Thermochemical studies of organometallic compounds

Joumul

of Organomerallic

EIscvier Sequoia

SA..

Chemistry

305

Lausannc

Primed inThe Ncthcrlmds

THERMOCHEMICAL STUDIES OF ORGANOMETALLIC II. THERMOCHEMISTRY OF METHYLCHLOROSILANES

S. N. HAJIEV

AND

Institure of P&sics,

COMPOUNDS

M. J. AGARUNOV

Acadenly of Sciences of the Azerbaijan SSR, Baku (U.S.S.R.)

(Received November 5th, 1969)

SUMMARY

This paper presents the results of the experimental determination of the heats of formation of trimethylchlorosilane and methyltrichlorosilane and the calculation for the heat of formation for a sixth ccmpound in this series-gaseous monomethylmonochlorosilane has been estimated_ In addition, the thermodynamic properties of the methylchlorosilanes have been listed. These were calculated on the basis of our enthalpies of formation and the entropies of these compounds cited in the literature.

INTRODUCTION

In an earlier paper’ we discussed a new method for the combustion of liquid organochlorosilanes in a calorimetric bomb, and the determination of the enthalpies of formation of liquid (CH3)2HSiCI, CH3HSiC12 and (CH&SiC12_ These were found to be -79.8+0.6, -105.9f0.8 and -118.4f0.5 kcal-mole-‘, respectively_ JZXPERIMENTAL

The parameters of these compounds, which were purilied by using a highly efficient rectification column of a special design, are summarized in Table 1. Under the conditions described previously’, methyltrichlorosilane burned with either a weak explosion or no explosion, thus resulting in incomplete combustion. To ensure a sufficiently strong explosion with methyltrichlorosilane, glass spheres’ of a larger diameter (44 mm) were used and the distance of the heater from the igniter was maintained in the range 28-30 mm. In other respects the experimental procedure, operating conditions, calorimeter parameters, and measuring circuit data were similar to those described in our previous paper’. RESULTS

The results of the calorimetric

experiments are presented in Tables 2 and 3. J. Organometal.

Chem., 22 (1970) 305-311

306

S.N.

HAJEV,

M. J. AGARIJNOV

In these Tables:

E tcont_J = energy equivalent of the bomb contents nl = rz2= a = i: =

number of moles of hydrazine dihydrochloride in reaction (lp number of moles of NH&l in reaction (2)* heat evolved according to reaction (I)* heat evolved according to reaction (2)* q3 = heat due to electric heating q4 = heat due to the bomb rotation q5 = heat of dissolution of CO, in water q6 = correction for reduction to HCl - 600 Hz0 q7 = heat evolved on HN03 formation qs = correction for admixtures. Correction for the heat of evaporation of water in the bomb is 1.33 cal for every experiment. Intrinsic energy change and combustion enthalpy hold for the reaction : (CHS),SiCl,_,(liq)+2n

O2 (gas)+(2402-602

n)H20

(liq) -

SiO, (amorph., hydrat.) + n CO, (gas) + (4-n) TABLE

[HCl - 600 Hz01 (liq)

1

PARAMFIERS

FOR PURE

Compound

D¶ETHYLCHLOROSILAN!3

B.p. CC) Actual

dzo 4

nF

(mm I-W

Recalcd. for 760 mm”

(CH&SiCl

57.6,(759)

57.7,

0.8590

CH3SiC13

66.4,(768)

66.1,

1.2761

Chlorine content of compound (%)

Composition” (%)

Found

Calcd.

1.3890

32.7

32.63

Basic compd. SiC14 CHsSiC13

99.76 0.17 0.07

1.4123

71.1

71.15

Basic compd. (CH&SiCl

99.80 0.20

DRecalculation for 760 mm was carried out according to ref. 2. * Determined by gas chromatography (see ref. 1). DISCUSSION

In order to calculate the heat of combustion of the methylchlorosilanes study, it was assumed3 that: A@’ CO2 (g) = -94.052+0.008

kcal*mole-’

A@

kcal-mole-’

H,O (1) = -68.315+0.009

AM,O[HCl in 600 H,O]

(1) = -39.823+0.01

* See ref. 1. J. Organomerai.Chem., 22 (1970) 305-311

kcal-mole-l

_

under

z

E

5 s

-9.0 -9,o -9.0 -8.7 -9.0

0.7362 0.8454 0.8122 0.6915 0.7807

-3.7 -9.0 -8.8 -8.8 -9.0

-1855.53 -2130.99 -2047.17 -1742.93 -1967.92

$$cg.-') Ax,,,

(&,I + &““J

IN ~14c IXTI~MINATI~N

(14 (Cal)

(13 (Cal)

(cd)

(IS

‘II $2)

$&

$11)

Ed)

0.000925 0.00012 123.90-0.42 462.7837.350.34 0.000863 0.00020 115.65-0.70 771.5437.350.34 0.001010 0.00023 135.21-0.81 690.1537.370.33 0.000724 0.00009 97.10 -0.32 397.1337.34 0.37 0.000885 0.00017 118.60-0.60 463.1837.350.34

(g-mole) ~-mole)y;ilI)

-

0.000474 0.00010 63.50 -0.35 11806337.370.78 0.000930 0.000255124.50-0.90 362.1637.330.86 0.000523 0.00022570.03 -0.79 158.5737.350.65 0.000830 0.00020 111.95-0.70 323,3937.340.67 0.000622 0.00014 83.33 -0.49 96.4437.350.83

(12 (Cal)

(CI-f3)$iCt (l.lOUll~)

(LIQUID) 0~ *rluz F~RMAIION ENT~IALPY 01: CH3SiC13

-2894.86 -3497.25 -2442.36 -2710.09 -3051.65

-Au~(mean)=1894.40~5.3Q cal*g-' -AU~=283.2&-0.8 kcal~mole-' -AH,O=283.8*0.8 kcal*molc-* -A.H~=147.21tO.9 kcal-mole-‘.

0.64442 0.63507 0.62022 0063647 0.71071

(6)

g a a” >

5! 3 .: E

gubstance) $&j

EXPERIMENTAL RIULTS

TABLE 3

9

3

1*1484 1.3874 0.9689 1.0753 1.2106

-Au~(mcan)=6618,95f7.80 calag-' -ALJ~=719.1-t_O.8 kcal*mok?-' -AH~=720.9_t0.8kcal~mole-' -A.N:=91,8f0,9 kcal*mole-'.

0.40417 0.44836 0.32824 0.33993 0.42756

EXPERIMENTAL RIZULTS IN TIE DETERMINATION OF THE FORMATION ENTIIALI’Y OI:

TABLE 2

-AU,

1891.87 1887989

(cal*g - ‘)

-2.37 -3.43 2.48 2.62 8.44 9.43

&I)

6610.59 6624073 6628,OO 6608.48 6628095

-Au, (cal$.')

1901.08 1890.90 1900.26

111)

199 2.21 1.62 L67 2.10

$1)

-3.50 2.76 8.55 -3.07 1.93 8.42 -3.87 3.31 8.23

741)

-0.97 3.47 - I,27 3.43 -0.69 1.37 -0.82 1.49 -1003 2.76

(Cal) &I)

(16

S. N. HAJIEV,

308

M. J. AGARUNOV

The selection of the enthalpy of formation for the amorphous, fmely dispersed hydrated silica which is formed when burning organosilicon compounds, is difficult. Although this value is fundamental for thermochemistry of organosilicon compounds, its selection has not yet been thoroughly anaIysed. Therefore, we think it necessary to give, on the basis of a survey of literature data, our own considerations about the selection of this value. The enthal_py of formation for or-quartz reported by Hubbard and co-workers4 as -217.72_+0.34 kcal-mole-’ is now universally accepted. This value is also confirmed by the results of Good’ (-217.5+0.5 kcal.mole-l) obtained by another independent method. Later, Hubbard and co-workers6 in another paper made a reliable determination of the enthalpy of amorphous glassy silica formation as -215.94+_0.31 kcal-mole-‘. The heats of formation of other SiO, modifications are calculated from the data on the heats of quartz transformation into these modifications, obtained by measuring the heats of dissolution of the Iatter in HF’- 14. The published results though not in good agreement, make it possible to suggest that the heat of quartz transformation into a certain amorphous modification will rise (in terms of absolute value) from 1.0 to 3.0 kcal- mole- ’ with increase in the degree of dispersion of amorphous Si02 (m ref. 10 AH”[SiO,(quartz)-+SiO,(amorph., finely dispersed)] =3.3 kcal - mole- ‘, but in view of the findings of other workers’ 1- l3 this value may be considered to be somewhat overestimated)_ Even less clearly understood are the solubility in water and hydration heat of various amorphous forms of Si02 in water. The value AH,,,,,: SiO,= + 1.5 kcal. mole- i , reported by Mulert’, is obsolete and erroneous. It is surpnsmg that it has been used by the authors of fundamental reference work3*’ 5_Thiessen and Koemer’ 6 have reported that the values for the heat of decomposition of hydrates, SiO, *2H20, 2Si02 -3Hz0 and SiO+ - Hz0 are in the range + 1.2 to + 1.5 kcal per mole SiO,, i.e., this value is opposite in sign to that of Mulert. Some otjer works”-” dealing with the silica hydration process also indicate that this process is exothermal. It can be readily concluded therefore that increased specific surface of the material results in increase of the hydration heat in terms of absolute value, i.e., it is expected to be greatest in finely dispersed SiO,. It has been shown’g that with increased specific surface of the compound, the hydration heat varies from - 0.2 to - 2.2 kcal mole- l_ Aconsiderationoftheabove work7-14~16-1g sh ows that the heats of formation of amorphous hydrated silica are independent of the type of amorphous silica, i.e., that AH,” Si02 (amorph., any type, hydrated) = AH: SiOt (quartz) + + AH0 [SiO, (quartz) -

SiOZ (amorph.)] + AHhOydra,. SiO, (amorph.) x const.

Moreover, this value is close to the heat of quartz formation, since the heat of quartz transformation into a certain amorphous silica and the hydration heat for an amorphous SiO, of the same type are approximately equal in magnitude but opposite in sign. Therefore, we decided to use the data of the particular SiOZ type for which the two values, AH: and ‘AHhOydrac_t had been accurately determined_ As mentioned above, A@ has been reliably determined for amorphous glassy silica only (AH; SiOZ (amorph., glassy) = - 215.94+ 0.3 1 kcal- mole- ’ “). The heat of hydration indicated for this type of silicaI - l8 is - l-5+0.5 kcal-mole-‘. For these reasons, we have selected the value AH! SiO-, (amorph., hydrat.)= -217.4+0.5 kcal.mole-‘. It should J. Organometal.

Chem., 22 (1970) 305-311

THJ5RMOCHEMICAL

STUDIES OF ORGANOMETALLIC

COMPOUNDS.

309

II

be noted that although this value is reasonable at the present time, it is far from being indisputably and ctinclusively established. A wholly reliable AH: Si02 (amorph., hydrat.) value can only be determined using special precision measurements which are planned for future studies. The experimental heat-of-formation values for five methylchlorosilanes, can be used to calculate the enthalpy of formation of the sixth compound in this series, gaseous CH,SiH$X A simple and rather accurate calculation method is that applied by Bernstein” to methane derivatives. The bond and interaction contributions (Table 4) necessary for this procedure were found from eqn. (1) (on the basis of the experimental values) using the least-squares method. The heats of evaporation of the methylchlorosilanes were taken from the literature ‘l -22. The enthalpy of formation of gaseous CH3SiH&l was found to be -60.2 kcal -mole- ‘. TABLE 4 BOND

AND

INTERACTION

CONTRIBUTIOKS

FOR

CALCULATING

THE

THERhlOCHEhlICAL

PROPERTIES

OF ‘METHYL-

CHLOROSILAh=

(for gas at 25”, in kcal~mole-‘) Interaction contributions

Bond contributions for AH,” (kcal)

for SO (e.u.)

Interaction

1.97

12.42

ACH(cH,)I

Si(CH3)

14.41

21.38

A(HCI)

SiCl

40.97

19.73

AL-CUCH,)J

Bond

SW

for S) 0.33 -0.02 0.26

for SO (e.u.) 0.03 1.28 0.8 1

The mean energies of bond cleavage calculated from our experimental using the least-squares method are: E(Si-CHJ E(Si-H)

data

= 74.8 kcal = 81.8 kcal

E(Si-Cl) = 98.0 kcal. The entropies ofmethylchlorosilanes have been studied by many authors23-26. To achieve good agreement these values were also handled using Bernstein’s method. The calculations were carried out with the following formula: P [SiH,(CH,),Cl

b_n_,Jgas)J

= n(SiH)+m

[Si(CHJJ+(4-n-m)(SiCl)+

+nmA[H(CHJJ+n(4--n--m)A(HCl)+m(4-~---m)A[(CH3)Cl],

(1)

where P is caIcuIated property of the molecule (in this instance the enthalpy of formation or entropy), the superior lines denote different bond contributions to the magnitude of this property and A denotes contributions of interactions between atoms two at time. J. Organometal.

Chem., 22 (1970) 305-311

310

S. N. HAJIEV,

M. J. AGARUNOV

Table 5 gives our recommended values of enthalpy of formation (AH:), entropies (So), entropies of formation (AS:) and isobaric potentials of formation (AGF) for the methylchlorosilanes in the gaseous state. TABLE

5

THERhfOCHEhllCAL PROPERTIESOF METHYLCHLOROSILANES (for gas at W) Compound

SiCHaCl, Si(CH&& Si(CH&Cl SiHCH,C12 SiH(CH&CI SiH2CHsCI

*

A@ (kcal-mole-

‘)

- 139.8 - 110.9 -84.6 -99.0 -73.0 -60.2

SO (e.u.)

AS,0 (e.u.)

83.0 85.5 86.3 77.5 77.9 69.4

- 49.6 -68.7 - 89.3 -44.1 - 65.2 -41.1

- 125.0 -90.4 -58.0 -85.9 -53.6 -47.9

CONCLUSIONS

1. The enthalpies of combustion of trimethylchlorosiIane and methyltrichlorosilane were measured using explosive combustion in oxygen in a rotating tantalum bomb; the foIlowing values were obtained: AH!(CH,),SiCl (liq) = - 720.9 +_O.Skcal - mole- ’ AHE CHsSiCl, (liq) = - 283.8 + 0.8 kcal - mole- I_ 2. The enthalpies of formation of the compounds under study were calculated : AH: (CHs)sSiCi (iiq) = -91.8 t-O.9 kcal -mole-’ AH: CH,SiCI, (liq) = - 147.2 +_0.9 kcal -mole- ’ . The value of the enthalpy of formation of trimethylchlorosilane obtained by Beezer and Mortimer27 using the hydrolysis method is AH~(CH&SiCl (liq)= - 91.9 + 0.8 kcal.mole- r. It is the only value given in the literature and is in a very good agreement with our data. 3. The enthalpy of formation of monomethylmonochlorosilane was calculated on the basis of our experimental data using Bernstein’s method as AH,” CH,SiH,Cl (gas)= -60.2 kcal*mole-‘. 4. The thermochemical parameters of methylchlorosilanes (AH:, So, AS,“, and AGF) were computed and tabulated. 5. The selection of the value AH: SiO, (amorph., hydrat.) = - 217.4-&0.5 kcalmole- l was substantiated by a literature analysis. . REFERENCES

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Chem., 22 (1970) 305-311

THERMOCHEMICAL

STUDIES OF ORGANOMETALLIC

COMPOUNDS.

II

311

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Nall.

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