Structural Exercises:

Gasoline is a mixture of relatively low boiling, volatile hydrocarbons. The octane rating for gasoline relates to the relative amount of isooctane, whose jmol structure is imaged in the Title Box, and compounds with similar combustion characteristics in the mixture of hydrocarbons that is sold as gasoline. The higher the content of these compounds the higher the octane rating and the better combustion performance of the fuel. Also, the more you pay per gallon of gasoline.

The capstone question that you are to answer after going throught the exercises is: Are more stable or less stable hydrocarbons better economically as automobile fuels?

Part I:

Experimental combustion data follows for three constitutional (structural) isomers of pentane in the condensed (liquid) phase from the NIST (National Institute of Standards) WebBook. The heat of combustion data lists the lowest and highest values from a series of different reported experimental values.
 

A) Complete and balance the chemical equation for the combustion reaction.

___ C₅H₁₂ (l) + ___ O₂ (g) → ________ + ________

Rank the alkanes in order of stability from most stable to least.

____ > ____ > ____

B) Draw an energy diagram that illustrates the combustion data for the three compounds.

C) Briefly explain why the compounds don't produce the same heats of combustion even though they have the same molecular formulas and same number and types of bonds.

D) Determine the one that you would want to burn in your car if they each sold at the same price per gallon. Use $4.50/gallon as a price. (Note: It is $ per unit of volume). 1) Calculate the amounts to complete the table. You will need the respective density for each isomer. They are quite easy to find using ChemFinder. Circle the most economical selection for your car (1), (2) or (3). 2) Show one calculation for $ /mol and another for $/kJ using one of the three constitutional isomers as an example. (Attach on a separate sheet if necessary.)

Part II:

Search for condensed (liquid) phase, and gas phase combustion data (Δ_cH) for C₅H₁₀ in the NIST database (WebBook). It can be accessed through the course Web resources page . You should find 14 or 15 structures in your search. (The last two or three are redundant! So there are actually twelve different constitutional or geometric isomers (E,Z or cis,trans). There will not be both gas and liquid data for all compounds.

A) Organize the 12 compounds into two groups: an acyclic group (6 compounds) and a cyclic group (6 compounds). Enter line structures for each in the table below; beginning with the largest ring structure for the cyclic and the least substituted for the acyclic. Add the respective combustion data for each compound to the table using data for combustion in either the condensed phase (liquid) or the gas phase DcH from the NIST WebBook.



Draw energy diagrams which illustrates the combustion data for each of the cyclic and acyclic compounds. (Separate those that you compare in the gas phase from those in the condensed phase, and compare separately.)

B) For the cyclic group (C) rank the compounds in order of stability from highest to lowest.:
Gas phase:

____ > ____ > ____ > ____ > ____ > ____

Condensed phase:

____ > ____ > ____ > ____ > ____ > ____

C) For the acyclic group (A) rank the compounds in order of stability from highest to lowest.:
Gas phase:
____ > ____ > ____ > ____ > ____ > ____

Condensed phase:

____ > ____ > ____ > ____ > ____ > ____

D) Analyze the data looking for patterns and trends. Explain any trends and what structural features could contribute to differences between the structures of the isomers and the energy produced from burning each of them.


E) From the data are cis or trans isomers more stable? Briefly explain why from a theoretical, molecular basis.


F) From the data are larger rings more or less stable? Briefly explain why from a theoretical, molecular basis.

Part III:

Consider your answers to questions IA) and ID). Are more stable or less stable hydrocarbons better economically as automobile fuels? Explain why by referring to specific examples from the series of componds you examined.

(Bonus Questions):

2-methylbutene produces 2-methylbutane on hydrogentation. 111.6 kJ are given off per mole of 2-methyl butene. Isomerization of n-pentane produces 2-methylbutane. The heat of reaction is - 7.78 kJ/mol.)

1) What is the energy difference in the stabilities of n-pentane and 2-methylbutane? Provide a value for the stability difference in kJ relative to the more stable isomer.

Double bonds in common ring systems are almost always cis, eg. cyclohexene. However, in cyclooctene, trans-cyclooctene is possible as well as cis-cyclooctene.

2) Explain what could possibly account for this apparent anomaly.

3) Which would be expected to be energetically more stable cis-1,2-dimethylcyclopropane or trans-1,2-dimethylcyclopropane? Briefly explain your answer and if the difference is significant.