2.1 Representing Molecules. 2.1 Representing Molecules. 2.2 Bond-line Structures. Chapter 2 Molecular Representations

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1 2.1 Representing Molecules Which representations are adequate to represent only isopropanol and not its constitutional isomers? Chapter 2 Molecular Representations Copyright 2014 by John Wiley & Sons, Inc Representing Molecules The antibiotic amoxicillin is a moderately complex molecule. Its Lewis structure looks cluttered, and it would be very time consuming to draw. 2.2 Bond-line Structures The bond-line structure is easier to read and to draw: To draw large molecules quickly, a different type of representation is needed. 3 4

2 2.2 Bond-line Structures Like Lewis structures, lines are drawn between atoms to show covalent bonds: 2.2 Bond-line Structures Double bonds and triple bonds are represented as you might expect: Atoms are bonded at angles (zigzag) that represent the actual bond angles. What is the bond angle for sp 3 hybridized carbon? Remember: 3D structures on 2D surface; branching. correct incorrect atoms are not shown, but we can assume there are enough to complete the octet (four bonds) for each carbon: Correct Incorrect Carbon atoms are not labeled, but a carbon is assumed to be located at every corner or endpoint on the zigzag. 5 6 Skillbuilder 2.2 Cl Skillbuilder 2.2 Cl Draw the complete Lewis structure of Diazepam N Draw the complete Lewis structure of Diazepam N N N 7 8

3 2.2 Bond-line Structures 2.2 Bond-line Structures eteroatoms (atoms other than C and ) should be labeled with all hydrogen atoms explicitly drawn. Single bonds can freely rotate: Respect the actual geometry: Draw alternative bond-line structures for: Correct Really Bad 9 10 Practice Problems Draw a bond-line structure of the following molecule: C C C C C C C C C C C Practice Problems For each of the following molecules, determine the number of carbon atoms present and then determine the number of hydrogen atoms connected to each carbon atom: Determine whether each transformation involves an increase, decrease, or no change in the number of atoms. Draw bond-line structures of all the constitutional isomers of composition C

4 2.3 Identifying Functional Groups 2.3 Identifying Functional Groups Bond-line structures allow chemists to quickly examine how a chemical reaction has changed a molecule. Bond-line structures allow chemists to quickly examine how a chemical reaction has changed a molecule. Compare the condensed formula with the bond-line structure below for the same reaction Identifying Functional Groups Functional groups - groups of atoms and bonds with predictable chemical behavior. Anything more complex than a saturated hydrocarbon is considered a functional group. 2.3 Identifying Functional Groups Functional groups - groups of atoms and bonds with predictable chemical behavior. Anything more complex than a saturated hydrocarbon is considered a functional group. Ester Carboxylic acid Amide Amine Aromatic ring 15 16

2.3 Identifying Functional Groups 2.3 Identifying Functional Groups 17 18 2.

Anything more complex than a saturated hydrocarbon is considered a functional group. 2.

5 2.3 Identifying Functional Groups 2.3 Identifying Functional Groups Identifying Functional Groups Functional groups - groups of atoms and bonds with predictable chemical behavior. Anything more complex than a saturated hydrocarbon is considered a functional group. 2.4 Formal Charges An atom that does not exhibit the appropriate number of valence electrons has a formal charge (Section 1.4) Are there any formal charges on these molecules? Ester Carboxylic acid Amide Amine Aromatic ring Conceptual checkpoints 2.12 and

5 Bond-line Structures and Lone Pairs 21 Isoelectronic Species of the Second Period 2.

6 2.4 Carbons With a Formal Charge A carbon atom with no formal charge will have four bonds Carbons with a formal charge (+/-) will have only three bonds. 2.5 Bond-line Structures and Lone Pairs xygen: C -e- C C Carbocation Nitrogen: C +e- C C Carboanion 2.5 Bond-line Structures and Lone Pairs 21 Isoelectronic Species of the Second Period 2.5 Bond-line Structures and Lone Pairs Formal charges and lone pair: you need one to determine the other. 22 Trivalent 3e - Tetravalent 4e - Trivalent 5e - Divalent Monovalent 6e - 7e - Zerovalent 8e - B B 3 B B 4 C C 3 C C 4 C C 3 N N 4 N N 3 N N Ambiguous usually means Wrong Always draw the formal charges; lone pairs only when needed. F 2 F F F F F Ne Skillbuilders 2.4 and

7 Practice Problems Complete the following structures with all lone pairs or formal charges as appropriate D Bond-line Structures It is difficult to represent a 3D molecule on a 2D piece of paper. Build: Cl Cl Cl Cl Cl Cl N 2 N N N N N D Bond-line Structures It is difficult to represent a 3D molecule on a 2D piece of paper. The thin lines are flat in the plane of the paper. ashed wedges (hash) show groups pointing away from you, into the plane of the paper. Solid wedges (wedge) show groups that point at you, out of the plane of the paper D Bond-line Structures It is difficult to represent a 3D molecule on a 2D piece of paper. The thin lines are flat in the plane of the paper. ashed wedges (hash) show groups pointing away from you, into the plane of the paper. Solid wedges (wedge) show groups that point at you, out of the plane of the paper. ash - pointing away from you Wedge - pointing towards you 27 28

2 29 30 2.7 Resonance In some other cases, a localized representation is utterly inadequate. 2.7 Resonance The allyl carbocation is both structures at the same time.

8 2.6 3D Bond-line Structures There are many different ways to show 3D structures: 2.7 Resonance Valence Bond (VB) theory and bond-line structures imply that bonding electrons are localized between two atoms. In some cases, this is an acceptable assumption. Medically Speaking Ch Resonance In some other cases, a localized representation is utterly inadequate. 2.7 Resonance The allyl carbocation is both structures at the same time. AND Allyl carbocation To represent this, VB theory invented the concept of resonance. Resonance contributors are linked by a double-headed arrow and enclosed in square brackets. The real structure is a resonance hybrid of the contributors. The pure p orbitals on each carbon are continuously overlapping, allowing the two π electrons to be associated with all three carbons

9 2.7 Resonance In the allyl carbocation, the π electrons are delocalized over the whole structure and the positive charge is equally distributed between the end carbons. This resonance delocalization increase the stability of the molecule as a whole. This is called resonance stabilization. Although a single representation is possible, the molecule is usually represented as the complete resonance hybrid. 2.7 Resonance Stabilization Resonance is stabilizing because of delocalization of charge: The charge is spread out over more than one atom. The resulting partial charges are more stable than a full localized charge. Resonance is stabilizing because of delocalization of orbitals: The more extended the orbital, the lower the energy of the electrons that occupy it. The more resonance contributors a structure has, the more stable it is. δ + δ + δ + δ + Resonance is not a physical phenomenon, it is a conceptual tool to explain chemical behavior. Resonance is not a physical phenomenon, it is a conceptual tool to explain chemical behavior Curved Arrows in Resonance rganic chemistry uses curved arrows as tools to construct structures with different electron distribution. Curved arrows will be used to construct: resonance contributors (ere) reaction mechanisms (Chapter 3) Curved arrows have a head and a tail: The arrow starts where the electrons are currently located (tail). The arrow ends where the electrons will be in the next structure (head). 2.8 Curved Arrows in Resonance Rules for using curved arrows to show resonance: 1. Avoid breaking a single bond 2. Never exceed an octet for second-period elements Tail (electrons start) ead (electrons end up) Bad arrow 35 36

10 2.8 Curved Arrows in Resonance Rules for using curved arrows to show resonance: Never exceed an octet for second-row elements (B, C, N, ). owever, they can sometimes have less than an octet. Drawing Resonance Arrows (in practice) Electrons flow from an electron source to an electron sink trough a conjugated system. Electron Source (start) Electron Flow Electron Sink (finish) Drawing Resonance Arrows (in practice) Electrons flow from an electron source to an electron sink trough a conjugated system. Drawing Resonance Arrows (in practice) Electrons flow from an electron source to an electron sink trough a conjugated system. Good electron sources (start): lone pairs Conjugated systems have alternating π bonds through which electrons can flow: R 2 N Good electron sinks (finish): electronegative atoms Electrons can stop on the way, and a π bond becomes a lone pair on an atom: N 39 40

11 2.9 Formal Charge in Resonance Resonance structures will often carry a formal charge that must be shown. Draw the resonance contributor indicated by the arrows below. Are any of the rules violated? Show any formal charges on the contributors. 2.9 Formal Charge in Resonance Skillbuilder (Unnecessary) Klein uses a pattern recognition system to draw resonance structures; there is no need to memorize these patterns, just be able to draw resonance structures in each case (there are good examples to practice with in this section). Allylic lone pairs Allylic positive charge Lone pair of electrons adjacent to a positive charge A π bond between two atoms with different electronegativities Conjugated π bonds in a ring Vinylic and allylic refer to positions directly bonded to or one atom away from a C=C double bond. Identify allylic lone pairs and draw the resonance structures: 43 44

12 45 46 Allylic positive charge: nly one curved arrow is needed. A lone pair adjacent to a positive charge: nly one arrow is needed. Explain how the formal charges are affected by the electron movement in the following examples. If there are multiple conjugated double bonds, push them over one at a time

13 Locate the lone pair adjacent to the positive charge and draw the resonance structures A lone pair adjacent to a positive charge: Nitro group. Why can t one arrow be used to cancel out the formal charge and create a resonance contributor? Draw all possible resonance contributors Conceptual Checkpoint A π bond between atoms of different electronegativity Conjugated π bonds in a ring Each atom in the ring has a pure p orbital that overlaps with its neighbors. This kind of electron delocalization around a ring is called aromaticity. The π electrons will be more attracted to the more electronegative atom. Explain the formal charges

14 (Unnecessary) Klein uses a pattern recognition system to draw resonance structures; there is no need to memorize these patterns, just be able to draw resonance structures in each case (there are good examples to practice with in this section). Allylic lone pairs Allylic positive charge Lone pair of electrons adjacent to a positive charge A π bond between two atoms with different electronegativities Conjugated π bonds in a ring 2.11 Stability of Contributors Not all valid resonance form are equally significant. In our nectarine analogy, we said a nectarine is 65% peach and 35% plum. What if we made a fruit that was 65% peach, 34% plum, and 1% kiwi? ow would that look? The kiwi character in not a significant contributor to the fruit. Similarly, there are many molecules that have multiple resonance structures, but not all contribute equally. The resonance hybrid will most resemble the more stable contributor(s) Stability of Contributors 2.11 Stability of Contributors More stable resonance forms are more significant contributors. There are 3 criteria to assess the stability of resonance contributors: 1. Structures in which electronegative atoms have complete octets 2. Structures that have the minimum charge separation 1. Structures with complete octets on electronegative atoms 3. Structures that have the maximum number of covalent bonds K K Non Significant 55 56

2.11 Stability of Contributors 2. Structures with minimum charge separation 2.11 Stability of Contributors 3. Structures with the maximum number of covalent bonds.

15 2.11 Stability of Contributors 2. Structures with minimum charge separation 2.11 Stability of Contributors 3. Structures with the maximum number of covalent bonds. Best K K Better K Exception: the nitro group K Non Significant K Delocalized vs. Localized Lone Pairs The nitrogen atom of an amide is not sp 3 but sp 2 hybridized. This allows the adjacent atomic p orbitals (N-C=) to overlap in the resonance structure Delocalized vs. Localized Lone Pairs For the lone pair to be delocalized, it needs somewhere to go and some way to get there. A lone pair adjacent to a π bond is the classical example. If all the adjacent atoms are sp 3 hybridized, the lone pair is stuck there (localized). The lone pair is delocalized (part on N and part on )

16 2.12 Delocalized vs. Localized Lone Pairs If there is no overlap, for geometric reasons for example, there can be no resonance. 61

2.1 Representing Molecules

2.1 Representing Molecules Notice that the molecular formula would be inadequate to distinguish between propanol and isopropanol. Practice converting from one type of representation to another with Skillbuilder