Chapter 2. The Lewis Model of Bonding:The Lewis Model of Bonding: Atoms bond together in such a way...

Chapter 2

• The Lewis Model of Bonding: The Lewis Model of Bonding: Atoms bond together in such a way that each atom participating in a chemical bond acquires a completed valence-shell electron configuration resembling that of the noble gas nearest it in the Periodic Table.

IntroductionIntroduction

• Ionic Bond – Ionic Bond – is a chemical bond resulting from the electrostatic attraction between an anion and a cation.

Na + F Na+ + F- .... .... ..

....

..

• Covalent Bond – Covalent Bond – is a chemical bond formed by the sharing of electron pairs between atoms.

2 H· H2

• Based on the the degree of electron sharing, covalent bonds can be divided into:

• Nonpolar covalent bondNonpolar covalent bond• Polar covalent bondPolar covalent bond

I. Polar Covalent BondsI. Polar Covalent Bonds I. Polar Covalent BondsI. Polar Covalent Bonds

A.A. ElectronegativityElectronegativity

B.B. Dipole MomentsDipole Moments

A.A. ElectronegativityElectronegativity

B.B. Dipole MomentsDipole Moments

• Polar Covalent Bond – Polar Covalent Bond – is a covalent bond with ionic character.

Bonding electrons are attracted more strongly by one atom than by the other

Electron distribution between atoms is not symmetrical

AA.. Polar Covalent Bonds: ElectronegativityPolar Covalent Bonds: Electronegativity AA.. Polar Covalent Bonds: ElectronegativityPolar Covalent Bonds: Electronegativity

• ElectronegativityElectronegativity (EN):(EN):

is a measure of the force of an atom’s attraction for electrons it shares in a covalent bond with another atom.

is an intrinsic ability of an atom to attract the shared electrons in a covalent bond

Bond Polarity and ElectronegativityBond Polarity and Electronegativity

• EN of an atom is related to its ionization energy (IE) and electron affinity (EA).

• Atom in ground state (g) + Energy Atom+ (g) + e- E = IEE = IE• Atom (g) + e- Atom- E = EAE = EA

• Differences in EN produce bond polaritybond polarity.

Bond Polarity ~ EN ~ EN

EN

Bond Polarity and ElectronegativityBond Polarity and Electronegativity

• EN is based on an arbitrary scale.

• The most widely used scale of EN was devised by Linus PaulingLinus Pauling in the 1930’s and is based on bond energies.

• Important EN values:

• F is the most electronegative (EN = 4.0)• Cs is the least electronegative (EN = 0.7)• C has an EN = 2.5

The Periodic Table and ElectronegativityThe Periodic Table and Electronegativity

The Periodic Table and ElectronegativityThe Periodic Table and Electronegativity

• Metals on left side of periodic table attract electrons weakly, lower EN

The Periodic Table and ElectronegativityThe Periodic Table and Electronegativity

• Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly, higher EN

Classification of Covalent BondsClassification of Covalent Bonds

Difference in EN Difference in EN between bonded Atomsbetween bonded Atoms Type of BondType of Bond

Similar ENSimilar EN Nonpolar CovalentNonpolar Covalent

Less than 2Less than 2 Polar CovalentPolar Covalent

Greater than 2Greater than 2 IonicIonic

Classification of Covalent BondsClassification of Covalent Bonds

Difference in EN Difference in EN between bonded Atomsbetween bonded Atoms Type of BondType of Bond

Less than 0.5 Less than 0.5 Nonpolar CovalentNonpolar Covalent

0.5 to 1.90.5 to 1.9 Polar CovalentPolar Covalent

Greater than 1.9Greater than 1.9 IonicIonic

Classification of Covalent Bonds: Classification of Covalent Bonds: ExamplesExamples

• C–H bonds are relatively nonpolar

EN = ENC – ENH = 2.5 - 2.1 = 0.4

• C-O and C-X bonds (more electronegative elements) are polar

EN = ENO – ENC = 3.5 - 2.5 = 1

• Bonding electrons are drawn toward electronegative atom

C acquires partial positive charge, + Electronegative atom acquires partial negative

charge, -

Bond Polarity and Inductive EffectBond Polarity and Inductive Effect

The crossed arrow indicates the direction of the electron displacement

• Inductive EffectInductive Effect – is the shifting of electrons in a bond in response to EN of nearby atoms

Metals (Li and Mg) inductively donate e-

Reactive nonmetals (O and Cl) inductively withdraw e-

Bond Polarity and Inductive EffectBond Polarity and Inductive Effect

Electrostatic Potential MapsElectrostatic Potential Maps

• Electrostatic potential maps show calculated charge distributions

• Colors indicate electron-rich (red) and electron-poor (blue) regions

Practice ProblemPractice Problem: Which element in each of the following pairs : Which element in each of the following pairs is more electronegative? is more electronegative?

a. Li or H

b. B or Br

c. Cl or I

d. C or H

Practice ProblemPractice Problem: Use the : Use the +/+/- convention to indicate the - convention to indicate the direction of expected polarity for each of the direction of expected polarity for each of the bonds indicated bonds indicated

a. H3C — Br

b. H3C — NH2

c. H3C — Li

d. H2N — H

e. H3C — OH

f. H3C — MgBr

g. H3C — F

Practice ProblemPractice Problem: Use the electronegativity values to rank the : Use the electronegativity values to rank the following bonds from least polar to most following bonds from least polar to most

polar: polar:

H3C — Li

H3C — K

H3C — F

H3C — MgBr

H3C — OH

Practice ProblemPractice Problem: Look at the following electrostatic potential : Look at the following electrostatic potential map of methyl alcohol, and tell the direction of map of methyl alcohol, and tell the direction of polarization of the C-O bond: polarization of the C-O bond:

BB.. PolarPolar Covalent Bonds: Dipole Moments Covalent Bonds: Dipole Moments BB.. PolarPolar Covalent Bonds: Dipole Moments Covalent Bonds: Dipole Moments

• Molecular PolarityMolecular Polarity

is the tendency of molecules as a whole to be polar

results from vector summation of individual bond polarities and lone-pair contributions

• ““Like dissolves like”Like dissolves like”

• Strongly polar substances are soluble in polar solvents like water; nonpolar substances are insoluble in water.

• To predict whether a molecule is polar, determine:

1. if the molecule has polar bonds, and

2. the arrangements of these bonds in space

• Dipole moment (Dipole moment ()) – is a measure of net molecular polarity, due to difference in summed charges

= = QQ rr

magnitude of charge magnitude of charge QQ at at either end of molecular dipoleeither end of molecular dipole

distance distance rr between charges between charges

• It is expressed in Debyes (D)• 1 D = 3.336 x 10-30 Coulomb meter

raverage covalent bond = 100pm

Qelectron = 1.60 x 10-19 C

The dipole moment (The dipole moment () of an average covalent ) of an average covalent bond is 4.80 Dbond is 4.80 D

Calculating the dipole moment of an average bond:Calculating the dipole moment of an average bond:

Given that r r C-Cl = 178 pm and assuming C-H is negligible, then

calculated CH3Cl = 178 pm x 4.80 D = 8.5 D

measured CH3Cl = 1.87D

% ionic character = measured / calculated x 100 = 22%

Calculating Ionic Character: CCalculating Ionic Character: Chloromethanehloromethane

• H2O and NH3 have large dipole moments:

ENO and ENN > ENH

Both O and N have lone-pair electrons oriented away from all nuclei

Dipole Moments in Water and AmmoniaDipole Moments in Water and Ammonia

Dipole Moments in Water and AmmoniaDipole Moments in Water and Ammonia

• H2O and NH3 have large dipole moments:

ENO and ENN > ENH

Both O and N have lone-pair electrons oriented away from all nuclei

Absence of Dipole MomentsAbsence of Dipole Moments

• In symmetrical molecules, the dipole moment of each bond has one in the opposite direction

• The effects of the local dipoles cancel each other

Practice ProblemPractice Problem: Carbon dioxide, CO: Carbon dioxide, CO22, has zero dipole moment , has zero dipole moment

even though carbon-oxygen bonds are even though carbon-oxygen bonds are strongly polarized. Explain. strongly polarized. Explain.

Practice ProblemPractice Problem: Make three-dimensional drawings of the : Make three-dimensional drawings of the following molecules, and predict whether following molecules, and predict whether each has a dipole moment. If you expect a each has a dipole moment. If you expect a dipole moment, show its direction. dipole moment, show its direction.

a. H2C = CH2

b. CHCl3

c. CH2Cl2

d. H2C = CCl2

II. Formal Charges and ResonanceII. Formal Charges and Resonance II. Formal Charges and ResonanceII. Formal Charges and Resonance

A.A. Formal ChargesFormal Charges

B.B. ResonanceResonance

A.A. Formal ChargesFormal Charges

B.B. ResonanceResonance

AA.. Formal ChargesFormal Charges AA.. Formal ChargesFormal Charges

• Formal Charge - Formal Charge - is the charge on an atom in a molecule or polyatomic ion

No formal charge

No formal charge

Comparing the bonding of the atom in the molecule to Comparing the bonding of the atom in the molecule to the valence electron structurethe valence electron structure

Calculating the formal chargeCalculating the formal charge

• If the atom has one more electron in the molecule, it is shown with a “-” charge

• If the atom has one less electron, it is shown with a “+” charge

• Neutral molecules with both a “+” and a “-” are dipolardipolar

Practice ProblemPractice Problem: Dimethyl sulfoxide, a common solvent, has : Dimethyl sulfoxide, a common solvent, has the structure indicated. Show why dimethyl the structure indicated. Show why dimethyl sulfoxide must have formal charges on S and sulfoxide must have formal charges on S and O. O.

Practice ProblemPractice Problem: Calculate formal charges for the nonhydrogen : Calculate formal charges for the nonhydrogen atoms in the following molecules: atoms in the following molecules:

a. Diazomethane,

b. Acetonitrile oxide,

c. Methyl isocyanide

Practice ProblemPractice Problem: Organic phosphates occur commonly among : Organic phosphates occur commonly among biological molecules. Calculate formal biological molecules. Calculate formal charges on the four O atoms in the methyl charges on the four O atoms in the methyl phosphate ion. phosphate ion.

BB.. Resonance Resonance BB.. Resonance Resonance

• Some molecules have Lewis structures that cannot be shown with a single representation

• In these cases we draw structures that contribute to the final structure but which differ in the position of the bond(s) or lone pair(s)

Resonance FormsResonance Forms

• Resonance formsResonance forms – are Lewis structures of the same molecule whose only difference is the placement of and nonbonding valence electrons (= delocalized).

• The atoms occupy the same place in the different forms• The connections between atoms are the same.

• The resonance forms are connected by a double-headed arrowdouble-headed arrow

Resonance HybridsResonance Hybrids

• A structure with resonance forms does not alternate between the forms

• Instead, it is a hybrid of the two resonance forms, so the structure is called a resonance hybrid

Resonance Hybrids: Resonance Hybrids: BenzeneBenzene

• For example, benzene (C6H6) has two resonance forms with alternating double and single bonds – In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single

Rules for Resonance FormsRules for Resonance Forms11

1.1. Individual resonance forms are Individual resonance forms are imaginaryimaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)

Rules for Resonance FormsRules for Resonance Forms22

2.2. Resonance forms differ only in the placement of their Resonance forms differ only in the placement of their or nonbonding electrons or nonbonding electrons

Rules for Resonance FormsRules for Resonance Forms33

3.3. Different resonance forms of a substance don’t have Different resonance forms of a substance don’t have to be equivalentto be equivalent

Rules for Resonance FormsRules for Resonance Forms44

4.4. Resonance forms must be valid Lewis structures: the Resonance forms must be valid Lewis structures: the octet rule appliesoctet rule applies

Rules for Resonance FormsRules for Resonance Forms55

5.5. The resonance hybrid is more stable than any The resonance hybrid is more stable than any individual resonance formindividual resonance form

• Resonance leads to stability. The larger the # of the resonance forms, the more stable the substance

Rules for Resonance FormsRules for Resonance Forms1. Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)

2. Resonance forms differ only in the placement of their or nonbonding electrons

3. Different resonance forms of a substance don’t have to be equivalent

4. Resonance forms must be valid Lewis structures: the octet rule applies

5. The resonance hybrid is more stable than any individual resonance form

Curved Arrows and Resonance FormsCurved Arrows and Resonance Forms

• We can imagine that electrons move in pairs to convert from one resonance form to another

• A curved arrow shows that a pair of electrons moves A curved arrow shows that a pair of electrons moves fromfrom the atom or the atom or bond at the tail of the arrow bond at the tail of the arrow toto the atom or bond at the head of the arrow the atom or bond at the head of the arrow

Different Atoms in Resonance FormsDifferent Atoms in Resonance Forms

• Sometimes resonance forms involve different atom types as well as locations

• The resulting resonance hybrid has properties associated with both types of contributors

• The types may contribute unequally

• The “enolate” derived from acetone is a good illustration, with delocalization between carbon and oxygen

Drawing Resonance FormsDrawing Resonance Forms

• Any three-atom grouping with a p orbital on each atom has two resonance forms

X, Y, Z can be C, N, O, P or S

Resonance Structures: Resonance Structures: 2,4-Pentanedione2,4-Pentanedione

• The anion derived from 2,4-pentanedione

– Lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O bond on the left and on the right

– Three resonance structures result

Relative Importance of contributing StructuresRelative Importance of contributing Structures

The most important contributing structures have:The most important contributing structures have:

1. filled valence shells

2. a maximum number of covalent bonds

3. the least separation of unlike charge, and

4. any negative charge on a more electronegative atom and/or any positive charge on a less electronegative atom

Practice ProblemPractice Problem: Draw the indicated number of resonance : Draw the indicated number of resonance structures for each of the following species: structures for each of the following species:

a. The nitrate ion, NO3- (3)

b. The allyl cation, H2C=CH-CH2+ (2)

c. Hydrazoic acid, (2)

d. (2)

III. Acids and BasesIII. Acids and Bases11 III. Acids and BasesIII. Acids and Bases11

A.A. The BrThe Brøønsted-Lowry Definitionnsted-Lowry Definition

B.B. Acid and Base StrengthAcid and Base Strength

C.C. Predicting Acid-Base Reactions Predicting Acid-Base Reactions from pKa from pKa ValuesValues

A.A. The BrThe Brøønsted-Lowry Definitionnsted-Lowry Definition

B.B. Acid and Base StrengthAcid and Base Strength

C.C. Predicting Acid-Base Reactions Predicting Acid-Base Reactions from pKa from pKa ValuesValues

III. Acids and BasesIII. Acids and Bases22 III. Acids and BasesIII. Acids and Bases22

D.D. Organic Acids and Organic BasesOrganic Acids and Organic Bases

E.E. The Lewis DefinitionThe Lewis Definition

D.D. Organic Acids and Organic BasesOrganic Acids and Organic Bases

E.E. The Lewis DefinitionThe Lewis Definition

The terms “acid”“acid” and “base”“base” can have different meanings in different contexts.

• For that reason, we specify the usage with more complete terminology

• There are three definitions of acid-base: Arrhenius, Bronsted-Lowry, and Lewis

IntroductionIntroduction

• According to the ArrheniusArrhenius definition:

• An Arrhenius acidAn Arrhenius acid - is a substance that donates HH++ in

aqueous solutions• An Arrhenius baseAn Arrhenius base - is a substance that donates OHOH--

in aqueous solutions

• The ArrheniusArrhenius definition is not useful in organic chemistry.

AA.. The BrThe Brøønsted-Lowry Definitionnsted-Lowry Definition AA.. The BrThe Brøønsted-Lowry Definitionnsted-Lowry Definition

• Brønsted–Lowry theoryBrønsted–Lowry theory defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors

• ““Brønsted-Lowry”Brønsted-Lowry” is usually shortened to “Brønsted”“Brønsted”

Brønsted Acids and BasesBrønsted Acids and Bases

• A Brønsted acid is a substance that donates a hydrogen ion (H+)

• A Brønsted base is a substance that accepts the H+

• “proton” is a synonym for H+ (i.e. Loss of the valence electron from a neutral H leaves only the hydrogen nucleus - a proton)

Brønsted acid–base ReactionsBrønsted acid–base Reactions

The Reaction of HCl with HThe Reaction of HCl with H22OO

• When HCl gas dissolves in water, a Brønsted acid–base reaction occurs

• HCl donates a proton to water molecule, yielding hydronium ion (H3O+) and Cl-

• The reverse is also a Brønsted acid–base reaction of the conjugate acid and conjugate base

Other Brønsted acid–base ReactionsOther Brønsted acid–base Reactions

Practice ProblemPractice Problem: Nitric acid (HNO: Nitric acid (HNO33) reacts with ammonia (NH) reacts with ammonia (NH33) )

to yield ammonium nitrate. Write the to yield ammonium nitrate. Write the reaction, reaction, and identify the acid, the base, the and identify the acid, the base, the conjugate conjugate acid product, and the conjugate base acid product, and the conjugate base product. product.

BB.. Acid and Base StrengthAcid and Base Strength BB.. Acid and Base StrengthAcid and Base Strength

• The equilibrium constant (Keq) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A-) is a measure related to the strength of the acid

• Stronger acids have larger Keq

KKaa – the acidity constant – the acidity constant

• The concentration of water as a solvent does not change significantly when it is protonated

Molecular weight (H2O) = 18 g/mol. Density (H2O) = 1000 g/l [H2O] ~ 55.6 M at 25°C

KKaa – the acidity constant – the acidity constant

• The acidity constant, Ka for HA is

• Ka ranges from 1015 for the strongest acids to very small values (10-60) for the weakest

Acid-Base StrengthAcid-Base Strength

• The “ability” of a Brønsted acid to donate a proton is sometimes referred to as the strength of the acid.

• The strength of the acid is measured with respect to the Brønsted base that receives the proton

• Water is used as a common base for the purpose of creating a scale of Brønsted acid strength

pKpKaa – The acid strength scale – The acid strength scale

• Acid strength is expressed using pKa values:

pKa = - log Ka

• The free energy in an equilibrium is related to -log of Keq:

G = -RT ln Keq

• A larger value of Ka indicates a stronger acid and is proportional to the energy difference between products and reactants

pKpKaa – The acid strength scale – The acid strength scale

• The pKa of water is 15.74

• The strongerstronger the acid, the weakeweaker its conjugate base. The weakerweaker the acid, the strongerstronger the conjugate base.

Practice ProblemPractice Problem: Formic acid, HCO: Formic acid, HCO22H, has pKa = 3.75, and H, has pKa = 3.75, and

picric acid, C picric acid, C66HH33NN33OO77, has pKa = 0.38. Which , has pKa = 0.38. Which

is the stronger acid? is the stronger acid?

Practice ProblemPractice Problem: Amide ion, H: Amide ion, H22NN--, is a much stronger base , is a much stronger base

than hydroxide ion, HO than hydroxide ion, HO--. Which would you . Which would you expect to be a stronger acid, NH expect to be a stronger acid, NH33 or H or H22O? O?

Explain. Explain.

CC.. Predicting Acid-Base Reactions from Predicting Acid-Base Reactions from pKa ValuespKa Values

CC.. Predicting Acid-Base Reactions from Predicting Acid-Base Reactions from pKa ValuespKa Values

• pKa values are related as logarithms to equilibrium constants

• The difference in two pKa values is the log of the ratio of equilibrium constants:

pKa = log Keq2/Keq1

pKa can be used to calculate the extent of H+ transfer

• H+ will always go from the stronger acid to the stronger base

• The product acid must be weaker and less reactive than the starting acid

• The product base must be weaker and less reactive than the starting base

• The stronger base holds the proton more tightly

Practice ProblemPractice Problem: Will either of the following reactions take : Will either of the following reactions take place as written, according to the pKa data? place as written, according to the pKa data?

a. HCN + CH3CO2- Na+ Na+ -CN + CH3CO2H

b. CH3CH2OH + Na+-CN CH3CH2O-Na+ + HCN

?

?

Practice ProblemPractice Problem: Ammonia, NH: Ammonia, NH33, has pKa = 36 and acetone , has pKa = 36 and acetone

has pKa = 19. Will the following reaction take has pKa = 19. Will the following reaction take place? place?

Practice ProblemPractice Problem: What is the Ka of HCN if its pKa = 9.31? : What is the Ka of HCN if its pKa = 9.31?

DD.. Organic Acids and Organic BasesOrganic Acids and Organic Bases DD.. Organic Acids and Organic BasesOrganic Acids and Organic Bases

• The reaction patterns of organic compounds often are acid-base combinations

• The transfer of a H+ from a strong Brønsted acid to a Brønsted base

– is a very fast process–will always occur along with other reactions

Organic AcidsOrganic Acids

• Organic acids lose a proton either from:

• O–H, such as methanol and acetic acid

• C–H, usually from a carbon atom next to a C=O double bond (O=C–C–H)

Organic AcidsOrganic Acids

• The conjugate base resulting from loss of H+ is stabilized by having its negative charge on a highly electronegative oxygen atom Acidity

Organic AcidsOrganic Acids

Organic BasesOrganic Bases

• Organic bases have an atom with a an atom with a lone pair lone pair of electronsof electrons that can bond to H+

• Nitrogen-containing compounds derived from ammonia are the most common organic bases

• Oxygen-containing compounds can react as bases when with a strong acid or as acids with strong bases

Organic BasesOrganic Bases

EE.. The Lewis DefinitionThe Lewis Definition EE.. The Lewis DefinitionThe Lewis Definition

• AA Lewis acidLewis acid is an electron pair acceptor; it has a low-energy empty orbital

• A Lewis baseA Lewis base is an electron pair donor

• Brønsted acids are not Lewis acids because they cannot accept an electron pair directly

–Only a protonproton would be a Lewis acidLewis acid

• The Lewis definition leads to a general description of many reaction patterns but there is no scale of strengths as in the Brønsted definition of pKa

Lewis AcidsLewis Acids

Lewis acids include:• HH++

• Metal cationsMetal cations, such as Mg2+: They accept a pair of They accept a pair of electrons when they form a bond to a baseelectrons when they form a bond to a base

• Group 3A elementsGroup 3A elements, such as BF3 and AlCl3: They They have unfilled valence orbitals and can accept electron have unfilled valence orbitals and can accept electron pairs from Lewis basespairs from Lewis bases

• Transition-metalTransition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4: They have unfilled valence orbitals They have unfilled valence orbitals and can accept electron pairs from Lewis basesand can accept electron pairs from Lewis bases

Lewis AcidsLewis Acids

Lewis AcidsLewis Acids

• Organic compounds that undergo addition reactions with Lewis bases are called electrophileselectrophiles and therefore Lewis AcidsLewis Acids

• The combination of a Lewis acid and a Lewis base can be shown with a curved arrow from base to acid

Illustration of Curved Arrows in Following Lewis Illustration of Curved Arrows in Following Lewis Acid-Base ReactionsAcid-Base Reactions

Lewis BasesLewis Bases

• Lewis bases can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted bases

• Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons

• Some compounds can act as both acids and bases, depending on the reaction

Lewis BasesLewis Bases

Practice ProblemPractice Problem: Using curved arrows, show how the species : Using curved arrows, show how the species in part (a) can act as Lewis bases in their in part (a) can act as Lewis bases in their reactions with HCl, and show how the species reactions with HCl, and show how the species in part (b) can act as Lewis acids in their in part (b) can act as Lewis acids in their reaction with OH reaction with OH- -

a. CH3CH2OH, HN(CH3)2, P(CH3)3

b. H3C+, B(CH3)3, MgBr2

Practice ProblemPractice Problem: Explain by calculating formal charges why the : Explain by calculating formal charges why the following acid-base reaction products have following acid-base reaction products have the charges indicated: the charges indicated:

a. F3B — O — CH3

| CH3

- +

b. Cl3Al — N — CH3

| CH3

CH3

|- +

Practice ProblemPractice Problem: The organic compound imidazole can act as : The organic compound imidazole can act as both an acid and a base. Look at the following both an acid and a base. Look at the following

electrostatic potential map, and identify the electrostatic potential map, and identify the most acidic hydrogen atom and the most most acidic hydrogen atom and the most basic nitrogen in imidazole basic nitrogen in imidazole

IV. Chemical StructuresIV. Chemical Structures IV. Chemical StructuresIV. Chemical Structures

A.A. Drawing Chemical StructuresDrawing Chemical Structures

B.B. Molecular ModelsMolecular Models

A.A. Drawing Chemical StructuresDrawing Chemical Structures

B.B. Molecular ModelsMolecular Models

AA.. Drawing Chemical StructureDrawing Chemical Structure AA.. Drawing Chemical StructureDrawing Chemical Structure

• Chemists use shorthand ways for writing structures.

• Organic molecules are usually drawn as:• Condensed structures• Skeletal structures

• C-H and C-C single bonds aren't shown but understood- If C has 3 H’s bonded to it, write CH3

- If C has 2 H’s bonded to it, write CH2; and so on.

• Horizontal bonds between carbons aren't shown in condensed structures—the CH3, CH2, and CH units are simply placed next each other but vertical bonds are added for clarity

Condensed StructuresCondensed Structures

Skeletal StructuresSkeletal Structures

• Minimum amount of information but unambiguous

• Rules for Skeletal StructuresRules for Skeletal Structures::

1.1. C’s are not shownC’s are not shown, assumed to be at each intersection of two lines (bonds) and at end of each line

2.2. H’s bonded to C’s aren't shownH’s bonded to C’s aren't shown – whatever number is needed will be there

3.3. All atoms other than C and H All atoms other than C and H areare shown shown

Practice ProblemPractice Problem: Tell how many hydrogens are bonded to each : Tell how many hydrogens are bonded to each carbon in the following compounds, and give carbon in the following compounds, and give the molecular formula of each substance: the molecular formula of each substance:

Practice ProblemPractice Problem: Propose skeletal structures for compounds : Propose skeletal structures for compounds that satisfy the following molecular formulas that satisfy the following molecular formulas (there is more than one possibility in each (there is more than one possibility in each case): case):

a. C5H12

b. C2H7N

c. C3H6O

d. C4H9Cl

Practice ProblemPractice Problem: The following molecular model is a : The following molecular model is a representation of representation of parapara-aminobenzoic acid -aminobenzoic acid (PABA), the active ingredient in many (PABA), the active ingredient in many sunscreens. Indicate the positions of the sunscreens. Indicate the positions of the multiple bonds, and draw a skeletal structure multiple bonds, and draw a skeletal structure (gray = C, red = O, blue = N, ivory = H) (gray = C, red = O, blue = N, ivory = H)

BB.. Molecular ModelMolecular Model BB.. Molecular ModelMolecular Model

• We often need to visualize the shape or connections of a molecule in three dimensions

• Molecular modelsMolecular models are three dimensional objects, on a human scale, that represent the aspects of interest of the molecule’s structure (computer models also are possible)

Space-filling

Framework

Space-filling

Framework

• Drawings on paper and screens are limited in what they can present to you

• Framework models (ball-and-stick) are essential for seeing the relationships within and between molecules – you should own a set

• Space-filling models are better for examining the crowding within a molecule

Chapter 2