Chapter 5: Rings

How much is the energy barrier separating a pair of puckered cyclobutanes?

1.4 kcal/mol

How far is the full boat from the twist and chair? Can it be isolated?

1.5 kcal/mol from twist; 7.0 kcal/mol from chair. Cannot be isolated; it is not an energy minimum.

What is the standard state of an element?

25 C, 1 atm. Delta Hf taken as zero.

Internal angles in triangle, square, octagon, hexagon

Ideal sp3 angle is 109.5 Triangle: 120 Square: 90 Pentagon: 108 Hexagon: 120 **larger than ideal sp3**

What happens with planar heptagons and octagons?

Internal angles are 129 and 135, respectively, thus have substantial angle strain. Have severe torsional strain in planar position; some stress is relieved in nonplanar forms, but not fully like in cyclohexane. There is no obvious low-energy solution.

Internuclear vs. interorbital angles in Cyclopropane

Internuclear is 60, interorbital is 104.

How does increased angle strain affect energy?

More angle strain = higher energy = less stable moleculeInt

van der Waals forces

A slight attraction that develops between the oppositely charged regions of nearby molecules

How do you calculate bond difference given Hf values of __ane's in a row?

Average of many bond differences.

Delta Hf trend for strain-free molecules cyclopropane --> infinity

Begins negative then gets more negative.

Difference in breaking central C-C bond in butane vs. cyclopropane

Butane requires 88 kcal/mol while Cyclopropane requires about 65 kcal/mol. We see that strain destabilizes cyclopropane by about 88-65 = 23 kcal/mol.

How does a chair convert to a twist-boat shape? What does each phase represent?

Chair: ideal. There are 2 forms. Half chair: Top of the energy "mountain pass;" transition state; unstable; many eclipsed C-H Twist: Molecular "valley," eclipsed become somewhat staggered, angle strain partially relieved.

Why is it difficult to measure the total strain of cyclopropane?

Composed to torsional plus angle strain; need calculation/experiment.

How do you tell the energy difference between two isomers that produce similar (sometimes identical) products?

Difference in their heats of combustion = difference in energy between them.

What are the two forms of comparable energy of planar cyclopentane?

Envelope and twist.

What is necessary for a chair to interconvert to other chair form?

Environment needs 10.8 kcal/mol. Easy under normal conditions, but impossible under freezing temps. One form will be isolated.

Why do half chair and full boat have so much strain?

Half chair: Much of the ring is planar. Both angle and torsional strain occur. Full boat: Has angle and torsional strain & H-H interaction that is destabilizing. **Van der Walls strain**

How far away are the half-chair and the twist from the chair in terms of energy?

Half-chair: 10.8 kcal/mol above chair. Twist: 5.5 kcal/mol above chair.

How do heat of combustion graphs show stabilities of molecules?

Higher heat of combustion = higher energy level of molecule = less stable.

Tortional strain (eclipsing strain) in cyclopropane

Hydrogens are stuck in eclipsed arrangement; no possible release, raising the energy of the cyclopropane

What is positive vs negative delta Hf?

More negative: More stable. Formation from constituent elements is exothermic (heat released, absorbs cold) More positive: Constituent elements are more stable than compound. Formation = endothermic, releases cold/absorbs heat.

How do you calculate strain E from delta Hc?

Multiple strain E per CH2 by how many carbons are in the compound name. I.e., for cyclopropane, multiple Strain E per CH2 8.8 x 3 carbons = 26.4 Strain EE from Delta Hc

Do chairs have to go through energy minimum full-boat conformation to interconvert?

No

What is the normal tetrahedron bond angle, and what makes it bigger or smaller?

Normal is 109.5 degrees. Small ring will constrict to less than 109.5 Large ring will expand to more than 109.5 **This is called "angle strain"**

What is the sign convention for energy as the product of a reaction vs. enthalpy?

Product of rxn = "+ energy" Enthalpy for exothermic = negative (system loses energy by releasing heat into universe) Enthalpy for endothermic = positive (system gains heat by absorbing heat from universe and emitting cold)

How does cyclobutane balance torsional and angle strain?

Reducing torsional: Changing 4-membered ring from planar to "puckered" (if this lowers the energy). Ring puckers around 34 degrees and C-C-C bond angles close to about 88 degrees.

Rotational barrier in ethane

Result of 3 pairs of eclipsed H; amounts to 3 kcal/mol. So, energy cost of each pair is likely 1 kcal/mol. Aka, a staggered conformation is about 3 kcal/mol lower in energy than eclipsed.

How does twisting planar cyclohexane change the torsional and angle strain?

Strain on BOTH decreases. Since 120 angle is made smaller, closer to ideal 109.5, angle strain decreases. Torsional strain from 12 C-H bonds contributes 12 kcal/mol of strain, which is decreased in nonplanar form. **Essentially all strain is gone!! Internal angle is about 111.5**

How to calculate Strain E per CH2 given the chart

Subtract given strain E per CH2 from ideal strain energy per CH2

Why doesn't cyclohexane follow the ethane eclipsed H rule?

The C-H bonds point away from each other more than ethane ones. Eclipsing strain must be significantly less than 6 kcal/mol.

Even when there are relatively small energy differences between isomers in equilibrium...

There is still an enormous preference for the more stable isomer of the pair, about 10^4 to 1.

What is the result of an infinitely large ring?

You can stagger all C-H bonds; there is no angle strain. This is the next strain-free species after cyclohexane.