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

Dr. Wolf's CHM 201 & 202 1- 4

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

Dr. Wolf's CHM 201 & 202 1- 6

1.81.8

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.”

Dr. Wolf's CHM 201 & 202 1- 8

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

Dr. Wolf's CHM 201 & 202 1- 9

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

....::.... ....

Dr. Wolf's CHM 201 & 202 1- 10

1.91.9

ResonanceResonance

Dr. Wolf's CHM 201 & 202 1- 11

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

Dr. Wolf's CHM 201 & 202 1- 12

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

........ ::.... ....

Dr. Wolf's CHM 201 & 202 1- 13

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

....::....

Dr. Wolf's CHM 201 & 202 1- 14

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

....::....

Dr. Wolf's CHM 201 & 202 1- 15

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

.... ::....++ ––

Dr. Wolf's CHM 201 & 202 1- 16

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

Dr. Wolf's CHM 201 & 202 1- 17

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

Dr. Wolf's CHM 201 & 202 1- 18

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?

Dr. Wolf's CHM 201 & 202 1- 19

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••••

••••••••••••••••––++

Dr. Wolf's CHM 201 & 202 1- 20

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••••

••••••••••••••••

–– ++

••••

Dr. Wolf's CHM 201 & 202 1- 21

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••••

••••••••••••••••

–– ++

••••

Dr. Wolf's CHM 201 & 202 1- 22

1.101.10The Shapes of Some Simple MoleculesThe Shapes of Some Simple Molecules

Dr. Wolf's CHM 201 & 202 1- 23

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

Dr. Wolf's CHM 201 & 202 1- 24

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

Dr. Wolf's CHM 201 & 202 1- 27

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

Dr. Wolf's CHM 201 & 202 1- 29

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

Dr. Wolf's CHM 201 & 202 1- 30

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

Dr. Wolf's CHM 201 & 202 1- 31

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

Dr. Wolf's CHM 201 & 202 1- 32

1.111.11Molecular Dipole MomentsMolecular Dipole Moments

Dr. Wolf's CHM 201 & 202 1- 33

++——

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

Dr. Wolf's CHM 201 & 202 1- 34

——++

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

Dr. Wolf's CHM 201 & 202 1- 35

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

Dr. Wolf's CHM 201 & 202 1- 36

OO CC OO

Carbon dioxide has no dipole moment; Carbon dioxide has no dipole moment; = 0 D = 0 D

Molecular Dipole MomentsMolecular Dipole Moments

Dr. Wolf's CHM 201 & 202 1- 37

= 1.62 D= 1.62 D = 0 D= 0 D

Carbon tetrachlorideCarbon tetrachloride DichloromethaneDichloromethane

Figure 1.7Figure 1.7

Dr. Wolf's CHM 201 & 202 1- 38

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

Dr. Wolf's CHM 201 & 202 1- 39

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

Dr. Wolf's CHM 201 & 202 1- 40

1.121.12

Acids and Bases:Acids and Bases:

The Arrhenius ViewThe Arrhenius View

Dr. Wolf's CHM 201 & 202 1- 41

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.

Dr. Wolf's CHM 201 & 202 1- 42

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..

..

..

..

Dr. Wolf's CHM 201 & 202 1- 43

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..

..

..

..

Dr. Wolf's CHM 201 & 202 1- 44

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 ++ ....

Dr. Wolf's CHM 201 & 202 1- 45

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

Dr. Wolf's CHM 201 & 202 1- 46

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

Dr. Wolf's CHM 201 & 202 1- 47

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

Dr. Wolf's CHM 201 & 202 1- 48

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

++ ++

Dr. Wolf's CHM 201 & 202 1- 49

[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

Dr. Wolf's CHM 201 & 202 1- 51

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

––

++ ++

Dr. Wolf's CHM 201 & 202 1- 52

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*

Dr. Wolf's CHM 201 & 202 1- 53

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!

Dr. Wolf's CHM 201 & 202 1- 54

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––