Transcript Document

Organic chemistry
• 100/11/11 期中考試
• 101/01/06 期末考試
講義位址 http://www.aqua.ntou.edu.tw/chlin/
Essential Organic Chemistry
2nd Edition
Paula Yurkanis Bruice
Pearson Education, Inc.
• What is organic chemistry
– Organic: available only from living organisms?
– Inorganic
• Why study organic chemistry
• About all of the molecules that make life
possible contain carbon
• Over 30 million organic compounds have
been identified
– about 1000 new ones are discovered or
synthesized and identified each day!
• Why carbon
• Small
• In the center of the 2nd row of elements
• can share 4 valence electrons and form 4
covalent bonds
• is able to form an immense diversity of
compounds, from methane to DNA
Chapter
1
Electronic Structure and
covalent Bonding
THE STRUCTURE OF AN ATOM
An atom consists of electrons, positively charged
protons, and neutral neutrons.
• Small dense nucleus, diameter 1014 - 10-15 m, which made of
neutrons and positively-charged
protons.
• Extranuclear space, diameter 2 x
10-10 m (0.2 nm, 200 pm, 2Å),
which contains negativelycharged electrons.
• Most of the mass of the atom is
contained in its nucleus.
• Electrons form chemical bonds
• Atomic number: numbers of protons in its
nucleus
• Mass number: the sum of the protons and
neutrons of an atom
– 12C, 13C, and 14C (same atomic number, 6 protons)
– Isotopes have the same atomic number but different
mass numbers (neutrons).
– 14C is radioactive, decaying with a half-life of 5730
years
• The atomic weight: the average weighted mass
of its atoms
• Molecular weight: the sum of the atomic
weights of all the atoms in the molecule
7
HOW THE ELECTRONS IN AN ATOM ARE
DISTRIBUTED
Electrons
– Occupying a set of shells that surround the nucleus
– The 1st shell is the smallest and the one closest to the nucleus;
the 2nd shell is larger and extends farther from the nucleus
– Each shell consists of subshells known as atomic orbitals
designated by the letters s, p, d, f........
Electrons
– Each shell can contain up to n2 atomic orbitals (n is the number
of shell= 1,2,3,4......)
– Each shell contains one s orbital.
– The atomic orbital closer to the nucleus has the lowest energy.
(1s2s2p3s 3p 4s 3d 4p 5s 4d 5p)
– Each atomic orbital can contain no more than 2 electrons.
•
The rules for determining the electronic configuration
1. An electron always goes to the available orbital with the
lowest energy
2. Only two electrons can occupy one atomic orbital and the
two electrons have opposite spin
3. Electrons will occupy empty degenerated orbitals before
pairing up in the same orbital
• Electrons in the inner shells are called core electrons.
• Electrons in the outermost shell are called valance
electrons
IONIC AND COVALENT BONDS
• An atom is most stable if its outer shell is either filled or
contains eight electrons and it has no electrons of higher
energy
– Octet rule: An atom will give up, accept, or share electrons
in order to achieve a filled outer shell or an outer shell that
contains eight (valence) electrons
• Ionic bond: a chemical bond resulting from the electrostatic
attraction of an anion and a cation (Na+ Cl-)
– an atom that gains electrons becomes an anion
– an atom that loses electrons becomes a cation
• Covalent bond: a chemical bond resulting from two atoms
sharing one or more pairs of electrons
• Lewis structure of an atom
– the symbol of an element surrounded by a number
of bots equal to the number of electrons in the
valence shell of the atom
Attractive forces between opposite charges are called
electrostatic attractions
• Covalent bond: a chemical bond resulting from two
atoms sharing one or more pairs of electrons
• A hydrogen atom can achieve a completely empty
shell by losing an electron and results in a positively
charged hydrogen ion– proton
• A hydrogen atom can achieve a filled outer shell by
gaining an electron, thereby forming in a negatively
charged hydrogen ion– hydride ion
Polar Covalent Bonds: Electronegativity
• The atoms that share the bonding electrons in F-F or
H-H covalent bond are identical—nonpolar covalent
bond
• The covalent bond between atoms of different
electronegativities—polar covalent bond
Cl-Cl
H-Cl
Na+-Cl-
Electrostatic Potential Maps
HOW THE STRUCTURE OF A COMPOUND
IS REPRESENTED
Lewis Structure
• Can show which atoms are bonded together
• Tell any atoms possess lone-pair electrons
(nonbonding electrons) or have a formal charge
Kekulé structures
Condensed structures
ATOMIC ORBITALS
An orbital tells us the volume of space around the nucleus
where an electron is most likely to be found
The s Orbitals
The 2nd shell contains three p orbitals
• A p orbital tells has two lobes
• The three 2p orbitals have the same energy
• The energy of 2p orbital is greater than that of a 2s
orbital
HOW ATOMS FORM COVALENT BONDS
A covalent bond forms when two atoms approach each other
closely and a singly occupied orbital on one atom overlaps
a singly occupied orbital on the other
The electrons are now paired in the overlapping orbitals and
are attracted to the nuclei of both atoms, thereby bonding
the atoms together
The covalent bond that is formed when the two orbitals
overlap is called sigma (s) bond
0.074 nm
1. Bond strength/bond dissociation:
energy required to break a bond
or energy released to form a
bond.
2. Every covalent bond has a
characteristic bond length and
bond strength
HOW SINGLE BONDS ARE FORMED IN
ORGANIC COMPOUNDS
The Bonds in Methane
1. The four sp3 orbitals adopt a spatial
arrangement that keeps them as far
from each other as possible
2. Methane forms covalent bonds using
4 equivalent sp3 orbitals is called a
tetrahedral carbon
3. Tetrahedral bond angle: 109.5°
The Bonds in Ethane
HOW A DOUBLE BOND IS FORMED:
THE BONDS IN ETHENE
HOW A TRIPLE BOND IS FORMED:
THE BONDS IN ETHYNE
BONDING IN THE METHYL CATION,
THE METHYL RADICAL, AND
THE METHYL ANION
The methyl cation (+CH3)
The methyl radical (·CH3)
The methyl anion (-:CH3)
THE BONDS IN WATER
THE BONDS IN AMMONIA
AND IN THE AMMONIUM ION
THE BOND IN A HYDROGEN HALIDE
2sp3
3sp3
SUMMARY: HYBRIDIZATION, BOND
LENGTH, BOND STRENGTHS, AND BOND
ANGLE
All single bonds are s bonds
All double bonds are composed of one s bond and one p bond
All triple bonds are composed of one s bond and two p bonds
Chapter
2
Acids and Bases
AN INTRODUCTION TO ACIDS AND
BASES
Brønsted–Lowry defined an acid as a species that donates
a proton, and a base as a species that accepts a proton
When a compound loses a proton (HCl), the resulting
species is called its conjugate base (Cl-).
When a compound accepts a proton (H2O), the resulting
species is called its conjugate acid (H3O+).
Water (H2O) can behave as either an acid or a base
Acidity is a measure of the tendency of a compound to
give up a proton
Basicity is a measure of a compound’s affinity for a
proton
A strong acid has a strong tendency to give up a proton
• The weaker the base, the stronger is its conjugate acid
• The stronger the acid, the weaker is it conjugate base
pKa AND pH
• When a strong acid such as hydrogen chloride is
dissolved in water, almost all the molecules dissociate.
• When a much weaker acid, such as acetic acid, is
dissolved in water, very few molecules dissociate.
Ka = 107
Ka = 1.74 x 10-5
An Acid/Base Equilibrium
[H3O+] [A-]
Ka =
[HA]
pKa = -log Ka
Very strong
moderately strong
Weak
Very weak
Extremely weak
pKa < 1
pKa = 1-3
pKa = 3-5
pKa = 5-15
pKa > 15
• Ka: The acid dissociation constant.
• The stronger the acid, the larger its Ka value and the
smaller its pKa value.
• pH is indicated the
concentration of positively
charge hydrogen ion [H+] in
the solution
• pH = -log [H+]
• The lower the pH, the more
acidic is the solution
• The pH scale is used to
describe the acidity of a
solution; the pKa is
characteristic of a particular
compound
ORGANIC ACIDS AND BASES
• The most common organic acids are carboxylic acids
– Compounds have a COOH group
– Have pKa values range from about 3-5
• Alcohols
– Compounds have an OH group
– Are much weaker acids than carboxylic acids
– With pKa values close to 16
• Water can behave both as an acid and as a base
• An alcohol behaves similarly
• A protonated compounds has gain an additional proton
– Very strong acids
HOW TO DETERMINE THE POSITION OF
EQUILIBRIUM
• Strong Acids / Bases React to Form Weak Acids / Bases
• Compare the pKa values of the acids
HOW THE STRUCTURE OF AN ACID
AFFECTS ITS pKa
• Two factors that affect the stability of a base are its size
and its electronegativity
• When atoms are similar in size, the strongest acid will
have its hydrogen attached to the most electronegative
atom.
• When atoms are very different in size, the strongest acid
will have its hydrogen attached to the largest atom.
HOW pH AFFECTS THE STRUCTURE OF
AN ORGANIC COMPOUND
• Whether an acid will lose a proton in an aqueous solution
depends on both the pKa of the acid and the pH of the
solution.
• pKa > pH; acidic form (with its proton)
• pKa = pH; acidic form = basic form
• pKa < pH; basic form (without its proton)
• Physiological pH (pH=7.3)
BUFFER SOLUTIONS
• A solution containing a weak acid (HA) and its
conjugated base (A-) is called a buffer solution.
• A buffer solution will maintain nearly constant pH when
small amounts of acid or base are added to it
LEWIS ACIDS AND BASES
• Lewis acid:
– Accepts a share in an electron pair
– non-proton-donating acid (AlCl3)
• Lewis base:
– donates a share in an electron pair
– all bases are Lewis bases