organic chemistry i chm 201

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Organic Chemistry I CHM 201 William A. Price, Ph.D.

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Organic Chemistry I CHM 201. William A. Price, Ph.D. Introduction and Review: Structure and Bonding. Atomic structure Lewis Structures Resonance Structural Formulas Acids and Bases. Electronic Structure of the Atom. - PowerPoint PPT Presentation

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Page 1: Organic Chemistry I CHM 201

Organic Chemistry ICHM 201

William A. Price, Ph.D.

Page 2: Organic Chemistry I CHM 201

Introduction and Review: Structure and Bonding

Atomic structureLewis Structures

ResonanceStructural Formulas

Acids and Bases

Page 3: Organic Chemistry I CHM 201
Page 4: Organic Chemistry I CHM 201
Page 5: Organic Chemistry I CHM 201

Chapter 1 5

Electronic Structure of the Atom

• An atom has a dense, positively charged nucleus surrounded by a cloud of electrons.

• The electron density is highest at the nucleus and drops off exponentially with increasing distance from the nucleus in any direction.

Page 6: Organic Chemistry I CHM 201

Orbitals are Probabilities

Page 7: Organic Chemistry I CHM 201

2s Orbital Has a Node

Page 8: Organic Chemistry I CHM 201

The p Orbital

Page 9: Organic Chemistry I CHM 201

The 2p Orbitals• There are three 2p

orbitals, oriented at right angles to each other.

• Each p orbital consists of two lobes.

• Each is labeled according to its orientation along the x, y, or z axis.

Chapter 1 9

Page 10: Organic Chemistry I CHM 201

px, py, pz

Page 11: Organic Chemistry I CHM 201

Electronic Configurations

• The aufbau principle states to fill the lowest energy orbitals first.

• Hund’s rule states that when there are two or more orbitals of the same energy (degenerate), electrons will go into different orbitals rather than pairing up in the same orbital.

Chapter 1 11

Page 12: Organic Chemistry I CHM 201

Electronic Configurations of Atoms• Valence electrons are electrons on the

outermost shell of the atom.

Chapter 1 12

Page 13: Organic Chemistry I CHM 201

Covalent Bonding• Electrons are shared between the atoms to complete

the octet.• When the electrons are shared evenly, the bond is

said to be nonpolar covalent, or pure covalent.• When electrons are not shared evenly between the

atoms, the resulting bond will be polar covalent.

Chapter 1 13

Page 14: Organic Chemistry I CHM 201

Lewis Dot Structure of Methane

C.. . .

carbon - 4 valence e hydrogen - 1 valence e

H.

1s22s22p2 1s

Page 15: Organic Chemistry I CHM 201

Tetrahderal Geometry

Page 16: Organic Chemistry I CHM 201

CH4 NH3

H2O Cl2

Lewis Structures

C

H

H

H

HN H

H

H

O HH ClCl

Carbon: 4 e4 H@1 e ea: 4 e 8 e

Nitrogen: 5 e3 H@1 e ea: 3 e 8 e

Oxygen: 6 e2 H@1 e ea: 2 e 8 e

2 Cl @7 e ea: 14 e

Chapter 1 16

Page 17: Organic Chemistry I CHM 201

Bonding PatternsValence

electrons (group #)

# Bonds # Lone Pair Electrons

C

N

O

Halides(F, Cl, Br, I)

4 4 0

5 3 1

6 2 2

7 1 3

Chapter 1 17

Page 18: Organic Chemistry I CHM 201
Page 19: Organic Chemistry I CHM 201

Bonding Characteristics of Period 2 Elements

Page 20: Organic Chemistry I CHM 201

Lewis structures are the way wewrite organic chemistry.

Learning now to draw themquickly and correctly will helpyou throughout this course.

HINT

Chapter 1 20

Page 21: Organic Chemistry I CHM 201

Multiple Bonding

• Sharing two pairs of electrons is called a double bond.• Sharing three pairs of electrons is called a triple bond.

Chapter 1 21

Page 22: Organic Chemistry I CHM 201

Convert Formula into Lewis Structure

• HCN• HNO2

• CHOCl• C2H3Cl

• N2H2

• O3

• HCO3-

• C3H4

Page 23: Organic Chemistry I CHM 201

Formal Charges

H3O+ NO+

OH H

H

Formal charge = [group number ] – [nonbonding electrons ] – ½ [shared electrons]

N O

6 – 2 – ½ (6) = +1 6 – 2 – ½ (6) = +1

5 – 2 – ½ (6) = 0+

+

• Formal charges are a way of keeping track of electrons.• They may or may not correspond to actual charges in the

molecule.

Chapter 1 23

Page 24: Organic Chemistry I CHM 201

Common Bonding Patterns

Chapter 1 24

Page 25: Organic Chemistry I CHM 201

Work enough problems tobecome familiar with thesebonding patterns so you canrecognize other patterns as

being either unusual or wrong.

HINT

Chapter 1 25

Page 26: Organic Chemistry I CHM 201

Electronegativity Trends Ability to Attract the Electrons in a Covalent Bond

Page 27: Organic Chemistry I CHM 201

Dipole Moment

• Dipole moment is defined to be the amount of charge separation (d) multiplied by the bond length (m).

• Charge separation is shown by an electrostatic potential map (EPM), where red indicates a partially negative region and blue indicates a partially positive region.

Chapter 1 27

Page 28: Organic Chemistry I CHM 201

Methanol

Page 29: Organic Chemistry I CHM 201

Dipole Moment (m) is sum of the Bond Moments

Page 30: Organic Chemistry I CHM 201

Nonpolar CompoundsBond Moments Cancel Out

Page 31: Organic Chemistry I CHM 201

Nitromethane

Page 32: Organic Chemistry I CHM 201

Nitromethane has 2 Formal Charges

CH3NO2 C N

O

O

H

HH

Formal Charge = [Group #] - [# bonds] - [# non-bonding electrons]

N = 5-4-0 = +1

O = 6-1-6 = -1

Page 33: Organic Chemistry I CHM 201

Both Resonance Structures Contribute to the Actual Structure

CH3NO2

C N

O

O

H

HHH

H

H

N

O

O

C

2 Equivalent Resonance Structures

Page 34: Organic Chemistry I CHM 201

Dipole Moment reflects Both Resonance Structures

C N

O

O

H

HHH

H

H

N

O

O

C

Resonance Hybrid

C

H

HH

O

O

N

d

d

Page 35: Organic Chemistry I CHM 201

Resonance Rules• Cannot break single (sigma) bonds• Only electrons move, not atoms

3 possibilities:– Lone pair of e- to adjacent bond position

• Forms p bond

p bond to adjacent atom p bond to adjacent bond position

Page 36: Organic Chemistry I CHM 201

Curved Arrow Formalism Shows flow of electrons

C N

O

O

H

HHH

H

H

N

O

O

C

Arrows depict electron pairs moving

Page 37: Organic Chemistry I CHM 201

Resonance Forms• The structures of some compounds are not

adequately represented by a single Lewis structure.• Resonance forms are Lewis structures that can be

interconverted by moving electrons only. • The true structure will be a hybrid between the

contributing resonance forms.

Chapter 1 37

Page 38: Organic Chemistry I CHM 201

Resonance Forms

Resonance forms can be compared using the following criteria, beginning with the most important:1. Has as many octets as possible.2. Has as many bonds as possible.3. Has the negative charge on the most

electronegative atom.4. Has as little charge separation as possible.

Chapter 1 38

Page 39: Organic Chemistry I CHM 201

Two Nonequivalent Resonance Structures in Formaldehyde

Page 40: Organic Chemistry I CHM 201

Major and Minor Contributors

• When both resonance forms obey the octet rule, the major contributor is the one with the negative charge on the most electronegative atom.

N C O N C OMAJOR MINOR

The oxygen is more electronegative,so it should have more of the negativecharge.

Chapter 1 40

Page 41: Organic Chemistry I CHM 201

Resonance Stabilization of IonsPos. charge is “delocalized”

CH

HC

C

H

H

H

CC

H

H

H

H

HC

H

HCd

d

H

H

H

CC

resonance hybrid

Page 42: Organic Chemistry I CHM 201

Solved Problem 2

Draw the important resonance forms for [CH3OCH2]+. Indicate which structure is the major and minor contributor or whether they would have the same energy.

The first (minor) structure has a carbon atom with only six electrons around it. The second (major) structure has octets on all atoms and an additional bond.

Solution

Chapter 1 42

Page 43: Organic Chemistry I CHM 201

Solved Problem 3Draw the resonance structures of the compound below. Indicate which structure is the major and minor contributor or whether they would have the same energy.

Both of these structures have octets on oxygen and both carbon atoms, and they have the same number of bonds. The first structure has the negative charge on carbon, the second on oxygen. Oxygen is the more electronegative element, so the second structure is the major contributor.

Solution

Chapter 1 43

Page 44: Organic Chemistry I CHM 201

Resonance Forms for the Acetate Ion

• When acetic acid loses a proton, the resulting acetate ion has a negative charge delocalized over both oxygen atoms.

• Each oxygen atom bears half of the negative charge, and this delocalization stabilizes the ion.

• Each of the carbon–oxygen bonds is halfway between a single bond and a double bond and is said to have a bond order of 1½.

Chapter 1 44

Page 45: Organic Chemistry I CHM 201

Condensed Structural Formulas Lewis Condensed

CH3CH3H C C H

H

H

H

H

1 2

• Condensed forms are written without showing all the individual bonds.

• Atoms bonded to the central atom are listed after the central atom (CH3CH3, not H3CCH3).

• If there are two or more identical groups, parentheses and a subscript may be used to represent them.

Chapter 1 45

Page 46: Organic Chemistry I CHM 201

Drawing Structures

CC

CC

H

H

H

H

HH H

HH

H

Butane, C4H10

CC

C

C

H

H H

HH

H H

HHH

Methylpropane, C4H10

CH3CH2CH2CH3 CH3CH(CH3)CH3

Page 47: Organic Chemistry I CHM 201

Octane Representations

CC

CC

CC

CC

H

HH

H H

H H

H H

H H

H H

HHH

HH

CH3CH2CH2CH2CH2CH2CH2CH3

CH3(CH2)6CH3

Lewis structure condensed structural formula

C8H18 is molecular formula but there are 18 possible structural isomers

Page 48: Organic Chemistry I CHM 201

Line-Angle Structures are Often Used as a Short-hand

Line-angle or "zig-zag" structures

Lewis structure

CC

CC

CC

CC

H

HH

H H

H H

H H

H H

H H

HHH

HH

Page 49: Organic Chemistry I CHM 201

Line-Angle Structures

CC

CC

CC

C

CC

C

CC

HH

H

H

H

H

H HH

H H

H

H

H

H

H

HH

HH

H

H H

HH

H

HH

H

H

H

H

H HH

H H

H

H

H

H

H

HH

HH

H

H H

HH

H

C12H26

Page 50: Organic Chemistry I CHM 201

HH

H

H

H

H

H HH

H H

H

H

H

H

H

HH

HH

H

H H

HH

H

At the end of every line and at the intersection of any lines there is a carbon atom with 4 bonds. Hydrogen atoms

are mentally supplied to fill the valency to 4.

Page 51: Organic Chemistry I CHM 201

Line-Angle structure Superimposed on Lewis Structure

CC

CC

CC

C

CC

C

CC

HH

H

H

H

H

H HH

H H

H

H

H

H

H

HH

HH

H

H H

HH

H

Page 52: Organic Chemistry I CHM 201

Line-Angle Drawings

H C C C

H

H

H

H

C

H

C

H

H

H H

H

C

O

H

O

H

1 2 3 4 5 61 2 3 4 5 6

• Atoms other than carbon must be shown.• Double and triple bonds must also be shown.

Chapter 1 52

Page 53: Organic Chemistry I CHM 201

For Cyclic Structures, Draw the Corresponding Polygon

Cyclohexane

CC

CC

C

C

HHH

HH

HHHH

HH

H

Page 54: Organic Chemistry I CHM 201

Some Steroids

HOCholesterol

C27H44O

OH

OTestosterone

C19H26O2

Page 55: Organic Chemistry I CHM 201

Definitions of Acids/Bases

Arrhenius acid - forms H3O+ in H2O

Bronsted-Lowry acid - donates a H+ (proton)

Lewis acid - accepts an electron pair to form a new bond

Arrhenius base - forms OH- in H2O

Bronsted-Lowry base - accepts a H+ (proton)

Lewis base - donates an electron pair to form a new bond

Page 56: Organic Chemistry I CHM 201

Dissociation in H2OArrhenius Acid forms H3O+

Bronsted-Lowry Acid donates a H+

Page 57: Organic Chemistry I CHM 201

Brønsted-Lowry Acids and BasesBrønsted-Lowry acids are any species that donate a proton.

Brønsted-Lowry bases are any species that can accept a proton.

Chapter 1 57

Page 58: Organic Chemistry I CHM 201

Conjugate Acids and Bases

• Conjugate acid: when a base accepts a proton, it becomes an acid capable of returning that proton.

• Conjugate base: when an acid donates its proton, it becomes capable of accepting that proton back.

Chapter 1 58

Page 59: Organic Chemistry I CHM 201

Acid Strength defined by pKa

HCl + H2O H3O + Cl

Keq = [H3O ][Cl ][HCl][H2O]

Ka = Keq[H2O] =[H3O ][Cl ]

[HCl]= 107

pKa = -log(Ka) = -7

Page 60: Organic Chemistry I CHM 201

Stronger Acid Controls Equilibrium

HCl + H2O H3O + Clacid base conjugate conjugate

acid basepKa = -7 -1.7

stronger weaker

Page 61: Organic Chemistry I CHM 201

Reaction Described with Arrows

H Cl + OHH O

H

HH+ Cl

Page 62: Organic Chemistry I CHM 201

Equilibrium Reactions

Page 63: Organic Chemistry I CHM 201

Identify the Acid and Base

CH3OH + OCOH

O

CH3O + HOCOH

O

Page 64: Organic Chemistry I CHM 201

Equilibrium Favors Reactants

CH3OH + OCOH

O

CH3O + HOCOH

O

acid base conj. base conj. acid

pKa 15.5 6.5

Page 65: Organic Chemistry I CHM 201

The Effect of Resonance on pKa

Page 66: Organic Chemistry I CHM 201

Effect of Electronegativity on pKa

• As the bond to H becomes more polarized, H becomes more positive and the bond is easier to break.

Chapter 1 66

Page 67: Organic Chemistry I CHM 201

Effect of Size on pKa

• As size increases, the H is more loosely held and the bond is easier to break.

• A larger size also stabilizes the anion.

Chapter 1 67

Page 68: Organic Chemistry I CHM 201

Lewis Acids and Lewis Bases

• Lewis bases are species with available electrons than can be donated to form a new bond.

• Lewis acids are species that can accept these electrons to form new bonds.

• Since a Lewis acid accepts a pair of electrons, it is called an electrophile.

Chapter 1 68

Page 69: Organic Chemistry I CHM 201

Nucleophiles and Electrophiles

• Nucleophile: Donates electrons to a nucleus with an empty orbital (same as Lewis Base)

• Electrophile: Accepts a pair of electrons (same as Lewis Acid)

• When forming a bond, the nucleophile attacks the electrophile, so the arrow goes from negative to positive.

• When breaking a bond, the more electronegative atom receives the electrons.

Chapter 1 69

Page 70: Organic Chemistry I CHM 201

Nucleophiles and Electrophiles

Chapter 1 70