Alkanes radical bromination

Answer In this problem, we are starting with an alkane. There are no leaving groups, so we cannot do a substitution or an elimination reaction. There are also no double bonds, so we cannot do an addition. It seems that we are stuck, with nothing to do. Clearly, our only way out of this situation is to introduce a functional group into the compound, via radical bromination. Radical bromination will place a Br at the most substituted position (the tertiary position), and then we can eliminate ... 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

Bromnation of alkanes. The combination of bromine and mercuric oxide is effective for free-radical bromination of alkanes ... 

To understand why both enantiomers are formed, we must look at th steps of the radical substitution reaction. In the first propagation step th ical removes a hydrogen atom from the alkane, creating a radical bromine rad-... 

SOLUTION We know that the first reaction has to be a radical halogenation because that is the only reaction that an alkane undergoes. Bromination will lead to a greater yield of the desired 2-halo-substituted compound than will chlorination because the bromine radical is more selective than a chlorine radical. To maximize the yield of substitution product (Section 11.8), the alkyl bromide is treated with acetate ion and the ester is then hydrolyzed to the alcohol, which forms the target compound when it is oxidized. 

Recently the relative reactivity of different alkanes in bromination and oxidation reactions was studied under Gif-conditions and compared with the corresponding radical chain processes (competitive oxidation of pairs of alkanes in CBrCl3 at reflux under Ar, initiated by dibenzoyl peroxide). The following hydrocarbons were studied and the data obtained (normalized per one C-H bond) is given below ... 

When this type of thermodynamic analysis is appHed to alkanes other than methane, similar results are obtained. For example, ethane will undergo both radical chlorination and radical bromination ... 

LEARN the skill Predict the stereochemical outcome of radical bromination of the following alkane ... 

Until now, we have focused on reactions of alkanes. Now let s consider the radical halogenation of alkenes. For example, consider what outcome you might expect when cyclohexene undergoes radical bromination. Begin by comparing all C—H bonds to identify the bond that is most easily broken. Specifically, compare the BDE for each type of C—H bond in cyclohexene. 

In truth, even radical bromination has very Hmited utility in synthesis. Its greatest utihty is to serve as a method for introducing a functional group into an alkane. When the starting material is an alkane, there is very Htde that can be done other than radical halogenation. By introducing a functional group into the compound, the door is opened for a wide variety of reactions ... 

Write a mechanism for the radical bromination of the hydrocarbon benzene, CgHg (for structure, see Section 2-4). Use propagation steps similar to those in the halogenation of alkanes, as presented in Sections 3-4 through 3-6. Calculate AH° values for each step and for the reaction as a whole. How does this reaction compare thermodynamically with the bromination of other hydrocarbons Data DH° (CgHs-H) = 112 kcal mol DH° (C Hs-Br) = 81 kcal mol . Note the Caution in Exercise 3-5. 

Br2, V Radical bromination Under these conditions, an alkane undergoes bromination, with installation of the Br at the most substituted position. 

Now let s work forward from the starting material. The starting material is an alkane (no functional group), so we must first install a functional group. Radical bromination will selectively install a bromine atom at a tertiary position ... 

This alkene can be made from the starting alkane in two steps (radical bromination, followed by elimination), giving the following synthesis ... 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

NBS can also be used to brominate alkanes. For example, cyclopropane, cyclopentane, and cyclohexane give the corresponding bromides when irradiated in a solution of NBS in dichloromethane. Under these conditions, the succinimidyl radical appears to be involved as the hydrogen-abstracting intermediate ... 

Of the reactions that involve carbon radicals, the most familiar- are the chlorination and bromination of alkanes (Sections 4.14 through 4.18) ... 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

The figures for fluorination reflect the weakness of the F—F [150kJ (36 kcal) mol ],and the strength of the H—F [560 kJ(134 kcal) mol" ], bonds. Fluorination normally requires no specific initiation (cf. p.324), and is explosive unless carried out at high dilution. That fluorination does proceed by a radical pathway, despite not requiring specific initiation, is demonstrated by the fact that chlorination may be initiated in the dark, and at room temperature, by the addition of small traces of F2. Bromination is a good deal slower than chlorination, under comparable conditions, as step (1)—H-abstraction by Br—is commonly endothermic. This step is usually so endothermic for I that direct iodination of alkanes does not normally take place. 

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