Química Orgânica I - w3.ualg.ptw3.ualg.pt/~abrigas/QOI2a_estrutura3D.pdf · Química Orgânica I...
Transcript of Química Orgânica I - w3.ualg.ptw3.ualg.pt/~abrigas/QOI2a_estrutura3D.pdf · Química Orgânica I...
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AFB QO I 2007/08 1
Química Orgânica I
Ciências Farmacêuticas
Bioquímica
Química
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Estrutura dos compostos orgânicos
� Estrutura 3D (rev)
� Propriedades (rev)
� Grupos funcionais
� Nomenclatura
� Análise conformacional
� Estereoquímica I
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Adaptado de:
� Jo Blackburn; 2006, Prentice Hall
� Organic Chemistry, 6th Edition; L. G. Wade, Jr.
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H2: s-s overlap
=>
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Cl2: p-p overlap
=>
Constructive overlap along the same
axis forms a sigma bond.
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HCl: s-p overlap
HCl
=>
2
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Pi Bonding
� Pi bonds form after sigma bonds.
� Sideways overlap of parallel p orbitals.
=>
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Multiple Bonds
� A double bond (2 pairs of shared electrons) consists of a sigma bond and a pi bond.
� A triple bond (3 pairs of shared electrons) consists of a sigma bond and two pi bonds.
=>
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Molecular Shapes
� VSEPR
� Hybridization
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sp Hybrid Orbitals
� 2 VSEPR pairs
� Linear electron pair geometry
� 180° bond angle
=>
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sp2 Hybrid Orbitals� 3 VSEPR pairs
� Trigonal planar e- pair geometry
� 120° bond angle
=>
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sp3 Hybrid Orbitals
� 4 VSEPR pairs
� Tetrahedral e- pair geometry
� 109.5° bond angle
=>
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Molecular Dipole Moments
� Depend on bond polarity and bond angles.
� Vector sum of the bond dipole moments.
=>AFB QO I 2007/08 14
Effect of Lone Pairs
Lone pairs of electrons contribute to the dipole moment.
=>
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Intermolecular Forces
� Strength of attractions between molecules influence m.p., b.p., and solubility, esp. for solids and liquids.
� Classification depends on structure.� Dipole-dipole interactions
� London dispersions
� Hydrogen bonding
=>
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Dipole-Dipole Forces
� Between polar molecules.
� Positive end of one molecule aligns with negative end of another molecule.
� Lower energy than repulsions, so net force is attractive.
� Larger dipoles cause higher boiling points and higher heats of vaporization.
=>
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Dipole-Dipole
=>
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London Dispersions
� Between nonpolar molecules
� Temporary dipole-dipole interactions
� Larger atoms are more polarizable.
� Branching lowers b.p. because of decreased surface contact between molecules.
=>
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Dispersions
=>
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Hydrogen Bonding
� Strong dipole-dipole attraction.
� Organic molecule must have N-H or O-H.
� The hydrogen from one molecule is strongly attracted to a lone pair of electrons on the other molecule.
� O-H more polar than N-H, so stronger hydrogen bonding. =>
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H Bonds
=>AFB QO I 2007/08 22
Boiling Points and Intermolecular Forces
CH3 CH2 OH
ethanol, b.p. = 78°C
CH3 O CH3
dimethyl ether, b.p. = -25°C
trimethylamine, b.p. 3.5°C
N CH3H3C
CH3
propylamine, b.p. 49°C
CH3CH2CH2 N
H
H
ethylmethylamine, b.p. 37°C
N CH3CH3CH2
H
CH3 CH2 OH CH3 CH2 NH2
ethanol, b.p. = 78°C ethyl amine, b.p. = 17 °C
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Solubility
� Like dissolves like.
� Polar solutes dissolve in polar solvents.
� Nonpolar solutes dissolve in nonpolarsolvents.
� Molecules with similar intermolecular forces will mix freely.
=>AFB QO I 2007/08 24
Ionic Solute with Polar Solvent
Hydration releases energy.
Entropy increases.=>
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Ionic Solute withNonpolar Solvent
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Nonpolar Solute withNonpolar Solvent
=>
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Nonpolar Solute with Polar Solvent
=>AFB QO I 2007/08 28
Classes of Compounds
� Classification based on functional group.
� Three broad classes� Hydrocarbons
� Compounds containing oxygen
� Compounds containing nitrogen.
=>
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Hydrocarbons
� Alkane: single bonds, sp3 carbons
� Cycloalkane: carbons form a ring
� Alkene: double bond, sp2 carbons
� Cycloalkene: double bond in ring
� Alkyne: triple bond, sp carbons
� Aromatic: contains a benzene ring
=>
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Compounds Containing Oxygen
� Alcohol: R-OH
� Ether: R-O-R'
� Aldehyde: RCHO
� Ketone: RCOR'
CH3CH2 C
O
H
CH3 C
O
CH3
=>
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Carboxylic Acids and Their Derivatives
� Carboxylic Acid: RCOOH
� Acid Chloride: RCOCl
� Ester: RCOOR'
� Amide: RCONH2
C
O
OH
C
O
Cl
C
O
O CH3C
O
NH2
=>AFB QO I 2007/08 32
Compounds Containing Nitrogen� Amines: RNH2, RNHR', or R3N
� Amides: RCONH2, RCONHR, RCONR2
� Nitrile: RCN
N
O
CH3
CH3 C N
=>
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Classification Review
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Alkane Formulas
� All C-C single bonds
� Saturated with hydrogens
� Ratio: CnH2n+2
� Alkane homologs: CH3(CH2)nCH3
� Same ratio for branched alkanes
=>
C
H
C
H
H
H C H
H
H
C H
H
H
Isobutane, C 4H10
C
H
C
H
H
H C C
H
HH H
H
H
Butane, C4H10
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Common Names
� Isobutane, “isomer of butane”
� Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain.
� Neopentane, most highly branched
� Five possible isomers of hexane,18 isomers of octane and 75 for decane!
=>AFB QO I 2007/08 36
Alkane Examples
=>
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IUPAC Names� Find the longest continuous carbon chain.
� Number the carbons, starting closest to the first branch.
� Name the groups attached to the chain, using the carbon number as the locator.
� Alphabetize substituents.
� Use di-, tri-, etc., for multiples of same substituent.
=>
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Longest Chain
� The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.)
� If there are two possible chains with the same number of carbons, use the chain with the most substituents.
C
CH3
CH2
CH3
CH CH2 CH2 CH3
CH CH2 CH3
H3C
H3C
=>
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Number the Carbons
� Start at the end closest to the first attached group.
� If two substituents are equidistant, look for the next closest group.
1
2
3 4 5
6 7
CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
=>
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Name Alkyl Groups
� CH3-, methyl
� CH3CH2-, ethyl
� CH3CH2CH2-, n-propyl
� CH3CH2CH2CH2-, n-butyl
CH3 CH CH2 CH3
sec-butyl
CH3 CH
CH3
CH2
isobutyl
C H3 C H C H3
isopropyl
CH3C
CH3
CH3
tert-butyl
=>
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Propyl Groups
C
H
H
H
C
H
H
C
H
H
H
n-propyl
C
H
H
H
C
H
C
H
H
H
isopropyl
H
A primary carbon A secondary carbon
=>
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Butyl Groups
C
H
H
H
C
H
C
H
H
C
H
H
H
C
H
H
H
C
H
C
H
HH
C
H
H
n-butyl sec-butyl
H
H
A primary carbon A secondary carbon
=>
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Isobutyl Groups
CH
H
H
C
C
H H
C
H
H
H H
CH
H
H
C
C
H H
C H
H
H
H
H
H
A primary carbon A tertiary carbon
=>
isobutyl tert-butyl
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Alphabetize
� Alphabetize substituents by name.
� Ignore di-, tri-, etc. for alphabetizing.
CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
3-ethyl-2,6-dimethylheptane=>
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Complex Substituents
� If the branch has a branch, number the carbons from the point of attachment.
� Name the branch off the branch using a locator number.
� Parentheses are used around the complex branch name.
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31-methyl-3-(1,2-dimethylpropyl)cyclohexane =>
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Physical Properties
� Solubility: hydrophobic
� Density: less than 1 g/mL
� Boiling points increase with increasing carbons (little less for branched chains).
Melting points increase with
increasing carbons (less for odd-
number of carbons).
•
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Boiling Points of AlkanesBranched alkanes have less surface area contact,so weaker intermolecular forces.
=>
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Melting Points of Alkanes
Branched alkanes pack more efficiently intoa crystalline structure, so have higher m.p.
=>
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Branched Alkanes
� Lower b.p. with increased branching
� Higher m.p. with increased branching
� Examples:
H
CH3CH
CH3
CH2 CH2 CH3
bp 60°Cmp -154°C
CH3CH
CH3
CHCH3
CH3
bp 58°Cmp -135°C
=>
bp 50°Cmp -98°C
CH3 C
C 3
CH3
CH2 CH3
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Major Uses of Alkanes
� C1-C2: gases (natural gas)
� C3-C4: liquified petroleum (LPG)
� C5-C8: gasoline
� C9-C16: diesel, kerosene, jet fuel
� C17-up: lubricating oils, heating oil
� Origin: petroleum refining
=>
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Reactions of Alkanes
� Combustion
CH3CH2CH2CH3 + O2 CO2 + H2Oheat
8 10132
long-chain alkanes catalyst
shorter-chain alkanes
CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +
heat or light
=>
• Cracking and hydrocracking (industrial)
• Halogenation
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Conformers of Alkanes
� Structures resulting from the free rotation of a C-C single bond
� May differ in energy. The lowest-energy conformer is most prevalent.
� Molecules constantly rotate through all the possible conformations.
=>
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Ethane Conformers
� Staggered conformer has lowest energy.
� Dihedral angle = 60 degrees
H
H
H
H
H H
Newman
projectionsawhorse
=>
modelAFB QO I 2007/08 54
Ethane Conformers (2)
� Eclipsed conformer has highest energy
� Dihedral angle = 0 degrees
=>
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Conformational Analysis
� Torsional strain: resistance to rotation.
� For ethane, only 12.6 kJ/mol
=>AFB QO I 2007/08 56
Propane Conformers
Note slight increase in torsional strain
due to the more bulky methyl group.
=>
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Butane Conformers C2-C3� Highest energy has methyl groups eclipsed.
� Steric hindrance
� Dihedral angle = 0 degrees
=>totally eclipsed
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Conformational Analysis
=>
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Higher Alkanes
� Anti conformation is lowest in energy.
� “Straight chain” actually is zigzag.CH3CH2CH2CH2CH3
C
H CC
CC
H H H H
H H
H H
HH H =>
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Cycloalkanes� Rings of carbon atoms (-CH2-
groups)
� Formula: CnH2n
� Nonpolar, insoluble in water
� Compact shape
� Melting and boiling points similar to branched alkanes with same number of carbons =>
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Naming Cycloalkanes
CH2CH3
CH2CH3
CH3
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Cis-Trans Isomerism
� Cis: like groups on same side of ring
� Trans: like groups on opposite sides of ring
=>
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Cycloalkane Stability
� 5- and 6-membered rings most stable
� Bond angle closest to 109.5°
� Angle (Baeyer) strain
� Measured by heats of combustion per -CH2 -
=>
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Heats of Combustion/CH2
Alkane + O2 → CO2 + H2O
Long-chain
658.6 kJ 658.6
697.1 686.1664.0 663.6 kJ/mol
=>
662.4
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Cyclopropane� Large ring strain due to angle compression
� Very reactive, weak bonds
=>AFB QO I 2007/08 66
Cyclopropane (2)
Torsional strain because of eclipsed hydrogens
=>
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Cyclobutane
� Angle strain due to compression
� Torsional strain partially relieved by ring-puckering
=>AFB QO I 2007/08 68
Cyclopentane� If planar, angles would be 108°, but all
hydrogens would be eclipsed.
� Puckered conformer reduces torsionalstrain.
=>
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Cyclohexane
� Combustion data shows it’s unstrained.
� Angles would be 120°, if planar.
� The chair conformer has 109.5° bond angles and all hydrogens are staggered.
� No angle strain and no torsionalstrain.
=>AFB QO I 2007/08 70
Chair Conformer
=>
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Boat Conformer
=>
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Conformational Energy
=>
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Axial and Equatorial Positions
=>
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Monosubstituted Cyclohexanes
=>
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1,3-Diaxial Interactions
=>
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Disubstituted Cyclohexanes
=>
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Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial around the ring.
=>
CH3
CH3
One axial, one equatorial
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Bulky Groups
� Groups like t-butyl cause a large energy difference between the axial and equatorial conformer.
� Most stable conformer puts t-butyl equatorial regardless of other substituents.
=>
14
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Bicyclic Alkanes� Fused rings share two adjacent carbons.
� Bridged rings share two nonadjacent C’s.
bicyclo[3.1.0]hexane=>
bicyclo[2.2.1]heptane
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Cis- and Trans-Decalin
� Fused cyclohexane chair conformers
� Bridgehead H’s cis, structure more flexible
� Bridgehead H’s trans, no ring flip possible.
H
H
cis-decalin
H
H
=>
trans-decalin
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Bicyclo[4.4.0]decane
=>AFB QO I 2007/08 82
STEREOCHEMISTRY I
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Stereochemistry 1
web.chem.ucla.edu/~harding/ppt/14C_stereo.ppt
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Stereochemistry: What is It?
Isomers
•Molecules with same chemical formula but different spatial
arrangement of atoms
Constitutional isomers
•Differ in sequence of atom connectivity
Urea
CH4N2O
H2N NH2
O
Ammonium cyanate
CH4N2O
NH
H
H
H
O C N
•Jöns Jakob Berzelius, 1830
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Stereochemistry: What is It?Isomers
•Same sequence of connectivity, but can be interconverted by rotation
around a single bond
rotate around
C2-C3 bond
Butane C4H10
Eclipsed conformation
Conformational isomers
Are other isomers possible? Tetrahedral carbon...
click on movie to play
Butane C4H10
Staggered conformation
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Historical BackgroundTimeline A: Light
1678: Christiaan Huygens discovers plane-polarized light
many vibrational planes
nonpolarized light
one vibrational plane
plane-polarized light
light beam
Iceland spar crystal
(natural CaCO3)
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Historical BackgroundTimeline A: Light
1815: Jean Baptiste Biot notes some natural substances rotate plane-polarized light
polarization plane shiftedtube of liquid organic
compound or solution
plane-polarized light
Optically active: the ability to rotate plane-polarized light
Optically inactive: does not rotate plane-polarized light
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Historical BackgroundTimeline A: Light
Dextrorotatory: rotates plane-polarized light in a clockwise direction (+)
Levorotatory: rotates plane-polarized light in a counterclockwise direction (-)
Optical activity
(-)-Nicotine
N
N
H
CH3
(+)-Methamphetamine
CH3
N
H3C H
H
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Historical BackgroundTimeline B: Tartaric Acid
Tartaric acid
Racemic acidOH
OH
HO
HO
O
O
1828: Joseph Louis Gay-Lussac shows tartaric acid and racemicacid are isomers
1819: Paul Kester isolates racemic acid from tartar
From Latin racemus: bunch of grapes
1769: Carl Wilhelm Scheele examines tartar (deposited in casks
during wine fermentation); isolates tartaric acid
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Historical BackgroundTimeline B: Tartaric Acid
1838: Biot notes racemic acid is optically inactive
1832: Jean Baptiste-Biot notes tartaric acid is optically active
1847: Louis Pasteur separates ammonium sodium salt of
racemic acid into (+) and (-) crystals
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Historical BackgroundTimeline B: Tartaric Acid
O Na
O NH4
HO
HO
O
O
Ammonium sodium racemateoptically inactive
(+)-tartaric acidoptically active
identical to Scheele’s tartaric acid
(-)-tartaric acidoptically active
Pasteur’s separation of racemic acid
Quantity: equal
Optical activity: equal but opposite
Conclusion: Racemic acid is a 1:1 mixture of two optically-active substances
separate crystals
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Historical BackgroundTimeline B: Tartaric Acid
1853: Pasteur investigates mesotartaric acid
Mesotartaric acid
OH
OH
HO
HO
O
O
•Artificial, optically-inactive isomer of tartaric acid
•Cannot be separated into (+) and (-) forms
•Tartaric acid isomers have different biological properties
1854: Pasteur notes a certain plant mold metabolizes (+) but not (-)-tartaric acid
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Historical BackgroundTimeline C: Tetrahedral Carbon
A molecule having a tetrahedral carbon atom with four unequal
attachments exists as a pair of isomers.
propose:
1874: Joseph Achille Le Bel (age 27) and Jacobus Henricus van’t Hoff (age 22)
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Historical BackgroundTimeline C: Tetrahedral Carbon
Example: 2-chlorobutane
CH3CH2
C
CH3
Cl H
Stereoisomers: isomers that differ only in the position of atoms in space,
and cannot be interconverted by rotation around a single bond
Constitutional isomers?
Conformational isomers? Cannot be made superposable by bond rotation
Verify with models
Identical? Not superposable Verify with models
Same atom connectivity sequence H C
H
H
C
H
H
C
H
Cl
C
H
H
H
C
CH2CH3CH3
H Cl