 : 20 ratio in favor of the primary bromide. Unfortunately, detailed conditions were not provided.
  5. Scalable anti-Markovnikov hydrobromination of aliphatic and aromatic olefins
    Marzia Galli, Catherine J. Fletcher, Marc del Pozo, and Stephen M. Goldup
    Org. Biomol. Chem., 2016, 14, 5622-5626
    DOI: 10.1039/C6OB00692B
    This is an interesting paper demonstrating the relevance of this chemistry in modern organic synthesis; it describes the rediscovery of simple scalable conditions for synthesis of primary bromides under “initiator free” conditions from alkyl and aryl alkenes.
A Fourth Alkene Addition Pattern - Free Radical Addition (2024)

FAQs

What is the free radical addition of an alkene? ›

The best known and most important use of free radical addition to alkenes is probably polymerization. Since the addition of carbon radicals to double bonds is energetically favorable, concentrated solutions of alkenes are prone to radical-initiated polymerization, as illustrated for propene by the following equation.

Is 1/4 addition reversible? ›

Conjugate addition can happen with a variety of nucleophiles such as enolates, organocuprates, thiols, amines, enamines and more. When both 1,2- and 1,4- addition are reversible, equilibrium tends to favor formation of the 1,4-product due to breakage of the weaker C-C (pi) bond.

What is the 1 4 addition reaction? ›

In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor (an enolate or other nucleophile) and a Michael acceptor (usually an α,β-unsaturated carbonyl) to produce a Michael adduct by creating a carbon-carbon bond at the acceptor's β-carbon.

Why is the 1/4-product more stable? ›

1) The 1,4- product is more thermodynamically stable because there are two alkyl groups on each side of the double bond and more substituted alkenes are more stable. Even though the cation would prefer to be in a secondary position in the transition state, the final product is less stable with a terminal alkene.

What are the steps of free radical addition? ›

In a free-radical addition, there are two chain propagation steps. In one, the adding radical attaches to a multiply-bonded precursor to give a radical with lesser bond order. In the other, the newly-formed radical product abstracts another substituent from the adding reagent to regenerate the adding radical.

What rule does a free radical addition reaction obey? ›

Markovnikov's Rule, also known as Markownikoff's rule, can be used to describe the outcome of some chemical addition reactions. The Russian chemist Vladimir Vasilyevich Markovnikov first formulated this rule in 1865.

What is the difference between 1 2 addition and 1,4-addition? ›

The 1,2-addition has a smaller activation energy than 1,4-addition - it occurs faster than 1,4 addition, because the bromide nucleophile is closer to carbon #2 then to carbon #4. However, the 1,4-product is more stable than the 1,2-product.

Which is more stable, 1/2 addition or 1/4 addition? ›

However, you might recall from Zaitsev's rule that the thermodynamic stability of alkenes increases as C-H bonds on the alkene are swapped for C-C bonds. Hence the 1,4 product (a di-substituted alkene) is more thermodynamically stable than the 1,2-product (a mono-substituted alkene).

Why is it called 1,4-addition? ›

The “1,4 Addition” Product Of Acids Adding To Butadiene

A new C-H single bond has formed on one end of the diene (C-1), and C-Br formed on the other end (C-4). Note that the C1-C2 and C3-C4 pi bonds are broken, and we've formed a new pi bond between C2 and C3. We call this “1,4 addition”.

What is the 1,4-addition also called? ›

Such a nucleophilic addition is called a nucleophilic conjugate addition or 1,4-nucleophilic addition. The most important active alkenes are the aforementioned conjugated carbonyls and acrylonitriles. Nucleophilic conjugate addition.

What are the 4 types of addition reactions? ›

There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition. Two non-polar addition reactions exist as well, called free-radical addition and cycloadditions. Addition reactions are also encountered in polymerizations and called addition polymerization.

Is thermodynamic 1/4 addition? ›

The 1,2 adduct is the kinetic product because it forms faster, but the 1,4 adduct is the thermodynamic product because it is more stable.

What is the mechanism of 1 4 conjugate addition? ›

Basic Reaction of 1,4 Conjugate Addition

In 1,4 addition, a nucleophile is added to the carbon β to the carbonyl while a hydrogen is added to the carbon α to the carbonyl. Overall, the carbonyl is unaffected by the nucleophilic addition.

What is the Markovnikov rule? ›

Markovnikov Rule predicts the regiochemistry of HX addition to unsymmetrically substituted alkenes. The halide component of HX bonds preferentially at the more highly substituted carbon, whereas the hydrogen prefers the carbon which already contains more hydrogens.

What is the free radical reaction for alkanes? ›

Free-radical Substitution of Alkanes with Halogens

In a substitution reaction, a reacting species is substituted (swapped) for a bonded species in a compound. Alkanes can react with halogens in the presence of ultraviolet (UV) light. A halogen is substituted for a hydrogen atom in the alkane to form a halogenoalkane.

What is the radical addition of Br2 to alkenes? ›

The addition of bromine on a double bond occurs without light. So the molecule Br2 is not broken into Bromine atoms. But it reacts with the alkene R1R2C=CR3R4, producing a triangular cation C2R1R2R3R4Br+. And this cation reacts later with the remaining Bromide ion to produce the saturated compound C2R1R2R3R4Br2.

Do alkenes have free radical substitution? ›

Alkanes undergo only substitution reactions but alkenes and alkynes undergo both substitution and addition reactions.

What is the free radical polymerization of ethene? ›

A Free Radical Addition Reaction

You will remember that during the polymeriation of ethene, thousands of ethene molecules join together to make poly(ethene) - commonly called polythene. The reaction is done at high pressures in the presence of a trace of oxygen as an initiator.

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