Chapter 2 Molecular Bonding and Acid, Base

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.

Strength of attractions between molecules influence m.p., b.p., and solubility; esp. for solids and liquids

Chapter 2 Intermolecular Bonding

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.

Zwitterion （两性离子）

Some important naturally occurring moleculescontain both groups – the amino acids.

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 bonddonor) the partially charged heteroatom of another molecule (theH 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

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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)

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.

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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

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）

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）

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• Occur between all neighboring molecules and arise because the electron distribution within molecules that are constantly changing

Dispersion Forces （色散力）

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 CH2 CH2 CH2 CH3

n-pentane, b.p. = 36°C

CH3 CH

CH3

CH2 CH3

isopentane, b.p. = 28°C

C

CH3

CH3

CH3

H3C

neopentane, b.p. = 10°C

oC oCoC

2.1 Intermolecular bonding

2.1.1 Ionic bonding

2.1.3 Dipole-Dipole

2.1.4 Dispersion Forces （ van der Waals interactions ）

2.1.2 Hydrogen Bond Forces

复习

2.2 Boiling Points and Intermolecular Forces

CH3 CH2 OH

ethanol, b.p. = 78°C

CH3 O CH3

dimethyl ether, b.p. = -25°C

trimethylamine, b.p. 3.5°C

N CH3H3C

CH3

propylamine, b.p. 49°C

CH3CH2CH2 N

H

H

ethylmethylamine, b.p. 37°C

N CH3CH3CH2

H

=>CH3 CH2 OH

ethanol, b.p. = 78°C ethyl amine, b.p. 17°C

CH3 CH2 NH2

oC oC

oC oC oC

oC oC

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 withNonpolar Solvent

=>

2.3.3 Nonpolar Solute withNonpolar 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 Solvent2.3.2 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–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

勃朗斯特 - 劳里

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

pKaAcids 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

CH3CH2NH2pKa = 40

CH3CH2OHpKa =16

Acid ?

Ethanoic acid (4.76), ethanol (16), and phenol (10)

Acid ?

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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

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2.4.2 Organic Acids and Organic Bases

• Organic Acids:- characterized by the presence of positively

polarized hydrogen atom

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.

2.4.3 Predicting Acid Strength from Inductive effect 诱导效应

• Ka & pKa & relative strength of acid

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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)

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

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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

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.

C N O F< < <Electronegativity:

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 4.74methylamine 3.36

basicities ？

pKa+ pKb= 14

Definition

an acid is a molecule which can provide a proton. a base is a molecule which can accept that proton.

复习

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

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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

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Illustration of Curved Arrows in Following Lewis Acid-Base Reactions

Lewis Bases

复习

Acids and Bases: The Lewis Definition

• Lewis acids are electron pair acceptors and Lewis bases are electron pair donors2.6.1 Illustration of Curved Arrows in Following Lewis Acid-Base Reactions