Download Acids and Bases

Acids and Bases
pH – a measure of acidity
The pH scale is useful in that it lets us express acidity by numbers
that are not exponentials
The strength of acids and bases
A strong acid is one that is completely ionized in water.
A weak acid is one that ionizes in water to the extent of less than 100%.
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1811 - Humphry Davy
Historical development
Questioned Lavoisier's theory, noting that hydrochloric
acid (HCl) did not contain oxygen yet is an acid.
Soon thereafter, several more acids without oxygen
were found. e.g. HBr, HF, HI
1661 - Robert Boyle
Characterized acids and alkalies (bases) as the following:
Acids:
Sour taste; Corrosive
Change litmus (石蕊) (dye extracted from lichens) from blue to red
Become less acidic when combined with alkalies.
Alkalies (Bases):
Feel slippery
Change litmus from red to blue
Become less alkaline when combined with acids.
1838 - Justig Liebig
Suggested that acids contain one or more hydrogen
atoms which can be replaced by metal atoms to
produce salts. e.g. HSCN is an acid because the H
atom can be replace by a metal to form a salt, such
as NaSCN.
1884 – Svante Arrhenius acids and bases:
1778 - Antoine Lavoisier (He stated the first version of the
law of conservation of mass)
Acids: compounds that produce hydrogen
ions (H+ or H3O+) in water
Bases: compounds that form OH- when
they dissolve in water
Believed that all acids contained oxygen after studying several acids
e.g. H2SO4 - sulfuric acid, HNO3 - nitric acid
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1
A base is a proton acceptor
1923 - Johannes Brønsted and Thomas Lowry
Bronsted-Lowry acids and bases
An acid is a proton donor
A base is a proton acceptor
hydronium ion
Identifying the acid-base pair
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ammonium ion
Questions:
A. Which of the following compounds can function as Bronsted acid?
NaH H2S
Fe(H2O)6]3+
B. Which of the following compounds can function as Bronsted base?
CH3OH
ClPPh3
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Water as an acid and a base
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2
Generalizations
The stronger the acid, the weaker the conjugate base.
Hydronium ion is the strongest acid that can exist in aqueous solution.
Hydroxide ion is the strongest base that can exist in aqueous solution.
The position of equilibrium lies to the side of the weaker acid
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Weak acids and acid ionization constants
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13
Weak bases and base ionization constant
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The relationship between the ionization
constants of acids and their conjugate bases
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4
Some typical Bronsted acids
Non-oxygen containing acids, e.g. HX (X = halide)
Aqua acids:
Fe(H2O)6]3+
Which one is a stronger Bronsted acid? (charge effect)
Ca(H2O)62+
v.s.
Mg(H2O)62+
Fe(H2O)62+
v.s.
Fe(H2O)63+
Oxoacids and hydroxoacids
O
O
O
S
OH
OH
Cl OH
O
O
OH
HO
Si
OH
OH
OH
B
HO
OH
H2SO4 and B(OH)3 (or Si(OH)4):
which one is more acidic?
Ka1
H2SO4 H+ + HSO4Ka2
HSO4- H+ + SO42-
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B
B
HO
OH
HO
OH
OH
O
O
O
S
Molecular
structures and the
strength of oxo
and hydroxoacids
O
OH
+
+
H
+
H+
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OH
O
S
O
OH
More oxo-ligands give higher acidity
OpE(OH)q Op+1E(OH)q-1- + H+
Stability of Op+1E(OH)q-1- increases when p ___
Pauling's rules
The pKa value of
H3PO3 was reported
to be 1.8. Predict its
structure.
(1) For the oxoacid OpE(OH)q, pKa ≈ 8-5p
(2) The successive pKa values of polyprotic acids (q>1), increase
by 5 units for each successive proton transfer.
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5
1923 - Gilbert N. Lewis
Lewis acids and bases
Some typical examples
Ag+
(1) Metal complexes
H+ + OH- H2O
Co2+ + 6 H2O [Co(H2O)6]2+
(2) A molecule with an incomplete octet can complete its octet
by accepting an electron pair.
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(3) Valence shell rearrangement
Hard and soft Lewis acids and bases
Mx+ + y X(4) Expand the octet
SiF4 + 2F- [SiF6]2(5) Charge transfer
I2 + I- I3-
I
[MXy]x-y
Equilibrium constant
I
I
I
I
K=
I
[MXy]x-y
[Mx+ ] [X-]y
C
O
To some Lewis acids (cations), the basicity order of halids is F- > Cl- > Br- > IBr
To others, the order is F- < Cl- < Br- <I-
Br
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1963 - Ralph G. Pearson
Hard soft acid-base principle (HSAB principle)
Hard acids/bases: hold electrons tightly and are not readily polarized
Soft acids/bases: do not hold electrons tightly. Their valence electron
cloud are easily distorted or polarized
The HSAB principle:
A hard acid prefers to combine with a hard base and a soft acid
prefers to combine with a soft base.
Some general observations:
(1) Hardness increases across a period and decreases down a group
(2) Hard bases are associated with O, N and F
(3) Hard acids are those with higher charges
(4) Many transition metals are soft acids
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Exercises
Why?
Hard acid-base interactions are predominantly electrostatic; soft acidbase interactions are predominantly covalent.
Lewis acids:
Ru2+ versus Os2+
Na+ versus Cu+
Os2+ versus Os3+
Mg2+ versus Ca2+
Lewis bases:
Cl- versus BrCO versus NH3
NH3 versus PH3
Chemical consequences of hardness
More examples
The tendency of soft acids to bond to soft bases and of hard acids to
bond to hard bases explains certain aspects of the Goldschmidt
classification of the elements into 4 types. (2 of the classes are the
lithophile elements and the chalcophile elements.)
(1) W(CO)6 is air stable but W(NH3)6 is unknown
Lithophile elements: Li, Mg, Ti, Al, Cr (hard cations which are found in
association to the hard base O2-)
Chalcophile elements: Cd, Pb, Bi, Ag (soft cations, which are found in
associaion with the soft bases S2-, Se2-)
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(2) ZnO + 2 LiMe ZnMe2 + Li2O
K > 1 or < 1 ?
(3) MeHgI + HCl MeHgCl + HI
K > 1 or < 1 ?
(4) AgCl2- + 2 CN- Ag(CN)2- + 2 Cl-
K > 1 or < 1 ?
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Organic chemistry and Polymers
What is Organic Chemistry?
It is defined as the study of hydrocarbons (compounds of hydrogen and
carbon) and their derivatives
7 million Organic Compounds
1.5 million Inorganic Compounds
Animal and plant matter, Foods, Pharmaceuticals, Cosmetics,
Fertilizers, Plastics, Petrochemicals, Clothing
Carbon, the element of life on earth
Has Four Bonding Electrons; Unique Strong Covalent Bonds;
Strong Single, Double and Triple Bonds
Average Bond Energies (kJ mol-1)
C-C
607
Si-Si
230
C-H
C-N
754
Si-N
470
C-O
416
336
Si-H
Si-O
323
368
O-Si-O = Sand and Rocks
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Hydrocarbons
CH3 CH3
Alkanes (saturated hydrocarbons), CnH2n+2
H
H C H
H
methane
CH3
H H
H C C H
H H
ethane
CH3CH3
CH3CH2CH2CH2CH CH CH CH3
H H H H
H C C C C H
H H H H
H H H H H
H C C C C C H
H H H H H
propane
butane
pentane
CH3CH2CH3
CH3CH2CH2CH3
H H H
H C C C H
H H H
CH2
CH2 CH CH3
CH3
CH3CH2CH2CH2CH3
butane
H
H
H C C
H
H
H
Ethane
Skeletal structure of only carbon atoms
propane
Rotation about single covalent bonds
occurs freely. The energy barrier is
small. The position of hydrogen
atoms relative to one is thus
constantly changing
C1 – C4 n-alkanes are all gases
Methane main component of natural gas
Propane and butane often stored as
compressed gases
3D – models show that because of
the tetrahedral carbon atoms the
chains are zig-zagged and not at all
straight
pentane
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Non-polar molecules, which are less dense than water.
Alkanes are immiscible with water making two layers.
Newman projections
Van-der Waals or dipole-dipole attractive forces, and not
H-bonding (as in polar molecules) are the main
intermolecular forces
Alkanes show regular increases in bpt and mpt as
molecular weight increases down the homolgous series
These weak intermolecular forces operate over small
distances, arising because the electron distribution within
molecules at any given instance is not uniform. Resulting in
tiny electrical attractions between molecules.
These temporary dipoles hold alkanes as liquids or solids,
and must be overcome in order to vaporize a liquid or
melt a solid (wax)
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Isomers – the have the same molecular formula, but a different structures
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Petroleum: a complex mixture of hydrocarbons (linear or branched)
Structural Isomers – same molecular formula, but atoms are bonded in
different orders.
Refining petroleum – fractional distillation – separation according to
boiling points
H
H 3C
C
CH 3
CH3
Fractional distillation of crude oil
C4H10 – has two isomers, n-butane and isobutane (2methylpropane)
Natural Gas (C1-C4)
Isobutane
H3 C
CH
Gasoline (C 4-C 12)
Bpt (40-200 ºC)
CH2 CH3
CH3
Isopentane
(2-methylbutane)
Has the same molecular formula as
n-pentane, C5H12
Petroleum
Kerosene (C12-C 16)
Bpt (200-250 ºC)
CH3
H3C
C
CH3
CH3
Have different Physical
Properties, Mpt, Bpt, densities,
Heating oil, Lubricating oil (C15-C 18)
Bpt (250-300 ºC)
Neopentane
(2,2-dimethylpropane)
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Octane rating
The octane rating is a measure of the autoignition resistance of
gasoline (petrol) and other fuels used in spark-ignition internal
combustion engines.
Straight-chain alkanes are a pure fuel,
because of engine knock.
2-methylheptane 23
39
n-hexane
25
2-methylhexane
44
1-heptene
60
n-pentane
62
1-pentene
84
n-butane
91
cyclohexane
97
benzene
101
toluene
112
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10
Cycloalkanes
CnH2n
Cyclohexane
H
cyclopropane
H
cyclobutane
H
H
H
H
H
H
chair conformation
H
H
cyclopentane
H equatorial
position
H
axial position
Alkyl halides (haloalkanes)
ring strain exists in these
three molecules
Cl
Br
cyclohexane (no ring strain)
1-Bromobutane
Cl
Cl
2-Chloropropane or
Isopropyl chloride
C
Cl
CH3 CH
Cl
CH2 CH2
Br
Tetrachloromethane
Cl
3-Bromo-1-chlorobutane
F
Cl
Cl
C
F
Cl
Trichlorofluoromethane
(Freon-11)
Cl
C
F
Cl
Chlorofluorocarbons (CFCs)
Refrigerant Gases, Ozone
Depletion, More H’s more
degradable
Dichlorodifluoromethane(F
reon-12)
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Alkenes – unsaturated compounds CnH2n
H2C CH2
Ethene
H3C CH CH2
H
Propene
H
1-Butene
1-Pentene
C C
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CisCis-Trans Isomerization of Retinal (視網膜)
H
H
1-Hexene
Bond rotation?
1,3-Butadiene
3-methyl-1,4-pentadiene
R
R
R
H
H
H
H
R
cis-
Opsin (視蛋白)
trans-
Present in Rhodopsin (視網膜色素)
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Reactivity
π−bond lobes represent areas of high electron density
Therefore, the π− bond is susceptible to attack by
electron deficient molecules, called electrophiles, E+
Mechanism
C
Slow
C
H
C
+ X
C
H
X
ADDITION REACTIONS
C
E+
+
C
A
B
A
C
C
fast
B
C
C
X
H
H
H
C
C
X
H
OSO3H
H
H
C
C
C
C
X
Alkyl Halides
H
C
C
Oxidation of alkenes
Alcohols
H2C CH2
X
X
X
C
C
X
X
The electrophile is a Lewis acid, its accepted a pair of electrons,
the simplest Lewis acid is H+
The nucleophile is a Lewis base, its donated a pair of electrons
OSO3H Alkyl hydrogen Sulfate
OH
C
H
X- is the nucleophile, and the carbocation is the electrophile
OH
H+
C
KMnO4, -OH, H2O
H2C CH2
+ MnO2
OH OH
Dihaloalkanes
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C
Hydrogenation
Slow
C
C
O
C
O
H
H
OSO2OH
Markovnikov’s Rule
H
C
H
Br
CH3
S
O
H2 adds to the C atoms of a double/triple bond.
A catalyst (e.g. Pt or Ni) is used to speed up the reaction.
H
O
Fast
H2C
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H2C
H
C
C
H
OSO3H
H atom adds to the carbon atom
which already has the most H
atoms
H
C
Adding H2 to double
bonds in vegetable
oils produces:
compounds with higher melting points.
solids at room temperature such as
margarine,
soft margarine,
and shortening.
CH3
Br Markovnikov addition
Product
Unsymmetrical Alkene
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12
H
Alkynes CnH2n-2
H3 C C
C H
C C H
Ethyne
(acetylene)
+ HBr
H3C
C C H
Propyne
H 3C
H
C C
H
Br
Markovnikov product
Alkynes are high energy compounds
H C C H
triglyceride
(三酸甘油酯 )
Vegetable oils
+
2 CO2 + H2O
2.5 O2
Welding (焊接 ) gas
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Benzene and aromatics
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Benzene and aromatics
H
H
H
C
C
C
C
C
C
H
H
H
Kekulé was the first to
suggest a sensible
structure for benzene.
Benzene is one of the main building blocks of
the petrochemical industry. The global
market grew from 4 million tonnes in 1960 to
World production (million tonnes)
over 30 million tonnes in 2000.
Today benzene is mainly used as an
intermediate to make other chemicals.
Its most widely-produced derivatives
include styrene, which is used to make
polymers and plastics, phenol for resins
and adhesives (via cumene), and
cyclohexane, which is used in the
manufacture of Nylon.
World production
30
Western Europe
7.53
North America
7.23
South America
0.89
Asia
8.49
Kekulé said that
he dreamt the
structure of
benzene – so
called Kekulé
structure of
benzene
(1865)
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High Carbon content – burn
with a smoky flame
Delocalisation, Resonance
-Stabilise molecules, so
make them less reactive
π-Electron Density Rings above and
below the plane of the ring –
Susceptible to electrophilic attack
Michael
Faraday
first
isolated
benzene
in 1825
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13
Bonding
53
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Derivatives of benzene
Aromatic compounds
Benzene and its derivatives are said to be aromatic - a term coined
because of the strong fragrance of some of the derivatives of benzene
1.
2.
3.
4.
p
O- H
N
H
HO
OH
O
2
3
m
Must be cyclic
Must be planar
Each atom of the ring must have a p orbital and these p
orbitals must be perpendicular to the plane of the ring
Must contain 4n+2 π− electrons (where n = 0, 1, 2, ...) –Hückel
Rule
N+
1
o
Rules for
Aromaticity
O
Br
CH3
4
Electrophilic Aromatic Substitution
H
E
H
H
H
H
E
X
H
H
H
H
+
H
H
X
H
Electrophilic attack – Slow Rate Determining Step
n = 1 , 6π
π electrons
E
H
Naphthalene
10 π
Anthracene
Phenanthrene
14 π
sp3
E
H
E
H
E
H
E
Transition State or Wheland Intermediate
Delocalised Cyclohexadienyl cation
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14
Alcohols, phenol and ethers
Halogenation: examples of electrophilic substitution
Cl
Cl2, AlCl3
Alcohols and Ethers can be regarded as derivatives of water in which
one or two of the H atoms has been replaced by an alkyl group
Br
Br2, FeBr3
Water, H2O
Br
Br
H
FeBr4
+
H
Br
FeBr3
O
0.96 Ao
Methanol, CH3OH
0.96 Ao
H
O
H C
H H
104.5o
H
1.43 Ao
Saturated molecules
are sp3 hybridized
108.5o
Methoxymethane, CH3OCH3
Nitration
NO2
1.43 Ao
NO2
H
NO2
H C
O
C H
H H H H
Nitrobenzene
109.5o
1.10 Ao
o
111.7
Alcohols are found to have much higher
bpt than those of alkanes or haloalkanes
of comparable size, e.g. Methanol (65oC),
Chloromethane and Methane are gases ;
Ethanol (78.5oC), Chloroethane (12oC)
and Ethane is a gas
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hydroxy or alcohol group
CH3
OH
CH3
Methyl alcohol
(methanol)
CH2
CH3
OH
Ethyl alcohol (ethanol)
CH3
CH3
CH OH
58
Phenols are stronger acids than alcohols
CH2
CH2
OH
OH
Propyl alcohol (propanol)
CH2
pKa = 18
CH CH2 CH3
H O
CH2 OH
CH3
Isopropyl alcohol
O
OH
pKa = 10
O
O
2-Ethyl-1-butanol
Polyhydroxy alcohols are alcohols that possess
more than one hydroxyl group
CH2 CH3
CH2
HO
OH
CH CH2
HO
OH
CH2
HO
HO
Cl F
OH
1,2-Ethanediol
1,2-Propanediol
1,2,3-Propanetriol
(ethylene glycol)
(propylene glycol)
(glycerol)
Extremely Toxic
Resonance Stabilised Phenoxide anion
CH CH2
Ethers
CH3 CH2
O CH2CH3
Diethyl Ether
F
H C C O C H
F F
F
Enflurane
NonFlammable
Anaesthetics
Harmless
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15
H
Aldehydes and Ketones
R
C O
R
R
H3C
Propanone
(ACETONE)
H
H 3C
H3C
H
C O
C O
H
Methanal
(formaldehyde)
H
C O
H3C
CH3CH2
δ−
O
C δ+
(butter flavour)
O H
C
C
H N R
H N R
H
O H
C O
Propanal
CH3
Butadione
O
RNH 2
C
C
CH3CH2CH2CH2
Ethanal
(acetaldehyde)
H3C
Carvone
Butanone
Carbonyls readily
undergo
Nucleophilic
Attack
al – aldehydes, one - ketones
H
H
C
H2
Most Reactive Group –
π− electrons + polarisation
C O
C O
H3C
CH3
(spearmint flavour)
Resonance Structures
H
CH3
Ketone
Aldehyde
O O
O
O
O
C O
Pentanal
Imine
N
H N R
R
- H2O
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Highly Polar; Low molecular weight acids show
Appreciable Solubility in Water. High Bpt –
Extensive H-bonds to themselves and water
Carboxylic acids and esters
pKa = 4 - 5 ,
O
C
C
O H
+
H2O
O
+
H
H2O
+
C
NaOH
HO
Red ants
O
O H
OH
CH3
Methanoic acid
H3O
O
C
C
OH
O
Na
pKa =
<
Cl
O
OH
0.7
Cl
O
C C
Cl
1.48
OH
<
C C
H
Cl
C C
H
2.86
O
H
<
Cl
Sodium Benzoate
H
OH
OH
O
O
HO
H C C
H
4.76
63
OH
Propanoic acid
OH
Br
4-Bromo-2-ethylpentanoic acid
O
( )n
OH
n = 1 = malonic acid
n = 2 = succinic acid
n = 3 = glutaric acid
OH
CH3CH2
Ethanoic acid
O
Ethanedioic acid (oxalic acid)
Benzoic acid
Cl
O
O
O
C
O
O
O
water = 14
HO2C
CO2H
Terephthalic acid
CO2H
CO2H
Phthalic acid
64
16
Esterification – condensation reaction, where H2O is lost
+ CH3CH2
OH
CH3
Acetic acid
(ethanoic acid)
OH
CH3
O CH2 CH3
Ethyl acetate
HCl or H2SO 4
H+(catalyst)
O
O
+
OH
Ph
皂化
O
O
soap
H3C OH
Benzoic acid
Ph
H+(catalyst)
O
O
O
O CH3
Methyl benzoate
O
Ethyl propanoate
Saponification is commonly used to refer to the reaction of a metallic
alkali (base) with a fat or oil to form soap. Saponifiable substances are
those that can be converted into soap.
H
O
Methyl formate
O
vinyl acetate
Alcohol part appears first in the name
65
66
How does soap work?
(1) Ester molecules cannot H-bond to each other,
because they do not have an –OH.
(2) Consequently, B.pt is much lower than that of
alcohols and acids of comparable mass
(3) H-bonding to water is possible - low mw esters are
soluble in water
(4) Solubility rapidly decreases with carbon chain length.
H O
H
Boiling points
Hexane = 69 ºC
Diethyl ether = 56 ºC
Ethanol = 78 ºC
Ethanoic acid = 118 ºC
Ethyl acetate = 77 ºC
67
O
R
O
H O
H
R
68
17
Omega 3 Fish Oil Fatty Acids
In chemistry, especially biochemistry, a fatty (脂肪 ) acid is a carboxylic
acid often with a long unbranched aliphatic tail (chain), which is either
saturated or unsaturated
Omega-3s are not just "good fats".
They are truly essential for health
and vitality
The major process for
transforming animal
and vegetable oils
and fats into
oleochemicals(人造奶
油) is hydrolysis, the
splitting of natural
triglycerides into
crude glycerine(甘油 )
and cruded mixed
fatty acids, under the
influence of water,
temperature and
pressure.
Essential Fatty Acids lift
depression, elevate mood, reduce
stress and hyperactivity, increase
focus, concentration and learning.
Healthy Skin & Coat Omega Fatty
Acids is an advanced veterinarian
formulated Essential Fatty Acid
(EFA) formula for dogs. It contains
the ratios of all natural Omega Fatty
Acids 3, 6 and 9 needed to assist in
reducing shedding and improving
your dogs overall skin, coat and
vascular health.
69
Amines
70
Lewis bases
where Et = CH2CH3
H2NEt
ethylamine
primary
NH2
H2N
HNEt2
NEt3
diethylamine triethylamine
secondary
tertiary
Putrescine
(found in decaying meat)
Amphetamine
(dangerous stimulant)
where Me = CH3
H2NMe
methylamine
primary
HNMe2
dimethylamine
secondary
NH2
NMe3
trimethylamine
tertiary
N
H
Piperidine
71
N
Isopropylamine
Triethylamine
NH2
72
18
Amides – not acids or bases
Cocaine
R'
C
N
R'
R
N
R
O
O
caffeine
C
Features of a Peptide Bond;
1. Usually inert
2. Planar to allow delocalisation
3. Restricted Rotation about the amide bond
4. Rotation of Groups (R and R’) attached to the amide
bond is relatively free
73
R
H2N C COOH
H
AMINO ACIDS
O
C
H
NH2
formamide
H 3C
O
C
Chirality – The Handedness of molecules
Many organic compounds can exist as mirror-image twins
A pair of mirror-image twins is called a pair of enantiomers, whose
molecules are nonsuperimposable mirror images of one another
O
H
NH 2
NH2
acetamide
74
Involves a tetrahedral sp3 atom
CH3
benzamide
C
CH2
CH3
OH
Chiral Centre
O
C
CH3
HO
H
C
H 2N
NH2
urea
All are high melting point solids, only
benzamide not soluble in water
CH3
H
C OH
2-Butanol
Interchanging any two groups at a
chiral centre (stereocentre) that
CH2
CH2
bears four different groups
CH3
CH3
converts one enantiomer into
another
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CH3
H C OH
CH3
CH3
HO C H
CH3
2-Propanol
O
If any of the groups
attached to the
tetrahedral atom are
the same, the centre
is achiral.
H
models of the molecule and its
mirror image and then determine
whether they are superimposable
H
S-(+)-Carvone
Principle component of
Caraway seed oil and
responsible for the
characteristic odour
The ultimate way to test for
molecular chirality is to construct
O
Screwdriver is achiral
Screws are chiral
Golf club is chiral
Gloves are chiral
R-(-)-Carvone
Principle component
of Spearmint oil and
responsible for the
characteristic odour
Spearmint
【植】綠薄荷
Receptor Sites in the Nose are Chiral
Caraway (用於調味或製藥的 )葛縷子籽
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Optical isomers
Nobel Prize 2001
Enantiomers differ only in optical properties – rotate plane-polarized light in
opposite directions.
Professor William Knowles
Professor Ryoji Noyori
Professor K. Barry Sharpless
For synthesis of optically active compounds –
asymmetric synthesis
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