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Title: The traditional Representation of chemical compounds


1
The traditional Representation of chemical
compounds
  • Goals
  • Name the compounds (XVIII century)
  • Describe them
  • Geometry and electron distribution
  • Global formulae, planar representation,
    representation in space
  • Single or multiple representations
  • different models, different ways of counting
    electrons

2
Name and Describe Naming is important Gods
power in genesis And God said, Let there be
light and there was light. Name and
Describe In chemistry we want to describe at the
same time. Ideograms are more descriptive than
names
??
Huàxué Chemistry Science of transformation
3
The domain of Chemistry that of Democritos
??µ????t?? , born 460 bc
Most of the understanding of chemistry goes back
before the knowledge of electrons, including the
discovery of the periodic classification.
What are the forces in physics? Which one
concerns Theoretical Chemistry?
4
The domain of Theoretical Chemistry of J. J.
Thomson
electron discovery in 1887
  • Domain of electrostatic(electromagnetic) forces.
    Coulomb Law
  • Nuclei will be treated as positively charged
    particles. This charge is equal to the atomic
    number of the atom, Z. It is a multiple of the
    charge of the proton, e (e 1.6 10-19 C). The
    electrons are particles with a charge -e. Between
    them exercised only electrostatic forces.

5
The atomic number, Z




Mass number

Atomic number
Isotopes are used by chemists for
characterization (physical chemistry) They should
not be distinguished for chemistry itself
(reactivity, exchanges of atoms)
6
Chemical formula AxByXz
  • This gives composition stoichiometry
  • Law of the defined proportions (1807) or the law
    of constant composition)
  • Defined does not mean unique NO2, NO and N2O
  • Law of multiple proportions. The relative number
    of different atoms is always a simple integer
    This is a quantum law.
  • Few exceptions CuyS.
  • As defined originally the ratio of atomic
    weights was approximate due to isotopic
    mixures.
  • A consequence about connectivity in AB2, B has
    twice neighbors than A (assuming only A-B bonds)

"Stoichiometry" is derived from the Greek words
st???e??? (element) and µet??? (measure.)
7
Molecular and structural formula
  • This system for writing chemical formulas was
    invented by the 19th-century Swedish chemist Jöns
    Jakob Berzelius.
  • A chemical formula supplies information about the
    types and spatial arrangement of bonds in the
    chemical, though it does not necessarily specify
    the exact isomer.
  • For polymers, parentheses are placed around the
    repeating unit. For example, a hydrocarbon
    molecule that is described as CH3(CH2)50CH3, is
    a molecule with 50 repeating units.

8
developed formula
  • This gives the connection between atoms.
  • Isomers of position CH2Cl-CH2-CH3 and
    CH3-CHCl-CH3
  • Skeleton sequence without monovalent atoms.
  • Coordination number of first neighboring atoms.

9
valence or valency number
William Higgins (1763-1825), an Irish chemist
  • a measure of the number of chemical bonds formed
    by the atoms of a given element.
  • This definition is ambiguous but historically
    very helpful.

The International Union of Pure and Applied
Chemistry (IUPAC) has made several attempts to
arrive at an unambiguous definition of valence.
The current version, adopted in 1994,4 The
maximum number of univalent atoms (originally
hydrogen or chlorine atoms) that may combine with
an atom of the element under consideration, or
with a fragment, or for which an atom of this
element can be substituted. This definition
reimposes a unique valence for each element at
the expense of neglecting, in many cases, a large
part of its chemistry.
10
valence or valency number
Column Valency number
1 and 17 H, Li, Na, F, Cl 1
2 and 16 Be, Mg, O, S 2
3 and 15 B, Al, N, P 3
14 C, Si 4
11
Which product is more likely ?
  • HOCl or HClO ?

12
Unsaturation Multiple bonds to respect valency
numbers
  • H2CO a double bond is necessary
  • N2 a triple bond is necessary
  • Never write a quadruple bond! (except for Cr2)
  • The chemical formula of a saturated hydrocarbon
    (alcane) is CnH2n2. For others, the number of
    unsaturations may be found from y in the formula
    CnH2n2-2y.
  • This is easily generalized
  • Replace every monovalent atom by H
  • Ignore divalent atoms
  • Replace NH by CH2

13
Unsaturations multiple bonds or rings
14
Planar representation of the molecule
  • A developed formula is inadequate to describe the
    electronic environment of the atoms in the
    molecule. Even before we talk about electron,
    from 1845, Auguste Laurent wrote "a system of
    such formulas is too absolute and if adopted, it
    would prevent finding a wealth of valuable
    reports. We do not know how atoms are really, but
    we already know that in such a compound the atoms
    are arranged like in another ... I can say that
    compounds have the same or different structures
    without knowing anything of their structure."

15
Planar representation Lewis structures 1916
The representation of Lewis gives the connection
between atoms and precise of every valence
electron.
  • Valence electron are described by
  • a dot (single electron)
  • a segment (electron pair or bonds)
  • a small square may indicate a vacant place
    available for extra electrons

Gilbert Newton Lewis, Berkeley (Octobre 23, 1875
- Mars 23, 1946)
16
History Lewis (1916), Lamgmuir (1919)
The first representation of the Lewis structure
used cubic atoms well adapted for the octet
rule. The binding did result from a pairing of
cubes through vertices, edges or faces according
to Abegg law.
17
Valence electrons, core electrons
  • Valence electrons are the least stable ones, core
    electrons the most stable ones.
  • Ionization potential for Valence electron is in
    the range of 10 eV or less, that for core
    electron in the range of 100 eV or more
  • Why to be interested in the least stable
    electrons? because they are involved in
    chemistry the others dont they are too stable.

18
Number of valence electrons
  • For main atoms the last digit of the number of
    the column in the periodic table.
  • For Transition metals, the number of the column.
  • The number of core electrons is the number of
    electrons of the last rare gas atom before.

2 8 8 18 18 32 0
? 2 ? 10 ? 18 ? 36 ?
54 ? 86 The main values correspond to
seps by 2, 8 and 18.
19
1 I  1 2 II  2 3 III  3 4 IV  4 5 V  5 6 VI  6 7 VII 7 8 VII 8 9 VII9 10 VII10 11 I B 11 12 II  12 13 III 3 14 IV  4 15 V  5 16 VI 6 17 VII 7 18 VII8
V
1 1 H COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER COLUMN VALENCY NUMBER 2 He
2 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne
3 11 Na 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar
4 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr
5 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe
6 55 Cs 56 Ba 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn
7 87 Fr 88 Ra 104 Rf 105 Db 106 Sg 107 Bh 108 Hs 109 Mt 110 Ds 111 Rg 112 Uub 113 Uut 114 Uuq 115 Uup 116 Uuh 117 Uus 118 Uuo
                                     
Lanthanides Lanthanides Lanthanides Lanthanides 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 71 Lu
Actinides Actinides Actinides Actinides 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md 102 No 103 Lr
20
What makes a formula a stable one?
  • A compound is stable when two rules are
    satisfied
  • the octet rule
  • and the electric neutrality.
  • If it not possible to satisfy both, obey first
    the octet rule unless excessive deviation of
    charge neutrality.
  • Molecules better have only electron pairs (closed
    shell molecules). No dots.

21
Choosing a central atom
  • There are no general recipe
  • For compounds containing multiple elements with
    only one atom in the formula, the central atom is
    the least electronegative single atom that is not
    hydrogen. For instance, in thionyl chloride
    (SOCl2), the sulfur atom is the central atom.
  • Do not connect similar atoms between them except
    C or Si (see Pauling remarks A-B stronger than
    (A-AB-B)/2)

22
The octet rule 2 (doublet rule), 8 or 18
(eighteen electron rule)
  • We are interested in the local environment of a
    given atom. How many electrons lie around on
    atom?
  • Warning! Be careful that the sum of the electrons
    for all the atoms is larger than the total number
    of valence electrons. Some atoms belong to two
    neighborhood and are counted twice!
  • A molecule is stable when for all of its atoms,
    the number of electrons in the atomic environment
    is 2, 8 or 18 (the number of valence electrons of
    the rare gas atom that follows).
  • Two is for hydrogen. The eighteen electron rules
    apply for transition metals.

23
Notice to find appropriate Lewis structures
  • Do not couple dots.
  • Start counting the total number of valence
    electrons.
  • Try to build a structure respecting the octet
    rule
  • If you fail, start from there and move electrons
    to adjust.

24
Lewis structures exceptions
  • Deficient atoms. BeH2, AlCl3 Lewis acids. We may
    indicate the lack of electrons by a square.
  • Hypervalent compounds. Rare gas atoms, large
    atoms, compromise for avoiding excessive charge
    (For H2SO4, charge -2 would appear).

25
Building Lewis structure
  • Start counting the total number of valence
    electrons.
  • Try to build a structure respecting the octet
    rule
  • If you fail, start from there and move electron
    to help.

26
Mesomery, Resonance
  • One structure may be not enough!
  • Symmetry imposes equivalence between atoms this
    is not possible using a single Lewis structure.

The neutral formula does not obey the octet
rule. One Lewis formula respecting the octet rule
does not verify symmetry several are then
needed. The arrow indicates mesomery. Both
representation contain vluable (different)
information.
27
p Electron
  • A requirement for conjugation is planarity.
  • Conjugation takes place for double bonds (or
    double bonds and electron pairs) that are on
    adjacent atoms (neither on the same atom nor on
    atoms separated by saturated atoms).

28
Conjugation is informative even when a unique
Lewis formula correctly describes the ground
state of a molecule. It indicates potential
electron localization occurring for reactivity.
The various Lewis structures for a given
structure globally describe a molecule in the
Valence Bond approach, VB theory. This theory
will attribute a weight to each structure.
29
Conjugation is informative even when a unique
Lewis formula correctly describes the ground
state of a molecule. It indicates potential
electron localization occurring for reactivity.



30
benzene naphtalene
6 p electrons 6 p electrons within
each rin 10 p electrons
total (two are shared)
Symmetry is the important concept Benzene is
D6h an hexagon with an edge of 1.40 Å.
31
s and p separation
  • It is strictly defined using symmetry (QM)
  • Molecule Atoms orbitals of
    H2

p orbitals of ethene
32
p orbitals of benzene
33
Butadiene p orbitals
34
Allene Why there is no conjugation between
double bonds on the same atom. Hyperconjugation
35
Mesomery summary
  • In some cases, several structures are necessary
    to represent together a molecule.
  • If these structures are equivalent (symmetry
    related) the account for the molecule together.
  • If these structures are different, one is more
    appropriate however the others are informative

36
Formal Charges
  • At variance with the octet rule, the counting of
    formal charge is a partition of electron. The sum
    of the electrons should be the total number of
    the valence electrons of the system. The sum of
    the charge should be the total charge of the
    system (0 for molecules, A value for ions).
  • These are two informative visions of the electron
    distribution.

37
Formal Charges, a democratic splitting
When distributing electrons on the atoms, one has
to split the contribution of electron pairs in
A-B bonds between A and B. In the formal charge
approach, the deal is one electron each even if A
and B differ. There are other models also useful
that we will see later on. The electronic density
on each atom is dSe The formal charge on each
atom is q-d, q being the atomic number of valence
electron of the atom. This definition matches the
Mulliken charge definition in theoretical
chemistry, which is a standard.
38
Formal Charges, a democratic splitting
  • A formal charge is a partial charge on an atom in
    a molecule assigned by assuming that electrons in
    a chemical bond are shared equally between atoms,
    regardless of relative electronegativity or in
    another definition the charge remaining on an
    atom when all ligands are removed homolytically.

39
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40
The case of carbon monoxide, CO
  • The total number of valence electron is 4610.
  • One Lewis structure satisfies the octet rule and
    not satisfies the electron neutrality whereas the
    other does the contrary.
  • The dipole moment is very weak it corresponds to
    the charged formula. Since O is more attractive
    to electrons than C, the formal charges however
    come out very small.

41
Several possible Lewis structures

Cl-Mg-Cl or ClMgCl

Neutral formula Charged
formula Satisfies the octet rule
Indicates p conjugation Shows Lewis acidity
does not
42
Electron count
  • This is very important. However there are many
    ways that differ all are informative
  • Isoelectronic system Isolobal
  • HClO HOCl
  • N2 CO
  • 3 main ways of distributing electrons
  • Formal charges, democratic
  • Oxidation numbers, not democratic everything for
    the most powerful atom
  • Electronegativity, intermediate

43
The isolobal analogy electron count
Roald Hoffmann Cornell, Ithaca NY Nobel 1981
44
Electronegativity
  • Partial charges. If A is stronger than B (more
    electronegative) A has more electrons.
  • Not to consider in cases of presence of formal
    charges.
  • There are many different scales Pauling,
    Mulliken, Allred and Rochow, Sanderson, Allen
  • Decomposition of dipole moments may generate
    partial charges on the atoms (however
    decomposition is also an oversimplification)

45
Pauling Electronegativity





46
Pauling Electronegativity
  • 1932
  • The covalent bond between two different atoms
    (AB) is stronger than would be expected by
    taking the average of the strengths of the AA
    and BB bonds.
  • The difference in electronegativity between atoms
    A and B is given by
  • where the dissociation energies, Ed, of the AB,
    AA and BB bonds are expressed in eV, the factor
    (eV)-½ being included to ensure a dimensionless
    result.

Linus Pauling, Stanford Nobel Chemistry
1954 Peace 1952
47
Mulliken Electronegativity
the arithmetic mean of the first ionization
energy and the electron affinity should be a
measure of the tendency of an atom to attract
electrons. c (PIEA)/2 As this definition is
not dependent on an arbitrary relative scale, it
has also been termed absolute electronegativity
with eV. However, it is more usual to use a
linear transformation to transform these absolute
values into values which resemble the more
familiar Pauling values. For ionization energies
and electron affinities in electronvolts      c
0.187(PIEA)0.17                              
Robert Sanderson Mulliken Nobel 1962
48
Oxidation numbers
  • It is also a partition of electronic density. The
    electrons from the bonds are attributed to the
    most electronegative atom. The model is extreme
    and purely ionic.
  • We can count electrons on the atoms in this doing
    so and oxidation numbers are the corresponding
    charge, q-d.

49
Oxidation numbers, N history
  • They have been defined relative to O (F would
    have been more appropriate)
  • 4 rules
  • N is 0 in elements whatever the allotropic form
    is.
  • For O, N-2 (except when bound to itself O2,
    O3,HOOH, peroxides)
  • H is 1 or -1 according to electronegativities
  • N for others is deduced so that the total sum is
    nil (molecules) or equal to the total charge
    (ions)

50
Octave rule
  • nmax -nmin 4-(-4) 8 for C
  • nmax -nmin 5-(-3) 8 for N
  • nmax -nmin 6-(-2) 8 for S
  • nmax -nmin 7-(-1) 8 for Cl

The oxidation numbers vary in a range of 8 By
losing electrons, they reach the number of
electrons of the preceding gas rare atom (number
of core electrons), by gaining them they acquire
the number of electrons of the following gas rare
atom (number of core electrons valence
electrons) the difference is eight (number of
valence electrons).
51
Valence and oxidation numbers
  • The valence is the maximum number for oxidation
    numbers ignoring sign
  • For electropositive atoms, it is the highest
  • 2 for Mg since they usually lose electrons
  • For the electronegative atoms, it is the
    smallest -3 (giving 3) for N since these atoms
    usually gain electrons

52
What are the isomers for C2H4O? Determine the
oxidation numbers for the individual atoms?
53
What are the isomers for C2H4O? Determine the
oxidation numbers for the individual atoms?
Un??/Insaturation 1 a ring or a double
bond Total number of electrons 2x44x1614 7
segments ²


Oxidation number for C -1
-3 1
-2 0
average is -1 in all cases -242x0
54
Spatial representation
  • VSEPR
  • Conformations and configurations
  • Chirality
  • Symmetries

55
VSEPR, Valence Shell Electronic Pairs Repulsion
  • The idea is to minimize the electron pair
    repulsion.
  • Largest distances between electron pairs on a
    sphere
  • Warning! The electron count differs
  • Pairs are
  • lone pairs
  • pairs associated with single bonds
  • Multiple bond electron pairs are neglected!

Ronald Gillespie Professor Emeritus Canada
56
VSEPR
2 180 sp linear 5 (by mistake) 90/120 Trigonal bipyramid
3 120 sp2 trigonal 6 90/180 Octahedral
4 109.4712 sp3 tetrahedral 7 72/90 Pentagonal bipyramid
57
VSEPR
  • Improvements
  • The lone pairs are closer to the nuclei than the
    bonding pairs so the angle between them is
    larger than expected and the angle between bonds
    decreases count 2 degrees per lone pair for that
    decrease
  • H-N-H 109-2107 H-O-H 109-2x2105
  • Multiple bonds have density that perturbs??.
  • There is no need to refine further the model or
    to apply it to exotic cases its main interest is
    simplicity!

58
Isoelectronic compounds, Isolobal analogy
  • H3O NH3 CH3-
  • Compounds with the same electron count have the
    same structure and often behave the same.
  • Of course charges differ and modify reactivity
    (modify the balance between the same set of
    iterations)

59
Conformations and Configurations
A conformation is a simple arrangement in space.
When it is possible to isolate a compound the
topology of the connections is called a
configuration. Flexibility makes that for a given
configuration, there are several
conformations. This notion depends on
temperature. At low temperature it is possible to
isolate frozen conformations that become
configurations. At high temperature it is
possible to convert a configuration to another
one and they became conformations. We will refer
to standard T.
60
Cis and Trans configurations
E
Z
  • In standard condition a double bond is rigid and
    it is not possible to convert an isomer to
    another without breaking or reforming a bond.
  • Names for configuration
  • Old Cis and Trans. This is ambiguous
  • New Z and E. Differents ligands are classified
    according to Z (R. S. Cahn, C. Ingold, V. Prelog)

61
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63
Cram Representation
A connection, between an atom in the plan and an
atom in front of the sheet, is represented by a
fatty feature or a full triangle, the point being
with dimensions atom in the plan, the base of
with dimensions of the other atom a connection,
between an atom in the plan and an atom behind of
the plan, is represented by a feature into dotted
or a hatched triangle whose point is with
dimensions atom in the plan, the base of with
dimensions of the other atom
Donald James Cram 1919-2001 Nobel 1997
64
Eclipsed (showing symmetry) Staggered
(most stable)
Full symmetry appears only in some conformation
65
eye
Ethane conformation
Rotation energy profile
Eclipsed Staggered Newman
representation
66
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67
BUTANE
Staggered left anti
right eclip
sed
68
D) Cyclohexane Chair and Boat conformations
Chair all staggered C
Boat 4 staggered C 2 eclipsed
69
Chirality
There are isomers D and L of the same compound
but with different spatial geometry that deviate
polarized light L to left, D to right. The
rigorous 50/50 (racemic) mixing of the two
compounds has no activity on light. These
correspond to crystal structures that are related
by a mirror symmetry.
Pasteur first work has been to separate Tartric
acids
Louis Pasteur 1922-1995 French (Arbois)
70
The mirror symmetry is present at the atomic
level and appears when a carbon atom has 4
different ligands.
71
Chirality Asymmetric carbon (1874)
Jacobus Henricus van't Hoff, 1852-1911
Dutch Nobel 1901
Joseph-Achille Le Bel 1847-1930 French
72
R and S
  • Instead of D and L nomenclature, asymmetric
    carbon atoms are named according to Z (R. S.
    Cahn, C. Ingold, V. Prelog).
  • Classify the atoms according to decreasing Z,
    place the smallest behind and see whether they
    are ordered clock-wise (R) or not (S).

73
Placing the smallest atom behind
?

74
  • Only one of the compounds below is a drug active
    against AIDS (Crixivan)
  • How may stereoisomer  correspond to this formula?
  • Tell whether the N lone pairs are s or p?

75
Only one of the compounds below is a drug active
against AIDS (Crixivan)





76
Several asymmetric carbons not related by
symmetry 2N isomers
enantiomers
diastereoisomers
enantiomers
By pairs there?? are related by a mirror
symmetry enantiomers Total exchange R
S and S R.
77
Several asymmetric carbons related by symmetry
lt 2N isomers
threo
Mirror symmetry
meso
A compound that possesses a mirror symmetry is
inactive on light
78
How many isomers are there for 1,2,3 trimethyl
cyclobutanes ? Which ones are active on light ?
79
How many isomers are there for 1,2,3 trimethyl
cyclobutanes ? Which ones are active on light ?
Inactive Inactive Active Active
80
Chirality originates from symmetry and does not
concern sp3 carbon only
rings
helicene
metallocene
81
SPIRO compounds
Plane of the screen
(two views of the same image)
Plane perpendicular
82
inactive active
active active

Left or right
Mirror plane in red

83
Camphor Tree ( Cinnamomum camphora )
active (camphor) ????
84
allene
85
Localized approaches
  • Some bond properties are transferable from one
    compound to another dissociation energies, bond
    lengths, dipole moments, spectroscopic shifts It
    is then useful to decompose molecules into bond
    contributions. Such reasoning seems reasonable
    knowing Lewis structure that electrons are
    attributed to bonds. It is productive leading to
    the concept of substituent.
  • However, this is conceptually wrong a major
    feature for a molecule is its symmetry and this
    requires considering the global molecule (several
    bonds together related by symmetry).

86
Units pm and kJ
Bond Length Energy Bond Length Energy
H--H 74 436 H--C 109 413
C--C 154 348 H--N 101 391
N--N 145 170 H--O 96 366
O--O 148 145 H--F 92 568
F--F 142 158 H--Cl 127 432
Cl-Cl 199 243 H--Br 141 366
Br-Br 228 193 H--I 161 298
I--I 267 151
C--C 154 348
C--C 154 348 CC 134 614
C--N 147 308 CºC 120 839
C--O 143 360
C--S 182 272 O--O 148 145
C--F 135 488 OO 121 498
C--Cl 177 330
C--Br 194 288 N--N 145 170
C--I 214 216 NºN 110 945
87
Dipole moment for a heteronuclear diatomic
molecule
  • If the center of mass of the positive charge of a
    neutral molecule differs from that of the
    negative charges, there is a dipole moment that
    is a molecular property.
  • It is a vector from to whose magnitude is qxd
    expressed in C.m.
  • Warning Never talk of dipole moment for charged
    species (not an intrinsic property it varies
    with the origin). Charge is then the significant
    property.

88
An old unit Debye
  • Two charges e and -e separated by a distance of
    1 Å lead to 4.8 Debyes.
  • (1 Debye equals 3.34 10-30 C.m).
  • Easy to use using proportionality.

89
Dipole moment for a molecule
  • A simple model is to attribute dipoles to bonds
    and sum up the contributions for each of them
    (sum of vectors)
  • This leads to partial charges (fraction of e).
    The total moment is on a symmetry axis of the
    molecule

m(Bond)
mT3 mN-H cos q 3 mN-H 1/3
m(molecule)
90
Main symmetries
  • There are 5 kinds of operations
  • 1. Identity
  • 2. n-Fold Rotations
  • 3. Reflection
  • 4. Inversion
  • 5. Improper n-Fold Rotation

91
Improper Rotations Sn n-fold rotation followed
by reflection through mirror plane perpendicular
to rotation axis n is always 3 or larger because
S1 s and S2 i. n3 Staggered ethane
92
  • If one of the many conformation has a mirror
    symmetry, the compound is inactive on light.

93
Improper Rotations S4
S4
94
Where symmetry is important?
  • For chirality chirality is bound to asymmetry
  • For dipole moments, the sum vector satisfies
    symmetry operations
  • For orbitals they should describe symmetry as a
    molecular property
  • For reactivity or excitation symmetry may be
    preserved during a process


95
In which field chemistry is acting?
Why to be interested in Chemistry?
96
In which field chemistry is acting?
  • Answer Everywhere
  • Food taste
  • Clothes colors
  • Perfumes smell
  • Medecine health
  • Gas energy
  • Materials
  • Environment

97
Clothing Art colors,
indigo, purple,..
98
Purple has played an important role in history
99
perfumes, smell
100
Medicine, health drugs
101
materials, pigments, dyes , plastics,
cements/concrete??

industry, building
102
environment, pollution, waste..
Chemists know how it is a matter of price  Can
we produce chemical...using gentle methods,
harmless and green, without using toxic
compounds, using less energy and few
non-renewable raw materials ? Answer is
Yes, we can, however this is more expensive  
Guy Ourisson
292ème conférence à lUTLS 18/10/2000
103
Frontiers between nature and artifact.
Nothing is lost, nothing is created, everything
is transformed
Lavoisier
Chemistry does not create anything, it merely
converts and copies nature. Substitution
reaction is the reaction archetype. Fear of
chemistry is that artifacts can go beyond nature
it is the fear that man assaults Gods powe
creation. No science better than chemistry has
defined its limits towards this fear.
104
  • Two main answers
  • Cooking
  • philosophy

Why be interested in Chemistry?
105
  • The combinatorial ability of elements was so
    little obvious property that it did not appear as
    an intrinsic property by observation but was
    revealed by a chemical analysis that destroyed
    the compound showing a property outside it.
  • Auguste Laurent (1845) wrote  that chemistry had
    become a science of bodies that do not exist "
    meaning that it was a property that was only
    revealed by destruction when these bodies no
    longer existed."

106
Chemistry is science of substance, of concepts.
  • Many have been discovered before the discovery of
    electrons
  • Lewis formula (1916), Kekulé formula (1856)
  • Periodic table.
  • Of course, this is much easier using Q.M.

107
Why is Chemistry more fundamental than physics?
Physics takes interest in primary aspects
Chemistry considers the substance itself.
108
Why is Chemistry more fundamental than physics?
Chemistry needs physics What is the most
important? The first step? Or moving further?
109
A harmful classification.
Auguste Comte, Maths gt Physicsgt Chemistry
110
Doctor ?
Physicist ?
111
  • WE ARE PERHAPS NOT FAR REMOVED FROM THE TIME
    WHEN WE SHALL BE ABLE TO SUBMIT THE BULK OF
    CHEMICAL PHENOMENA TO CALCULATION.
  • -- JOSEPH LOUIS GAY-LUSSAC
    MEMOIRES DE LA SOCIETE D'ARCUEIL, 2, 207 (1808)

112
??
World of representation
This is not a pipe
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