Relative Stability of Isomers

Dehydration of 1-butanol and the Relative Stability of Alkenes

Dehydration of an alcohol such as 1-butanol produces a mixture of butenes: 1-butene, cis- and trans- 2-butene.

The relative amounts of each of these products can be determined experimentally. By computing the energies of each isomer, we can estimate their relative stabilities. If the thermodynamic stability of the isomers is in agreement with the experimentally observed product distribution, the reaction is said to be under thermodynamic control. If not, it is said to be under kinetic control.

Procedure

Construct 1-butene, then model build and do a geometry optimization. Record the energy for this structure. Repeat for cis- and trans- 2-butene.

Notice that 1-butene has two single C-C bonds. The HyperChem model builder and Geometry Optimizer do not necessarily optimize torsion angles (optimization finds the local minimum); in fact, the model builder occasionally assigns energetically unfavorable values. For this reason, it is a good idea to adjust some of the angles before concluding that a structure is stable.

View the molecule along the C-C single bonds and look for strained (eclipsed) conformers. To change the value of a torsion angle, select the four atoms defining the angle. From the Edit menu, choose Set bond torsion and enter the desired value. For Mouse-controlled rotation about a bond: with the Select tool, double-click on the rotation bond. This selects the bond and everything to one side it. Using the z-rotate tool, R-drag to rotate the selection about the rotation bond. Reoptimize the geometry and save the structure of the most stable conformer of each isomer.

Both the MM Energy and the heat of formation from a semi-empirical calculation can be used to determine relative stabilities of isomers. The heat of formation can also be compared directly with the experimental value (see table).

From your results, predict which isomer should predominate in the product mixture. If you have carried out the experiment and performed a GC analysis of the product, comment on the extent of agreement between your predictions and the experimental results. Is this reaction under thermodynamic or kinetic control?

To automate optimization and report the heat of formation and dipole moment directly to the HyperChem workspace: Enter the following text using any text editor, and save it as c:\hyper\yourfile.scr. To run the script from HyperChem you choose Script/Open Script and select yourfile.scr. The optimized strucuture is saved as chem.hin.

Lines beginning with a semicolon are for documentation purposes.

; Geometry optimization of structure in active window, using

; current setup values and computational method. Detailed

; output will go to chem.log.

 

; Open log file "chem.log"

; "no" = do not append to previous file

start-logging chem.log no

 

do-optimization

append-omsgs-to-file chem.log

stop-logging

 

; Save resulting structure in chem.hin

write-file chem.hin

 

; Confirm convergence and

; send values of heat of formation and dipole moment to screen.

; For molecular mechanics energy contributions, substitute

; total-energy, stretch-energy, bend-energy, torsion-energy, etc.

 

omsgs-not-to-file

query-value optim-converged

query-value heat-of-formation

query-value dipole-moment

exit-script