Chapter 2 Molecular Representations: Bond-line structures Resonance 2-1 Representing Molecules There are many ways to represent molecules For a large molecule with 20 or more atoms, to draw a structure that gives you the most information about the structure, we need a more concise and efficient way.
2-2 Representing Molecules Antibiotic Amoxicillin: Lewis structure vs. Line structure 2-3 Bond-line structure for Organic Rxn Bond-line structures clearly show how chemical reaction has changed a molecule Condensed formula vs. Bond-line structure:
2-4 Bond-line Structures Hydrogen atoms on Carbon are omitted Carbon atoms are not labelled. Atoms are bonded at angles (zigzag) that represent the actual geometry of the bond angles Bond angle for sp3 hybridized carbon= ___________ Carbon atoms are at every corner or endpoint on the zigzag. 2-5
Bond angles for Double/Triple Bonds Double bonds (sp2 or sp) and triple bonds (sp) are represented as you might expect C C Why is a triple bond written without zigzagging? correct 2-6
incorrect C H atoms in Bond-line Structures Assume there are enough to complete the octet (4 bonds) for each carbon #H atoms = 4 (#Bonds shown in the BL structure) H atom locations interpreted
2-7 Practice: How many H atoms? 3 0 0 1
2 HS Cl 2-8 Draw Bond-line Structures Given Lewis structure or condensed structure, draw the corresponding bond-line structure Represent the bond angles (VSEPR) with zigzags Follow VSEPR and spread out the electron pairs on a
central atom 2-9 Bond-line Structures Single bonds are axes of rotation, so be aware that they can rotate Practice: Give alternative bond-line structures for the molecule below. Hint: How many bonds can rotate? 2-10
Bond-line Structures Heteroatoms (atoms other than C and H) should be labeled with all hydrogen atoms and lone pairs attached NEVER draw a carbon with more than 4 bonds!! Practice with SkillBuilder 2.3 2-11 Practice: Bond-line Structures Draw bond-line representations for the following Lewis structures
H H H H Cl C C C C C C C H H H H H H HH C H C C C H H HH
H C C H HH HH H C C H C C C H H O H H
2-12 What are the Functional Groups When certain atoms are bonded together in specific arrangements, they undergo specific chemical reactions Different from Polyatomic Groups (normally do NOT change) 2-13
Functional Groups More functional groups are listed in table 2.1 2-14 Practice: Formal Charge in Line Structures H O O S 1
2 O 3 2 N N 1
2-15 +1; 0; 0; 0; -1; +1 Carbocation (C+): sp2 hybridization Carbocation: Sometimes carbon will have a +1 charge. In such cases, the carbon will only have 3 bonds. CARBOCATIONS are generally unstable. 2-16 Carbanion (C-) : sp3 hybridization
Sometimes carbon will have a -1 charge, named as CARBANIONS. In such cases, the carbon will only have 3 bonds, plus a lone pair electrons. CARBANIONS are generally unstable. Either carbocation or carbanion need to be shown in the bond-line structure 2-17 O Formal charges need to be shown in the Bond-line Structures
If lone pairs are omitted from bond-line structures. For example (not sufficient) The formal charge on the N atom depends on how many electrons there are on the N It could be N 2-18 Find Lone Pair Electrons If the formal charge is indicated on an atom, you can
determine how many lone pairs are present To calculate the number of lone pair electrons (#LPE) for an atom, compare the number of valence electrons (#VE) of the atom to the number of valence electrons that are actually associated with an atom (section 1.4) Based on OCTET rule Remember Formal charge (FC) = #VE ( #BE + #LPE) #LPE = #VE FC #BE 2-19 Practice: Final Lone Pair Electrons How many lone pairs are on the oxygen atom below?
Oxygen should have #VE = 2-20 More on Formal Charge: Oxygen FCO = 6 - #BE/2 - #LPE Determine the formal charge on an O atom by matching its bonding pattern with its formal charge 2-21 Formal charge on Nitrogen FCO = 5 - #BE/2 - #LPE
The formal charge on a N atoms can be calculated the same way or by matching its bonding pattern with its formal charge in the table below 2-22 3D Bond-line Structures How to show 3D object without 3D goggles in OChem? Use dashed and solid wedges to show groups that point back into the paper or out of the paper 2-23
More 3D Bond-line Structures Here are some other ways to show 3D structure O OH H OH H
OH HO OH OH 2-24 Resonance Drawing lines between atoms inadequately represents covalent bonds in molecules with resonance Resonance:
Consider the allyl carbocation: 2-25 Resonance: Hybridization of Orbitals Valence bond theory on the bonding in the allyl carbocation Calculate the steric number (# of bonds + lone pairs) When the steric number is 3, it is sp2 hybridized. The 3 sp2 hybrid orbitals forms single bonds with carbon and hydrogen
2-26 Resonance If all of the carbons have unhybridized p orbitals, they can overlap All three overlapping p orbitals allow the electrons to move throughout the overlapping area simultaneously Thats RESONANCE 2-27 Formal Charge in Resonance 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 2-28 Resonance Hybrid The average or hybrid is much more appropriate vs. +
+ resonance hybrid two contributors + means partial positive charge. 2-29 Resonance Resonance makes a molecule MORE stable Delocalization of electrons Electrons exist in orbitals that span a greater distance giving the electrons more freedom minimizing repulsions
Electrons spend time close to multiple nuclei all at once maximizing attractions Delocalization of charge The charge is spread out over more than one atom. The resulting partial charges are more stable than a full +1 charge. + + resonance hybrid 2-30
Curved Arrows Curved arrows to show electron pair movement The arrow starts where the electrons are currently located The arrow ends where the electrons will end up after the electron movement From left to right, resonance structure can be formed using curved arrow 2-31
Change in Formal Charge in Resonance: Lone Pair With the arrow moving away, Lone pair electrons migrate away to add bond, leaving the original atom have more positive charge O O 2-32
Change in Formal Charge in Resonance: Multiple Bond With the arrow moving away, Bonding pair electrons (from double or triple bond) migrate away to reduce the original bond, leaving the atom that loses an electron have more positive charge 2-33 Change in Formal Charge in Resonance: At the End Where the arrow terminally lands, Arrow landing on an atom will lead to more negative charge on
the end atom. Make sure to show the lone pair! Arrow landing on a single bond will lead to double bond O O 2-34 Curved Arrows in Resonance Rules for using curved arrows to show RESONANCE 1. Avoid breaking a single bond Single bonds can break, but NOT in RESONANCE
Resonance occurs for electrons existing in overlapping p orbitals, while electrons in single bonds are overlapping sp, sp2, or sp3 (sigma) orbitals. 2-35 Curved Arrows in Resonance Rules for using curved arrows to show RESONANCE 2. Never exceed an octet for 2nd row elements (B, C, N, O, F) Atoms in the 2nd row can only have four 2nd energy level orbitals holding a max. of 8 electrons Examples of arrows that violate rule 2.
How does this arrow look? 2-36 Curved Arrows in Resonance Rules for using curved arrows to show RESONANCE 3. 2nd row elements (B, C, N, O, F) will rarely but sometimes have LESS than an octet O - The resonance hybrid
+ 2-37 Formal Charge in Resonance When using curved arrows to show RESONANCE, often structures will carry a formal charge that must be shown Draw the resonance contributor indicated by the arrows below Show any formal charges on the contributors 2-38
Formal Charge in Resonance In the resonance, the arrows tell us how to move the electrons to create the other contributor Draw arrows showing the resonance in the reverse direction arrows as showing the direction that charge will flow 2-39 Patterns in Resonance There are 5 main bonding patterns in which resonance
occurs. Recognize these patterns to predict when resonance will occur X Allylic lone pairs X Allylic positive charge Lone pair of electrons adjacent to a positive charge A pi bond between two atoms with different electronegativities X Y 5. Conjugated pi bonds in a ring
1. 2. 3. 4. X 2-40 Y Y
X Vinyl vs. Allyl 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 Cl Cl Cl Cl
Cl 2-41 Resonance for Allylic Lone Pair 1. Identifying allylic lone pairs Circle all of the allylic lone pairs Draw arrows on each structure to show resonance
2-42 Allylic Lone Pair 1. Identifying allylic lone pairs For each, show the resulting resonance contributor and all formal charges 2-43 Allylic cation 2. Dealing with allylic positive charge Only 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 Draw a resonance hybrid Practice with conceptual checkpoint 2.26
2-44 LP X + 3. A lone pair adjacent to a positive charge: Example A: Only one arrow is involved 2-45 LP X
+ 3. If ONE arrow results in violation of octet rule: Example B: NITRO group (-NO2) 2-46 X Y X
Y 4. A pi bond between atoms of different electronegativity The pi electrons will be more attracted to the more electronegative atom Formal charges are created by the electron movement in the following examples 2-47 Conjugated bonds in a ring
Each atom in the ring MUST have an unhybridized p orbital that can overlap with its neighbors Electrons can be shown to move clockwise or counterclockwise 2-48 Practice: Patterns in Resonance 2-49
Practice: Patterns in Resonance Find all of the resonance contributors. All lone pairs need to be shown when drawing resonance structures. N N O 2-50 How to Draw Resonance Hybrid Find ALL significant resonance contributors
For each bond that changes order in resonance contributors, add - - - above the lowest bond type, such as or For each atom that changes formal charge, add + or - ) based on the most frequent charge it has among the contributors. 2-51
Stability of Contributors Similarly, not all resonance structures contribute equally for a specific molecule The resonance hybrid will most resemble the more stable contributor(s) General Rules in the stability of resonance contributors? 1. Formal charge generally DECREASES stability, especially a +1 charge on an electronegative atom or -1 on a low electronegativity atom. Example:
O 2. COMPLETE OCTETS INCREASE stability H 2-52 Stability of Contributors 1. 2. 3. Formal charge generally DECREASES stability + on an electronegative atom or - on a low electronegativity atom are
unstable COMPLETE OCTETS INCREASE stability Draw and Assess resonance contributors for CH3CONH2 2-53 More on Stability of Contributors 1. 2. 3.
Formal charge generally DECREASES stability A +1 charge on an electronegative atom or -1 on a low electronegativity atom is especially unstable COMPLETE OCTETS INCREASE stability Draw resonance contributors and resonance hybrid for the acetate ion 2-54 Octet rule > Formal Charge
The octet rule is usually a bigger factor than formal charge when assessing stability For each structure, assess the stability of each contributor, and draw a resonance hybrid H O N 2-55
Practice with SkillBuilder 2.8 Delocalization of electrons Localized electrons are NOT in resonance Delocalized electrons ARE in resonance
Delocalization increases stability Ways for electrons to be delocalized through resonance 1. To be delocalized, electrons must exist in an unhybridized p orbital that can overlap with p orbitals on neighboring atoms 2. To be delocalized, electrons must be on an sp or sp2 hybridized atom 2-56 Delocalized vs. Localized
Resonance may require change in hybridization state (such as sp3 hybridization sp2 hybridization) The hybridization for N atom of an amide = If the N atom were sp3, its lone pair of electrons couldnt engage in resonance
The N atom of an amide will be sp2 hybridized In this case, the stability of the delocalization outweighs the stability difference between sp2 and sp3 sp2 generally less stable than sp3 ? 2-57 Delocalized vs. Localized
The sp2 hybridization of the N atom causes it to be trigonal planar rather than tetrahedral To be delocalized, all three atoms involved MUST have p orbitals overlapping Experimental Data: CNR + RNR + CNR = ~360 (sp2 hybridization) (327 for pure sp3)
CNR 2-58 Lone pair may not ALWAYS be Delocalized Generally, lone pairs adjacent to a C=C double bond are capable of resonance, but not in this case.
The electron movement above does not violate any of our rules, but why not possible? 2-59 Localized Lone Pair Recall that delocalized electrons must exist in an unhybridized p orbital overlapping with p orbitals on neighboring atoms
The Nitrogens lone pair is positioned perpendicular to the plane where the other electrons reside 2-60 Molecular Orbital Theory on Resonance When 3 unhybridized p orbitals overlap, three MOs are formed:
Bonding; Nonbonding; Antibonding #electrons in each of the MOs: Bonding: 2 Nonbonding: 1 Antibonding: 0 2-61 Formal charge in Resonance The allyl carbocation (CH2=CHCH2+) has a charge of +1, total of TWO pi electrons
Where is the positive charge located? Occupancy of each MO: Bonding: 2 Nonbonding: 0 Antibonding: 0 2-62
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