Chapter 2 Polar Covalent Bonds Acids and Bases
Chapter 2 Polar Covalent Bonds Acids and Bases
Chapter 2 Polar Covalent Bonds Acids and Bases
1 Polar Covalent Bonds: Electronegativity
Chemical bonds
Ionic bonds
Ions held together by electrostatic attractions between unlike
charges
CHAPTER 2 Bond in sodium chloride
Sodium transfers an electron to chlorine to give Na+ and Cl‐
Polar Covalent Bonds; Acids Nonpolar Covalent bonds
and Bases Two electrons are shared equally by the two bonding atoms
Topics such as bond polarity, the acid‐base Carbon‐carbon bond in ethane
behavior of molecules, and hydrogen‐ Symmetrical electron distribution in the bond
bonding are a particularly important
part of the foundation for understanding
the specific reactions in the following *** Most bonds are neither fully ionic or co‐valent
chapters.
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Polar Covalent Bonds: Electronegativity
2.1 Polar Covalent Bonds: Electronegativity
2.2 Polar Covalent Bonds: Dipole Moments Polar covalent bonds
A covalent bond in which the electron distribution between atoms is
2.3 Formal Charges unsymmetrical
2.4 Resonance
2.5 Rules for Resonance Forms
2.6 Drawing Resonance Forms
2.7 Acids and Bases: The Brønsted–Lowry Definition
2.8 Acid and Base Strength
2.9 Predicting Acid–Base Reactions from pKa Values
2.10 Organic Acids and Organic Bases
2.11 Acids and Bases: The Lewis Definition Bond polarity is due to difference in electronegativity (EN)
2.12 Noncovalent Interactions between Molecules
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Polar Covalent Bonds: Electronegativity Polar Covalent Bonds: Electronegativity
Electronegativity (EN) Electrostatic potential maps
The intrinsic ability of an atom to attract the shared electrons in a Show calculated charge distributions
covalent bond Colors indicate electron‐rich (red; ‐) and electron‐poor (blue;
Electronegativity generally increases across the periodic table from +) regions
left to right and from bottom to top Methanol, CH3OH, has a
polar covalent C‐O bond,
and methyllithium has a
polar covalent C‐Li bond
A crossed arrow
is used to indicate direction
of bond polarity
Electrons are displaced
in the direction of the arrow
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Polar Covalent Bonds: Electronegativity Polar Covalent Bonds: Electronegativity
Bonds between atoms whose electronegativities differ by less than An atom’s ability to polarize a bond is known as the inductive
0.5 are nonpolar covalent (<0.5) effect
Bonds between atoms whose electronegativities differ by 0.5 to
2.0 are polar covalent (0.5 – 2.0)
Inductive effect
Bonds between atoms whose electronegativities differ by more
than 2.0 are largely ionic (>2.0) The electron‐attracting or electron‐withdrawing effect transmitted
through bonds. Electronegative elements have an electron‐
Carbon hydrogen bonds are nonpolar withdrawing inductive effect
Bonds between carbon (EN = 2.5) and more electronegative Metals inductively donate electrons, the electron‐donating effect
elements, such as oxygen (EN = 3.5) and nitrogen (EN = 3.0) are Reactive nonmetals inductively withdraw electrons
polar covalent bonds with the bonding electrons drawn towards
the more electronegative atoms Inductive effects play a major role in understanding chemical
Chemical reactivity of the covalent bond reactivity
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2.2 Polar Covalent Bonds: Dipole Moments Polar Covalent Bonds: Dipole Moments
Molecules as a whole are often polar Factors affecting dipole moments
Molecular polarity results from the vector summation of all individual Lone‐pair electrons on oxygen and nitrogen project out into space away
bond polarities and lone‐pair contributions in the molecule from positively charged nuclei giving rise to a considerable charge
Strongly polar substances are soluble in polar solvents like water, separation and contributing to the dipole moment
whereas nonpolar substances are insoluble in water Symmetrical structures of molecules cause the individual bond polarities
and lone‐pair contributions to exactly cancel
Dipole moment ()
Magnitude of charge Q at either end of molecular dipole times distance r
between charges
= Q r, in debyes (D)
1e‐ = 1.6 x 10‐19C
1 D = 3.336 1030 coulomb meter (C • m)
A measure of the net polarity of a molecule
Arises when the centers of mass of positive and negative charges within a
molecule do not coincide
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Polar Covalent Bonds: Dipole Moments 2.3 Formal Charges
Formal charge
The difference in the number of electrons owned by an atom in a molecule and by the
same atom in its elemental state
Formal charges do not imply the presence of actual ionic charges
Device for electron “bookkeeping”
Assigned to specific atoms within a molecule
Dimethyl sulfoxide CH3SOCH3
Sulfur atom has three bonds rather than the usual two and has a formal positive
charge
Oxygen atom has one bond rather than the usual two and has a formal negative
charge
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Formal Charges Formal Charges
Formal Charge Determination
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Formal Charges 2.4 Resonance
Two different ways to draw the acetate ion
Double bond placement
Neither structure correct by itself
True structure is intermediate between the two
Two structures are known as resonance forms
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Resonance 2.5 Rules for Resonance Forms
Resonance forms Rule 1 – Individual resonance forms are imaginary, not real
Individual line‐bond structures of a molecule or ion that differ Real structure is a composite
only in the placement of and nonbonding valence electrons
Rule 2 – Resonance forms differ only in the placement of their or
Indicated by “ ”
nonbonding electrons
Resonance forms
contribute to a single,
unchanging structure that
is the resonance hybrid of
the individual forms and
exhibits the characteristics
of all contributors
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Resonance Rules for Resonance Forms
Benzene has two equivalent resonance forms electrons in double bonds of benzene can move
The true structure of benzene is a hybrid of the two individual Electron movement is indicated by curved arrow formalism
forms, and all six carbon‐carbon bonds are equivalent Curved arrows indicate electron flow, not the movement of atoms
Symmetrical distribution of electrons is evident in an electrostatic A curved arrow indicates that a pair of electrons moves from the
potential map of benzene atom or bond at the tail of the arrow to the atom or bond at the
head of the arrow
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Rules for Resonance Forms 2.6 Drawing Resonance Forms
Rule 3 – Different resonance forms of a substrate do not have to be In general any three‐atom grouping with a p orbital on each atom has two
equivalent resonance forms:
The atoms X,Y, and Z in the general structure might be C,N,O,P,or S
The asterisk (*) on atom Z for the resonance form on the left might mean that
the p orbital is:
Vacant
Contains a single electron
Contains a lone pair of electrons
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Rules for Resonance Forms Drawing Resonance Forms
Rule 4 – Resonance forms obey normal rules of valency (following Reaction of pentane‐2,4‐dione with a base
the octet rule) H+ is removed
An anion is formed
Resonance of the anion product:
Rule 5 – The resonance hybrid is more stable than any individual Reactivity
resonance form
Resonance leads to stability
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2.7 Acids and Bases: The Brønsted‐Lowry Acids and Bases:
Definition The Brønsted‐Lowry Definition
Two frequently used definitions of acidity Water can act either as an acid or as a base
The Brønsted‐Lowry definition
Lewis definition
Brønsted‐Lowry acid
A substance that donates a hydrogen ion (proton; H+) to a base
Brønsted‐Lowry base
A substance that accepts a hydrogen ion (proton; H+) from an acid
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H3O+ A -
K eq =
HA H2O
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Acid and Base Strength Acid and Base Strength
The concentration of water, [H2O], remains nearly constant at 55.5
M at 25 ˚C
Can rewrite equilibrium expression using new quantity called the
acidity constant Ka
Acidity constant Ka
A measure of acid strength in water
For any weak acid HA, the acidity constant is given by the
expression Ka
A - + H3O+
HA + H2O
H O+ A -
K a = K eq H2O = 3
HA
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Acid and Base Strength Acid and Base Strength
Equilibria for stronger acids favor the products (to the right) and Strong acid (BrØnsted‐Lowry)
thus have larger acidity constants One that loses H+ easily
Equilibria for weaker acids favor the reactants (to the left) and Conjugate base holds on to the H+ weakly (weak base)
thus have smaller acidity constants Strong acid has weak conjugate base
A - + H3O+
HA + H2O
Acid strengths are normally expressed using pKa values
pKa
The negative common logarithm of the Ka Weak acid (BrØnsted‐Lowry)
pKa = ‐log Ka One that loses H+ with difficulty
Stronger acids (larger Ka) have smaller pKa Conjugate base holds on to the H+ strongly (strong base)
Weaker acids (smaller Ka) have larger pKa Weak acid has strong conjugate base
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2.9 Predicting Acid‐Base Reactions from pKa Values 2.10 Organic Acids and Organic Bases
An acid will donate a proton to the conjugate base of a weaker Most biological reactions involve organic acids and organic bases
acid Organic acid
The conjugate base of a weaker acid will remove the proton Positively polarized hydrogen atom
from a stronger acid Two main kinds of organic acids
1. Contains a hydrogen atom bonded to an oxygen atom (O‐H)
2. Contains a hydrogen atom bonded to a carbon atom next to a C=O double
bond (O=C‐C‐H)
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Methanol
Acetic Acid
Acetone
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Organic Acids and Organic Bases Organic Acids and Organic Bases
Conjugate bases from methanol, acetic acid, and acetone Organic bases
The electronegative oxygen atoms stabilize the negative charge in Characterized by the presence of an atom with a lone pair of
all three electrons that can bond to H+
Nitrogen‐containing compounds are common organic bases and
are involved in almost all metabolic pathways
Oxygen‐containing compounds can act both as acids and as bases
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Organic Acids and Organic Bases 2.11 Acids and Bases: The Lewis Definition
Carboxylic acids The Lewis definition is broader than the Brønsted‐Lowry definition
Contain the –CO2H grouping Lewis acid – an electrophile
Occur abundantly in all living organisms A substance with a vacant low energy orbital that can accept an electron
pair from a base
Involved in almost all metabolic pathways
All electrophiles are Lewis acids
At cellular pH of 7.3 carboxylic acids are usually dissociated and
exist as their carboxylate anions, – CO2‐ Lewis base – a nucleophile
A substance that donates an electron lone pair to an acid
All nucleophiles are Lewis bases
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Acids and Bases: The Lewis Definition Acids and Bases: The Lewis Definition
Lewis Acids and the Curved Arrow Formalism Further examples of Lewis acids
To accept an electron pair a Lewis acid must have either:
A vacant, low‐energy orbital
A polar bond to hydrogen so that it can donate H+
Various metal cations, such as Mg2+, are Lewis acids because they
accept a pair of electrons when they form a bond to a base
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Acids and Bases: The Lewis Definition Acids and Bases: The Lewis Definition
Compounds of group 3A elements, such as BF3 and AlCl3 are Lewis acids Lewis bases
which have unfilled valence orbitals and accept electron pairs from Lewis
bases A compound with a pair of nonbonding electrons that it can use in
Many transition metals, such as TiCl4, FeCl3, ZnCl2, and SnCl4 are Lewis acids bonding to a Lewis acid
Curved arrow formalism Definition of Lewis base similar to Brønsted‐Lowry definition
Indicates the direction of electron flow from the base to the acid
H2O acts as a Lewis base
Always means that a pair of electrons moves from the atom at the tail of the
arrow to the atom at the head of the arrow Has two nonbonding electrons on oxygen
For the reaction of boron trifluoride, a Lewis acid, with dimethyl ether, a Lewis
base. All movement of electrons from the Lewis base to the Lewis acid is
indicated by a curved arrow
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2.12 Noncovalent Interactions between
Acids and Bases: The Lewis Definition Molecules
Most oxygen‐ and nitrogen‐ containing organic compounds are Noncovalent Interactions
Lewis bases Also called intermolecular forces or van der Waals forces
They have lone pair electrons Types of noncovalent interactions include:
Dipole‐Dipole forces
Occur between polar molecules as a result of electrostatic interactions
among dipoles
Forces are either attractive or repulsive
Attractive Repulsive
Attractive geometry is lower in energy and therefore predominates
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Acids and Bases: The Lewis Definition Noncovalent Interactions between Molecules
Some compounds can act as both acids and bases Dispersion forces
Some compounds have more than one atom with a lone pair of Attractive dispersion forces in nonpolar molecules are caused by temporary
dipoles
electrons
One side of the molecule may have a slight excess of electrons relative to the
Reaction normally occurs only once in such instances opposite side, giving the molecule a temporary dipole
The more stable of the two possible protonation products is formed Temporary dipole in one molecule causes a nearby molecule to adopt a
Occurs with carboxylic acids, esters, and amides temporarily opposite dipole resulting in a small attraction between the two
molecules
Arise because the electron distribution within molecules is constantly changing
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Noncovalent Interactions between Molecules Noncovalent Interactions between Molecules
Hydrogen Bonds Hydrophilic (water‐loving)
A weak attraction between a hydrogen atom bonded to an
Dissolves in water
electronegative O or N and an electron lone pair on another O or N atom
Strong dipole‐dipole interaction involving polarized O‐H and N‐H bonds
Table sugar
Important noncovalent interaction in biological molecules Has ionic charges, polar –OH groups, in its structure
Hydrophobic (water‐fearing)
Does not dissolve in water
Vegetable oil
Does not have groups that form hydrogen bonds
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Noncovalent Interactions between Molecules
Effects of Hydrogen Bonding
Causes water to be a liquid rather than a gas at room temperature
Holds enzymes in the shapes necessary for catalyzing biological
reactions
Causes strands of deoxyribonucleic acid (DNA) to pair up and coil
into a double helix
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