2. Polar Covalent Bonds: Acids and Bases Flashcards
Covalent bonds can have ionic character
Polar Covalent bonds
Polar Covalent bonds
Bonding electrons attracted more strongly by one
atom than by the other
Electron distribution between atoms is not
symmetrical
Difference in EN of atoms < 2
intrinsic ability of an atom to attract the shared electrons in a covalent bond
Electronegativity
Metals on left side of periodic table attract electrons weakly, lower EN
Halogens and other reactive nonmetals on right side
of periodic table attract electrons strongly, higher
electronegativities
atoms with similar EN
Nonpolar covalent bonds
Ionic Bonds
Difference in EN > 2
shifting of electrons in a bond in
response to EN of nearby atoms
Inductive effect
show calculated
charge distributions
Colors indicate electron-rich (red) and electron-poor (blue) regions
Arrows indicate direction
of bond polarity
Electrostatic potential
maps
Net molecular polarity, due to difference in
summed charges
Dipole moment
In symmetrical molecules, the dipole moments of each bond has one in the opposite direction. The effects of the local dipoles cancel each other
Neutral molecules with both a “+” and a “-” are called?
Dipolar
Formal charge formula:
Valence electron - ( Bonding + Dots)
it is a hybrid of the two resonance forms
Resonance hybrid
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Goal: NO REMEDIAL!!!
The resonance hybrid is more stable than any
individual resonance form would be
shows that a pair of electrons moves
from the atom or bond at the tail of the arrow to the
atom or bond at the head of the arrow
A curved arrow
Any three-atom grouping with a multiple bond
has two resonance forms
a substance that donates a hydrogen ion (H+)
Bronsted acid
a substance that accepts
the H+
Bronsted base
“proton” is a synonym for H+
- loss of an
electron from H leaving the bare nucleus—a
proton
The equilibrium constant (Keq) for the reaction of an
acid (HA) with water to form hydronium ion and the
conjugate base (A-
) is a measure related to the
strength of the acid
High pKa = weak acid, strong base
Low pKa = strong acid, weak base (is proportional to the energy difference between products and reactants)
Stronger acids have larger Keq
pKa = -log Ka
pKa
of water is 15.74
The stronger base holds the proton more tightly
characterized by the presence of positively
polarized hydrogen atom
Organic Acids
Have an atom with a lone pair of electrons that can bond to H+
Organic Bases
Nitrogen-containing compounds derived from
ammonia are the most common organic bases
Oxygen-containing compounds can react as bases
when with a strong acid or as acids with strong bases
are electron pair acceptors; electron deficient; + charged; boron
Lewis acids
are electron pair donors; - charged; lone pairs
Lewis bases
Brønsted acids are not Lewis acids because they
cannot accept an electron pair directly (only a proton
would be a Lewis acid)
The Lewis definition of acidity includes metal cations, such as Mg2+
- They accept a pair of electrons when they form a bond to a base
Group 3A elements, such as BF3 (boron trifluoride)
and AlCl3 (aluminum chloride) , are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases
Transition-metal compounds, such as TiCl4 (titanium tetrachloride), FeCl3 (ferric chloride or iron (III) chloride), ZnCl2 (zinc chloride), and SnCl4 (tin (IV) chloride or stannic chloride), are Lewis acids
can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted bases
Lewis Bases
Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons
Noncovalent Interactions
- Dipole-dipole forces
- Dispersion forces
- Hydrogen bonds
Occur between polar molecules as a result of electrostatic interactions
among dipoles; Forces can be attractive of repulsive depending on orientation of the molecules
Dipole-dipole
Occur between all neighboring molecules and arise because the
electron distribution within molecules that are constantly changing
Dispersion Forces
Most important noncovalent interaction in biological molecules
Hydrogen bond forces
The strength Brønsted acid is related to the -1 times the
logarithm of the acidity constant, pKa. Weaker acids have
higher pKa’s
From strongest to weakest, it would be ion-dipole, hydrogen bonding, dipole-dipole, dipole-induced dipole, london dispersion forces.
Bronsted acid = H+ donors
Bronsted base = H+ acceptors
Lewis acid = e- acceptors
Lewis base = e- donors
