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Strain and Stability
Chapter 2- Reactivity of a new molecule^ - Prediction of the lowest energy conformation of a new molecule^ - -> a rapid evaluation of strains and stabilizing effects2.1 Thermochemistry of stable molecules (strain and stability)2.1.1 The concepts of internal stain and relative stabilityStrain:Is typically associated with a conformational distortion or nonoptimal bonding situationrelative to standard organic structures. The reference structure lacks the particularstrain.Internal energy:It is the energy held or stored within a molecule. Part of this energy can be releasedwhen given an outlet such as a chemical reaction.When a molecule has a higher potential energy (internal energy), it is less stable and/ormore strained.
2.1.2 Types of energyGibbs free energy (
oΔ G ): It is change in
o^ Δ G between two different chemical states that
determines the position of equilibrium between these states.^ A^
←B→ R = 4.184 kJ/molo Δ H : enthalpy (kcal/mol)o Δ S : entropy (cal/mol·K)
K is influenced by temperatureeq^ Exergonic, when the Gibbs free energy of B is lower than A, spontaneous conversionEndergonic, when the Gibbs free energy of B is higher than A
2.1.3 Bond dissociation energiesIs defined as
o Δ H. (bond strength) Homolytic cleavage A larger BDE implies a less stable radical.
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, (m
+m^ )/m^ m^1
Infrared spectroscopy
v^ = 1/(2π) √^ k /μ k = force constant μ^ = reduced mass, (m
+m^ )/m^ m^1 v^ = 1/λ^ =^ v/ c = 1/(
πc) √^ k /μ Wavenumber(cm frequency-1^ ) C-C C=C^
C≡C
-1^ 450-500 cm
-11617-1640 cm 2100-2260 cm
C=O
-1^ 1690 cm
-11730 cm - O O + less double character
k m m^1 2 Hooke’s law v = 1/(2π) √^ k/μ
- wavenumber^
the largest^ the lowest
middle O Cl R
O MeO R
O Me R
2.2.2 Radicals 1. BDE; methane > ethane > propane > isobutane <- radical stability 3
o^ o^ o^ > 2> 1> methyl
- Allyl and benzyl radicals -> substantially stabilized (resonance effect)
rotation barrier ~ 15.7 kcal/mol (resonance structure) Allylic radical 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),
o Δ H
A larger HIA implies a less stable carbocation.
- Planarity and pyramidalization
Carbocation: planar Ring constraints preventthe ion from achievingPlanarity. But 3
o^ cation
o^ Planar but 2cation
relatively small difference in HIVs (9 kcal/mol)Lifetimes of carbocationso^3 carbocations: 10
-10^ s in water o^2 carbocations: 10
-12^ s in water In solutionCarbocations are formed in solution by SbF
(Olah, 1994, Nobel Prize) 5
2.2.4 Carbanions
Stability of carbanions is related to p
K values.a^ The smaller p K
value implies a stronger acid.a
aromaticanti-aromatic
Barrier height
Similar;consider size and bond lengthLower than C-C, lone pair < C-H 1,3Allylic (A) strain
Cyclobutane and cyclopentane
puckered conformations
Strain energy: 26.5 kcal/mol very small barrier(1.45 kcal/mol)
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’ 1’4’2’ O N^ NH NNNH^2 O H (OH)OH HO5' 1'2'3'4'
