Thermochemistry of Stable Molecules

Question 1

About BDEs

Answer 1

-direct measure of bond strength

Question 2

BDE trends

Answer 2

• more polar bonds = stronger bonds = increased BDE
• large atoms have poor orbital overlap = weaker bond = lower BDE
• More %s = stronger bond = higher BDE

Question 3

Stability

Answer 3

• relative term comparing DG* of two species

- thermodynamic property

Question 4

Persistance

Answer 4

• describes lifetime of reactive species
• kinetic property (described by DG^[double dagger])
• sterics can dictate persistance

Question 5

Radical Stability

Answer 5

• BDE are good measure of this

- look for stabilization through hyperconjugation, delocalization/resonance, hybridization effects

Question 6

Carbocation stability

Answer 6

-Use hydride ion affinity to determine (HIA)
R-H —–>R+ + H-
-larger HIA = less stable carbocation
carbocations are more stable in planar sp2 geometry
-stabilizing features: hyperconjugation, resonance, hybridization effects

Question 7

Carbanion stability

Answer 7

-use the pKa of the conjugate acid to determine carbanion stability
R-H —-> R- + H+
-higher pKa = less stable carbanion
Stabilizing features: resonance, hybridization
-NOT hyperconjugation b/c cannot donate more e- density
-high s-character carbanion is more stable
-Higher pKa = more basic