Transcript Orbitals - drjosephryan.com

Chapter 2
Polar Covalent Bonds
Acids and Bases
Equilibrium
Chemical Reaction Analysis
Why This Chapter?
 Description of the ways chemists account for
chemical reactivity.
 Establish a foundation for understanding specific
reactions discussed in subsequent chapters.
Polar Covalent Bonds:
Who is more negative?
 Polar covalent bonds


Bonding electrons attracted more strongly to one
atom than the other
Electron distribution between atoms is not
symmetrical
The Periodic Table and
Electronegativity
Bond Polarity




Nonpolar: atoms with similar EN
Polar: Difference in EN of atoms 0.5 to 2
Ionic Bonds: Difference in EN > 2
Bonding electrons pulled toward electronegative atom
 Lower EN acquires partial positive charge, +
 Higher EN acquires partial negative charge, -
Inductive Effect
 Inductive effect: shifting of electrons in a “sigma”
bond in response to EN of nearby atoms
Dipole Moments - Observable
 Molecules as a whole are often polar from vector summation of
individual bond polarities and lone-pair contributions
 Strongly polar substances are soluble in polar solvents like water;
nonpolar substances are insoluble in water.
 Dipole moment () - Net molecular polarity


Can be measured
Physical Chemists Love to try and calculate it with models
 Polar
stuff like polar things
 Non-polar stuff like non-polar things
Formal Charges
 Sometimes it is necessary to have structures with
formal charges on individual atoms
 We compare the bonding of the atom in the molecule to
the bonding the neutral atom would have
Formal Charges
Resonance Structures
 Some molecules have structures that cannot be shown with a





single representation
In these cases, we draw structures that contribute to the final
structure but which differ in the position of the  bond(s) and/or
lone pair(s)
Such a compound has delocalized pi electrons and is represented
by resonance forms or resonance structures
IT’S THE p ORBITALS DOING THIS PEOPLE!
The resonance forms are connected by a double-headed arrow
Acetate Ion Example
Resonance Structures
 Meaning what exactly?
Resonance Hybrids
 A structure with resonance forms is a hybrid of the resonance forms,
so the structure is called a resonance hybrid
 For example, benzene (C6H6) has two resonance forms with
alternating double and single bonds
 In the resonance hybrid, the actual structure, all its C-C bonds
are equivalent, midway between double and single
Rules for Resonance Forms
 Individual resonance forms are imaginary
 Resonance forms differ only in the placement of their 
or nonbonding electrons
 Different resonance forms of a substance do not have to
be equivalent
 Resonance forms must be valid Lewis structures
Curved Arrows – Electron Movement
 We can imagine that electrons move in pairs to convert
from one resonance form to another
 A curved arrow 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
Curved Arrows – Electron Movement
 Practice
Drawing Resonance Forms
 Any three-atom grouping with a p orbital on each
atom has two resonance forms
Different Atoms
 Sometimes resonance forms involve different atom types
 The resulting resonance hybrid has properties associated with both types of
contributors
 The types may contribute unequally
2,4-Pentanedione
 The anion derived from 2,4-pentanedione
 Lone pair of electrons and a formal negative charge on
the central carbon atom, next to a C=O bond on the left
and on the right
 Three resonance structures result
Brønsted Acids and Bases
 “Brønsted-Lowry” is usually shortened to “Brønsted”
 A Brønsted acid is a substance that donates a hydrogen cation (H+)
 A Brønsted base is a substance that accepts the H+

“proton” = naked hydrogen atom … always has to be handed off.
Cannot be “passed” down field
Conjugate “What?”
Acid Strength
 Stronger acids have large Keq = Ka
 Weak acids have small Ka
Everything is listed as pKa values
Base Strength
 Stronger bases have large Keq = Kb
 Weak bases have small Kb
pKa’s of Some Common Acids
Let’s Work a Problem
Rank the following substances in order of
increasing acidity.
Answer
Do not make things too hard if we don’t have
to. The lower the value of the pKa, the more
acidic the molecule is:
But Why? Explain!
Predicting Acid–Base
Reactions from pKa Values
 The difference in two pKa values is the log of the ratio of equilibrium
constants, and can be used to calculate the extent of transfer
 The equilibrium lies on the side where the weaker acid has the
proton.
 The extent of the reaction equilibrium can be calculated by the
difference of the two pKa values or the ratio of the Ka
values.
Organic Acids and Organic Bases
 Which H is acidic?
Organic Bases
 Have an atom with a lone pair of electrons
 Nitrogen-containing compounds are the most common
organic bases
 Oxygen-containing compounds can accept a proton but
are less basic than nitrogen
Lewis Acids and Bases
 Lewis acids are electron pair acceptors and Lewis bases
are electron pair donors
 The Lewis definition leads to a general description of
many reaction patterns but there is no scale of strengths
as in the Brønsted definition of pKa
Illustration of Curved Arrows in
Following Lewis Acid-Base Reactions
Lewis Bases
 Lewis bases can accept protons as well as Lewis acids, therefore
the definition encompasses that for Brønsted bases
 Most oxygen- and nitrogen-containing organic compounds are
Lewis bases because they have lone pairs of electrons
 Some compounds can act as both acids and bases, depending on
the reaction
Noncovalent Interactions
Between Molecules
 Several types:
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Dispersion forces
Dipole-dipole forces
Hydrogen bonds
- Used in purification
-
To the extent that your products and reactants are
different, you can purify one form the other!
-
Boiling points
Melting points
Solvent solubility
Dispersion Forces

Occur between all neighboring molecules and arise
because the electron distribution within molecules that
isconstantly changing
Dipole-Dipole
 Occur between polar molecules as a result of
electrostatic interactions among dipoles
 Forces can be attractive or repulsive depending on
orientation of the molecules
Hydrogen Bond Forces
 Most important noncovalent interaction in biological
molecules
 Forces are result of attractive interaction between a
hydrogen bonded to an electronegative atom and an
unshared electron pair on another O or N atom