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Allylic and benzylic bromination[edit] Standard conditions for using NBS in allylic and/or benzylic bromination involves refluxing a solution of NBS in anhydrous CCl4 with a radical initiator—usually azobisisobutyronitrile (AIBN) or benzoyl peroxide, irradiation, or both to effect radical initiation.[5][6] The allylic and benzylic radical intermediates formed during this reaction are more stable than other carbon radicals and the major products are allylic and benzylic bromides. This is also called the Wohl–Ziegler reaction.[7][8]

The carbon tetrachloride must be maintained anhydrous throughout the reaction, as the presence of water may likely hydrolyze the desired product.[9] Barium carbonate is often added to maintain anhydrous and acid-free conditions. In the above reaction, while a mixture of isomeric allylic bromide products are possible, only one is created due to the greater stability of the 4-position radical over the methyl-centered radical. Chapter 11 : Arenes and Aromaticity Benzylic systems 

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As we saw in chapter 10, the positions adjacent to C=C in alkenes is referred to as the allylic position. They often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent π system. Similarly, the positions adjacent to a benzene ring, known as the benzylic position also show enhanced reactivity compared to simple alkanes.

Students often confuse the term benzyl with phenyl, for example compare the location of the bromine atoms in bromobenzene and benzyl bromide:

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Bromobenzene (phenyl bromide) Benzylic carbocations

Benzyl bromide

The p system of a benzene ring can stabilise an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilising effect. Note that in the resonance forms of the benzylic cation, the positive charge is located on the ortho and para positions of the benzene ring, but not the meta positions. This is reflected in the resonance hybrid. Due to the stability of these benzylic cations, they are readily formed as intermediates during chemical reactions, for example SN1 reactions of benzylic halides. Note that 2-chloro-2phenylpropane is 600 times more reactive that the 2-methyl analogue. Benzylic radicals Benzyl radicals can also be stabilised by resonance in the same manner as shown above for carbocations.

© Dr. Ian Hunt, Department of Chemistry

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Chapter 15. Reactions of Free Radicals and Radical Ions

15.2: Allylic and Benzylic Halogenation 

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When halogens are in the presence of unsaturated molecules such as alkenes, the expected reaction is addition to the double bond carbons resulting in a vicinal dihalide (halogens on adjacent carbons). However, when the halogen concentration is low enough, alkenes containing allylic hydrogens undergo substitution at the allylic position rather than addition at the double bond. The product is an allylic halide (halogen on carbon next to double bond carbons), which is acquired through aradical chain mechanism.

Why Substitution of Allylic Hydrogens? As the table below shows, the dissociation energy for the allylic C-H bond is lower than the dissociation energies for the C-H bonds at the vinylic and alkylic positions. This is because the radical formed when the allylic hydrogen is removed is resonance-stabilized. Hence, given that the halogen concentration is low, substitution at the allylic position is favored over competing reactions. However, when the halogen concentration is high, addition at the double bond is favored because a polar reaction outcompetes the radical chain reaction.

Radical Allylic Bromination (Wohl-Ziegler Reaction) Preparation of Bromine (low concentration) NBS (N-bromosuccinimide) is the most commonly used reagent to produce low concentrations of bromine. When suspended in tetrachloride (CCl4), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction.

Allylic Bromination Mechanism Step 1: Initiation Once the pre-initiation step involving NBS produces small quantities of Br2, the bromine molecules are homolytically cleaved by light to produce bromine radicals.

Step 2: Propagation One bromine radical produced by homolytic cleavage in the initiation step removes an allylic hydrogen of the alkene molecule. A radical intermediate is generated, which is stabilized by resonance. The stability provided by delocalization of the radical in the alkene intermediate is the reason that substitution at the allylic position is favored over competing reactions such as addition at the double bond.

The intermediate radical then reacts with a Br2 molecule to generate the allylic bromide product and regenerate the bromine radical, which continues the radical chain mechanism. If the alkene reactant is asymmetric, two distinct product isomers are formed.

Step 3: Termination The radical chain mechanism of allylic bromination can be terminated by any of the possible steps shown below.

Radical Allylic Chlorination Like bromination, chlorination at the allylic position of an alkene is achieved when low concentrations of Cl2 are present. The reaction is run at high temperatures to achieve the desired results. Industrial Uses Allylic chlorination has important practical applications in industry. Since chlorine is inexpensive, allylic chlorinations of alkenes have been used in the industrial production of valuable products. For example, 3-chloropropene, which is necessary for the synthesis of products such as epoxy resin, is acquired through radical allylic chlorination (shown below).

Problems (Answers are attached as a file) 1. Cyclooctene undergoes radical allylic bromination. Write out the complete mechanism including reactants, intermediates and products. 2. Predict the two products of the allylic chlorination reaction of 1-heptene. 3. What conditions are required for allylic halogenation to occur? Why does this reaction outcompete other possible reactions such as addition when these conditions are met? 4. Predict the product of the allylic bromination reaction of 2-benzylheptane. (Hint: How are benzylic hydrogens similar to allylic hydrogens?) 5. The reactant 5-isopropyl-1-hexene generates the products 3-bromo-5-isopropyl-1-hexene and 1-bromo5-isopropyl-2-hexene. What reagents were used in this reaction?

References 1. Djerassi, Carl. "Brominations with N-Bromosuccinimide and Related Compounds - The Wohl-Ziegler Reaction." Chemical Reviews 43 (1948):271-314. 2. Easton, Christopher J., Alison J. Edwards, Stephen B. McNabb, Martin C. Merrett, Jenny L. O'Connell, Gregory W. Simpson, Jamie S. Simpson, and Anthony C. Willis. "Allylic halogenation of unsaturated amino acids." Organic and Biomolecular Chemistry (2003). RSC Publishing. 9 June 2003. Royal Society of Chemistry. 25 Feb. 2009. 3. Kent, Doug. Allylic Bromination. Chem 118B Workshop. Learning Skills Center. 3 Feb. 2009. 4. Li, Chao-Jun, and Tak-Hang Chan. Comprehensive Organic Reactions in Aqueous Media. New York: Wiley-Interscience, 2007.

5. Vollhardt, Peter C., and Neil E. Schore. Organic Chemistry: Structure and Function. 5th ed. New York: W.H. Freeman and Company, 2007.

Outside Links   

http://en.wikipedia.org/wiki/N-Bromosuccinimide#Preparation http://en.wikipedia.org/wiki/Wohl-Ziegler_reaction http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ref/ch10allylic.html

Further Reading Carey 5th Ed Online Radical halogenation of Allylic Systems Chemtube3D Allylic bromination MasterOrganicChemistry Allylic bromination  o

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15.1: Free Radical Halogenation of Alkanes o

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15.3 Radical Addition to Pi Systems Unless otherwise noted, the StatWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at [email protected].

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SN Reactivity of Benzyl and Allyl Systems Benzylic and allylic halides undergo both SN1 and SN2 reactions more readily than purely alkyl halides. The reason is the same in both cases: stabilization by the -orbitals of the double bond or benzene ring. In the SN1 reaction the carbocation empty p orbital interacts:

In the SN2, it is the p orbital of the transition state.