8. The bond angles in ammonia and water are somewhat reduced. This suggests that the electron-electron repulsions between
unshared pairs are greater than for electrons in bonds to hydrogen. In other words, the unshared pairs occupy somewhat larger
orbitals.
The double and triple bonds are conceived as arising from the overlap of the unhybridized p orbitals on adjacent atoms. These
bonds have a nodal plane and are called π bonds. Because the overlap is not as effective as for sp3 orbitals, these bonds are
somewhat weaker than σ bonds
Hybridization
Qualitative application of VB theory to molecules containing second-row elements such as carbon, nitrogen, and oxygen involves the concept of
hybridization.
9. Electronegativity and Polarity:
* Electronegativity is the power of an atom in a molecule to attract electrons to it.
* Electronegativity = ability of an atom to bear negative charge effectively or= ability
to pull electron density towards itself
* In general, electronegativity increases L to R and Down to Up in the Periodic table.
•The more electronegative element takes on the partial negative charge.
10. •More polarizable electrons are more easily attracted, hence C=O has a larger
dipole than C-O.
Note that the negative charge on fluorine becomes smaller as the
electronegativity of the central atom increases.
•The electronegativity sp3 <sp2 <sp.
•The acidity increases with the electronegativity of the carbon group.
pKa 4.87 4.25 1.84
11. Polarizability, Hardness, and Softness:
Polarizability: is the tendency of the electron shell of an atom/molecule to become deformed in an
electric field
Polarizability can be described in terms of hardness and softness.
•A relatively large atom or ion with a small effective nuclear charge is relatively easily distorted (polarized) by an
external charge and is called soft.
•A more compact electron distribution resulting from a higher net nuclear charge and less effective screening is
called hard.
Hardness increases with electronegativity and with positive charge.
* For the halogens the order is F− > Cl−> Br− > I−
* For second-row anions, F− >HO−>H2N− > H3C−.
*For cations, hardness decreases with size and increases with positive charge, so that H+ > Li+> Na+ > K+.
* Metal ion hardness increases with oxidation state as the electron cloud contracts with the removal of each
successive electron
12. * For polyatomic molecules and ions, hardness and softness are closely related to the HOMO
and LUMO energies.
- The larger the HOMO-LUMO gap, the greater the hardness
- In general, chemical reactivity increases as LUMO energies are lower and HOMO energies
are higher. The implication is that softer chemical species, those with smaller HOMO-LUMO
gaps, tend to be more reactive than harder ones.
13. In the case of molecules, when a Lewis acid reacts with a Lewis base, electrons from the highest occupied
molecular orbital (HOMO) of the Lewis base are transferred to the lowest unoccupied molecular orbital (LUMO)
of the Lewis acid. How well this transfer of electron density works depends on the relative energies of the acid
LUMO and the base HOMO. The stability of the reaction product will increase with the energy difference
between its HOMO and LUMO
The HSAB theory predicts that hard Lewis bases react preferentially with hard acids, and that likewise soft bases
form stronger bonds with soft acids. For example, Li+LiX+ and BF3BFX3 are hard acids
and F−FX− and NH3NHX3 are hard bases due to their small size and high charge density/low polarizability of
the ions. BF3BFX3 therefore readily forms Lewis acid-base adducts with fluoride and ammonia, and LiFLiF is
less soluble in water than LiClLiCl and LiBrLiBr.
Ag+AgX+ is an example for a soft acid (large ion with low charge), and the halide ions become increasingly
softer bases with increasing ionic radius. Silver therefore forms stronger bonds with heavier halides
like I−IX− and Br−BrX−, and this also explains the decreasing solubility of the silver halides in water, which is
classified as a hard solvent. In the case of soft acids and bases, the difference of electronegativity tends to be
smaller, and the bonds have more covalent character (e.g. silver iodide) than those between hard acids and
bases (e.g. lithium fluoride) with larger difference in electronegativity (source).
14. •Lewis acid-base interactions in that hard acid prefer hard bases and soft acids prefer soft bases.
Expressed in terms of reaction energetics, ΔE is usually negative for
** Polarizability measures the response of an ion or molecule to an electric field.
- Polarizability increases with atomic or ionic radius; it depends on the effectiveness of nuclear screening and
increases as each valence shell is filled.
* Methane is the least polarizable hydrocarbon and polarity increases with size. Polarizability is also affected by
hybridization, with ethane > ethene > ethyne and propane > propene > propyne.
- Polarizability is also directly related to the dipole moment induced by an electric field. The greater of the
polarizability of a molecule the larger the induced dipole.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25. • Distance a in 33 is 1.45 Å,
whereas b is 1.35 Å. This
phenomenon can be explained
if the oxygen atoms of the p-
nitro group are in the plane of
the ring and thus in resonance
with it, so that b has partial
double-bond character, while
the oxygen atoms of the o-nitro
groups are forced out of the
plane by the large iodine atom.
the difference in bond length is
associated with
hyperconjugative effects,
represented by the canonical
form.
52. • A nucleophile is
a chemical
species that
donates
an electron
pair to form
a chemical
bond in relation
to a reaction
53. • This free-energy relationship relates the pseudo first order reaction rate constant (in water at
25 °C), k, of a reaction, normalized to the reaction rate, k0, of a standard reaction with water as
the nucleophile, to a nucleophilic constant n for a given nucleophile and a substrate
constant s that depends on the sensitivity of a substrate to nucleophilic attack (defined as 1
for methyl bromide).
54.
55. • Halogens aren't
nucleophilic in their
diatomic form (e.g. I2 is
not a nucleophile),
their anions are good
nucleophiles.
• In polar, protic
solvents, F− is the
weakest nucleophile,
and I− the strongest;
this order is reversed in
polar, aprotic solvents
