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

化学Huàxué : Chemistry

Science of transformation

Name and Describe

Naming is important: God’s 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

3

The domain of Chemistrythat of Democritos

Δημόκριτος , 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

-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.

The domain of Theoretical Chemistry of J. J. Thomson

electron discovery in 1887

5

The atomic number, Z

C126

C136 C14

6 O168

O178 O18

8

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 στοιχειον (element) and μετρον (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

• a measure of the number of chemical bonds formed by the atoms of a given element.

• This definition is ambiguous but historically very helpful.

William Higgins (1763-1825), an Irish chemist

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

• H2C=O 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 CnH2n+2. For others, the number of unsaturations may be found from y in the formula CnH2n+2-2y.

• This is easily generalized: – Replace every monovalent atom by H– Ignore divalent atoms

– Replace NH by CH2

13

Unsaturations: multiple bonds or rings

CHH H

CHH H

CH H

CHH

Ring Closure

-H2

CHH H

CHH H

CH

2 CH

2

CHH H

CHH H

HC = CH

Double bond

-H2

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

Gilbert Newton Lewis, Berkeley(Octobre 23, 1875 - Mars 23, 1946)

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

16

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.

History: Lewis (1916), Lamgmuir (1919)

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 don’t; 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 +320 → 2 → 10 → 18 → 36 → 54 → 86

The main values correspond to seps by 2, 8 and 18.

19

1I 1

2II 2

3III 3

4IV 4

5V  5

6VI 6

7VII7

8VII8

9VII9

10VII10

11I B11

12II 12

13III 3

14IV 4

15V 5

16VI 6

17VII 7

18VII8

V

11H

COLUMN

VALENCY NUMBER

2He

23Li

4Be

5B

6C

7N

8O

9F

10Ne

311Na

12Mg

13Al

14Si

15P

16S

17Cl

18Ar

419K

20Ca

21Sc

22Ti

23V

24Cr

25Mn

26Fe

27Co

28Ni

29Cu

30Zn

31Ga

32Ge

33As

34Se

35Br

36Kr

537Rb

38Sr

39Y

40Zr

41Nb

42Mo

43Tc

44Ru

45Rh

46Pd

47Ag

48Cd

49In

50Sn

51Sb

52Te

53I

54Xe

655Cs

56Ba

* 72Hf

73Ta

74W

75Re

76Os

77Ir

78Pt

79Au

80Hg

81Tl

82Pb

83Bi

84Po

85At

86Rn

787Fr

88Ra

**104Rf

105

Db

106

Sg

107

Bh

108

Hs

109Mt

110

Ds

111

Rg

112

Uub

113

Uut

114

Uuq

115

Uup

116

Uuh

117

Uus

118

Uuo

 

* Lanthanides57La

58Ce

59Pr

60Nd

61Pm

62Sm

63Eu

64Gd

65Tb

66Dy

67Ho

68Er

69Tm

70Yb

71Lu

** Actinides89Ac

90Th

91Pa

92U

93Np

94Pu

95Am

96Cm

97Bk

98Cf

99Es

100Fm

101Md

102No

103Lr

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-A+B-B)/2)

22

The octet rule2 (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.

B-F+ B-

F+

F

F

F

F B-

F+

F

F

BF

F

F

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

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.

C C C C C C C C

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

C l C l

+

C l

+

C l

+C l

++ +

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 electrons 6 electrons within each rin

10 electrons total (two are shared)

Symmetry is the important concept Benzene is D6h: an hexagon with an edge of 1.40 Å.

31

and separation

It is strictly defined using symmetry (QM)

orbitals of ethene

Molecule Atoms

orbitals of H2

32

orbitals of benzene

33

Butadiene 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 d=eThe 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

40

The case of carbon monoxide, CO

• The total number of valence electron is 4+6=10.

• 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.

:C≡O: or :C=O ¨

¨

41

Neutral formula Charged formula

Satisfies the octet rule Indicates conjugation

Shows Lewis acidity does not

Several possible Lewis structures

:Cl-Mg-Cl: or Cl=Mg=Cl ¨ ¨ ¨ ¨

¨ ¨ ¨ ¨

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

1932 The covalent bond between two different

atoms (A–B) is stronger than would be expected by taking the average of the strengths of the A–A and B–B bonds.

The difference in electronegativity between atoms A and B is given by:

where the dissociation energies, Ed, of the A–B, A–A and B–B bonds are expressed in eV, the factor (eV)−½ being included to ensure a dimensionless result.

Pauling Electronegativity

2/))()(()( BBEAAEBAE ddd Linus Pauling,

Stanford

Nobel

Chemistry 1954

Peace

1952

47

Mulliken Electronegativitythe arithmetic mean of the first ionization energy and the electron affinity should be a measure of the tendency of an atom to attract electrons.

= (PI+EA)/2As 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     

= 0.187(PI+EA)+0.17                             

Robert Sanderson MullikenNobel 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:

1) N is 0 in elements whatever the allotropic form is.

2) For O, N=-2 (except when bound to itself: O2, O3,HOOH, peroxides)

3) H is +1 or -1 according to electronegativities

4) 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 bondTotal number of electrons 2x4+4x1+6=14 7 segments

²

Oxidation number for C

-1 -3 +1 -2 0

average is -1 in all cases -2+4+2x=0

54

Spatial representation

• VSEPR

• Conformations and configurations

• Chirality

• Symmetries

55

Ronald GillespieProfessor EmeritusCanada

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!

56

VSEPR

2 180

sp

linear

5 (by mistake)

90/120Trigonal

bipyramid

3 120

sp2

trigonal

6 90/180Octahedral

4 109.4712

sp3

tetrahedral

7 72/90Pentagonal

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 = 109-2=107 H-O-H = 109-2x2=105• 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 Transconfigurations

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)

Z E

61

62

63

Cram Representation

Donald James Cram1919-2001 Nobel 1997

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:

C CH

HH

HH

H

64

Full symmetry appears only in some conformation

Eclipsed (showing symmetry) Staggered (most stable)

C C

H H

F F

Cl Cl

C C

H

HF

Cl

Cl

F

65

Ethane conformation Rotation energy profile

Eclipsed StaggeredNewman representation

eye

66

67

Staggered left anti right

eclipsed

BUTANE

68

D) CyclohexaneChair and Boat conformations

Chair: all staggered C

Boat: 4 staggered C

2 eclipsed

69

Pasteur first work has been to separate Tartric acidsLouis Pasteur

1922-1995

French (Arbois)

ChiralityThere 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.

70

The mirror symmetry is present at the atomic level

and appears when a carbon atom has 4 different ligands.

71

Jacobus Henricus van't Hoff, 1852-1911 DutchNobel 1901

Chirality: Asymmetric carbon (1874)

Joseph-Achille Le Bel 1847-1930French

72

R and SInstead 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).

A B

C

D

B A

C

D

RECTUS SINISTER

73

Placing the “smallest” atom behind

HCO2H

OH

R

C*

(4)(1)

(3)

(2)OH

HCO2H

R

C*

(1)(2)

(3)

(4)

→

74

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

1) How may stereoisomer  correspond to this formula?

2) Tell whether the N lone pairs are or ?

75

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

* * **

*

76

Several asymmetric carbonsnot related by symmetry

2N isomersOHH

HO H

CHO

CO2H

R

R Thréo

Erythro

HHO

H OH

CHO

CO2H

S

S

OHH

H OH

CHO

CO2H

HHO

HO H

CHO

CO2H

S

R

R

S

diastereoisomers

enantiomers

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 < 2N isomers

OHH

HHO

CO2H

CO2H

HHO

OHH

CO2H

CO2H

OHH

OHH

CO2H

CO2H

miroir

R

R

R

S

S

SThréo

méso

Mirror symmetry

A compound that possesses a mirror symmetry is inactive on light

meso

threo

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 ?

R

R

S

S

S

S

R

R

Inactive

Inactive

Active

Active

80

Chirality originates from symmetry and does not concern sp3 carbon only

helicene

rings

metallocene

81

Plane of the screen

O

O

OO

OO (two views of the same image)

Plane perpendicular

SPIRO compounds

82

OO

O

inactive active active active

Mirror plane in red

Left or right

O

83

Oactive (camphor)

樟脑樟脑

Camphor Tree ( Cinnamomum camphora )

84

allene

85

Localized approachesSome 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).

86Units: 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 C=C 134 614

C--N 147 308 CC 120 839

C--O 143 360

C--S 182 272 O--O 148 145

C--F 135 488 O=O 121 498

C--Cl 177 330

C--Br 194 288 N--N 145 170

C--I 214 216 NN 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 moleculeA 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

N

H

H

H

+

moment de liaison

momentrésultant

(Bond)

(molecule)

T=3 N-H cos = 3 N-H 1/3

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: Snn-fold rotation followed by reflection through mirror plane perpendicular to rotation axis n is always 3 or larger because S1 = s and S2 = i. n=3 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

Answer : EverywhereFood tasteClothes colorsPerfumes smellMedecine healthGas energyMaterials…Environment

In which fieldchemistry is acting?

97

Clothing; Art colors,

indigo, purple,..

98

Purple has played an important role in history

99

perfumes, smell

100

Medicine, health drugs

101materials, 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 l’UTLS 18/10/2000

103

Nothing is lost, nothing is created, everything is transformed

Lavoisier

Frontiers between nature and artifact.

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 God‘s 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.

O H

H

108

Why is Chemistry more fundamental than physics?

Chemistry needs physics

What is the most important? The first step? Or moving further?

O H

H

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Auguste Comte,

Maths > Physics> Chemistry

A harmful classification.

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Doctor ? Physicist ?

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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)

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World of representation

This is not a pipe

化学