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Chapter 2 Molecular Bonding and Acid,
Base
Chapter 2 Intermolecular Bonding
Strength of attractions between molecules
influence m.p., b.p., and solubility; esp.
for solids and liquids
Definition
Intermolecular bonding （分子间作用力）is the bonding
interaction which takes place between different molecules.
This can take the form of
ionic bonding（离子键）
hydrogen bonding（氢键）
dipole–dipole interactions （偶极-偶极作用）
van der Waals interactions （范德华作用力）
These bonding forces are weaker than the covalent bonds
, but they have an important influence on the physical and
biological properties of a compound.
2.1.1 Ionic bonding （离子键）
Ionic bonding takes place between molecules having
opposite charges and involves an electrostatic
interaction（静电作用） between the two opposite
charges.
Some important naturally occurring molecules
contain both groups – the amino acids.
Zwitterion（两性离子）
electrostatic interaction 静电作用
2.1.2 Hydrogen Bonding
• Strong dipole-dipole attraction
• Organic molecule must have N-H or O-H.
• The hydrogen from one molecule is strongly
attracted to a lone pair of electrons on the
other molecule.
• O-H more polar than N-H, so stronger
hydrogen bonding
=>
H Bonds
=>
2.1.2 Hydrogen bonding（氢键）
Hydrogen bonding can take place when molecules have
a hydrogen atom attached to a heteroatom such as
nitrogen or oxygen.
Hydrogen bonding is possible due to the polar
nature of the N–H or O–H bond.
the partially charged hydrogen
of one molecule (the H bond
donor) the partially charged
heteroatom of another
molecule (the
H bond acceptor)
Most important noncovalent interaction in biological molecules
alcohols, phenols, carboxylic acids, amides, and
amines
donor
acceptor
举一些其它氢键的例子
2.1.2 Hydrogen Bond Forces
•
• Forces are result of attractive interaction between a hydrogen bonded
to an electronegative O or N atom (or F atom) and an unshared
electron pair on another O or N atom (or F atom)
11
2.1.3 Dipole–dipole interactions （偶极-偶极作用）
Dipole–dipole interactions are possible between
polarized bonds other than N–H interactions or O–H
bonds.
2.1.3 Dipole-Dipole
• Occur between polar molecules as a result of
electrostatic interactions among dipoles
• Forces can be attractive of repulsive depending on
orientation of the molecules
13
2.1.3 Dipole-Dipole
=>
举一些其它氢键的例子
2.1.4 van der Waals interactions（范德华作用力）
Dispersion Forces（色散力）
van der Waals interactions are the weakest of the
intermolecular bonding forces interactions and
involve the transient existence of partial charges
in a molecule.
Alkane molecules can interact in this way and the
strength of the interaction increases with the size
of the alkane molecule. van der Waals interactions
are also important for alkenes, alkynes and aromatic
rings.
Hydrophobic molecules can dissolve in nonpolar,
hydrophobic solvents due to van der Waals interactions
Dispersion Forces（色散力）
• Occur between all neighboring molecules and
arise because the electron distribution within
molecules that are constantly changing
16
2.1.4 Dispersions
=>
预测一下溶解性
=>
2.1.4 Dispersions
•
•
•
•
Between nonpolar molecules
Temporary dipole-dipole interactions
Larger atoms are more polarizable(Br,I.)
Branching lowers b.p. because of decreased
surface contact between molecules.
CH3
CH3
CH2
CH2
CH2
n-pentane, b.p. = 36°CoC
CH3
CH3 CH CH2 CH3
oC
isopentane, b.p. = 28°C
CH3
H3C
C CH3
CH3
oC
neopentane, b.p. = 10°C
=>
复习
2.1 Intermolecular bonding
2.1.1 Ionic bonding
2.1.2 Hydrogen Bond Forces
2.1.3 Dipole-Dipole
2.1.4 Dispersion Forces
（van der Waals interactions）
2.2 Boiling Points and
Intermolecular Forces
CH3
CH2
ethanol, b.p. = 78°CoC
H3C N CH3
CH3
OH
O CH3
dimethyl ether, b.p. = -25°CoC
CH3CH2
CH3CH2CH2
N CH3
N H
H
oC
trimethylamine, b.p. 3.5°CoC ethylmethylamine, b.p. 37°CoC propylamine, b.p. 49°C
H
CH3
CH3
CH2
OH
oC
ethanol, b.p. = 78°C
CH3
CH2
NH2
ethyl amine, b.p. 17°CoC
=>
2.3 Solubility
• Like dissolves like
• Polar solutes dissolve in polar solvents.
• Nonpolar solutes dissolve in nonpolar
solvents.
• Molecules with similar intermolecular forces
will mix freely.
=>
2.3.1 Ionic Solute with
Polar Solvent
Hydration releases energy.
Entropy increases.
=>
2.3.2 Ionic Solute with
Nonpolar Solvent
=>
2.3.3 Nonpolar Solute with
Nonpolar Solvent
=>
2.3.4 Nonpolar Solute
with Polar Solvent
=>
复习
2.2 Boiling Points and Intermolecular Forces
2.3 Solubility
2.3.1 Ionic Solute with Polar Solvent
2.3.2 Ionic Solute with Nonpolar Solvent
2.3.3 Nonpolar Solute with Nonpolar Solvent
2.3.4 Nonpolar Solute with Polar Solvent
2.4 Acids and Bases: The Brønsted–
勃朗斯特-劳里
Lowry Definition
The terms “acid” and “base” can have different meanings
in different contexts
• For that reason, we specify the usage with more complete
terminology(术语)
•
The idea that acids are solutions containing a lot of “H+”
and bases are solutions containing a lot of “OH-” is not
very useful in organic chemistry
• Instead, Brønsted–Lowry theory defines acids and bases
by their role in reactions that transfer protons (H+)
between donors and acceptors
•
2.4 Brønsted Acids and Bases
“Brønsted-Lowry” is usually shortened to “Brønsted”
• A Brønsted acid is a substance that donates a hydrogen
ion (H+)
• A Brønsted base is a substance that accepts the H+
•
•
“proton” is a synonym for H+ - loss of an electron
from H leaving the bare nucleus—a proton
The Reaction of HCl with H2O
Acids are shown in red, bases in blue. Curved arrows go from
bases to acids
2.4.1 Quantitative Measures of Acid
Strength
•
•
Stronger acids have larger Ke
Note that brackets [ ] indicate concentration, moles per
liter, M.
pKa – the Acid Strength Scale
• pKa = -log Ka
• A larger value of pKa indicates a stronger acid and is
proportional to the energy difference between products
and reactants
• The pKa of water is 15.74
pKa
Acids can be described as being weak or strong and the
pKa is a measure of this.
Keq is normally measured in a dilute aqueous solution of
the acid and so the concentration of water is high and
assumed to be constant. Therefore, we can rewrite the
equilibrium equation in a simpler form where Ka is the
acidity constant and includes the concentration of pure
water (55.5 M).
pKa=- log10 Ka
pKa
The strongest acid has the lowest pKa value.
the stronger the acid, the higher the value of Ka, and the
lower the value of pKa
2.4.2 Predicting Acid–Base Reactions
from pKa Values
pKa = 40
CH3CH2NH2
pKa =16
CH3CH2OH
Acid ?
Ethanoic acid (4.76), ethanol (16), and phenol (10)
Acid ?
2.4.2 Predicting Acid–Base Reactions
from pKa Values
•
•
•
•
pKa values are related as logarithms to equilibrium constants
Useful for predicting whether a given acid-base reaction will take
place
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 stronger base holds the proton more tightly
38
2.4.2 Organic Acids and Organic Bases
• Organic Acids:
- characterized by the presence of positively
polarized hydrogen atom
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2.4.3 Predicting Acid Strength from Inductive effect
诱导效应
Inductive electron withdrawal increases the acidity
of a conjugate acid; Inductive effect: shifting of
electrons in a bond in response to EN of nearby
Atoms.
• Ka & pKa & relative strength of acid
Organic Acids
• Those that lose a proton from O–H, such as
methanol and acetic acid
• Those that lose a proton from C–H, usually from a
carbon atom next to a C=O double bond (O=C–C–H)
42
a simple acid/base reaction
Acid ? base? Nucleophile? Electrophile?
Mineral acids(无机酸）
Functional groups contain hydrogens which are potentially
acidic
>
1.
electronegative
>
F
>
O
>
N
Fluorine strongly polarizes the H–F bond such that the
hydrogen becomes highly electron deficient and is easily
lost. Once the proton is lost, the fluoride ion can stabilize
the resulting negative charge. nitrogen which is less
electronegative than fluorine.
2
完全电离
部分电离
部分电离
复习
•A Brønsted acid is a substance that donates a
•
hydrogen ion (H+)
2.4.1 Quantitative Measures of Acid Strength
2.4.2 Predicting Acid–Base Reactions from
pKa Values
2.4.3 Predicting Acid Strength from Inductive effect
2.5 Organic Bases
A Brønsted base is a substance that accepts
the H+
“proton” is a synonym for H+ - loss of an
electron from H leaving the bare nucleus—
a proton
• Bronstered-Lowry definition
•
•
Donates/accepts a hydrogen ion (H+)
Conjugate A/B
The Reaction of Acid with Base
•
•
•
Hydronium ion, product when base H2O gains a proton
HCl donates a proton to water molecule, yielding hydronium ion
(H3O+) [conjugate acid] and Cl [conjugate base]
The reverse is also a Brønsted acid–base reaction of the
conjugate acid and conjugate base
51
2.6 Acids and Bases: The Lewis
Definition
• Lewis acids are electron pair acceptors
and Lewis bases are electron pair donors
2.6.1 Illustration of Curved Arrows in Following
Lewis Acid-Base Reactions
• 2.6.2 Organic bases (red)
•
•
Have an atom with a lone pair of electrons that can bond to H+
-O & -N
Base is a molecule which can form a bond to a proton.
include negatively charged ions with a lone pair of electrons
Electronegativity(电负性）
Electronegativity has an important influence to play on basic
strength.
Electronegativity:
C <N<O< F
Strongly electronegative atoms are able to stabilize a
negative charge making the ion less reactive and less basic.
These neutral molecules are much weaker bases than their
corresponding anions.
Anitrogen atom can stabilize a positive charge better
than a fluorine atom since the former is less
electronegative
amines act as weak bases in aqueous solution and are
partially ionized. Alcohols only act as weak bases in
acidic solution. Alkyl halides are essentially nonbasic
even in acidic solutions.
pKb
pKb is a measure of basic strength.
If methylamine is dissolved in water, an equilibrium is set up
Acid ? base? Nucleophile? Electrophile?
the equilibrium constant
Keq is normally measured in a dilute aqueous solution
of the base and so the concentration of water is high
and assumed to be constant.
Kb is the basicity constant and includes the concentration
of pure water (55.5 M).
pKb=- Log10Kb
Alarge pKb indicates a weak base.
Ammonia 4.74
methylamine 3.36
basicities ？
pKa+ pKb= 14
复习
Definition
an acid is a molecule which can provide a
proton. a base is a molecule which can
accept that proton.
2.6.3 Lewis Acids
• 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 and AlCl3, are Lewis acids
because they have unfilled valence orbitals and can accept
electron pairs from Lewis bases
• Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and
SnCl4, are Lewis acids
• Organic compounds that undergo addition reactions with
Lewis bases (discussed later) are called electrophiles and
therefore Lewis Acids
亲电试剂
Lewis Acids and the Curved Arrow
Formalism （格式，形式）
•
•
•
•
•
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 and AlCl3, are Lewis acids
because they have unfilled valence orbitals and can accept
electron pairs from Lewis bases
Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and
SnCl4, are Lewis acids
Organic compounds that undergo addition reactions with Lewis
bases (discussed later) are called electrophiles and therefore
Lewis Acids
The combination of a Lewis acid and a Lewis base can shown
with a curved arrow from base to acid
65
Illustration of Curved Arrows in Following Lewis
Acid-Base Reactions
66
Lewis Bases
复习
Acids and Bases: The Lewis Definition
• Lewis acids are electron pair acceptors
and Lewis bases are electron pair donors
2.6.1 Illustration of Curved Arrows
in Following Lewis Acid-Base
Reactions