Radiation Chemistry-II

24

Liquid-Phase I o n - M o l e c u l e Reactions:

The

I o n Injection M e t h o d A p p l i e d to Isobutylene N. S. VISWANATHAN and L. KEVAN

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch024

University of Kansas, Lawrence, Kan. 66044 tert-Butyl carbonium ion is produced by vapor phase photolysis of isobutylene with a krypton resonance lamp. The tert-C4H9+ is separated from its concomitant electron and injected into liquid isobutylene by an electric field. Only C and C products are observed; the relative C and C12 yields are dose dependent. The C products are 2,2,4-tri8

12

8

8

methylpentane (4%); 2,4,4-trimethylpentene-2 (49%); 2,2,3trimethylpentane(19%)and 3,4,4-trimethylpentene-2 (28%). Proton transfer is the most important neutralization step but hydride transfer also occurs. Thetert-C4H9+is vibrationally excited and reacts equally at both double-bond carbons in isobutylene. Added argon deexcitestert-C4H9+which then primarily reacts at the terminal double-bond carbon in isobutylene. TJeactions of specific hydrocarbon ions in the liquid phase are difficult to study directly. Ions may be produced in the liquid by direct liquid radiolysis, but the situation is complicated because many other reactive species such as electrons, radicals, and excited states are produced simul taneously. This complex situation may be simplified by producing spe cific ions in the vapor phase and injecting them by means of an electric field into a liquid or solid matrix. Under such conditions the positive ion is separated from its concomitant electron and is accelerated into the liquid or solid alone. We call this the ion injection method. It shows considerable promise for studying specific ion-molecule reactions in the liquid phase and should allow new types of studies on positive ion trapping in inert matrices to be made. Here we describe a successful ion injection method for one com ponent systems which is based on previous experiments by Schlag and Sparapany (7,8). By this method ierf-butyl carbonium ion is formed in 361 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

362

RADIATION

CHEMISTRY

Π

the vapor from photoionization of isobutylene. After injection into liquid isobutylene the reactions of terf-butyl carbonium ion with isobutylene to form C and C carbonium ions have been ascertained. It has also been possible to study the effect of vibrational excitation on the reaction selectivity of feri-butyl carbonium ions. 8

12


Experimental Techniques Resonance Lamps. Krypton resonance lamps were constructed of borosilicate glass as shown i n Figure 1. A n O-ring joint is provided to attach to the sample cell. A 2 mm. thick by 20 mm. diameter L i F window (Optovac, Inc.) was attached to each lamp with Torr-Seal (Varian Vacuum Products Division). Torr-Seal was applied so as to not be ex posed to the inside of the lamp. Lamps were attached to a greaseless vacuum line used only for lamp filling and were evacuated to about 10" torr for 24 hours while being heated with heating tape to about 300°C. The lamps were then cooled and filled with about 1.0 torr of research grade krypton to provide maximum photon output (6) and sealed off. The lamp discharge is maintained by a 125 watt Raytheon 2450 Mc./sec. microwave generator; it is initiated with a Tesla coil. The lamp was operated with one end i n a liquid nitrogen trap to remove condensable impurities. 5

O R I N G JOINT

- 0 RING STOPCOCK GROUND

/

loi

0 RING MICROWAVE EXCITATION APPLIED H E R E

-400 V

COLD FINGER

REACTION

Figure 1.

CELL

Right angle reaction cell and vacuum ultraviolet photolysis lamp

Under normal operating conditions about 40% of full power from the microwave generator was used. The discharge had to be i n contact with the L i F window for maximum intensity ( 9 ) ; if not the intensity dropped b y about tenfold. Good lamps lasted for 60-90 hours of opera tion before the impurity level increased so that the lamp would not operate.

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

24.

VISWANATHAN

AND KEVAN

363

Ion-Molecule Reactions

Current-voltage saturation curves for krypton resonance lamps were determined i n a special cell with two internal rectangular (1.1 X 1.7 cm.) nickel electrodes 1.6 cm. apart. The L i F window extended between the electrodes. For 1.3 torr N O the saturation current was typically about 2.5 /xamp. The lamp intensities were usually about 10 quanta/sec. Lamp intensities were monitored at about 10 hour intervals of operation. The spectral output from several used lamps was examined on a McPherson vacuum ultraviolet monochromator. Strong krypton resonance lines were observed at 1236 A . and 1165 A . with relative intensities of about 4 to 1 respectively. A number of much weaker lines at longer wavelengths were observed which were mostly attributable to H2O. Some H 0 was apparently released from the borosilicate glass walls during the operation of the lamp. As w i l l be seen later a weak lamp intensity at wavelengths longer than 1236 A . does not affect the ion chemistry of the isobutylene system. 15


2

TOP

ELECTRODE

CONFIGURATIONS

Figure 2. Electrode configuration in right angle reaction cell Cell Designs. Several different reaction cell designs were tested. Two basic designs (right angle and direct) with various electrode configurations in each were finally used. Most of the experiments were performed i n the right angle cell shown in Figure 1. O-ring joints and greaseless stopcocks were used to minimize contamination. The electrodes (1-2 sq. cm.) were made of nickel and in early experiments were attached external to the cell with Silastic R T V 731 ( D o w Corning C o r p . ) . It is interesting to note that the ion injection method w i l l work with both electrodes external. However, to measure reaction cell characteristics the top electrode was normally internal as shown in Figure 2. The bottom electrode was also normally internal. In some experiments a mesh electrode made of electroformed nickel mesh with 8 5 % optical transmittance


364

RADIATION CHEMISTRY

was used between the top and bottom solid nickel electrodes. The positioning of the electrodes for the right angle reaction cell is illustrated i n Figure 2. A copper foil guard ring was attached externally and grounded to minimize surface leakage currents. Sample Treatment. Research grade isobutylene (99.5 mole % purity) was used after degassing by the freeze-pump-thaw method for most experiments. The only impurity detected by gas chromatographic analysis was 0.02% isobutane. C and C i compounds which were used as gas chromatographic standards were obtained from the Chemical Samples C o . (Cleveland). Research grade rare gases were used. A block diagram of the experimental set-up is shown i n Figure 3. The lower part of the reaction cell which held the liquid matrix extended into a Dewar. The temperature was controlled to ± 2 ° C . by a cooled air stream which was regulated by a differential flow controller and flowmeter. The temperature could be decreased by increasing the flow rate. The temperature was monitored by a platinum resistance probe attached to a digital readout unit (Digitec M o d e l 531). 8


II

2

DIGITAL

ELECTROMETER

THERMOCOUPLE

H'" VOLTAGE

C 0 L

SUPPLY

i " M !

LIQUID

Figure 3.

-GLASS N

WOOL

2

Experimental set-up for the ion injection method

Voltage from a Keithley 240A power supply was applied to the electrodes during photolysis and the current was monitored by a Keithley 610B electrometer. Normal experiments were carried out with a temperature of —128°C, —400 volts applied to the bottom electrode and 10" to 10" amp of current. The experimental procedure was as follows. Isobutylene was expanded into a glass vacuum system and condensed into the reaction cell with liquid N . FV relations were used to calculate the amounts taken. Photolysis was usually carried out for 120 minutes although times as short as 30 minutes were also used. After photolysis the cell was warmed to 0°C. and the isobutylene was expanded back into a vacuum system of known volume. The remaining high molecular weight products i n the reaction cell were dissolved in 1 ml. of hexane and subjected to gas chromatography. In the experiments involving added rare gases a measured amount of isobutylene was condensed into the cell first. Then the rare gas was added ( A r at —150°C. and Ne at —196°C.) to the desired pressure. 7

8

2


24.

viswANATHAN AND KEVAN


365

In those experiments which incorporated a mesh electrode the mesh blocked access to the sample for gas chromatographic sampling. Therefore, the entire reaction mixture was distilled out of the reaction cell into a detachable cold finger on the vacuum line at —196 ° C . The isobutylene was then evacuated at 0 ° C , and 1 ml. hexane was added to dissolve the C and C products. This procedure discriminated against complete C i product recovery and invalidated C / C i ratios. However, the relative amounts of C products were still accurate. Gas chromatography was carried out on an Aerograph Model 202 with thermal conductivity detection and helium carrier gas at 45 ml./min., and on an Aerograph Model 600D with flame ionization detection and nitrogen carrier gas at 24 ml./min. C and C groups were separated at 90°C. on a 6 feet 6 inches long by 1/4 inch o.d. SE30 (25% w / w on Chromosorb W ) column. C products were resolved at 50°C. on a 14 feet 9 inches long by 3/16 inch o.d. SE30 (30% w/w on Chromosorb W ) column and at 20 °C. on a 14 feet 2 inches long by 1/4 inch o.d. A g N O / benzyl cyanide column. C products were analyzed on the same columns at 150°C. (SE30 column) and at 20°C. ( A g N 0 column). For mass spectral measurements products were trapped from the gas chromatographic stream on silica sand at —196 °C. The trapped samples were degassed and injected into a Nuclide 12-90G mass spectrometer for analysis. 8

12

2

8

2

8

8

i 2


8

a

J 2

3

Results Reaction Cell Characteristics—Gas Phase. The right angle reaction cell with an applied electric field is effectively an ionization chamber. Current-voltage curves for different electrode configurations are shown in Figure 4 for 2 torr isobutylene at room temperature. Good saturation currents are achieved with internal electrodes. W h e n the bottom electrode was external to the reaction cell, no saturation current plateau was found. The different electrode connections are top to bottom ( T - B ) , top to mesh ( T - M ) and top to mesh - f bottom ( T - M B ) . The average ion current values at —400 volts and 2 torr are 2.8, 5.8 and 7.5 tiamp respectively for the different electrode connections. The mesh electrode collects a large fraction of the total current. Ideally the T-B current plus the T - M current should equal the T - M B current. The results show that the T - M B current is a little lower than expected. This is probably because of an artificially high T - M current caused by some transmitted ions being attracted back to the mesh electrode. Thus, in the T-B or T - M B connections 2.8/7.5 = 38% of the ions are transmitted through the mesh electrode. Figure 5 shows the dependence of saturation current in isobutylene vs. pressure in the reaction cell. The saturation current increases to a plateau at 4 torr. Above 4 torr all of the incident photons are absorbed


366

RADIATION CHEMISTRY

II

by isobutylene. A t pressures above 8 torr the saturation current decreases because of incomplete ion collection by the electrodes.

Ρ C

= 4

H

2

TORR

8

®

TOP-BOTTOM

®

TOP-MESH

©

T O P -MESH

a


BOTTOM

APPLIED

Figure 4.

VOLTAGE

(ΧΙ00)

Current-voltage saturation curves for different electrode configura tions in 2 torr isobutylene

50

40 CO Q_ «

30

'o

X 20

10 0

2

4

6

8

10

12

14

TORR

Figure 5.

Saturation current vs. isobutylene pressure



24.

367


VISWANATHAN AND KEVAN

Reaction Cell Characteristics—Liquid Phase. In order to determine the reaction cell characteristics under actual experimental conditions, saturation current measurements were made at —128 °C. with about 1 mm. of liquid isobutylene covering the bottom electrode. The results for different electrode configurations are shown i n Figure 6. Saturation current curves analogous to those for the gas phase are observed. The magnitudes of the saturation currents are 15-fold smaller in the liquid case because the vapor pressure of isobutylene at —128 °C. is only 40 microns. The current magnitudes are quite similar to those observed for gaseous isobutylene at 40 microns at room temperature. Thus, the pres ence of an isobutylene layer over the bottom electrode has little effect on the magnitude or shape of the current-voltage curves obtained in the reac tion cell. The saturation current plateau begins at about 400 volts so this voltage was chosen for most experiments.

® ®

Τ 0 Ρ - BOTTOM Τ0Ρ-

MESH

Τ0Ρ-

MESH

RIGHT A N G L E

CELL

a

BOTTOM

£

'ο

—·—^-^®

50

< 00 40

y

χ

s oc oc o

ο

ο

30 20

10

/°

-

ο

ο

O

y l 1

1 2

3

I

ι 4

5

6

APPLIED

Figure 6.

I

ι

7 VOLTAGE

· "® 8

I 9

I

I

10

II

(Χ 100)

Current-voltage saturation curves for different electrode configura tions at —128°C. with liquid isobutylene present

The sum of the saturation currents for the T - M and T-B electrode connections is about 10% greater than the saturation current for the T - M B connection. The T-B over T - M B current ratio is 4 1 % and is taken as the percent of ions that are transmitted through the mesh electrode into the liquid. This percentage is the same as found for the all gas system and indicates that essentially all ions injected into the liquid are collected at the bottom electrode. Therefore, the bottom electrode can be used


368

RADIATION CHEMISTRY

II

just as reliably as the mesh electrode to monitor the number of ions injected into the liquid. The remaining purpose of the mesh electrode is to provide a field-free region between the mesh and bottom electrodes for the T - M B connection. It was found that the presence of a field-free region did not affect the product distribution and that it lowered the total product yield by the fraction of ions collected on the mesh electrode. Thus, a mesh electrode was only i n place for part of the experiments. Product Identification. Gas chromatography showed that C and C compounds comprised all detectable products. N o products between C and C12 were observed. A careful search was made for C i o compounds but none could definitely be detected. Material balance calculations showed indeed that the C and C i compounds detected constituted over 95% of the reacted isobutylene. It seems that a l l important products were detected and that C i e products, if present, are minor. The C products were analyzed on a A g N 0 / b e n z y l cyanide column which discriminates between alkenes and alkanes, and on a SE-30 column which discriminates according to boiling point. These two columns clearly distinguish the alkenes from the alkanes. The products were identified by retention time comparison on both columns with C standards (Table I ) . Identification of the more abundant olefins was confirmed by mass spectrometry. 8

i2

8


8

2

3

8

8

Table I. Gas Chromatographic Retention Times of C Products from ferf-C Hg* Reaction with Liquid Isobutylene 8

4

Retention Time* AgNO Column 20°C.

SE-30 Column 50°C.

Product 2,2,4-Trimethylpentane 2,4,4-Trimethylpentene-l 2,4,4-Trimethylpentene-2 2.2.3-TrimethyIpentane 3.4.4- Trimethy lpentene-2

19.7 23.7 27.8 30.3 35.8

s

7.8 12.0 11.8 10.0 15.0

min. min. min. min. min.

min. min. min. min. min.

i=0.3 min.

Since the amount of 2,4,4-trimethylpentene-l was only 5 % of the amount of 2,4,4-trimethylpentene-2, resolution was not normally achieved. The relative C yields at 120 minutes of photolysis at —128 ° C . are given in Table III. It w i l l be seen that the individual C isomers reveal the probable structure of the C carbonium ions and how they are neutralized. The relative C yields are only slightly dependent on dose (see the section on dose effects). The C12 products were not resolved into separate isomers or clearly identified. The gas chromatographic retention times for the C i peak 8

8

8

8

2


24.



369

and several known standards are shown i n Table II. O n a A g N 0 column n-alkanes have longer retention times than isomeric branched alkanes. Furthermore, highly branched alkenes have shorter retention times than linear alkenes. Therefore, the C12 product seems to consist of one or more highly branched alkenes. 3

Table II. Gas Chromatographic Retention Times of Standards and C12 Products from teri-C H Reaction with Liquid Isobutylene 4

+

9

Retention Time AgNO Column 20°C.

SE-30 Column 150°C.

s


Compound 2-Dodecene Methylundecene-2 Dodecane C product

126 118 94 104

1 2

min. min. min. min.

49 min. — — 37 min.

Table III. Phase and Temperature (Vapor Pressure) Effects on C Products from £er£-C H Reaction with Isobutylene 8

4

9

+

Vapor Percent 224Pressure Temp. C in TMP Microns) CC.) Products' (%ofC ) b

8

1

540 280 140 80 40 6 4 1

-110 -115 -120 -124 -128 -138 -140 -145" r

75 70 74 73 76 76 80 91

120 min. photolysis time. 2,2,4-trimethylpentane, ± 2,4,4-trimethylpentene-2, 2,2,3-trimethylpentane, ± ' 3A4-trimethylpentene-2, Freezing point. ' Solid.

8

4 4 5 5 4 5 6 7

244TMP-2 (% ofC ) C

8

344TMP-2 (%ofC )

223TMP (%ofC ) d

e

8

8

27 28 29 27 28 21 6 6

20 16 17 18 19 14 4 3

49 52 49 50 49 60 84 84

α 6 c

d

1%. ± 4% (includes 2,4,4-trimethylpentene-l to about 5%). 2%. =*= 3%.

1

Dose Effects. The percent conversion of isobutylene is linear to at least 120 minutes of photolysis time (Figure 7 ) . The conversion rate is typically 0.01%/min. The relative amounts of C and C products change with photolysis time (Figure 8). Thus, the C12 products seem to be secondary i n nature. This observation is consistent with the absence of observable amounts of C i products. The relative yields of C products change only slightly with dose. A t longer doses the total percentage of C alkenes drops by a few percent. 8

6

i2

8

8


370

RADIATION CHEMISTRY 1.6 ISOBUTYLENE 1.4

^ I R U N

1.2

+co ο

1.0

/ Γ 12 R U N S

-

ο


g

0.8

0.6

cr 0.4

0.2

TEMP-I26±4°C

f

0.0

1 30

ι

I

ι

ι

60

90

120

150

PHOTOLYSIS

Figure 7.

TIME

180

IN M I N S

Isobutylene conversion vs. krypton resonance photolysis time

tert-BUTYL C A R B O N I U M ION R E A C T I O N P R O D U C T S 100

co

ζ

C

80

8

HYDROCARBONS

60

UJ ο

40

I

20

h

C,

60

90

120

2

HYDROCARBONS

150

180

P H O T O L Y S I S T I M E IN M I N S .

Figure 8.

Rehtive C and C product yields from isobutylene vs. krypton resonance photolysis time 8

12


II

24.

371



Reversed Field Experiments. The experiments are normally per formed with a negative voltage applied to the bottom electrode to inject positive ions into the liquid. W h e n the field is reversed and +400 volts are applied to the top electrode so that positive ions are not injected into the liquid, no C or C i products were found. Very low yields of iso butylene fragmentation products were observed but were not measured quantitatively. The C yields, observed in normal field experiments, were 20-100 times greater than these yields. P V measurements showed that the total isobutylene decomposition for the reversed field ( + 4 0 0 volts at top electrode) was only about 5 % of the decomposition for the normal field (—400 volts at bottom electrode). These reversed field experiments support the contention that the C and C products are formed from positive ions in the liquid phase. 8

2

8


8

i

2

Table IV. Effect of Added Argon and Neon on C Products from i e r / - C H Reaction with Liquid Isobutylene 8

4

9

+

β

Added Gas Type None

C Product Distribution 8

Pressure (torr) 224-TMP

(%)

b

244-TMP-2

223-TMP

4

r

d

344-TMP-2 28

e

f

49

19

Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar

0.05 0.10 0.20 0.50 1.0 2.0 3.0 5.0 10.0 12.0 15.0

4 4 3 3 4 4 4 5 4 4 5

55 59 62 67 65 71 76 76 78 77 76

16 14 13 12 11 10 7 8 8 7 8

25 23 22 18 18 15 13 11 10 12 11

Ne Ne Ne Ne

1.0 2.0 5.0 10.0

5 5 4 4

52 55 51 49

17 15 17 18

26 25 28 29

"Conditions: —400 volts, 120 minutes photolysis, —128°C. (0.04 ton isobutylene). Relative C yield and C / C i ratio is constant for all experiments. 2,2,4-trimethylpentane, ± 1%. 2,4,4-trimethylpentene-2, ± 4% (includes 2,4,4-trimethylpentene-l to about 5%). 2,2,3-trimethylpentane, ± 2%. 3,4,4-trimethylpentene-2, ± 2 % . 6

8

8

2

e

d

β

1

Applied Field Dependence. The magnitude of the applied voltage is expected to change the energy of the positive ions striking the liquid. The applied voltage was varied from —100 to —1200 volts at the bottom electrode. N o change was seen in the relative yields of C to C products or i n the distribution among the C products. Above —400 volts the total 8

i

2

8


372

RADIATION CHEMISTRY

II

yield of products was also constant. So in the voltage range investigated no applied field effects are observable. Gas Phase Photolysis. Two experiments were performed in the gas phase at 23° C. at a pressure of 5 torr isobutylene, 120 minute photolysis time and —400 volts applied field. N o C or C products were observed. Temperature and Phase Effects. As the liquid temperature of isobutylene is lowered, the vapor pressure is decreased. The freezing point of isobutylene is — 140°C. Table III shows the effects of temperature and phase on the percentage of C products and on the C product distribution. The apparent decrease in the percentage of C products is artificial; the decrease simply corresponds to a lower conversion of isobutylene and does not reflect a phase effect. The C product distribution remains about constant from —110° to —128°C. A t lower liquid temperatures and in the solid phase the percentage of C products with a 2,2,4-structure increases with respect to the C products with a 2,2,3-structure. Added Rare Gases. Added rare gases can collide with the gas phase ions and de-excite any excited ions. The effect of added argon and neon on the C product distribution is shown as a function of rare gas pressure in Table IV. Krypton was not used because it would absorb the photons from the krypton resonance lamp. Neon has little effect on the C product distribution, but argon causes the C products with a 2,2,4-structure to increase relative to the C products with a 2,2,3-structure. 8

12

8

8

8


8

8

8

8

8

8

8

Discussion Nature of Ion Injected into Liquid Isobutylene. The krypton resonance lamp emits photons at 1236 A . (10.0 e.v.) and 1165 A . (10.6 e.v.) with relative intensities of 1.00 and 0.28 respectively (5). The ionization potential of isobutylene is 9.4 e.v., and the lowest appearance potential for a fragment ion from isobutylene is 11.3 e.v. ( C H ) ( I ) . Therefore, the only ion produced is the parent C H \ Several mass spectrometric examinations of ion-molecule reactions in isobutylene have been carried out (2, 4, 10). Reaction 1 constitutes 76% of the total cross section; the secondary ions C H and C , H each 4

4

7

8

4

C H 4

8

+

+ C H = C H 4

8

+

4

9

+

+ C H 4

7

+

r

9

+

(1)

7

constitute about 12% of the total cross section (4). The C H ion is considered to have the tertiary butyl cation structure because of its great stability. The C H w i l l be formed about 5 cm. above the liquid isobutylene in an electric field of 60 volts/cm. at 400 volts applied so on the average it w i l l take 2 X 10" sec. to reach the liquid. The vapor pressure of isobutylene at —128 °C. is 0.04 torr; at this pressure collisions occur about 4

4

8

+

e


9

+

24.

viswANATHAN A N D KEVAN

373


every 2 X 10"° sec. Hence, the C H ion w i l l make about one collision before reaching the liquid which w i l l allow Reaction 1 to occur. The cross section for Reaction 1 is undoubtedly dependent on the kinetic energy of C H . However, the liquid phase products show no applied field effect between 100-1200 volts and are characteristic of C H reactions. W e conclude that most of the ions striking the liquid are ter£-C H \ There may also be small amounts of C H , C H , and 4

4

4

8

8

+

+

+

9

4

9

4

8

+

4

7

+

CBH . 9

+

Reactions of tert-C H \ The major product of terf-C H in liquid isobutylene is 2,4,4-trimethylpentene-2. ter£-C H can react with the two double-bonded carbons in i s o - C H . The reaction with the terminal car bon i n isobutylene is shown in Reaction 2. This reaction is estimated to be 4

4

9

4


4

ι

9

8

CH

CH3

CH3

3

CH3

Γ

Γ

ι

H C — C + C H = C -> H C — C — C H — C — C H I I I 3

+

2

3

CH3

+

9

+

2

CH3

(2)

3

CH3

about 15 kcal./mole exothermic from extrapolated heats of formation, and it gives the correct carbon skeleton for the major product. There are two basic neutralization reactions for C carbonium ions: one is hydride transfer to the carbonium ion to yield an alkane plus C H and the other is proton transfer from the carbonium ion to give an alkene plus C H . It is also possible that proton transfer occurs to trace amounts of water. Both the 2,2,4-alkene and alkane are observed; their relative abundances show that proton transfer predominates by 10:1. 8

4

7

+

4

9

+

A n alternative possibility for alkane formation is neutralization of the C ion on the internal electrode, followed by Η abstraction to produce an alkane. However, the alkane/alkene ratio is the same for internal or external bottom electrodes. 8

The other two C products indicate somewhat surprisingly that tertC H also reacts at the tertiary carbon is isobutylene. This yields the 2,2,3,3-tetramethylbutyl primary carbonium ion shown in Reaction 3. 8

4

9

+

CH3 C H

CH CH

3

3

= C H -> H C — C 3

CH3

C

3

CH3

3

CH

2

+

(3)

CH3 CH3

4 CH

S

H C—C 3

CHo

CH •C C

3

Η CH CHo


(4)

374

RADIATION CHEMISTRY

II

Primary carbonium ions as in Reaction 3 are expected to rearrange rapidly as in Reaction 4 to give the tertiary 2,2,3-trimethylpentyl structure because the tertiary carbonium ion is so much more stable. Proton transfer from and hydride transfer to this carbonium ion lead to the observed 3,4,4alkene and 2,2,3-alkane respectively. F o r this carbonium ion the results show that proton transfer again predominates, but only by 2:1. The principal neutralization reaction to form C products from both carbonium ions is proton transfer. The proton transfer Reaction 5 is 8

ferf-C H


8

+ i - C H -> terf-C H

+

17

4

8

4

+

9

+

terf-C H 8

(5)

ie

estimated to be exothermic by a few kcal./mole. This exothermicity arises from the conversion of an i s o - C olefin to a tert-C olefin; the greater branching i n the tertiary structure lowers its heat of formation compared with a secondary structure. Reaction 5 produces a chain reaction. Non-chain propagating neutralization can take place by reaction at the walls, at the electrode, to form alkane, or with trace water impurities. 4

8

It is striking that the total reactivity of terf-C H at both the primary and tertiary carbons in isobutylene is about 50%. Since fer£-C H shows so little selectivity i n this situation it was suggested that it was vibrationally excited (11). Evidence supporting this supposition is discussed later. 4

+

9

4

9

+

The C12 products have not been characterized as completely as the C products. Reactions 6 and 7 are both possible pathways to C i formation. 8

2

terf-C H 8

+

17

terf-C H 4

+

9

+ i - C H -> tert-C H + 4

8

12

+ f e r f - C H -> 8

ie

(6)

25

terf-C H 12

+

25

(7)

The retention time data on a A g N 0 / b e n z y l cyanide gas chromatographic column suggests that only C i olefins are formed. Thus, proton transfer seems to predominate as the neutralization reaction. Since no resolution of the C gas chromatographic peak has been obtained, it is possible that the C i product is largely one compound. In Reaction 6 it is expected that the £ e r f - C H i * ion w i l l be a thermal ion since it is formed i n the liquid, and that it w i l l react preferentially at the terminal carbon of isobutylene to give one olefinic product. However, i n Reaction 7 tertC H is vibrationally excited (see last section of Discussion) and should react at both double-bond carbons in C H i to give two olefinic products. The dose effects discussed i n the next section do not clearly distinguish between Reactions 6 and 7 since tert-CgHu* can be formed from tertC H i e by proton transfer. Both Reactions 6 and 7 are estimated to be exothermic reactions by about 15 and 7 kcal./mole respectively. O n the basis of existing evidence either or both Reactions 6 and 7 contribute to C formation. 3

2

J

2

2

8

4

9

7

+

8

6

8

i 2


24.

VISWANATHAN

AND KEVAN

375


Dose Effects on Molecular Weight. A dose effect is clearly exhibited in Figure 8. As the photolysis time increases the percent of C products decreases linearly while the percent of C i products increases linearly. The C 's extrapolate to 100%v and the d ' s extrapolate to 0 % at 10 minutes of photolysis time. The data do not guarantee that 10 minutes of photolysis time is significantly different from 0 minutes, but they do indicate that an induction period is present before the dose dependence of the product yields begins. The relative rise of C i products together with the decrease in C products can be explained by Reaction 7 or b y Reaction 8. The relationship shown in Figure 8 8

2

8

2

2

8

terf-C H Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch024

12

4- terf-C H

+

25

8

-> terf-C H

16

8

+

17

+

terf-C H 12

(8)

24

indicates that low concentrations of tert-CsHiQ can compete with i - C H for carbonium ions. Similar dose effects are noted in the ionic polymerization of irradiated ethylene in the solid phase (12). 4

8

Evidence for Excited terf-C H \ Vibrationally excited ions exhibit less selectivity in their reactions than thermal ions. Ions produced in the liquid phase are expected to be thermalized rapidly at the time of formation. However, vibrationally excited ions are commonly produced in the gas phase both by electron or photon impact and by ion-molecule reactions. In the ion injection method ions are produced in the gas phase and injected into the liquid. Thus, the possibility arises for injecting excited ions into the liquid and for examining whether excited ions w i l l show reactive selectivity i n the liquid phase. The formation and reaction of terf-C H in isobutylene by the ion injection method illustrate these possibilities. 4

4

9

+

9

The C reaction products in Table III illustrate that fer£-C H reacts nonselectivity at both the primary and tertiary carbons i n isobutylene. £er£-C H is formed b y Reaction 1 which is estimated to be 10-20 kcal./mole exothermic in the gas phase. It is postulated that vibrationally excited £erf-C H is formed i n the gas phase and exhibits this by nonselective reactivity in the liquid phase. T w o independent sets of experiments support this. W h e n ter£-C H * is de-excited in the gas phase by collisions with added argon, its reaction selectivity towards liquid isobutylene increases. When Reaction 1 takes place i n the liquid phase, nonexcited fer£-C H is formed which exhibits considerable reaction selectivity towards 2-butene. The reaction selectivity of terf-C H can be measured quantitatively from the C products. The percent of 2,2,4-trimethylpentane plus 2,4,4trimethylpentene-2 represents the percentage of attack of £erf-C H at the primary double-bond carbon i n isobutylene. A n d the percent of 2,3,3-trimethylpentane plus 3,4,4-trimethylpentene-2 represents the percentage of attack of t e r t - C H at the tertiary double-bond carbon i n 8

4

9

4

9

+

+

4

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+

4

4

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9

+

4

+

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8

4

4

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+


9

+

376

RADIATION CHEMISTRY

II

isobutylene. Thermal ions preferentially attack at the primary doublebond carbon. Argon and neon were tested as de-exciters for terf-C H . Both can de-excite vibrational levels of molecules as well as act as moderators for translational energy (3). Table I V shows that reaction selectivity for the primary carbon increases from 53% with no added argon to 76% with 3 torr and above added argon. However, from 1 to 10 torr of neon shows no effect on the C distribution. If neon and argon were acting entirely as translational energy moderators 3 torr A r would be equivalent to 4.5 torr neon. +

4

9

8

The absence of ion translational energy effects on the C distribution is also indicated by the lack of an applied field effect. W e conclude that vibrational de-excitation is occurring and that argon is much more effective for vibrational de-excitaton than is neon. Such a large difference in vibrational deactivation efficiency between neon and argon is somewhat surprising. However, deactivation of ions may be more sensitive to polarizability effects than is deactivation of neutral molecules.


8

The de-excitation experiments were done at a liquid isobutylene temperature of —128 °C. where the vapor pressure is 0.04 torr. C H makes 1-2 collisions before reaching the liquid at this pressure. Maximum effective de-excitation by argon occurs at 3-4 torr which means that about 100 collisions of C H with argon are necessary for maximum reaction selectivity. The selectivity of de-excited C H never rises above 78% whereas percentages approaching 90-100% are expected. The limiting observed value of 78% may be caused by those C H ions that are formed in a narrow layer above the liquid and are not de-excited before being injected. Table III shows that the reaction selectivity of tert-C H can also be increased by lowering the liquid temperature. Lowering the temperature lowers the isobutylene vapor pressure. A t 0.04 torr and above C H makes one or more gas-phase collisions with isobutylene to form vibrationally excited ter£-C H \ However, below 0.04 torr more and more of the C H ions are directly injected into the liquid. Reaction 1 then occurs in the liquid phase and the product ter*-C H ion is vibrationally deexcited. A t vapor pressures of only 0.004 torr the reaction selectivity reaches 90%. This suggests that thermal energy terf-C H ions are indeed quite selective in their reaction with isobutylene. In fact, 2,4,4trimethylpentene-2 accounts for 84% of all the C products. 4

9

9

9

8

+

+

4

4

4

+

+

4

8

9

+

4

4

4

9

+

9

+

4

+

9

4

+

9

8

Acknowledgment This research was supported by the A i r Force Rocket Propulsion Laboratory. W e thank R. Koob for help with lamp evaluation, J. Futrell


24.



377


for the mesh electrode suggestion, and S. Lipsky for use of a vacuum ultraviolet monochromator. Literature Cited (1) Field, F. H., Franklin, J. L., "Electron Impact Phenomena," Table 45, Academic Press, New York, 1957. (2) Fuchs, R., Z. Naturforsch 16a, 1026 (1961). (3) Kondrat'ev, V. N., "Chemical Kinetics of Gas Reactions," Chap. 6, Addison-Wesley Inc., Reading, Mass., 1964. (4) Koyano, I.,J.Chem..Phys. 45, 706 (1966). (5) McNesby, J. R., Okabe, H., "Advances in Photochemistry," p. 157, Vol. 3, W. A. Noyes, G. S. Hammond, J. N. Pitts, eds., Wiley Interscience, New York, 1964. (6) Okabe, H.,J.Opt. Soc. Am. 54, 478 (1965). (7) Schlag, E. W., Sparapany, J. J.,J.Am. Chem. Soc. 86, 1875 (1964). (8) Sparapany, J. J.,J.Am. Chem. Soc. 88, 1357 (1966). (9) Stief, L. J., Mataloni, R. J., Appl. Opt. 4, 1674 (1965). (10) Talroze, V. L., Lyubimova, A. K., Dokl. Acad. Nauk. SSSR 86, 909 (1952). (11) Viswanathan, N. S., Kevan, L.,J.Am. Chem. Soc. 89, 2482 (1967). (12) Wagner, C. D.,J.Phys. Chem. 66, 1158 (1962). RECEIVED January 8, 1968.


Radiation Chemistry-II

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