These are the homework exercises to accompany the Textmap for McMurry's Organic Chemistry textbook.
Name the following compounds:
Draw the following structures
1 = 1,2-dimethylcyclohexane
2 = 2-cyclopropyl butane
3 = 1,2,3-trimethyl cyclopentane
1 = 1-cyclopropyl cyclopropane
2 = 1-isopropyl cyclohexane
3 = 2-propenyl cyclopentane
Draw the following molecules:
trans-1,2-Dimethylcyclobutane is more stable than cis-1,2-dimethylcyclobutane. Explain this observation.
The trans form does not have eclipsing methyl groups, therefore lowering the energy within the molecule. It does however have hydrogen-methyl interactions, but are not as high in energy than methyl-methyl interactions.
If cyclobutane were to be planar how many H-H eclipsing interactions would there be, and assuming 4 kJ/mol per H-H eclipsing interaction what is the strain on this “planar” molecule?
In the two conformations of trans-cyclopentane one is more stable than the other. Explain why this is.
There are 8 eclipsing interactions (two per C-C bond). The extra strain on this molecule would be 32 kJ/mol (4 kJ/mol x 8).
The first conformation is more stable. Even though the methyl groups are trans in both models, in the second structure they are eclipsing one another, therefore increasing the strain within the molecule compared to the first structure where the larger methyl groups are anti to one another.
Consider the conformations of cyclohexane, chair, boat, twist boat. Order them in increasing strain in the molecule.
Chair < Twist Boat < Boat (most strain)
Draw two conformations of cyclohexyl amine (C6H11NH2). Indicate axial and equatorial positions.
Draw the two isomers of 1,4-dihydroxylcyclohexane, identify which are equatorial and axial.
In the following molecule, label which are equatorial and which are axial, then draw the chair flip (showing labels 1,2,3).
Original conformation: 1 = axial, 2 = equatorial, 3 = axial
Flipped chair now looks like this.
In the molecule, cyclohexyl ethyne there is little steric strain, why?
The ethyne group is linear and therefore does not affect the hydrogens in the 1,3 positions to say to the extent as a bulkier or a bent group (e.g. ethene group) would. This leads to less of a strain on the molecule.
For the following molecules draw the most stable chair conformation and explain why you chose this as an answer
1 = trans-1,2-dimethylcyclohexane
2 = cis-1,3-dimethylcyclohexane
1 – The most stable conformation would be to have the methyl groups equatorial reducing steric interaction
2 – The most stable conformation would be to have the groups equatorial this would reduce the strain if they were axial
Someone stated that trans-decalin is more stable than cis-decalin. Explain why this is incorrect.
Cis-decalin has fewer steric interactions than trans-decalin.