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Transcript of 1 The traditional Representation of chemical compounds Goals: Name the compounds (XVIII century)...
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.
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Why is Chemistry more fundamental than physics?
Physics takes interest in primary aspects Chemistry considers the substance itself.
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Why is Chemistry more fundamental than physics?
Chemistry needs physics
What is the most important? The first step? Or moving further?
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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