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Home , Cyclodecane, Strain (chemistry)

Chapter 2 Strain and Stability

- Reactivity of a new molecule -Prediction of the lowest conformation of a new molecule --> a rapid evaluation of strains and stabilizing effects 2.1 Thermochemistry of stable molecules (strain and stability) 2.1.1 The concepts of internal stain and relative stability

Strain: Is typically associated with a conformational distortion or nonoptimal bonding situation relative to standard organic structures. The reference structure lacks the particular strain. : It is the energy held or stored within a molecule. Part of this energy can be released when given an outlet such as a chemical reaction. When a molecule has a higher (internal energy), it is less stable and/or more strained. 2.1.2 Types of energy (ΔGo): It is change in ΔGo between two different chemical states that determines the position of equilibrium between these states.

A ← → B

R = 4.184 kJ/mol ΔHo : (kcal/mol) ΔSo : (cal/mol·K)

Keq is influenced by

Exergonic, when the Gibbs free energy of B is lower than A, spontaneous conversion Endergonic, when the Gibbs free energy of B is higher than A Enthalpy (ΔHo): The change in enthalpy is defined as the change in heat between two different compositions of an ensemble of molecules at constant pressure if no work is done. Exothermic, negative ΔHo Endotherimic, positive ΔHo

Entropy (ΔSo): is a measure of the disorder of the system. -> degrees of freedom the more degrees of freedom, the greater the entropy. There are three different kinds of degrees of freedom: translational, rotational and vibrational. Translational and rotational refer to the translation of the molecule throughout space and tumbling of the molecule, respectively. Vibrational refers to every kind of internal motion of the molecule, such as bond stretches, bond rotations and various forms of bond angle vibrations. While entropy is certainly important for significant changes in chemical structure (such as cyclization), often when comparing two similar structures, the difference in will be fairly small. Thus, ΔHo is mainly considered in reactions.

2.1.3 Bond dissociation

Is defined as ΔHo. (bond strength)

Homolytic cleavage

A larger BDE implies a less stable radical. 1. F > OH > NH2, the larger the electronegativity difference, the stronger the bond. 2. Shorter bonds are stronger bonds; O-H > N-H> C-H 3. F > Cl > Br > I; move down the periodic table, the valence orbitals of X get progressively larger. The larger orbital size leads to a size mismatch with the carbon valence orbitals, and this weakens the bond by decreasing orbital overlap. 4. Hybridization C(sp)-H > C(sp2)-H > C(sp3)-H, more s character in a hybrid orbital makes the group more electronegative and decreases the bond length. 5. Resonance

PhCH2-H (88 kcal/mol) and CH2=CH-CH2-H (86 kcal/mol) 6. O-O bonds of peroxides: generally very weak 2.1.4 An introduction to potential functions and surfaces: bond stretches

Vibrational energy states

X. + Y.

Anharmonic oscillator X-Y

E = (n + ½)hv (n=0, ZPE) frequency = v = 1/(2π) √ k/μ k = force constant

μ = reduced mass, (m1 +m2)/m1m2 Infrared spectroscopy k v = 1/(2π) √ k/μ k = force constant m1 m2 μ = reduced mass, (m1 +m2)/m1m2 Hooke’s law v = 1/(2π) √ k/μ frequency v = 1/λ = v/c= 1/(2πc)√k/μ Wavenumber(cm-1)

1) C-C C=C C≡C 450-500 cm-1 1617-1640 cm-1 2100-2260 cm-1

2) C=C-C=O C=O 1690 cm-1 1730 cm-1 O O- + less double character 3) O O O

Cl R MeO R Me R

O- O- + MeO R +Cl R wavenumber the largest the lowest middle 2.2 Thermochemistry of reactive intermediates 2.2.1 Stability vs persistence 1. stable; thermodynamic notion <-> unstable 2. persistent; kinetic notion (kinetically inert) <-> labile (reactive) 참조

1,3- ethylene more stable less stable <- extra orbital interaction between C2 and C3 more labile less labile HOMO energy (butadiene) > HOMO energy (ethylene) LUMO energy (butadiene) < LUMO energy (ethylene)

ψ 4 Π* ψ3 ψ 2 Π ψ1 1,3-butadiene ethylene

Stability: determined by the energies of all the filled orbitals Lability (reactivity): must consider the energy of HOMO or LUMO 2.2.2 Radicals

1. BDE; methane > > propane > isobutane <- radical stability 3o > 2o > 1o > methyl 2. Allyl and benzyl radicals -> substantially stabilized (resonance effect)

Allylic radical rotation barrier ~ 15.7 kcal/mol (resonance structure) In many cases, radical species are unstable but in some cases there are stable radical species.

Commercially available 2.2.3 Carbocations

Hydride ion affinity (HIA), ΔHo

A larger HIA implies a less stable carbocation. 1. Heteroatom effects

+ + + stability: NH2CH2 > HOCH2 > FCH2 .. + + X-CH2 X=CH2 Consider both inductive and resonance effects 2. Hybridization effects stability: sp3 > sp2 > sp Consider electronegativity

vinyl cation and phenyl cation: ~287 kcal/mol HIA verse Ethyl cation ~ 273 kcal/mol, propyl cation ~266 kcal/mol HIA propargyl cation ~270 kcal/mol verse allyl cation ~256 kcal/mol

3. Aromaticity and antiaromaticity

stability: >>

HIA 201 212 258 aromaticnon-aromatic antiaromatic resonance effect 4. Planarity and pyramidalization Carbocation: planar

Ring constraints prevent Planar but 2o cation the ion from achieving Planarity. But 3o cation

relatively small difference in HIVs (9 kcal/mol)

In solution

Carbocations are formed in solution by SbF5 (Olah, 1994, Nobel Prize)

Lifetimes of carbocations

3o carbocations: 10-10 s in water 2o carbocations: 10-12 s in water 2.2.4 Carbanions

Stability of carbanions is related to pKa values. The smaller pKa value implies a stronger acid.

aromatic

anti-aromatic 2.3 Relationships between structure and energetics -basic conformational analysis 2.3.1 Acyclic systems-torsional potential surfaces

rotation barrier: 3kcal/mol ethane t1/2 = ln2/k Barrier height

Similar; consider size and bond length

Lower than C-C, lone pair < C-H

Allylic (A1,3) strain 2.3.2 Basic cyclic systems

Cyclic propane

Bent bonds (sp4~sp5)

115o larger than H-C-H (106o) (sp~sp2)

C-C-C, more p character to reduce bond angle (sp 180o, sp2 120o, sp3 109.5o) -> C-H more s character -> more acidic than C-H

Strain energy of : 27.5 kcal/mol (results from deviation of bond angles from normal values and eclipsing C-H interactions) and

very small barrier (1.45 kcal/mol)

Strain energy: 26.5 kcal/mol puckered conformations

Strain energy: 6.2 kcal/mol

Two forms are very nearly equal in energy and they interconvert very rapidly (the barrier is < 2 kcal/mol) 5’ 3’

4’ 1’ 2’

O

N NH

HO 5' N N NH2 O 3' 2' 1' 4' OH H (OH)

Newman projection : ΔGo of two structures (axial and equatorial)

Not much difference similar R R consider size and bond length H

R H R Conformational interconversion of cyclohexane Larger rings

~ 3 kcal/mol more stable cyclodecane Bicyclic ring systems

spiro: a molecule has two rings and two share only one carbon in common Strain energy 2.4 Electronic effects

2.4.1 interactions involving π systems

Substitution on :

The more substitution, the more stable; interaction of a filled π(CH3) orbital with the π* orbital.

π* - H H+ π(CH ) H H 3 H H H : no-bond resonance

trans-alkenes are more stable than cis-ones.

Stability, CH2=CH2 < CH2=CHMe < cis-CHMe=CHMe < trans-CHMe=CHMe

< CMe2=CMe2 < CHMe=CMe2 steric effect Conformations of substituted alkenes: Eclipsed conformers are more stable than staggered ones.

2 kcal/mol more stable although there is steric hindrance.

Repulsive interactions filled π filled π

filled π(CH3) H H filled π(CH3) eclipsed staggered Repulsive interactions between filled π(CH ) and filled π Attractive interaction 3 π∗ C1 C2-C3: strengthened filled π(CH ) C2 3 C1-C2:weakened C3 H Allylic (A1,3) strain

eclipsed; more stable A1,3 strain H C R H3C R 3 R = CH ; ~90% 3 H CH3 R = TMS; ~98% OH HO CH3 H the most stable conformer

H3C R OH H H3C H C R H3C R 3 H3C R VO(acac)3 O O + H H H tBu-OOH CH CH3 HO 3 HO CH3 hydroxyl group directing HO epoxidation 90% 10% R = H acac: acetoacetate Carbonyl compounds

1 kcal/mol more stable

1-methylallyl cation Conjugation

preferred steric effect

Diels-Alder rxn

5 kcal/mol more stable Aromaticity Planar, cyclic, fully conjugated π systems Homoaromatic (4n+2)π electrons: aromatic, 4nπ e-: antiaromatic

Ha Hb Ha Hb

Homoaromatic: systems in which a stabilized cyclic conjugated system is formed by bypassing a saturated atom.

- 6π e- 6π e

μ = 0.8 D (HBr; 0.828 D) Aromatic compounds: ring current, downfield shift at 1H NMR spectra

H downfield (~7 ppm)

H H H H

H H H H H 6H: -3 ppm H H 12H: 9.3 ppm H H H

H H H H Antiaromaticity Planar, cyclic, fully conjugated π systems 4nπ e-

Cyclobutadiene: antiaromatic

not square but rectangular structure

Cyclooctatetraene: non-aromatic nonplanar Cl SbCl 5 - + SbCl6 aromatic

SbCl5 Cl

extremely slow antiaromatic

H pKa = 16

aromatic Ph Ph

Ph Ph pKa = 50 Ph Ph antiaromatic 2.4.2 Effects of multiple heteroatoms Bond length effects

The more substitution, the more stable; interaction of a filled π(CH3) orbital with the π* orbital.

C-O and C-N are shorter than C-C, leading to increased steric strain. C-S is significantly longer than C-C.

O O 2-position 5-position R no 1,3-diaxial interaction Orbital effects

.. + DC DC - A A D: donors lone pair > bonding pairs, F < O < N A: acceptors F > O > N antiperiplanar

σ* CH2FNH2 n n -> σ* interaction CH3OCH2Cl

H3C O Cl O Cl H3C H H one antiperiplanar H H two antiperiplanar interactions -> more stable conformer Anomeric effect Pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when substituent has an axial orientation rather than an equatorial one.

antiperiplanar

axial

n -> σ* interaction

axial O OR equatorial Example)