Structures and Models of Bonding

Question 1

Bond Length and molecular properties

Answer 1

• more s-character results in a shorter bond
• this is bc the s-orbital (bonding electrons) are closer to the nucleus
• CH bond lengths sp3>sp2>sp

Question 2

CH bond strength, BDE, stretching force constants

Answer 2

• shorter bonds are stronger
• CH bond strengths sp>sp2>sp3
• BDE increases with increases %s character
• stronger bonds result in higher force constants

Question 3

Ring strain

Answer 3

methylenes (CH2) give a nearly constant decrease in DHf (approx -4.93 kcal/mol per CH2)

Question 4

CH pKa values

Answer 4

• acidity correlates with s-character of CH orbital (the s-orbital/bonding e- are closer to the nucleus)
• with a higher %s, the pKa is lower.

Question 5

1H-13C NMR coupling constant

Answer 5

-J(13C-1H) coupling constants scale with % s character
-s-orbital/bonding electrons closer to nucleus
-Only s orbitals on C have density at the nucleus to affect the neighboring H atoms
Predict: J(13C-H) = 500Hz/(i+1)

Question 6

C-C bond lengths

Answer 6

• more % s-character = shorter C-c bonds

- Dewar eqn: L=1.692-0.0062(average %s character)

Question 7

Ionization Potential and Basicity

Answer 7

R ====> R+ +e- DH = IP

• IP (ionization potential) increases with %s
• Lone pairs in high %s orbitals are less basic, as the LP are held closer to the nucleus, less likely to share w H+

Question 8

Gas phase basicity

Answer 8

• solvation effects can obscure trends in pKa

- GAS PHASE pKa determined through: DG=-RTln(Keq)

Question 9

Polar Covalent Bonding

Answer 9

• polar bonds generally stronger
• lower energy valence orbitals = more electronegative atom
• can use partial charges to predict reactivity

Question 10

Types of electronic effects (3)

Answer 10

Resonance
Inductive
Field

Question 11

Resonance effects

Answer 11

Delocalization of π through conjugated system

• describe a single hybrid structure, not interchanging structures
• more resonance structures = more stable species

Question 12

Inductive Effects

Answer 12

-electron push/pull through sigma bonds

Question 13

Electron donating groups through induction

Answer 13

O-
COO-
CR3
CHR2
CH2R
CH3

Question 14

Electron Withdrawing through induction

Answer 14

NR3+
SR2+
NO2
CN
SO2R
COOR
OR
COR
SR
C(triple bond)CR
Ar
F, Cl, Br, I
CR=CR2

Question 15

Field effects

Answer 15

Electron push/pull through space

Question 16

Polarizability

Answer 16

• distortion of electron cloud in response to electric field
• larger volume occupied by electrons = more polarizable atom
• roughly correlates with electronegativity