dr. wolf's chm 201 & 202 1- 1 1.7 structural formulas of organic molecules
TRANSCRIPT
Dr. Wolf's CHM 201 & 202 1- 1
1.71.7
Structural Formulas of Organic MoleculesStructural Formulas of Organic Molecules
Dr. Wolf's CHM 201 & 202 1- 2
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.
Dr. Wolf's CHM 201 & 202 1- 3
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
Dr. Wolf's CHM 201 & 202 1- 5
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
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.
Ammonium cyanate is “inorganic.” Urea is Ammonium cyanate is “inorganic.” Urea is “organic.” Wöhler is credited with an important “organic.” Wöhler is credited with an important early contribution that helped overturn the early contribution that helped overturn the theory of “vitalism.” theory of “vitalism.”
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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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)
Recall from Table 1.5Recall from Table 1.5
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.
Table 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How 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.
Table 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How 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.
Table 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How 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.
Table 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How to Write Lewis StructuresTable 1.5 How 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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1.101.10The Shapes of Some Simple MoleculesThe Shapes of Some Simple Molecules
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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°
Table 1.6 Table 1.6 MethaneMethane
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tetrahedral geometrytetrahedral geometryeach H—C—H angle = 109.5°each H—C—H angle = 109.5°
Table 1.6 Table 1.6 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
::
Table 1.6 Table 1.6 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
::
Table 1.6 Table 1.6 AmmoniaAmmonia
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F—B—F angle = 120°F—B—F angle = 120°trigonal planar geometry trigonal planar geometry
allows for maximum separationallows for maximum separationof three electron pairsof three electron pairs
Table 1.6 Table 1.6 Boron TrifluorideBoron Trifluoride
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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
Table 1.6: Table 1.6: FormaldehydeFormaldehyde
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O—C—O angle = 180°O—C—O angle = 180°linear geometrylinear geometry
OO CC OO
Table 1.6 Table 1.6 Carbon DioxideCarbon Dioxide
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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. = e x d= e x d
(expressed in Debye units)(expressed in Debye units)
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. = e x d= e x d
(expressed in Debye units)(expressed in Debye units)
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 moment; Carbon dioxide has no dipole moment; = 0 D = 0 D
Molecular Dipole MomentsMolecular Dipole Moments
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= 1.62 D= 1.62 D = 0 D= 0 D
Carbon tetrachlorideCarbon tetrachloride DichloromethaneDichloromethane
Figure 1.7Figure 1.7
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Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
= 0 D= 0 D
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
Figure 1.7Figure 1.7
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Resultant of theseResultant of thesetwo bond dipoles istwo bond dipoles is
= 1.62 D= 1.62 D
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.
Figure 1.7Figure 1.7
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1.121.12
Acids and Bases:Acids and Bases:
The Arrhenius ViewThe Arrhenius View
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DefinitionsDefinitions
ArrheniusArrheniusAn acid ionizes in water to give protons. A An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions.base ionizes in water to give hydroxide ions.
Brønsted-LowryBrønsted-LowryAn acid is a proton donor. A base is a proton An acid is a proton donor. A base is a proton acceptor.acceptor.
LewisLewisAn acid is an electron pair acceptor. A base An acid is an electron pair acceptor. A base is an electron pair donor.is an electron pair donor.
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Arrhenius Acids and BasesArrhenius Acids and Bases
An acid is a substance that ionizes to give An acid is a substance that ionizes to give protons when dissolved in water.protons when dissolved in water.
AA––HH++
HH AA ++ ....
A base is a substance that ionizes to give A base is a substance that ionizes to give hydroxide ions when dissolved in water.hydroxide ions when dissolved in water.
MM++ ++ OHOH––.... ..
..
..
..
MM OHOH..
..
..
..
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Arrhenius Acids and BasesArrhenius Acids and Bases
Strong acids dissociate completely in water. Strong acids dissociate completely in water. Weak acids dissociate only partially.Weak acids dissociate only partially.
AA––HH++
HH AA ++ ....
Strong bases dissociate completely in water. Strong bases dissociate completely in water. Weak bases dissociate only partially.Weak bases dissociate only partially.
MM++ ++ OHOH––.... ..
..
..
..
MM OHOH..
..
..
..
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Acid Strength is Measured by pAcid Strength is Measured by pKKaa
KKaa ==[H[H++][A][A––]]
[HA][HA]
ppKKaa = – log= – log1010KKaa
AA––HH++
HH AA ++ ....
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Brønsted-Lowry definitionBrønsted-Lowry definitionan acid is a proton donoran acid is a proton donor
a base is a proton acceptora base is a proton acceptor
1.131.13
Acids and Bases:Acids and Bases:
The Brønsted-Lowry ViewThe Brønsted-Lowry View
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HH AABB .... BB HH AA––....
++
A Brønsted Acid-Base ReactionA Brønsted Acid-Base Reaction
A proton is transferred from the acid to the A proton is transferred from the acid to the base.base.
++ ++
basebase acidacid
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HH AABB .... BB HH AA––....
++
A Brønsted Acid-Base ReactionA Brønsted Acid-Base Reaction
A proton is transferred from the acid to the A proton is transferred from the acid to the base.base.
++ ++
basebase acidacid conjugate conjugate acidacid
conjugate conjugate basebase
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hydronium ionhydronium ion
HH BrBrOO
HH
HH
.... ....
HH
HH
.... OO HH BrBr––
.... ............
..
..
........
++
Proton Transfer from HBr to WaterProton Transfer from HBr to Water
basebase acidacid conjugate conjugate conjugate conjugate acid acid base base
++ ++
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[H[H33OO++][Br][Br––]]
[HBr][HBr]KKaa = =
HH BrBrOO
HH
HH
.... ....
HH
HH
.... OO HH BrBr––
.... ............
..
..
........
++++ ++
Equilibrium Constant for Proton TransferEquilibrium Constant for Proton Transfer
Takes the same form as for Arrhenius Takes the same form as for Arrhenius KKaa, but , but
HH33OO++ replaces H replaces H++. H. H33OO++ and H and H++ are considered are considered
equivalent, and there is no difference in equivalent, and there is no difference in KKaa
values for Arrhenius and Brønsted acidity.values for Arrhenius and Brønsted acidity.
Dr. Wolf's CHM 201 & 202 1- 50
[H[H33OO++][Br][Br––]]
[HBr][HBr]KKaa = =
HH BrBrOO
HH
HH
.... ....
HH
HH
.... OO HH BrBr––
.... ............
..
..
........
++++ ++
ppKKaa = – log= – log1010 KKaa
Equilibrium Constant for Proton TransferEquilibrium Constant for Proton Transfer
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HH OHOHNN
HH
HH
.... ....
HH
HH
.... NN HH OHOH––............
..
..
....
Water as a Brønsted AcidWater as a Brønsted Acid
basebase acidacid conjugate conjugate conjugate conjugate acid acid base base
––
++ ++
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strong acids are stronger than hydronium ionstrong acids are stronger than hydronium ion
weaker weaker acidacid
stronger stronger acidacid
AAcciidd ppKKaa CCononjj.. BBaasese
HHII II––
HBHBrr BBrr––
HClHCl ClCl––
HH33OO++
-10.4-10.4
-5.8-5.8
-4.8-4.8
-3.9-3.9
-1.7-1.7 HH22OO
HH22SSOO44 HSHSOO44––
Dissociation Constants (pDissociation Constants (pKKaa) of Acids*) of Acids*
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The stronger the acid, the weaker the conjugate base.The stronger the acid, the weaker the conjugate base.
weaker weaker acidacid
stronger stronger acidacid
AAcciidd ppKKaa CCononjj.. BBaasese
HHII II––
HBHBrr BBrr––
HClHCl ClCl––
HH33OO++
-10.4-10.4
-5.8-5.8
-4.8-4.8
-3.9-3.9
-1.7-1.7 HH22OO
HH22SSOO44 HSHSOO44––
Important Generalization!Important Generalization!
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Dissociation Constants (pDissociation Constants (pKKaa) of Acids*) of Acids*
weak acids are weaker than hydronium ionweak acids are weaker than hydronium ion
AAcciidd ppKKaa CCononjj.. BBaasese
HH33OO++ ––11.7.7 HH22OO
HHFF 3.3.55 FF––
CHCH33CCOO22HH 4.4.66 CHCH33CCOO22––
NHNH44++ 9.9.22 NHNH33
HH22OO 1155.7.7 HHOO––
Dr. Wolf's CHM 201 & 202 1- 55
Dissociation Constants (pDissociation Constants (pKKaa) of Acids*) of Acids*
alcohols resemble water in acidity; their conjugatealcohols resemble water in acidity; their conjugatebases are comparable to hydroxide ion in basicity bases are comparable to hydroxide ion in basicity
AAcciidd ppKKaa CCononjj.. BBaasese
CHCH33OOHH CHCH33OO––
CHCH33CHCH22OOHH ~~1166 CHCH33CHCH22OO––
((CHCH33))22CHCHOOHH ~~1177 ((CHCH33))22CHCHOO––
((CHCH33))33CCOOHH ~~1188 ((CHCH33))33CCOO––
HH22OO 1155.7.7 HHOO––
1155.2.2
Dr. Wolf's CHM 201 & 202 1- 56
Dissociation Constants (pDissociation Constants (pKKaa) of Acids*) of Acids*
ammonia and amines are very weak acids;ammonia and amines are very weak acids;their conjugate bases are very strong basestheir conjugate bases are very strong bases
AAcciidd ppKKaa CCononjj.. BBaasese
NHNH33 ~~3366 NHNH22––
((CHCH33))22NHNH ~~3366 ((CHCH33))22NN––
Dr. Wolf's CHM 201 & 202 1- 57
Dissociation Constants (pDissociation Constants (pKKaa) of Acids*) of Acids*
Most hydrocarbons are extremely weak acids.Most hydrocarbons are extremely weak acids.
AcidAcid ppKKaa CCononjj.. BBaasese
2626HCHC CHCH
4343
4545
6262CHCH33CHCH33
HH22CC CHCH22
HH
HHHH
HH
HH HH
HCHC CC ––
HHHH
HH
HH HH
––
HH22CC CHCH––
CHCH33CHCH22––