Polar Covalent Bonds; Acids and Bases

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1 2 Polar Covalent Bonds; Acids and Bases پیوندهای کوواالنسی قطبی اسیدها و بازها Dr Morteza Mehrdad University of Guilan, Department of Chemistry, Rasht, Iran Based on McMurry s Organic Chemistry, 7 th edition

2 چرا این فصل Description of basic ways chemists account for chemical reactivity. ت صیف شی ای ت یادی ج ت دسک واکنش پذیزی شیویایی Establish foundation for understanding specific reactions discussed in subsequent chapters. شال د دسک واکنشهای ههن ه سد تحث دس فصل ای آی ذ 2

3 2.1 Polar Covalent Bonds: Electronegativity Covalent bonds can have ionic character These are polar covalent bonds Bonding electrons attracted more strongly by one atom than by the other Electron distribution between atoms is not symmetrical 3

4 Bond Polarity and Electronegativity Electronegativity (EN): intrinsic ability of an atom to attract the shared electrons in a covalent bond الکتزونگاتیوی جزب تشای اتن یک راتی ت ا ایی :(EN) الکتش ای است. ک اال سی پی ذ یک دس هشتشک Differences in EN produce bond polarity 4

5 The Periodic Table and Electronegativity F is most electronegative (EN = 4.0), Cs is least (EN = 0.7) Metals on left side of periodic table attract electrons weakly, lower EN Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly, higher electronegativities EN of C = 2.5 5

6 Bond Polarity and Inductive Effect قطبش پیوند و اثر القایی Nonpolar Covalent Bonds: atoms with similar EN Polar Covalent Bonds: Difference in EN of atoms < 2 Ionic Bonds: Difference in EN > 2 C H bonds, relatively nonpolar C-O, C-X bonds (more electronegative elements) are polar Bonding electrons toward electronegative atom C acquires partial positive charge, + Electronegative atom acquires partial negative charge, - Inductive effect: shifting of electrons in a bond in response to EN of nearby atoms اثز القایی: هجا س است. ا تقال الکتش ى ای یک پی ذ دس اثش الکتش گاتی ی اتن ای 6

7 Electrostatic Potential Maps نقشه پتانسیل الکترواستاتیکی Electrostatic potential maps show calculated charge distributions Colors indicate electronrich (red) and electronpoor (blue) regions Arrows indicate direction of bond polarity 7

8 2.2 Polar Covalent Bonds: گشتاور دوقطبی Dipole Moments Molecules as a whole are often polar from vector summation of individual bond polarities and lone-pair contributions ه لک ل ای آلی اغلة ت اسط حاصل جوع تشداسی قطثیت یکایک پی ذ ا ست ذ. قطثی ت ا الکتش ای جفت Strongly polar substances soluble in polar solvents like water; nonpolar substances are insoluble in water. ه اد قطثی هعو ال دس حالل ای قطثی هثل آب حل هی ش ذ اها ه اد غیش قطثی دس آب ا حالل اپزیش ذ. 8

9 شبیه شبیه را در خود حل می کند Like dissolves Like

end of molecular dipole times distance r between charges - ت ص ست ا ذاص تاس Q دس یک سش د

10 Dipole moment ( ) - Net molecular polarity, due to difference in summed charges گشتاور دوقطبی ( ) حاصل جوع تاس ا است قطثیت خالص ه لک لی اشی اص تفا ت دس - magnitude of charge Q at end of molecular dipole times distance r between charges - ت ص ست ا ذاص تاس Q دس یک سش د قطثی ه لک لی ضشتذس فاصل r تیي تاس است: = Q r, in debyes دتای (D), ) 1 D = coulomb meter ( 10

11 the unit charge on an electron is C. Thus, if one positive charge and one negative charge are separated by 100 pm (a bit less than the length of a typical covalent bond), the dipole moment is C m, or 4.80 D. 11

12 Dipole Moments in Water and Ammonia Large dipole moments EN of O and N > H Both O and N have lone-pair electrons oriented away from all nuclei 12

13 Absence of Dipole Moments In symmetrical molecules, the dipole moments of each bond has one in the opposite direction The effects of the local dipoles cancel each other 13

14 )بار قراردادی( 2.3 Formal Charges Sometimes it is necessary to have structures with formal charges on individual atoms We compare the bonding of the atom in the molecule to the valence electron structure If the atom has one more electron in the molecule, it is shown with a - charge If the atom has one less electron, it is shown with a + charge Neutral molecules with both a + and a - are dipolar 14

15 Building Lewis (ELECTRON DOT) Structures of Molecules Example: HCN Step 1. Count the total number of valence electrons H has 1 C has 4 N has = Total of 10 Step 2. Place one e - pair between each BONDED atom _ H C N We have 10-4 = 6 e - left All atoms must have an octet or duet _ 15

16 Step 3. Add remaining electrons to terminal atoms first Add 6 electrons in pairs to give the N an octet. _ H C N Step 4. Add any electrons left over to central atom We have none left! _ Step 5. Check for an acceptable Lewis Structure Do all atoms have an octet? IN THIS CASE 16

17 Step 5. Check for an acceptable Lewis Structure bring electron pairs from outer N atom to form shared pairs to give C its octet!!! H C N Still no octet on C Do it again!!!! _ H C = N H C N three electron pairs between the C and N... 17

18 H C N H C N Another possible structure is. H N C How can we choose? FORMAL CHARGE 18

19 All possible Lewis structures with stable electronic configurations for HCN and HNC. H C N H N C Formal charge = #valence electrons _ { #unshared } electrons Calculate formal charge for this one _ { 1/2#shared } electrons H C N FC on C = /2 (8) = 0 FC on N = /2 (6) = 0 Hydrogen is zero 19

20 All possible Lewis structures with stable electronic configurations for HCN and HNC. H C N H N C WE CHOOSE THE STRUCTURE WITH THE FORMAL CHARGES CLOSEST TO ZERO AND ANY NEGATIVE FORMAL CHARGES ON THE MOST ELECTRONEGATIVE ELEMENTS 20

21 Formal Charge for Dimethyl Sulfoxide Atomic sulfur has 6 valence electrons. Dimethyl suloxide sulfur has only 5. Oxygen atom in DMSO has gained electron and has (-) charge. It has lost an electron and has positive charge. 21

22 22

2.4 Resonance Some molecules are have structures that cannot be shown with a single representation In these cases we draw structures that contribute to the final structure but which

23 2.4 Resonance Some molecules are have structures that cannot be shown with a single representation In these cases we draw structures that contribute to the final structure but which differ in the position of the bond(s) or lone pair(s) Such a structure is delocalized and is represented by resonance forms The resonance forms are connected by a doubleheaded arrow 23

24 هیبرید رزونانسی Resonance Hybrids A structure with resonance forms does not alternate between the forms یک ساختاس تا فزم های رسونانسی دس سفت تشگشت یست ک صها ی ت شکل یک فشم صها ی ت شکل فشم دیگش دسآیذ. Instead, it is a hybrid of the two resonance forms, so the structure is called a resonance hybrid تلک یثشیذی اص د فشم سص ا سی است ک هیبزید رسونانسی اهیذ هی ش د. 24

25 سرافه یا شتزگاوپلنگ 25

26 For example, benzene (C 6 H 6 ) has two resonance forms with alternating double and single bonds In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single 26

2.5 Rules for Resonance Forms Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure) ای

27 2.5 Rules for Resonance Forms Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure) ای فشم سص ا سی خیالی ست ذ اقعی Resonance forms differ only in the placement of their or nonbonding electrons فشم ای الکتش ای سص ا سی فقط اپی ذی تفا ت اص لحاظ داس ذ استقشاس هحل الکتش ای یا 27

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29 قواعد فرم های رزونانسی - بقیه -2.5 Different resonance forms of a substance don t have to be equivalent سص ا سی ای فشم یک هختلف تشکیة حتوا تاش ذ اسص ن ثایذ 29

30 قواعد فرم های رزونانسی - بقیه -2.5 فشم آ ا Resonance forms must be valid Lewis structures: the octet rule applies ای سص ا سی هعتثش تاشذ وا ذ تایذ ساختاس ای دس شتایی قاعذ ل ئیس: The resonance hybrid is more stable than any individual resonance form would be یثشیذ سص ا سی پایذاستش اص ش یک اص فشم ای سص ا سی است شچ تعذاد فشم ای سص ا سی تیشتش تاشذ تشکیة پایذاستش است. 30

31 Curved Arrows and Resonance Forms We can imagine that electrons move in pairs to convert from one resonance form to another A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow 31

32 2.6 Drawing Resonance Forms Any three-atom grouping with a multiple bond has two resonance forms ش گش س تایی اص اتن ای داسای یک پی ذ د گا د فشم سص ا سی داسد 32

33 Different Atoms in Resonance Forms Sometimes resonance forms involve different atom types as well as locations The resulting resonance hybrid has properties associated with both types of contributors The types may contribute unequally The enolate derived from acetone is a good illustration, with delocalization between carbon and oxygen 33

34 2,4-Pentanedione The anion derived from 2,4-pentanedione Lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O bond on the left and on the right Three resonance structures result 34

35 2.7 Acids and Bases: The Brønsted Lowry Definition The terms acid and base can have different meanings in different contexts»تاصاسیذ«دس صهی ای هختلف هعا ی هتفا تی داس ذ. The idea that acids are solutions containing a lot of H + and bases are solutions containing a lot of OH - is not very useful in organic chemistry Instead, Brønsted Lowry theory defines acids and bases by their role in reactions that transfer protons (H + ) between donors and acceptors نظزیه اسید و باس بزونستد-لوری: تیاى قش اسیذ تاص دس اک ش ا تقال پش ت ى تیي پزیش ذ د ذ ) + (H 35

36 Brønsted Acids and Bases Brønsted-Lowry is usually shortened to Brønsted A Brønsted acid is a substance that donates a hydrogen ion (H + ) A Brønsted base is a substance that accepts the H + proton is a synonym for (H + ) - loss of an electron from H leaving the bare nucleus a proton»پش ت ى«هتشادف تا ) + H) است تا حزف یک الکتش ى اص H فقط ست یذس طى ( یع ی پش ت ى( تاقی هی ها ذ. 36

37 The Reaction of Acid with Base Hydronium ion (H 3 O + ), product when base H 2 O gains a proton HCl donates a proton to water molecule, yielding hydronium ion (H 3 O + ) [conjugate acid] and Cl [conjugate base] The reverse is also a Brønsted acid base reaction of the conjugate acid and conjugate base 37

38 2.8 Acid and Base Strength The equilibrium constant (K eq ) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A - ) is a measure related to the strength of the acid Stronger acids have larger K eq Note that brackets [ ] indicate concentration, moles per liter, M. 38

39 K a the Acidity Constant ثابت قدرت اسیدی The concentration of water as a solvent does not change significantly when it is protonated The molecular weight of H 2 O is 18 and one liter weighs 1000 grams, so the concentration is ~ 55.4 M at 25 The acidity constant, K a for HA K eq times 55.6 M (leaving [water] out of the expression) K a ranges from for the strongest acids to very small values (10-60 ) for the weakest 39

40 pk a the Acid Strength Scale pk a = -log K a The free energy in an equilibrium is related to log of K eq (DG = -RT log K eq ) A smaller value of pk a indicates a stronger acid and is proportional to the energy difference between products and reactants The pk a of water is ثاتت حاصلضشب ی ی آب 40

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2.9 Predicting Acid Base Reactions from pk a Values pk a values are related as logarithms to equilibrium constants Useful for predicting whether a given acid-base reaction will take

42 2.9 Predicting Acid Base Reactions from pk a Values pk a values are related as logarithms to equilibrium constants Useful for predicting whether a given acid-base reaction will take place The difference in two pk a values is the log of the ratio of equilibrium constants, and can be used to calculate the extent of transfer The stronger base holds the proton more tightly 42

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44 2.10 Organic Acids and Organic Bases Organic Acids: - characterized by the presence of positively polarized hydrogen atom تا اتن یذس طى قطثیذ هثثت )ت س گ آتی( هشخص هی ش ذ - 44

45 Organic Acids Those that lose a proton from O H, such as methanol and acetic acid Those that lose a proton from C H, usually from a carbon atom next to a C=O double bond (O=C C H) 45

46 Organic Bases Have an atom with a lone pair of electrons that can bond to H + داسای یک اتن تشکیل پی ذ تا س گ قشهض( )ت سا داسد الکتش ى جفت داسای ت ا ایی ک اپی ذی Nitrogen-containing compounds derived from ammonia are the most common organic bases Oxygen-containing compounds can react as bases when with a strong acid or as acids with strong bases H + 46

47 2.11 Acids and Bases: The Lewis Definition Lewis acids are electron pair acceptors and Lewis bases are electron pair donors Brønsted acids are not Lewis acids because they cannot accept an electron pair directly (only a proton would be a Lewis acid) The Lewis definition leads to a general description of many reaction patterns but there is no scale of strengths as in the Brønsted definition of pk a 47

48 Lewis Acids and the Curved Arrow اسیدهای لوئیس و شیوه استفاده از پیکان خمیده Formalism The Lewis definition of acidity includes metal cations, such as Mg 2+ They accept a pair of electrons when they form a bond to a base Group 3A elements, such as BF 3 and AlCl 3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases Transition-metal compounds, such as TiCl 4, FeCl 3, ZnCl 2, and SnCl 4, are Lewis acids Organic compounds that undergo addition reactions with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids The combination of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid 48

49 Illustration of Curved Arrows in Following Lewis Acid-Base Reactions 49

50 50

51 Lewis Bases Lewis bases can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted bases تاص ای ل ئیس داسای یک جفت الکتش ى اپی ذی تشای تشکیل پی ذ تا یک اسیذ ل ئیس ست ذ تعزیفی شبیه به باسهای بزونستد یع ی آب تا د جفت الکتش ى اپی ذی تش س ی اتن اکسیظى تا دادى یک جفت + H تشکیل ی ى یذس ین H قش تاص ل ئیس سا داسد. 3 O + الکتش ى ت 51

52 Lewis Bases Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons Some compounds can act as both acids and bases, depending on the reaction 52

53 2.12 Molecular Models Organic chemistry is 3-D space Molecular shape is critical in determining the chemistry a compound undergoes in the lab, and in living organisms Space-filling models Ball-and stick models مدل گوی و میله مدل فضا پرکن 53

54 2.13 Noncovalent Interactions برهم کنش های غیر کوواالنسی Several types: - Dipole-dipole forces نیزوهای دوقطبی دوقطبی نیزوهای پزاکنش - Dispersion forces پیوندهای هیدروژنی - Hydrogen bonds 54

55 Dipole-Dipole نیروهای دوقطبی دوقطبی Occur between polar molecules as a result of electrostatic interactions among dipoles Forces can be attractive of repulsive depending on orientation of the molecules 55

56 نیروهای پراکنش Dispersion Forces Occur between all neighboring molecules and arise because the electron distribution within molecules that are constantly changing دس تیي توام ه لک ل ای وسای الکتش ای یک ه لک ل است هتغیش ت صیع اص اشی داسد ج د 56

57 Hydrogen Bond Forces Most important noncovalent interaction in biological molecules Forces are result of attractive interaction between a hydrogen bonded to an electronegative O or N atom and an unshared electron pair on another O or N atom 57

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59 Summary Organic molecules often have polar covalent bonds as a result of unsymmetrical electron sharing caused by differences in the electronegativity of atoms The polarity of a molecule is measured by its dipole moment,. (+) and ( ) indicate formal charges on atoms in molecules to keep track of valence electrons around an atom Some substances must be shown as a resonance hybrid of two or more resonance forms that differ by the location of electrons. A Brønsted( Lowry) acid donates a proton A Brønsted( Lowry) base accepts a proton The strength Brønsted acid is related to the -1 times the logarithm of the acidity constant, pka. Weaker acids have higher pka s 59

60 Summary (cont d( A Lewis acid has an empty orbital that can accept an electron pair A Lewis base can donate an unshared electron pair In condensed structures C-C and C-H are implied Skeletal structures show bonds and not C or H (C is shown as a junction of two lines) other atoms are shown Molecular models are useful for representing structures for study Noncovalent interactions have several types: dipole-dipole, dispersion, and hydrogen bond forces 60

2. Polar Covalent Bonds: Acids and Bases

2. Polar Covalent Bonds: Acids and Bases Based on McMurry s Organic Chemistry, 6 th edition, Chapter 2 2003 Ronald Kluger Department of Chemistry University of Toronto 2.1 Polar Covalent Bonds: Electronegativity!