3.5.2 2,2,5-Trimet hylhexane

Initially, the mechanism for the 2,2,5-trimethylhexane system included all of the reactions except the isomerization reactions. Once again, comparisons of tlie predicted and observed yields of the major products formed (ace- t!one and 3,3 dimethylbutyraldehyde) indicate that the mechanis~n without alkoxy radical isomerization overpredicts the observed yields. Once isomer- ization react,ion steps corresponding to 1,s-H aton1 shifts are added to the mechanism, the experimental results are more closely represented. The pre- dicted yield for 3,3-dimethylbutyraldehyde is appr~xima~tely 22% larger than the observed yield. However, the mechanism predictions for acetonr are

still approximately 150% larger than the experimental results. This observa- tion indicates either that additional and significant isomerization reactions other than 1,5-H shifts are possible with the radicals formed in the pho- too~ida~tion of larger alkanes, rate constants corresponding to 1,5-I-I atom shifts for branched compounds are too low, or decomposition and O2 re- action rate constants for the alkoxy radicals are too high. Because there wags excellent agreement between observed and predicted concentrations for the case of 2,2,4-trimethylpentane using rate constant data available for 1,s- isomerization, decomposition, and O2 reactions of the alkoxy radicals, we must conclude that there are additional isomerization pathways for 2,2,5- trimet hylhexane that are insignificant for 2,2,4-trimethylpent ane. Therefore, 1.4-isomerization alkoxy radical reactions were included in the mechanisms, and the results compared to the observed data. The rate constants used for the 1,4-isomerizations were those determined from fits of the 2-methyl- I -propanal data from the photooxidation of 2,2,4-trimethylpentane (see the previous section). Inclusion of l,4-isomerization steps, where applicable, pro- vide a better fit to the acetone data. The predicted acetone yield is only 18% larger than the observed yield. The 3,3-dimet hylbutyraldehyde data re- nlained unchanged.

In

all cases, however, the formaldehyde and a~et~aldehyde yields are underpredicted. Experimental data and mechanism predictiorls for 3,:3-dimethylbutyraldehyde, acetone, acetaldehyde, and formaldehyde appear in Figures 3.14-3.1'7. respectively.

The underpredicted yields for a-cetaldehyde and formaldehyde, as in the case of 2,2,4- trimet hylpent ane, is likely a result of reactions of i~omerizat~ion products from the alkoxy radicals. To examine this, we used the predicted

2

^loo

Figure 3.14: Depicted are the observed and the predicted concentrations for 3.3- dinlethylbutyraldehdye from 2,2,5-trimethylhexane photooxidation. The p e d i ct,ed concent rations are based on mechanisms that exclude isomeriza- t)ions and include both 1,4- and 1,5- isomerizations.

0 200 400 600 800 loo0 1 200 1.360

A [2,2,5 Trimethylhexme], ppb

Figure 3.15: Observed and predicted concentrations for acetone from 2?2,5- trimethylhexane photooxidation. The predicted concentrations are based on mechanisms that exclude all alkoxy radical isomerizations, include only 1,s- isomerizations, and include both 14- and 1,s-isomerizations.

Figure 3.16: Observed and predicted concentrations for acetaldehyde from 2.2,s-trimethylhexane photooxidation.

The

line labeled "Correction Esti- mated" takes into account the formation of acetaldehyde from the reactions of alkoxy radical isomerization products.

200

la,

0

Figure 3.17: Observed and predicted concentrations for formaldehyde from 2,2,5-trimet hylhexane photooxidat ion . The line labeled "Correction Esti- mated" takes into account the formation of formaldehyde from the secondary react ions of alkoxy radical isomerization products.

concentrations of products resulting from the alkoxy radical 1,4- and 1,5- isomerization steps to calculate the amount of HCHO that would result from continued reaction of these products. Most of the pathways that are capable of undergoing 1,4- and 1,s-isomerizations lead to the direct formation of at least one molecule of HCKO. In addition, acetaldehyde is generated from the reaction of the 1,s-isomerization product resulting from the alkoxy radical,

TMHZA-0

(see Figure

3.7).

Assuming that only one HCHO molecule is formed in any of the reactions of the products of alkoxy radical isomerizations, a lower bound estimate for the production of HCHO is possible, and included

in Figure 3.1'7. The yield of the isomerization products from the single alkoxy radical isomerization reaction producing

CH3CH0

as a product was used to correct the (:H3CH0 yield, and the result appears in Figure 3.16. The predicted yields based on the formation of only one HCHO or CH3CH0 molecule per reaction of alkoxy radical isomerization product are still lower than the observed yields, but it is important to note that we have not taken into account the full reaction mechanisms of these products.

As discussed earlier, the neopentyl radicals are not expected to react to form 2.2-dimet hyl- 1-propanal in the presence of

NO,.

Regardless of whet her or not isonclerization reactions are included in the overall mechanisnls for 2,2 4-trimethylpentane or 2,2,5-trimethylhexam, the predicted yields of 2,2- d i ~ n e t hyl- 1-propanal are negligible. This result lends credence to the sup- positiorl that the 2,2-dimethyl-1-propanal detected in the photooxidation of 2.2.5-trimethylhexam is a result of secondary reaction of OH with 3,3- dirnethylhutyraldehyde, rather than primary formation from the neopentyl radical.

3.6 Conclusions

Experiments have been performed t o investigate the photooxidation of two relatively large alkanes, 2,2,4- trimethylpentane and 2,2,5-trimethylhexane.

The first kinetic study of the OH

+

2,2,5-trimethylhexane reaction resulted in a rate constant of (5.267f 0.147)xlO-l2 em3 molecule-' s-', in excellent agreement with a predicted value of 5.7 x10-l2 cm3 molecule-' s-', us- ing Atkinson's structure reactivity relationship[l5]. Carbonyl products have been identified from laboratory experiments with each alkane, and the major products observed are consistent with those expected from the mechanisms.

Mechanisms for each of the alkanes were developed to examine the effect of alkoxy radical isomerization reactions on predicted carbonyl yields. Using the mechanism for %,2,4-trimethylpentane photooxidation, upper limits for the rate constants of 1,4-H atom isomerization with abstraction from

-CH3

and

-CHz

groups have been determined as 1 x 1 0 ~ and 1 x 1 0 ~ s-', respectively.