Report a) ike. NEW ENG OF CH
•
OCIATION EACHERS
CHEMISTRY IN SOLUTION
FREE RADICAL
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ROY J. GRITTER University of Connecticut, Storrs
1 hree reactive intermediates which have a valence of three for carbon are known: the earbonium ion, where the carbon undergoing reaction has only six electrons and a formal positive charge; the free radical, in which the reactive carbon has seven electrons (one unpaired and thus “free”) and is neutral; and the carbanion, where the reactive carbon has eight electrons and a formal negative charge. These reactive intermediates have certain similarities and dissimilarities, and some of these will be illustrated by a comparison of the free radical and the ionic intermediates. The earbonium ions and free radicals are similar in that the relative stability sequence for each is: tertiary > secondary > primary. The aliphatic carbanion stability is just the opposite: primary > secondary > tertiary. However, when an aromatic ring is attached to the reactive carbon, the relative stability sequence for all three intermediates is tertiary > secondary > primary. GENERAL CHARACTERISTICS OF FREE RADICAL REACTIONS IN SOLUTION
Other comparisons can be made between the ionic and neutral intermediates when the general characteristics of all free radical reactions in solution are considered. These five characteristics and subsequent details on free radical reactions will be presented in outline form and can be illustrated with the following reaction scheme. This shows the (erf-butyl peroxide catalyzed free radical addition of methyl malonate to 1-octene. (CH3)3CO—OC(CH,)3
2(CH3)sCO-
(CH3)3CO- + CH2(C02CHs), (CH3)3COH + •CHfCOsCHsk
(1)
—
(2a)
(I) or
(CH3),CO- + 0Ha(C02CH3)2 — CH, + CH3COCHs +
•
CH(C02CHs)s
(26)
(I) CH(C02CHj)2 + CcH13CH=CH2
-*
CbH13CH— CHs—CH(C02CH3)s
(3)
_(II)_ Presented at the joint meeting of the New England Association of Chemistry Teachers and the Connecticut Valley Section of the American Chemical Society, University of Connecticut, Storrs, Connecticut, April 12, 1958. 1
VOLUME 35, NO. 9, SEPTEMBER, 1958
C6H,3CH— CH2—CH(C02CH:,)2 + CH2(C02CH3)2 — •CH(C01CH3)2 + Cf.H.sCHs CH2—CH(C02CHj)2 -
(4)
(HI) CeHlsCH—CH2—GH( C02CH3 )2 -f C6H13CH=CH2 CeHlsCH -CH. I
2
CH(C02CH3)a
-»
(IV)
CeHuCH—CH2—C H( COjCHs )2
(5)
(CHjOsOjCH—CH(COsCHj).. (V)
(6)
Acceleration by Known Radical Sources. The above reaction and all free radical reactions in solution are accelerated by the known sources of free radicals: peroxides and the absorption of ultraviolet light. The peroxide used above is (erf-butyl peroxide (equation 1) which is a convenient one to employ, since it is extremely stable (can be distilled at 110° at atmospheric pressure without decomposition), commerieally available, does not undergo an induced decomposition, and does not add to an olefin, A good example of acceleration with ultraviolet light is the side chain halogenation of toluene in the presence of light to produce benzyl-, benzal-, and benzotri-halides. When these two free radical catalyzed reactions are caused to react under ionic reaction conditions, methyl malonate will not add to an olefin and halogenation of toluene gives nuclear halogenation. Chain Reaction Which Is Started with a Catalytic Amount of Initiator. Free radical and ionic reactions have some similarities in this characteristic; however, it has been shown that the amount of peroxide does not have any effect on the product produced in the free radical reaction. In the reaction scheme above, the same product (III) and yield are obtained when varying amounts of peroxide are used. In many ionic reactions, especially in the cationic chain polymerization of olefins and the Friedel and Crafts reaction, the amount of catalyst can affect both the type and yield of product. Inhibition with a Compound Which Will Form a Stable Free Radical, A free radical reaction will either be completely stopped or stopped for some length of time when a compound which will form a stable free radical is added to the reaction mixture. Normally, this compound will have a negligible effect on an ionic process. This inhibition can be illustrated as follows with diphenylamine as the added compound. 475
(CHAC—O- A (C6H5)2N--H
-*
(CH3)aC—O—H A (C6HAN-
(7a)
(V) or
CH3- A (CeH6)4N—H
CH4 + (C6Hs)2N-
—
(76)
(V) or
CH(C02CH3)2 + (CbH5)sN—H — CHs(COsCH.0s A (CbH6)2N
(7c)
(V)
The structure of the intermediate diphcnylamino radical (V) is such that it is very stable and will not undergo chain transfer. Other inhibitors will undergo a similar chain transfer reaction or will form an addition compound with the intermediate free radical to form a stable free radical. Anti-Markownikoff Addition to Double Bonds. Equation (3) indicates that an intermediate free radical adds to the end carbon of a terminal double bond which places the hydrogen atom on carbon 2 (equation 4). This product is the most reasonable, since a secondary free radical is more stable than a primary. This is also exemplified in the reaction of hydrogen bromide with ally! bromide to give 1,3-dibromopropane under free radical reaction conditions: CH2=CHCH2Br + HBr
CTPBr—OTBCTPBr
-*
(8a)
and 1,2-dibromopropane under ionic reaction conditions :
CHf=CHCH5Br + HBr
CHs—CHBrCH2Br
—
(86)
Ionizing Power of the Solvent. The decomposition of ferf-butyl peroxide (equation 1) is a typical homolytic reaction, and it can be compared to Insensitivity to the
the formation of a ferf-butyl carbonium ion which is an example of a heterolytic (ionic) reaction. Evidence is given in Tables and 2. 1
TABLE
1
Decomposition of ferf-Butyl Peroxide
at 145°
(equation
Solvent
k (hr.-1)
E*(kcal./mole)
Isopropylbenzene feri-Butylbenzene
0.56 ± 0 05 0.54 ± 0.08 0.58 ± 0.07 0.41
38 38
Tributylaraine Vapor phase
TABLE
Formation
1).
39.1,36 and 38
2
of the ferf-Butyl Carbonium from tert-Butyl Bromide (CH,),C- -Br - (CHAC-1 -f Br-
Energies of formation: Solvent
Water Vapor phase Relative solvolysis rates (rate of ionization): Solvent
Ethanol 50% Ethanol-water Water Formic acid
E*(kcal./mole) 24
TYPES OF FREE RADICAL REACTIONS
To further clarify free radical chemistry in solution, the types of free radical reactions will be considered in outline form. These types of reactions are illustrated in the reaction scheme above, and reference will be made to them and other free radical reactions. Radical Forming Reactions. There are a number of methods of initiating free radical reactions in solution. The first is the thermal cleavage of covalent bonds which appears in Table 3. This illustrates the value TABLE
Bond Dissociation
D (kcal./mole)
Rond
(CHACO—OC(CHA
0—0 HO—H (CHACO
-
3
Energies to Free Radicals
38
118.2 120 110 102
H
OH:,- H CHs—CHj CH;,0 -N()2
84.2 40
of ferf-butyl peroxide as a free radical source, because its oxygen-oxygen bond is readily broken (half-life of about twelve hours at 125°) to form two ferf-butoxy radicals. The table also depicts why the intermediate ferf-butoxy radical is an efficient promoter of a free radical reaction, for the oxygen-hydrogen bond is 8 kcal./mole stronger than a carbon-hydrogen bond. In comparison, the bond dissociation energies for other peroxides are about 30 kcal./mole or less; thus, the other peroxides are not as stable as (erf-butyl peroxide. The oxygen-nitrogen bond in methyl nitrate is included to show that little energy would be needed to decompose glycerol trinitrate. Photochemical Cleavage of Bonds. The second method of producing free radicals in solution is the photochemical cleavage of covalent bonds. This has been used in the addition of methyl malonate to 1-octene, and is also used in many halogenation reactions which proceed by free radical and halogen atom intermediates. The bromination of toluene is given as follows: + sunlight or U.V. lamp —> 2 BrCtJhCIIi + BrCJhCH, + HBr
(10)
CsH,CH,Br A Br-
(11)
Br2
—
132
CeH6CH2-
Relative rate 1
58
1,440 130,000
It is quite evident that the effect of ionic strength, dielectric constant, and other similar properties of the 476
solvent is negligible on a free radical reaction, and that an ionic reaction is affected greatly by these solvent properties. There is a difference in the rate constants of the decomposition of the peroxide in the solvents and the vapor phase; however, an examination of the energies of activation (E*) indicates that this difference is negligible compared to the ionization reaction in water and the vapor phase. These five characteristics of free radical reactions in solution are sufficient to indicate whether or not a reaction goes by a free radical mechanism.
+ Br,
—
(9)
A third method uses an oxidation-reduction process to give a free radical which will initiate a homolytic reaction. The oldest method is that of Gomberg: (ChHACCl A Ag
—
Another initiator which Fenton’s reagent:
(CbHAC- A Ag+ A Cl~
(12)
is useful in aqueous media is
JOURNAL OF CHEMICAL EDUCATION
H2Oi + Fe++
HO- + OH" + Fe+3
—
(13)
The hydroxyl radical will be very reactive because of the strength of the oxygen-hydrogen bond in water, as indicated in Table 2. The final method for initiating a free radical reaction utilizes the high energy sources: a, (3, and 7-rays. These are of extreme interest in recent years and most evidence indicates the formation of free radicals. Radical Transformations. Equation (26) depicts a free radical decomposition reaction in which the tertbutoxy radical attacks the substrate with concurrent formation of acetone and methane. Apparently acetone and a methyl radical forms and the latter abstracts a hydrogen atom from the methyl malonate to give methane and the intermediate malonate radical. Another example would be the breakdown of the propyl radical to produce ethene and the methyl radical : CHjCH2CH2
CHa- + CH2=CH2
-*
(14)
A further typo of radical transformation is the rearrangement of an intermediate free radical. Rearrangements in ionic reactions occur frequently, but are uncommon in reactions which proceed by free radical intermediates. Two examples of free radical rearrangements are the Wittig: (C6H5)sC—O
