Organic Chemistry Second Edition - PowerPoint Presentation

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Organic Chemistry Second Edition - PPT Presentation

Chapter 2 Molecular Representations David Klein Copyright 2015 John Wiley amp Sons Inc All rights reserved Klein Organic Chemistry 2e 21 Representing Molecules There are many ways to represent molecules ID: 757312

organic chemistry wiley amp chemistry organic amp wiley reserved rights sons 2015 john copyright bond resonance line structures klein electrons formal charge

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Slide1

Organic Chemistry

Second Edition

Chapter 2Molecular Representations

David Klein

Klein, Organic Chemistry 2e Slide2

2.1 Representing Molecules

There are many ways to represent moleculesIf you were representing a large molecule with 20 or more atoms, which structure would be most time consuming to draw?Which structures give you the most information about the structure?

2Klein, Organic Chemistry 2e Slide3

2.1 Representing Molecules

Given that there are three isomers of propanol (below), which structures above are adequate to represent only isopropanol and not its isomers?

2-3

Klein, Organic Chemistry 2e Slide4

2.1 Representing Molecules

To draw large molecules quickly, a different type of representation is neededConsider the antibiotic Amoxicillin. Its Lewis structure looks cluttered, and it would be very time consuming to draw

2.2 Bond-line Structures

2-5Klein, Organic Chemistry 2e Slide6

2.2 Bond-line Structures

It may seem like a foreign language at first, because many of the atoms are not labeledThis type of representation is THE main way that chemists communicate, so it is a language you MUST master to be successful in organic chemistry

2.2 Bond-line Structures

Like Lewis structures, lines are drawn between atoms to show covalent bonds Atoms are bonded at angles (zigzag) that represent the actual geometry of the bond anglesCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

2-7Klein, Organic Chemistry 2e Slide8

2.2 Bond-line Structures

Like Lewis structures, lines are drawn between atoms to show covalent bondsCarbon-Hydrogen bonds are omitted. WHY?If the H atoms are omitted, how will we know how many H atoms are attached to a carbon?Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

2-8Klein, Organic Chemistry 2e Slide9

2.2 Bond-line Structures

Practice identifying the location of and counting the number of carbon and hydrogen atoms in the structures below

9Klein, Organic Chemistry 2e Slide10

2.2 Bond-line Structures

2-10Klein, Organic Chemistry 2e Slide11

2.2 Bond-line Structures

You should practice bond-line structures until it becomes natural for you to see all of the carbon and hydrogen atom locations. What’s the molecular formula of the following molecule?

Klein, Organic Chemistry 2e Slide12

2.2 Bond-line Structures

If you are given a Lewis structure or condensed structure, you must also be able to draw the corresponding bond-line structureRepresent the bond angles with zigzagsFollow VSEPR and spread out the electron pairs on a central atom

12Klein, Organic Chemistry 2e Slide13

2.2 Bond-line Structures

Single bonds are axes of rotation, so be aware that they can rotateGive alternative bond-line structures for the molecule below

Klein, Organic Chemistry 2e Slide14

2.2 Bond-line Structures

Heteroatoms (atoms other than C and H) should be labeled with all hydrogen atoms and lone pairs attachedNEVER draw a carbon with more than 4 bonds!!

2-14Klein, Organic Chemistry 2e Slide15

2.2 Bond-line Structures

Draw bond-line representations for the following Lewis structures

Klein, Organic Chemistry 2e Slide16
Slide17

2.2 Bond-line Structures

Draw bond-line representations for 3 possible isomers given the formula: C5H9ClODraw bond-line structures for 3 different rotational conformations for the molecule: CH2CH(CH2)4CH3

Klein, Organic Chemistry 2e Slide18

2.3 Indentifying Functional Groups

Bond-line structures allow chemists to quickly examine how a chemical reaction has changed a moleculeCompare the condensed formula with the bond-line structure below for the same reactionWhich representation makes it more apparent that the H2 is reacting to convert the double bond to a single bond?

2-18Klein, Organic Chemistry 2e Slide19

2.3 Indentifying Functional Groups

When certain atoms are bonded together in specific arrangements, they undergo specific chemical reactions

2-19Klein, Organic Chemistry 2e Slide20

2.3 Indentifying Functional Groups

2-21Klein, Organic Chemistry 2e Slide22

2.4 Bond-line Structures with Formal Charge

Formal charge (section 1.4) affects the stability and reactivity of molecules, so you must be able to identify formal charges in bond-line representationsLabel all of the formal charges in the following moleculePractice with conceptual checkpoints 2.12 and 2.13

Klein, Organic Chemistry 2e Slide23

2-23Klein, Organic Chemistry 2e Slide24

2.4 Bond-line Structures with Formal Charge

Most carbon atoms will have 4 covalent bonds and no lone pairs to avoid carrying a formal chargeSometimes carbon will have a +1 charge. In such cases, the carbon will only have 3 bonds.

Klein, Organic Chemistry 2e Slide25

2.4 Bond-line Structures with Formal Charge

Most carbon atoms will have 4 covalent bonds and no lone pairs to avoid carrying a formal chargeSometimes carbon will have a -1 charge. If carbon carries a charge in a molecule, the charge MUST be shown on the bond-line structure

25Klein, Organic Chemistry 2e Slide26

2.5 Bond-line Structures and Lone Pair Electrons

Sometimes lone pairs are omitted from bond-line structures. For example…You can’t determine the formal charge on the N atom unless you know how many electrons there are on the NIt could be…You must ALWAYS draw formal charges on a bond-line structure to eliminate confusion

Klein, Organic Chemistry 2e Slide27

2.5 Bond-line Structures and Lone Pair Electrons

If the formal charge is indicated on an atom, you can determine how many lone pairs are presentTo calculate the number of lone pair electrons for an atom, compare the number of valence electrons that should be associated with the atom to the number of valence electrons that are actually associated with an atom (section 1.4)

2.5 Bond-line Structures and Lone Pair Electrons

How many lone pairs are on the oxygen atom below? Oxygen should have 6 valence e-s assigned to it, because it is in group VIA on the periodic table.HOW many lone pairs should it have?

2-28Klein, Organic Chemistry 2e Slide29

2.5 Bond-line Structures and Lone Pair Electrons

You can also determine the formal charge on an O atom by matching its bonding pattern with its formal charge according to table 2.2Practice with SkillBuilder 2.4

29Klein, Organic Chemistry 2e Slide30

2-30Klein, Organic Chemistry 2e Slide31

2.5 Bond-line Structures and Lone Pair Electrons

The formal charge on a N atoms can be calculated the same way or by matching its bonding pattern with its formal charge according to table 2.3Practice with SkillBuilder 2.5

Klein, Organic Chemistry 2e Slide32

2-32Klein, Organic Chemistry 2e Slide33

2.6 3D Bond-line Structures

The vast majority of molecules are 3-dimensional, but it is difficult to represent a 3D molecule on a 2D piece of paper or blackboardWe will use dashed and solid wedges to show groups that point back into the paper or out of the paper

2-33

Klein, Organic Chemistry 2e Slide34

2.6 3D Bond-line Structures

Here are some other ways to show 3D structure

2-34Klein, Organic Chemistry 2e Slide35

2.7 Resonance

Drawing lines between atoms inadequately represents covalent bonds in molecules with resonanceRemember from General Chemistry, what is resonance?Consider the allyl carbocation:How is the bond-line structure inadequate in representing the allyl carbocation’s TRUE structure?

35Klein, Organic Chemistry 2e Slide36

2.7 Resonance

Let’s look at the hybridization of the carbons in the allyl carbocationCalculate the steric number (# of σ bonds + lone pairs)When the steric number is 3, it is sp

2 hybridized

2-36

Klein, Organic Chemistry 2e Slide37

2.7 Resonance

If all of the carbons have unhybridized p orbitals, they can overlapAll three overlapping

p orbitals allow the electrons to move throughout the overlapping area simultaneouslyThat’s RESONANCE

2-37Klein, Organic Chemistry 2e Slide38

2.7 Resonance

How do we represent the complete picture of the allyl carbocation provided by valence orbital and MO theories using a bond-line structure?The pi electrons can exist on both sides of the molecule, so we can use two resonance contributors to represent the structure

The brackets indicate that both resonance contributors exist simultaneously

2-38

Klein, Organic Chemistry 2e Slide39

2.7 Resonance

Resonance makes a molecule MORE stableDelocalization of electronsElectrons exist in orbitals that span a greater distance giving the electrons more freedom minimizing repulsionsElectrons spend time close to multiple nuclei all at once maximizing attractionsDelocalization of chargeThe charge is spread out over more than one atom. The resulting partial charges are more stable than a full +1 charge.

δ+ δ+ resonance hybrid

Klein, Organic Chemistry 2e Slide40

2.8 Curved Arrows in Resonance

Throughout Organic Chemistry, we will be using curved arrows to show electron movementCurved arrows generally show electron movement for pairs of electronsThe arrow starts where the electrons are currently locatedThe arrow ends where the electrons will end up after the electron movementWe will explore curved arrows to show other reactions in Chapter 3

Klein, Organic Chemistry 2e Slide41

2.8 Curved Arrows in Resonance

Rules for using curved arrows to show RESONANCEAvoid breaking a single bond

Resonance occurs for electrons existing in overlapping p orbitals, while electrons in single bonds are overlapping sp, sp2, or sp3 (sigma) orbitals.

Klein, Organic Chemistry 2e Slide42

2.8 Curved Arrows in Resonance

Rules for using curved arrows to show RESONANCENever exceed an octet for 2nd row elements (B, C, N, O, F)Atoms in the 2nd row can only have four 2

nd energy level orbitals holding a max. of 8 electronsExamples of arrows that violate rule 2.

Klein, Organic Chemistry 2e Slide43

2.8 Curved Arrows in Resonance

Rules for using curved arrows to show RESONANCE2nd row elements (B, C, N, O, F) will rarely but sometimes have LESS than an octet

43Klein, Organic Chemistry 2e Slide44

2-44Klein, Organic Chemistry 2e Slide45

2.9 Formal Charge in Resonance

When using curved arrows to show RESONANCE, often structures will carry a formal charge that must be shownDraw the resonance contributor indicated by the arrows belowAre any of the rules violated?Show any formal charges on the contributors

Klein, Organic Chemistry 2e Slide46

2.9 Formal Charge in Resonance

In the resonance, the arrows tell us how to move the electrons to create the other contributorDraw arrows showing the resonance in the reverse directionYou can also think of the arrows as showing the direction that charge will flowPractice with SkillBuilder 2.7

46Klein, Organic Chemistry 2e Slide47

2.10 Patterns in Resonance

There are 5 main bonding patterns in which resonance occurs. Recognize these patterns to predict when resonance will occurAllylic lone pairsAllylic positive chargeLone pair of electrons adjacent to a positive charge

A pi bond between two atoms with different electronegativitiesConjugated pi bonds in a ringWe will see many examples in the next few slides

Klein, Organic Chemistry 2e Slide48

2.10 Patterns in Resonance

Vinyl and allyl refer to positions directly bonded to or one atom away from a C=C double bond

Label the vinylic chlorides and the allylic chlorides

Klein, Organic Chemistry 2e Slide49

2.10 Patterns in Resonance

Identifying allylic lone pairs

Circle all of the allylic lone pairs

Draw arrows on each structure to show resonance

2-49Klein, Organic Chemistry 2e Slide50

2.10 Patterns in Resonance

Identifying allylic lone pairsFor each, show the resulting resonance contributor and all formal charges

Klein, Organic Chemistry 2e Slide51

2-51Klein, Organic Chemistry 2e Slide52

2-52Klein, Organic Chemistry 2e Slide53

2.10 Patterns in Resonance

Dealing with allylic positive chargeOnly one curved arrow is needed

If there are multiple double bonds (conjugated), then multiple contributors are possible. Show the resonance contributors and curved arrows below

2-53

Klein, Organic Chemistry 2e Slide54

2-54Klein, Organic Chemistry 2e Slide55

2.10 Patterns in Resonance

A lone pair adjacent to a positive chargeOnly one arrow is neededExplain how the formal charges are affected by the electron movement in the following examples

2-55Klein, Organic Chemistry 2e Slide56

2.10 Patterns in Resonance

A lone pair adjacent to a positive chargeConsider the resonance in the NITRO groupDraw all possible resonance contributors

Klein, Organic Chemistry 2e Slide57

2-57Klein, Organic Chemistry 2e Slide58

2.10 Patterns in Resonance

A pi bond between atoms of different electronegativityThe pi electrons will be more attracted to the more electronegative atomExplain how the formal charges are created by the electron movement in the following examples

2-58

Klein, Organic Chemistry 2e Slide59

2-59Klein, Organic Chemistry 2e Slide60

2.10 Patterns in Resonance

Conjugated pi bonds in a ringEach atom in the ring MUST have an unhybridized p orbital that can overlap with its neighbors

2-60Klein, Organic Chemistry 2e Slide61

2.10 Patterns in Resonance

Summary figure 2.5

2-61

Klein, Organic Chemistry 2e Slide62

2-62Klein, Organic Chemistry 2e Slide63

2-63Klein, Organic Chemistry 2e Slide64

2-64Klein, Organic Chemistry 2e Slide65