Lecture 2 - Chemical Bonding
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Transcript of Lecture 2 - Chemical Bonding
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General
Organic ChemistryTwo credits
Second Semester 2009
King Saud bin Abdulaziz University for Health Science
Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. CareyThird Edition
Instructor: Rabih O. Al-Kaysi, PhD.
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Chapter 1Chapter 1
Chemical bonding
Lecture 2
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Structural Formulas of Organic Structural Formulas of Organic MoleculesMolecules
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ConstitutionConstitution
The order in which the atoms of a The order in which the atoms of a molecule are connected is called its molecule are connected is called its constitutionconstitution or or connectivityconnectivity..
The constitution of a molecule must be The constitution of a molecule must be determined in order to write a Lewis determined in order to write a Lewis structure.structure.
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Condensed structural formulasCondensed structural formulas
Lewis structures in which many (or all) Lewis structures in which many (or all) covalent bonds and electron pairs are covalent bonds and electron pairs are omitted.omitted.
HH
OO
CC CC CC
HH HH HH
HH
HHHH :: ::
HH
can be condensed to:can be condensed to:
CHCH33CHCHCHCH33
OHOH
(CH(CH33))22CHOHCHOHoror
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Bond-line formulasBond-line formulas
Omit atom symbols. Represent Omit atom symbols. Represent structure by showing bonds between structure by showing bonds between carbons and atoms other than carbons and atoms other than hydrogen.hydrogen.
Atoms other than carbon and hydrogen Atoms other than carbon and hydrogen are called are called heteroatomsheteroatoms..
CHCH33CHCH22CHCH22CHCH3 3 is shown asis shown as
CHCH33CHCH22CHCH22CHCH22OHOH is shown asis shown as
OHOH
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Bond-line formulasBond-line formulas
Omit atom symbols. Represent Omit atom symbols. Represent structure by showing bonds between structure by showing bonds between carbons and atoms other than carbons and atoms other than hydrogen.hydrogen.
Atoms other than carbon and hydrogen Atoms other than carbon and hydrogen are called are called heteroatomsheteroatoms..
HH ClClCC
CC
HH22CC
HH22CC
CHCH22
CHCH22
HHHH
is shown asis shown as
ClCl
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Constitutional IsomersConstitutional Isomers
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Constitutional isomersConstitutional isomers
Isomers are different compounds that Isomers are different compounds that have the same molecular formula.have the same molecular formula.
Constitutional isomers are isomers Constitutional isomers are isomers that differ in the order in which the that differ in the order in which the atoms are connected.atoms are connected.
An older term for constitutional An older term for constitutional isomers is “structural isomers.”isomers is “structural isomers.”
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A Historical NoteA Historical Note
In 1823 Friedrich Wöhler discovered that In 1823 Friedrich Wöhler discovered that when ammonium cyanate was dissolved in hot when ammonium cyanate was dissolved in hot water, it was converted to urea.water, it was converted to urea.
Ammonium cyanate and urea are Ammonium cyanate and urea are constitutional isomers of CHconstitutional isomers of CH44NN22O.O.
NHNH44OCNOCN
Ammonium cyanateAmmonium cyanateHH22NCNHNCNH22
OO
UreaUrea
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NitromethaneNitromethane Methyl nitriteMethyl nitrite
.... ::
HH CC
OO
OO
NN
::
::....
––
++
HH
HH
Examples of constitutional isomersExamples of constitutional isomers
Both have the molecular formula CHBoth have the molecular formula CH33NONO22 but but
the atoms are connected in a different order.the atoms are connected in a different order.
....CC OO NN OOHH
HH
HH
....::.... ....
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ResonanceResonance
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two or more Lewis structures may be two or more Lewis structures may be written for certain compounds (or ions)written for certain compounds (or ions)
ResonanceResonance
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If an atom lacks an octet, use electron If an atom lacks an octet, use electron pairs on an adjacent atom to form a pairs on an adjacent atom to form a double or triple bond.double or triple bond.
Example:Example:Nitrogen has only 6 electrons in the Nitrogen has only 6 electrons in the structure shown. structure shown.
How to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis Structures
....CC OO NN OOHH
HH
HH
........ ::.... ....
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If an atom lacks an octet, use electron If an atom lacks an octet, use electron pairs on an adjacent atom to form a pairs on an adjacent atom to form a double or triple bond.double or triple bond.
Example:Example:All the atoms have octets in this Lewis All the atoms have octets in this Lewis structure. structure.
How to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis Structures
........
CC OO NN OOHH
HH
HH
....::....
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Calculate formal charges.Calculate formal charges.
Example:Example:None of the atoms possess a formal None of the atoms possess a formal charge in this Lewis structure.charge in this Lewis structure.
How to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis Structures
........
CC OO NN OOHH
HH
HH
....::....
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Calculate formal charges.Calculate formal charges.
Example:Example:This structure has formal charges; is This structure has formal charges; is less stable Lewis structure.less stable Lewis structure.
How to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis StructuresHow to Write Lewis Structures
........
CC OO NN OOHH
HH
HH
.... ::....++ ––
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same atomic positionssame atomic positions
differ in electron positionsdiffer in electron positions
more stable more stable Lewis Lewis
structurestructure
less stable less stable Lewis Lewis
structurestructure
........
CC OO NN OOHH
HH
HH
.... ::....++ ––
........
CC OO NN OOHH
HH
HH
....::....
Resonance Structures of Methyl NitriteResonance Structures of Methyl Nitrite
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same atomic positionssame atomic positions
differ in electron positionsdiffer in electron positions
more stable more stable Lewis Lewis
structurestructure
less stable less stable Lewis Lewis
structurestructure
........
CC OO NN OOHH
HH
HH
.... ::....++ ––
........
CC OO NN OOHH
HH
HH
....::....
Resonance Structures of Methyl NitriteResonance Structures of Methyl Nitrite
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Electrons in molecules are often delocalizedElectrons in molecules are often delocalizedbetween two or more atoms.between two or more atoms.
Electrons in a single Lewis structure are Electrons in a single Lewis structure are assigned to specific atoms-a single Lewis structure assigned to specific atoms-a single Lewis structure is insufficient to show electron delocalization.is insufficient to show electron delocalization.
Composite of resonance forms more accurately Composite of resonance forms more accurately depicts electron distribution.depicts electron distribution.
Why Write Resonance Structures?Why Write Resonance Structures?
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Ozone (OOzone (O33))
Lewis structure of Lewis structure of ozone shows one ozone shows one double bond and double bond and one single bondone single bond
Expect: one short bond and one Expect: one short bond and one long bondlong bond
Reality: bonds are of equal length Reality: bonds are of equal length (128 pm)(128 pm)
ExampleExample
OO OO••••
OO••••
••••••••••••••••––++
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Ozone (OOzone (O33))
Lewis structure of Lewis structure of ozone shows one ozone shows one double bond and double bond and one single bondone single bond
Resonance:Resonance:
ExampleExample
OO OO••••
OO••••
••••••••••••••••––++
OO OO••••
OO••••
••••••••••••••••––++
OO OOOO••••
••••••••••••••••
–– ++
••••
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Ozone (OOzone (O33))
Electrostatic potentialElectrostatic potentialmap shows both endmap shows both endcarbons are equivalentcarbons are equivalentwith respect to negativewith respect to negativecharge. Middle atomcharge. Middle atomis positive.is positive.
ExampleExample
OO OO
••••OO••••
••••••••••••••••––++
OO OOOO••••
••••••••••••••••
–– ++
••••
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The Shapes of Some Simple The Shapes of Some Simple MoleculesMolecules
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The most stable arrangement of groups The most stable arrangement of groups attached to a central atom is the one that has attached to a central atom is the one that has the maximum separation of electron pairsthe maximum separation of electron pairs(bonded or nonbonded). (bonded or nonbonded).
Valence Shell Electron Pair RepulsionsValence Shell Electron Pair Repulsions
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tetrahedral geometrytetrahedral geometryH—C—H angle = 109.5°H—C—H angle = 109.5°
MethaneMethane
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tetrahedral geometrytetrahedral geometryeach H—C—H angle = 109.5°each H—C—H angle = 109.5°
MethaneMethane
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bent geometrybent geometryH—O—H angle = 105°H—O—H angle = 105°
but notice the tetrahedral arrangement but notice the tetrahedral arrangement of electron pairsof electron pairs
OO
HH
....
HH
::
WaterWater
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trigonal pyramidal geometrytrigonal pyramidal geometryH—N—H angle = 107°H—N—H angle = 107°
but notice the tetrahedral arrangement but notice the tetrahedral arrangement of electron pairsof electron pairs
NNHH
HH
HH
::
AmmoniaAmmonia
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Four-electron double bonds and six-electron Four-electron double bonds and six-electron triple bonds are considered to be similar to a triple bonds are considered to be similar to a two-electron single bond in terms of their spatialtwo-electron single bond in terms of their spatialrequirements. requirements.
Multiple BondsMultiple Bonds
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H—C—H and H—C—OH—C—H and H—C—Oangles are close to 120°angles are close to 120°trigonal planar geometry trigonal planar geometry
CC OOHH
HH
FormaldehydeFormaldehyde
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O—C—O angle = 180°O—C—O angle = 180°linear geometrylinear geometry
OO CC OO
Carbon DioxideCarbon Dioxide
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Molecular Dipole Molecular Dipole MomentsMoments
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++——
not polarnot polar
A substance possesses a dipole moment A substance possesses a dipole moment if its centers of positive and negative charge if its centers of positive and negative charge
do not coincide.do not coincide.
Dipole MomentDipole Moment
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——++
polarpolar
A substance possesses a dipole moment A substance possesses a dipole moment if its centers of positive and negative charge if its centers of positive and negative charge
do not coincide.do not coincide.
Dipole MomentDipole Moment
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molecule must have polar bondsmolecule must have polar bonds
necessary, but not sufficientnecessary, but not sufficient
need to know molecular shapeneed to know molecular shape
because individual bond dipoles can cancel because individual bond dipoles can cancel
OO CC OO++-- --
Molecular Dipole MomentsMolecular Dipole Moments
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OO CC OO
Carbon dioxide has no dipole momentCarbon dioxide has no dipole moment
Molecular Dipole MomentsMolecular Dipole Moments
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Carbon tetrachlorideCarbon tetrachloride DichloromethaneDichloromethane
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Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
Carbon tetrachloride has no dipoleCarbon tetrachloride has no dipolemoment because all of the individualmoment because all of the individualbond dipoles cancel.bond dipoles cancel.
Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
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Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
The individual bond dipoles do notThe individual bond dipoles do notcancel in dichloromethane; it hascancel in dichloromethane; it hasa dipole moment.a dipole moment.