Chapter 1 Introduction and Review

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1 Chapter 1 Introduction and Review

2 Concept to review: It is your responsibility to review the following concepts before the first class to ensure success in understanding new concepts: Atomic structure (1-2) Bond formation: octet rule (1-3) Lewis structure (1-4) Multiple bonds (1-5) Electronegativity and bond polarity (1-6) Formal charges (1-7) 2

3 Resonance (1-9) Sometimes the structure of a compound cannot be represented by a single Lewis structure. In this case, more than one structure is used. The actual structure of the compound is somewhere between the resonance forms and is called the resonance hybrid. C + N C N + = C N resonance forms of a resonance hybrid resonance hybrid combined representation 3

4 For example, when acetic acid is deprotonated by a base, a carboxylate anion is formed for which 2 resonance forms can be drawn. Remember that in resonance hybrid, only electrons can move, not atoms 4

5 Rules: Individual resonance forms are imaginary. Resonance forms differ only in the placement of their π or non-bonding electrons. (see example of the carboxylate anion). 5

6 Resonance forms of a compound do not have to be equivalent Example: acetone anion 6

7 Resonance forms must be valid Lewis structures and obey the normal rules of valency. Example: Acetate anion O O O - O 3 C O O - _ 3 C O - 3 C O 3 C O not a valid Lewis Structure 7

8 The resonance hybrid is more stable than any individual resonance form, ie, resonance leads to stability. Rule: the larger the number of resonance forms, the more stable a species will be. 8

9 Often resonance structures have major and minor contributors. While both are always considered, in these cases the major contributor usually predominate and is used to describe the compound. To determine the major/minor contributors of in resonance structures, simply verify if the octet rule is respected and whether there is charge separation or not. 9

10 Problem:1-8 (g & h) For each of the following compounds, draw the important resonance forms and indicate which structures are major/minor contributors. 10

11 Condensed/Line-Angle Formulas (1-10) Because of the complexity of some organic molecules (ex: vitamin A has a formula C 20 30O and has 51 different chemical bonds), a simply notation that follows the rules below was adopted: Skeletal Structures 11

12 Rule 1 Carbon atoms are not shown. It is assumed that a carbon atom is present at each intersection of 2 lines, and at the end of each line Example: 12

13 Rule 2 ydrogen atoms bonded to carbon are not shown. Since carbon will normally form 4 bonds, we mentally supply the correct number of s for each carbon. butane 13

14 Rule 3 Atoms other than C and are shown. If those atoms bear hydrogen atoms as well, those are also shown. 14

15 Problem: Draw a line-angle structure for each of the following compounds. C 3 C 3 C 2 C 2 C 2 CC 2 C 2 COO C 3 C=CC 2 Cl C 3 C(C 3 ) 2 C(O)C 2 C 2 O 15

16 Bronsted/Lowry Acids and Bases (1-13) Acidity and basicity are two important aspects of organic reactions. While we will not calculate K a or pk a of acids in this course, many organic reactions will occurs because the compounds is either acidic or basic. Therefore, it is very important to be able to determine the acidity of organic molecules. There are 2 important definitions to remember: Bronsted-Lowry Acids: are substances that donate + (a proton). Bronsted-Lowry Bases: are substances that accept + 16

17 You should remember the definition of pk a as: pk a = - log 10 K a and that the smaller the value of the pk a, the most acidic is the compound 17

18 For example, Cl is considered a strong acid and completely dissociate into its ions in water solution: The pk a of Cl is -7 18

19 Similarly, methanol can be deprotonated by water to give the associated conjugate acid and base. owever, the pka of methanol is only 15.7 (similar to that of water) and methanol is therefore considered a weak acid. 19

20 The relationship between an acid and its conjugate base is inversely proportional. For example what would you answer if I ask you which of Cl - or C 3 O - is the strongest base? Since Cl - is the conjugate base of Cl (pka -7) and C 3O - is the conjugate base of C 3 O (pka ~16), than your answer would be C 3 O - is the strongest base. Or, a strong acid has a weak conjugate base and a weak acid has a strong conjugate base. 20

21 Example 2: O O O- Oacetic C3 C O C3 C O- C3 C O + O acid 21

22 Therefore, it is important to know some of the standard pka values encountered in organic chemistry. ere are a few: acid conjugate base pka Cl Cl - -7 C 3 COO C - 3 COO 5 C 3 O C 3 O - 16 N 3 - N 2 33 C 4 - C 3 >40 22

23 Problem: Using the previous table as a guide, classify the following compounds in order of increasing (least to most) acidity. O O Br O A B C D 23

24 Lewis Acids and Bases (1-14) This is different concept that is used to describe organic reaction. Lewis Acids: substances that accepts an electron pair. It is called: electrophile Lewis bases: substances that give an electron pair. It is called: nucleophile 24

25 In a reaction, the nucleophile is always the attacking species, giving electrons to the electrophile. This is represented by curved arrows: 25

26 This representation is what organic chemists called: mechanisms For example, the mechanism of formation of dimethyl ether from chloromethane and methoxide ion. 26

27 Problem: 1-19 (b) For the following systems, identify the electrophile/nucleophile, draw curved arrows to show the movement of electrons leading to the product shown. 27

28 Problem: 1-19 (e) For the following systems, identify the electrophile/nucleophile, draw curved arrows to show the movement of electrons leading to the product shown. 28

29 Study Problems 1-36: Determine whether the following pairs of compounds are different compounds or resonance forms of the same compound? 29

30 1-37 Draw the important resonance forms showing the delocalization of the charge of the following ion. 30

31 1-45 Label the reactants of the following acid/base reactions as electrophile (E) or nucleophile (Nu). 31

Chapter 01 Structure Determines Properties part 2

Chapter 01 Structure Determines Properties part 2 CEM 341: Spring 2012 Prof. Greg Cook Resonance Section 1.8 Resonance Some molecules may have more than one correct Lewis structure These are NT isomers.