Alkylation and Related Processes of Modern Petroleum Practice F R A N K C.
WHITMORE
J. ο UNDERSTAND the carbonium km and
its behavior we need to review some simple electronic conceptions of the structure and · chemical action of some simpler atoms. The nuclear charges and the ar rangement of extranuclear electrons for some of these follow: Na (H-ll)2e)Se)le H (-hl)le S (-f-16)2e)Se)6e C (-f6)2e)4e CI (-hl7)2c)8e)7e •N (+7)2e)5e Ο (-|-8)2e)6e The electrons of the outermost energy levels, the valence electrons, are repre sented by the usual dots in the following electronic formulas: Η Methane H : C : H Steam H : 0 : Η Η Hydrogen chloride :C1:H Sodium ion :Na:
In each case a satisfied arrangement is indicated by a complete octet or four pairs of electrons. Of course, we under stand that these electrons are not static and are doing nothing as simple as fol lowing planetary orbits. Since we shall be discussing the carbonium ion so intensively, a word of cau tion may be in order about ionization. The conversion of inert hydrogen chloride to reactive hydrochloric acid by solution in water is no longer regarded as a "magi cal" influence of the latter. It may in volve steps such as: i-
+ :G: II
Ϊ1
+ H:0: 11
:C1 :11 : 0 : H . H
11 + + :Ô:H H The initial step in the ionization may be an addition of the two molecules through a hydrogen bond. The H may be pictured as leaving its attachment to the CI and attaching itself to the O, thus forming a hydronium ion, H3O4·, and leaving the Cl as a chloride ion. A distinctly polar compound such as NaCl, when dissolved or melted, gives the two kinds of ions each having its own stable octet of electrons. On the other hand, a compound like CH3CI cannot undergo so simple a change because the C and CI are held by a nonpolar linkage consisting of a shared electron pair: :C*1:
Η
668
11 \\:C:C\: Ï1 If, by any means, the CI is converted to a chloride ion with a complete octet, the remainder would be II H:C
Η Ammonia H : N : H
Chloride ion :C1:
:Cl: Η
D i m c r i z a l i o n a n d c o d i m e r i z a t i o x i , r e a r r a n g e m e n t a n d i so r n e r i z a t i o n , c a t a l y t i c depolymerizati.011., a n d a l k y l a t i o n s h a v e h e e n s h o w n t o h e intimately related processes. T h e m a s t e r key to t h e m all is t h e o n c e vigorously clispiited h u t n o w generally a c cepted " c a r b o n i u m ion." Because o f its manifold applications i n h y d r o c a r b o n c h e m i s t r y i t is p r o p e r t o g i v e d e t a i l e d c o n s i d e r a t i o n t o t h i s s t r a n g e arad i m p o r t a n t c h e m i c a l e n t i t y .
•
a The residue after the removal of the chloride ion would thus have only six electrons instead of a stable octet. This simplest carbonium ion may be abbreviated as CH3*** or as M e + . In the case of such a carbonium ion the plus sign indicates a shortage of a pair of electrons below t h a t needed for a complete octet. It must be emphasized t h a t because of this unstable structure the carbonium ion lias no existence like that of other -oniam ions. Typical stable -onium ions having complete octets are the following: Ammonium ion
^
Hydronium ion
11 H:N:Ii H Γ H:0:EL
•H
l-H
•
L [i J
Oxonium. ions 1
R:Ô:rI 1
Sulfonium ion
fi
R:5:H
narily it simply cannot exist but must undergo an immediate stabilizing change. I jokingly estimate the half life of an ordinary carbonium ion as perhaps 10~ 6 seconds. The formation and reaction of a carbonium ion are peculiar in ways corresponding to its electronic instability. Only a few compounds such as triphenylmethyl chloride, trityl chloride, (C 6 H$)ÎCC1 give carbonium ions on mere solution. I n the good old days when I used to spend a couple of weeks on Daytona Beach every other year, I saw what might be an excellent analogy for a carbonium ion. A steady wind had drifted sand up against a log until a cross section of the pile of sand and the log looked like this.
I noted that the resulting system was about as stable as that illustrated by the electronic formula for methyl chloride above. If the log were removed, however, the sand would slide to a new position of equilibrium. If the log could be removed quickty enough and an instantaneous photograph taken before the sand could start sliding, the result would be a pile of sand with a concave face.
J.L * J
|"":R:S:R
r
To repeat, each of these ions has a complete and stable octet of electrons, whereas the carbonium ion has only six electrons and is consequently both unstable and reactive to an extraordinary degree. OrdiCHEMICAL
Such an unstable pile of sand would correspond roughly in stability and halflife to a carbonium ion. Whenever we see the plus sign on a carbonium ion we should recall this hypothetical pile of sand with a concave face. Of course, if the sand were wet and had frozen, such a AND
ENGINEERING
NEWS
JLiast June the editors of C & E N were corresponding with Frank C. Whitmore on the publication of a paper which he had presented to a meeting of the Esso Research Club (affiliated tvith the Society of Sigma Xi). Before the questions in connection tvith the article were completely settled we9 along tvith so many other friends and associates of Dean Whitmore9 were shocked to hear of his sudden death on June 24. At the request of a number of his associates and former students and with the assistance of Mrs. Frank C . Whitmore9 we have gone ahead with the publico" tion of his last paper and present it in this issue. Frank Whitmore'*s ability^ enthusiasm9 sense of humor, boundless energy9 and capacity for friendship place him high in the ranks of chemists and in the memories of his many friends. In a way we look upon the publication of this paper as a remembrance of one of the outstanding mem bers of the American chemical profession. - T h e Editors
concave face would be possible. Perhaps we have such a situation in the trityl ion, (CeH6)3C+. In spite of the brief half-life of the carbonium ion it serves as a useful guide to the course of many reactions. Its modes of formation and reactions will be briefly considered. Throughout, the plus sign will indicate a carbon with only six electrons. /.
Formation of Carboniufn Ions 1. From alkyl halides. As indicated above, ordinary ionization of such com pounds is rare. Even the ionization of trityl halides can be increased by removal of the halogen with its complete octet by means of some reagent such as stannic chloride: (C6H5)3CC1 -f SnCU =5=^ SnCU" -J- (CoH4)aC+ Anhydrous aluminum halides produce similar changes with practically all or ganic halides. RCi 4- A1C13 ^=±: AlCU- + R + 2. From alcohols by acid reagents or catalysts. The initial addition of a proton from the reagent forms a relatively stable oxonium compound. Decomposition of compound by heat or by collision with a suitable reagent gives the carbonium ion: Acid reagent · τ> .fVTT . or catalyst ^ x t . U . X l .
:R:0:H^=*= R+ + H 2 0
As will be seen later, the reversibility of these changes is of the utmost impor tance. We cannot repeat too often that R + immediately after or even simultane ously with this formation must change to a more stab*e entity (the sand pile can not stay with a hole in its side!). Inorganic esters can similarly give carbonium ions. Again, an acid agent or catalyst supplies the protons. V O L U M E
2 6, N O . 1 0
.
.
»
R:0:S0 3 I1 4- H+ ^=5= R : 0 : S 0 3 H H
#
R+ + H2SO4 Conversely, carbonium ions formed in the presence of sulfuric acid or its equiva lent can reverse the above process. This is very important in polymerizations a n d . alkylations because the esters can act as reservoirs for carbonium ions. Esters are often assumed as intermedi ates in various processes of the types in which we are interested. I t should be noted, however, that any use of a pure neutral ester is naturally ineffective since the ester by itself probably has no more tendency to give ions than does water by itself or hydrogen chloride by itself. On the other hand, the ester in the presence of a suitable donor of protons can form carbonium ions. 3. From primary amines by the action of nitrous acid. This change may be followed in terms of the electrons which ap pear in the inorganic products: R : N : H -+- H+ + H : 0 : N : : 0 > H : N : : : N : + 2H:Ô: + R + H The 26 electrons required for the stable products, N 2 and 2H 2 0, leave the alkyl group electronically deficient, as a carbonium ion. I n this case the latter is formed by a decomposition process which bears not the slightest resemblance t o any ordinary process of ' 'ionization." The varied changes which t h e carbonium ion can undergo explain the notoriously poor yields of ROH obtainable by the action of nitrous acid upon RNH2. 4. Addition to an olefin of a proton from an acid agent, for our discussions, is the most important means of formation of a carbonium ion, n o t any separation M A R C H
8, 1 9 4 8
process such as those in the above three cases. The addition takes place with the activated or polarized form of the olefin in which the pi M electrons have "moved" away from one member of the unsaturated group. HH + II II H II C: : C V :C:C-^1I:C:CHH 11 H H 11 In this case the first product is an ethyl carbonium ion. Its formation could be abbreviated a s : CH 2 : :CII 2 + H + =?=^ CH 3 CH 2 + For our purposes, the most readily formed and most important carbonium ion is that of tertiary butyl: Me 2 C: :CH 2 + H + ^=±: Me 2 C + CH 3 This, important carbonium ion will be abbreviated as Me 3 C + or i-Bu4" 5. By addition to an olefin of an "electronically deficient" molecule such as BF3 or A1X3. Such molecules are "electronically deficient" in that the central atom has only six electrons in the monomerie form. Thus, although they are electrically neutral, they can combine with shifted pi electrons from a double bond. Thus, :F: Me :F:B-f- :CH 2 :CMe 2 ^=±= F 3 B : C H 2 : 6 + :F:
Me
Such a carbonium ion is apparently much more stable than one of the purely hydrocarbon type. The above reaction is much less readily reversed than in the case of the addition of a proton to an olefin. There may be a statistical effect here. Thus, the (CH 3 ) 3 C + can revert to isobutylene by the removal of any one of the nine protons without its electron pair. The peculiarities of what might be called 669
the heterocarbonium ions may be responsible for the complexity of the polymers produced by such reagents as BF*. It is noteworthy that such a heterocarbunium ion is a positive ion formed by the union of two neutral molecules, the pi electrons of the olefin having shifted to form a coordinate link between C and the hetero atom. This is in contrast to the formation o, a positive tert-butyl earbonium ion by the union of a positive proton from the acid agent with the neutral olefin molecule, the pi electrons of which have shifted to form a true valence link be-, tween C and II. 6. By addition to an olefin of "atomic oxygen" from an oxidizing agent. :Ô -f- :CH 2 :CMe 2
^ :0:CH2:CMe2
This accounts for the oxidation of olefins to acids having the same number of carbon atoms without the usual splitting of the carbon chain. In this case the final oxidation product of isobutylene is isobutyric acid instead of acetone. The carbonium ion formed by the addition of the Ο "atom" with its sextet of elec trons undergoes a shift of an electron pair with its proton to give isobutyraldehyde. 7. By addition to a carbonyl group of a proton from a suitable agent. This change will not be considered in detail here be cause it is not specially applicable to our problem. It is, however, of the greatest importance in general organic reactions. A good example is Hammett's discovery of the cryoscopic behavior of benzoic acid in 100% sulfuric acid in which the freezing point depression is twice that cal culated from the amount of benzoic acid added. C 6 H 5 : C : : 0 + H 2 S0 4 χ - ^ :Ô: H C C H S :C:Ô:H + H S 0 4 :Ô:**
ii In this case the sulfuric acid is ionized by the benzoic acid in much the way that HC1 and H 2 0 achieve ionization.
//.
Reactions of Carbonium
Ions
As might be expected from its electronically deficient structure, the reactions of this ion are not like those of ordinary ions which are dependent usually on the rate of addition of a reagent. Simultaneously with the formation of a carbonium ion or immediately thereafter it must undergo a change. (The concave face of the dry sand pile must slide.) If the carbonium ion cannot satisfy its electronic deficiency intermolecularly, it must change intramolecularly and a rearrangement occurs. I t should be noted that the order of stability of these very unstable fragments is as follows: tertiary ^> secondary > primary 670
This order corresponds to the rapidity of formation of RC1 from the three classes of alcohols. 1. Union with an ion or molecule with a complete octet of electrons but having at least one free pair. This might appear to be the easiest change for a carbonium ion. Thus, a methyl carbonium ion formed in aqueous methanol by the action of nitrous acid with methylamine is immediately stabilized by union with one of t he free pairs of electrons of the solvent Me* -f :0:Me
ii
^=±i
ΓΜΘ:0:ΜΟ~Γ
L
H
J
The product is the more stable oxonitim ion. This can give a proton to the me dium and thus become dimethyl ether, the chief product of the reaction. If the action of methj'laminc with nitrous acid is carried out in aqueous solu tion, the products include methanol and •dimethyl ether, the former being the re sult of coordination of the methyl car bonium ion with water, followed by loss of a proton t o the medium. This con ception of the coordination of the car bonium ion with the medium is much more plausible than the ordinary idea that the carbonium ion unites with hydroxyl ions from the water or mcthoxyl ions from methanol. The reversible addition of R + to sul furic acid to give an ester as mentioned above is another important intramolecular stabilization of the carbonium ion. The carbonium ion can also unite with ordinary anions. This is well illustrated in the action of n-butylamine with nitrous acid during which the B u + ion forms not only BuOH but also BuCl and BuONO with the chloride and nitrite ions in the solution. The larger amount of the first product is not due t o any action of hy droxyl ions but to the coordination of the carbonium ion with the water. In many cases, the carbonium ion stabilizes itself intramolecularly. The electronically deficient carbon atom takes an electron pair from an adjacent atom. The mechanism of this apparently simple trans fer of an electron pair from one atom to the adjacent atom is now being attacked by mathematical physical chemists. D e pending on the structure of the molecule, this transfer of an electron pair may take place either with or without the proton or the group attached to the moving electron pair. 2- An adjacent multiple linkage may furnish the needed electron pair, giving • an allylic shift. This change is not im portant in our processes but will be con sidered briefly because the usual peculiari ties of the carbonium ion appear. The replacement by bromine of hydroxyl in crotyl alcohol is a good example. The OH is removed i n the usual way probably through an oxonium. compound. The re sulting crotyl carbonium ion then under goes the simplest possible internal shift of an electron pair. CHEMICAL
MeCH: : C H : C H 2 ' i ^ M e C H : C l I : :CH 2 Only the essential electrons are indi cated. As usual the change is reversible. The product is formed by the union of these two carbonium ions with bromide ions to give a mixture of crotyl bromide and methylvinylbromomethane. 3. The electron pair may take with it an attached proton. A good example of this type of change is the replacement by CI of OH in 2-methyl-3-butanol. All re agents which bring about this change give only the 2-chloro compound instead of the 3-chloro compound. As usual, the first change is undoubtedly the formation of an oxonium compound from the alcohol. This then gives the corresponding second ary carbonium ion. The shift of an elec tron pair with its attached proton converts the secondary carbonium ion to the more stable tertiary one which then combines with the chloride ion to give the observed product. H „ ... H :H shift
+
+
Me 2 C—CHMe ττ"^ Me 2 C—CHMe Only the electrons involved in the shift, are indicated. The desired 2-methyl-3chlorObutane can be obtained in 5 0 % yield by adding dry HC1 to isopropylethylene in the cold. Even under these conditions, an equal amount of the rearranged 2-chloro compound is obtained. I t is important to note that the latter is not formed by rearrangement of the 3-chloro compound which is completely stable under the conditions used. The steps involved in the formation of the two products are in structive. We shall indicate only the essential electrons. ? HCl Me 2 C— CH : : CH 2 > Η Me 2 C—CH : CH 2 : II : CI : II
/
MeaC—CHCH» add C I - /
φ McoCH—CHCH 3
\
: Η shift
\*** Cl" Me.C—CH 2 CH 3
k
The initial attack of the dry hydrogen chloride on the dry olefin may take place through a hydrogen bond involving the shifted pi electrons of the double linkage. It would thus resemble the mechanism of the ionization of HCl in water. 4. The moving electron pair may carry an attached organic group with it to give ordinary 1,2-rearrangement. In gen eral, the rearrangement of an electron pair with the attached organic group (:R) takes place more readily than does that of :H. Usually the larger the group, the more readily it rearranges. Perhaps the most notorious example of 1,2-rearrangement is the action of neopentyl alcohol with HBr to give rearrange ment products and only 5 % of neopentyl AND
ENGINEERING
NEWS
bromide. In this case the alcohol ab sorbs almost exactly 1.0 mol. of HBr in the cold. This H B r can be removed by a stream of inert gas o r - b y washing with water to leave the original alcohol with practically unchanged freezing point (50°). On the other hand, heating the oxonium compound in a sealed tube or letting it stand a long time gives mainly teri-amyl bromide, isoamylenes, and 5 % neopentyl bromide. The relatively stable oxonium compound decomposes to give water and the neopentyl carbonium ion. Only 5 % of these ions stabilize themselves by union with the bromide ions without rearrangement. M
e
Me:C:CH2 + Me
ι
.__ . . . • Me shift + >Me:C:CH2:Me Me Br :shift
~/
I
Br" Br / (without H) Me 3 CCH 2 Br Me 2 CCH 2 Me M e 2 C : : C H M e etc. 5% 95 C: :C + most readily part with an electron pair | | | without gp. | | and its attached proton. MeMeMe Me M e Me Me:C:H Me Me : shift Me +C:b:H ->- H+ + C : : Ô H:0:C—CMe2 Me
I
··
:Me
without H
Me
I
Me
The important new t y p e of change here illustrated is the movement of the electron pair away from the tertiary group to the electronically deficient carbon. This type of change is involved in the fission (not cracking) of higher branched olefins. We found this change and realized its significance during the vigorous dehydration of di-teri-butylcarbinol to give isobutylene and isoamylenes instead of the expected nonenes. T h e following steps are involved. Again,-only the essential electrons will be indicated. M A R C H
8,
1948
The conditions must be such as to prevent other changes and favor this one. The n e t result is a n important new type of reaction. R + -h M e 3 C : H — > - R H 4- M e 3 C This important change and its application will be discussed later under alkylation. I t should again be emphasized that all the reactions of the carbonium ion, even this last and very remarkable one, involve the same fundamental change— namely, :C + : — > - :C: 671
Application
of tlie Carbonium
Ion
T w o points should be re-emphasized. 1. Almost all of t h e changes studied are reversible. 2. The nature of a given step is usu ally decided by two processes of the pushpull type giving the same direction to the reaction. This will be especially evident in the isomerization of the branched hexylenes. ///.
Intramolecular
Rearrangements
W e have already discussed several of these. It m a y be of interest t o touch on some rearrangements from the new field of organosiiicon compounds. The prin ciples involved will be found t o be the same. We have recently studied the sili con analog of neopentyl chloride. A comparison of the reactions of these t w o substances is instructive. Me
Me
Me—C—CH 2 C1 Me NaOH AgNOs Nal
N o action
A1CU
polymers
Me—Si—CH 2 C1 Me N o action »» >» forms iodide, no rearrangement : M e shift
>Me MeSiCH 2 Me
A
!1 T h e comparable inertness of the t w o compounds to the first two reagents is understandable. T h e difference in the behavior with sodium iodide is interesting. The iodide ion apparently can attack the rear of the CH2 group in the silicon com pound and expel the CI b y a Walden t y p e mechanism, whereas the more sterically hindered neopentyl chloride will not allow such an attack. T h e action of the anhydrous aluminum chloride starts with the *'ionization" of the CI by the formation of the AICI4"" ion and the neopentyl carbonium ion and its silicon analog. The former undergoes the usual shift of :Me t o give a terJ-amyl car bonium ion which t h e n gives polymers in the usual way, via isoamylenes formed b y the shift of : without its attached pro ton. The corresponding carbonium ion . containing Si as its central member under goes this same shift of :Me but no double bond is possible between the large Si a t o m and the small C a t o m . Consequently, no polymers result. I t is noteworthy that siliconeopentyl chloride reacts with aluminum chloride m u c h more readily and more vigorously t h a n does neopentyl chloride. J u s t as fission proved a special case of satisfying an electronically deficient car bon in t h e reactions of certain glycols, alcohols, and olefins, w e have found simi lar changes w i t h compounds having o x y gen o n a carbon in t h e beta position to silicon. Such compounds are highly sen 672
sitive t o acid reagents. T h e latter render the beta carbon electronically deficient by the usual mechanisms. T h e needed electron pair is readily supplied by silicon because of i t s highly "positive" character. The shift of the pair from Si forms a double linkage between t h e former alpha and beta carbons. T h u s , 1-trimethylsilyl-2-propanol (trimetnyl-2-hydroxypropylsilane) acts readily w i t h even dilute acids t o give propylene. Me3Si:CH2:CHCH3 Me3Si- +
shift without MeaSi
CH,::CHCHi
MeaSiOH, etc. Trimethylsilylacetic acid a n d trimethylsilylacetone react e v e n w i t h water t o give acetic acid a n d acetone respectively, prob ably t h e m o s t expensive k n o w n prepara tion of t h e s e two c o m m o n chemicals. These apparently startling changes begin with a proton attack by t h e water on the shifted electron pair of the carbonyl link age. Here, a s with the carbonium ion obtained by the action of acid ο η Ί - t r i methylsilyl-2-propanol, the electronically deficient carbon is in a position beta t o silicon and so can acquire t h e needed elec tron pair t o form a 1-olefinic linkage. In the last t w o cases, the primary prod ucts of fission arc the enol forms of acetic acid and acetone. > Me3Si:CH2:C:Ô:H Me 2 Si +
+
CII2::C:0:H
>=^CH3C::0
\ Me 3 SiOH, etc. In honor of the unity of nature, it might be recalled t h a t enolization and its reversal are n o t a jumping of H from C t o Ο and back again. Instead i t goes by way of a proton attack o n the carbonyl Ο to form a carbonium ion which behaves as such a n ion should. Conversely the proton a t t a c k takes place a t the end of the enolic double linkage to give the same carbonium ion. The fissions of Si-C c i t e d above re call a n old mystery presented by organomercury compounds in m y earlier y o u t h . This can be intimated b y t w o equations CHaCH 2 HgCl + HC1
>H g C l j - f CHaCH,
HOCH 2 CH 2 HgCl + HC1 > HgCl 2 + C H S = C H 1 +
H20
T h e latter reaction gives n o trace of the expected HOCH 2 CH 3 . W e can now see how t h i s fission occurs. T h e proton from the acid attacks the O H forming the usual oxonium compound which decomposes to give a carbonium ion. I n this case t h e electronic problem of the carbon is ridicu lously simple because of t h e ease with which t h e metallic atom g i v e s up an electron pair. C H E M I C A L
C l H g : C H 2 : C H 2 + U ^ Î C l H g + - r - C H 2 : :CH a All of t h e above material on silicon compounds and .mercury compounds is dragged in t o emphasize the importance of our olefin fissions and the fact that they are n o t lone or unique reactions. IV. Isomerization of Olefins Changes possible with the highly branched hexylenes are typical of many changes occurring in industrial · petroleum processes. T h e y involve the usual formation and reactions of carbonium ions. Tetramethylethylene (b. 7 3 ° ) can be converged completely to 1,1-methylisopropylethylene (b. 5 6 ° ) by boiling i t with 3 0 % sulfuric acid under an effective column set for suitable reflux. Me Me H 2 C : C : : C M e 2 + H * ^-^ H,C:C:CMe2 H H * H
•.
: shift
Me H2G::C:CHMe2-r- H + T h e medium supplies the protons for the first change and removes protons i n the second change. Again, we have the push-pull combination. T h e constant re. mov a l of t h e lower boiling olefin constantly shifts both equilibria. W h e n the lower boiling of these hexylenes is stirred w i t h 5 0 t o 6 9 % sulfuric acid a t 20° the above changes are reversed to give about S 5 % conversion t o tetramethylethylene. Stirring teri-butylethylene with 6 9 % sulfuric acid a t 20 ° gives a 5 0 % conversion to% tetramethylethylene. H H H + -r- H 2 C : : C : C M e 2 ^ = ^ H 2 C : C : C M e 2 Me j # H +Me :Me shift
Η C H 3 : C : C M e 2 ^ = ^ - H + + Me 2 C::CMe2 Me V,
Dimerisation
of
Olefins
T h e simplest case is that of the readily polymerized isobutylene. T h e acid cata lyst adds a proton t o give a ί-Bu4* ion which adds t o another molecule of iso butylene in the same manner. T h e resuiting C 8 carbonium ion can stabilize itself b y the shift of a n electron pair with out t h e attached proton. 2 M e 2 C : : C H 2 ·+· H + ^ = ^ Me3C -r-CH2::CMe2Me 3 CCH 2 CMe 2 : H + + Me 3 CCH 2 C: :CH a (I) M e (II) Η + - Γ - M e 3 C C H : : C M e 2 (III) Compounds I I and I I I are the well known diisobutylenes, the low and high boiling forms respectively. Each i s con verted to t h e equilibrium mixture (4.5:1) by acid agents. Addition of a proton t o either gives the carbonium ion (I) which AND
ENGINEERING
NEWS
reverts to the equilibrium mixture. The four triisobutylenes result from the loss of protons from C12 carbonium ions produced by union of (u) I I with £-Bu + and (b) I with isobutylene. These two changes take place in about 9:1 ratio. Apparently f-Bu + does not add to I I I . It is easier for III to go through I t o II which, adds ί-Bu4" readily. I t is important t o re member that all these changes are re versible. . Tetramethylethylene, with acid cata lysts, gives more than 2 0 dimers. The composition of the resulting mixture is very sensitive to conditions. Thus, 8 0 % sulfuric acid gives mainly the skeleton C
c—c—c=c i c fc while S4% acid gives mainly C C
c—c—c=c—c—c—c These and the other score of dimers all contain groupings related to tert-butylethylene, the least stable of the isomerization products of tetramethylethylene. This is an excellent example of t h e im portant practical significance of the re versibility of the changes involved in such reactions as the dimerization of olefins and, as we shall see later, in alkylation. T h e equilibria involving tetramethyl ethylene, 1,1-methylisopropylethylene, and teri-butylethylene probably never in volve more than 5% of the last olefin. In spite of this, feri-butylethylene is in volved in the formation of all of the known dimers of tetramethylethylene. T h e com plex results possible from combinations of the simple changes which we have been studying m a y well be illustrated by the following chart. T y p i c a l D i m e r s of T e t r a m e t h y l e t h y lene 8 0 % Acid. 8 4 % Acid.
carbon, followed b y the shift of an elec tron pair without its proton from the third carbon to form the 3-olefin would give the? main products. The preponder ance of the 3-isomers is surprising. The other dLmers are formed in much smaller amounts by a larger number of steps. The dinnerization of tetramethylethylene is a real gold mine of what we need most— namely,* specific information about which of the many possible steps and the multi tude of combinations of those steps actu ally tak« place under given conditions. VI.
AMkyhtion
As ca-rried out with a n olefin and a very large excess of a tertiary hydrocarbon in the presence of concentrated sulfuric acid or othex equivalent catalyst, this process involves the following steps which have already been described : 1. Addition of a proton to the olefin. 2. Ftevcrsible union with the sulfuric acid of the carbonium ion thus formed. 3. Addition to the olefin of the small R + as tihe latter is formed or, more prob ably, a s obtained from the ''reservoir" formed in 2. The result is a larger carbo niurm ion, B / + . 4. Reaction of the larger carbonium ion w i t h Me 3 C: H or other tertiary hydro carbon to give R ' H , a final alkylation product;, and Μ β 3 0 + or other tertiary carbonium ion which starts the chain again. 5. A t all stages, the carbonium ions can undergo the usual rearrangements. G, T h e loss of a proton by a carbo nium i o n to give t h e corresponding olefin is apparently repressed b y the concentrated sulfuric acid which combines with the carbonium ion until such time as one of the large number of tertiary hydrocarbon molecu-lcs enters t h e reaction. These principles may be illustrated b y the action of isobutylene with a large ex cess of isobutane in'the presence of con centrated sulfuric acid. M C ï C=CIT 2 + I 1
+
Mt*C+- + H 2 S 0 4 -
Me 3 C +
^=
:Me>COSO s H (reservoir)
Me,C4--l-CH2=CMei
HH
rMe3CCH..CiMe2
:ii shift Me3C:il : M e shift
Me 2 èCHCHMe 2 M e 3 C C H 2 C I I M e 2 + M e 3 C + C shift ,
:c-
