ChemActivity 4: Polar Bonds, Polar Reactions

Transcription

1 hemactivity 4: Polar Bonds, Polar eactions PAT A: BILING A LIQUID T M A GAS (ow is the structure of a molecule related to its melting point or boiling point?) Model 1: ovalent Bonds vs. Intermolecular orces In the previous hemactivities we have explored covalent bonds (e.g.,,,, etc.), which are strong attractive forces that hold atoms together to form molecules. Now we will explore the comparatively weak attractive forces that make molecules clump together to form a liquid or solid. These are called intermolecular forces (some books call them van der Waals forces). The word forces is used to distinguish them from their much stronger counterparts, bonds. Unfortunately, by convention the strongest of these intermolecular forces are called hydrogen bonds. Watch out for this! To summarize: ovalent Bonds = strong attractions between atoms (e.g., and ) within a molecule (e.g., 2 ) Intermolecular orces = weak attractions between separate molecules (e.g., two 2 molecules) = = = = igure 4.1: Molecular View of Liquid Water igure 4.2: Molecular View of Solid Water (ice) = = igure 4.3: Molecular View of Gaseous Water (water vapor or steam) ead the Model once through and begin answering the TQ s on the next page. opyright oughton Mifflin arcourt ompany. All rights reserved.

2 36 hemactivity 4: Polar Bonds, Polar eactions ritical Thinking Questions 1. (E) What common molecule is represe nted nine separate times in each igure in Model 1? 2. igure 4.1 is a cartoon depiction of liquid water at the molecular level. a. (E) Label at least one representation of a covalent bond and at least one representation of an intermolecular force found in igure 4.1. b. (E) Which are shorter and stronger: covalent bonds or intermolecular forces [circle one]? 3. According to Model 1, in going from liquid water to gaseous water a. What percentage of the covalent bonds were broken? b. What percentage of the intermolecular forces were broken? Model 2: Polar Bonds The bond is called a nonpolar bond because the bonding electrons are shared equally. is an example of a polar bond. luorine has a much higher core charge than hydrogen, and so the atom pulls the bonding electrons toward itself and away from the atom. Note: and have slightly different affinities for electrons, but because these affinities are very close, organic chemists consider a typical bond to be nonpolar. An ionic bond such as in Nal is the extreme case of a polarized bond. The Na atom has a very low affinity for electrons and completely gives up its share of the bonding electrons to the l. electron "cloud" equally distributed "Non-polar ovalent Bonds" electron "cloud" nearly equally distributed lack of electrons extra electrons electron "cloud" skewed toward fluorine "Polar ovalent Bond" no bonding electrons Na l electron "cloud" localized on l "Ionic Bond" igure 4.4: Map of Electron Density for 2,, and Nal rganic chemists think of electrons in a bond as forming a fine mist of negative charge. This electron cloud can assume many different shapes. When this cloud is mapped (as in igure 4.4) the atom with more electron mist near it is said to have a greater electron density. ritical Thinking Questions 4. n the map of electron den sity for Nal, add a to one atom and a to the other indicating the formal charge on each atom. 5. In, neither nor holds a full formal charge of +1 or -1. rganic chemists represent a partial charge using the Greek letter delta (δ). n the electron density map of the molecule above, add a δ+ to one atom and a δ to the other to indicate which way the bond is polarized. opyright oughton Mifflin arcourt ompany. All rights reserved.

3 hemactivity 4: Polar Bonds, Polar eactions 37 Model 3: Electronegativity (EN) and Dipole Moment The electronegativity (EN) of an atom is a measure of its ability to attract electrons toward it and can be used to predict the polarization of a bond in a molecule. In this book, we will deal almost exclusively with the shaded elements. Table 4.1: Electronegativities (EN) of Elements in the irst ive ows of the Periodic Table 2.1 Li 1.0 Na 0.9 K 0.8 b 0.8 Be 1.6 Mg 1.2 a 1.0 Sr 1.0 Sc 1.3 Y 1.2 Ti 1.5 Zr 1.4 V 1.6 Nb 1.6 r 1.6 Mo 1.8 Mn 1.5 Tc 1.9 e 1.8 u 2.2 o 1.9 h 2.2 Ni 1.9 Pd 2.2 u 1.9 Ag 1.9 Zn 1.6 d 1.7 B 2.0 Al 1.5 Ga 1.6 In Si 1.8 Ge 1.8 Sn 1.8 N 3.0 P 2.1 As 2.0 Sb S l Se Br Te I e Ne Ar Kr Xe l 3 l l 3 Li δ+ δ- δ+ δ- δ+ δ- δ- δ+ δ+ δ- igure 4.5: Dipole Moments and alculated EN Differences for Selected Bonds 2 2 The polarity of a bond is often represented with a dipole moment arrow. The tail is crossed to make a + sign to remind you the head of the arrow points to the more negative end (the end with more electrons). The size of the arrow is proportional to the EN difference = EN of atom one EN of atom two. ritical Thinking Questions 6. (E) alculate the EN difference for the Li bond in 3 Li a. (E) Add the missing dipole moment arrow to the structure of 3 Li in igure 4.5. b. (E) Add δ+ and δ to the and Li as appropriate. c. In what way does the in 3 Li differ from all the other carbons found in igure 4.5? 7. Summing all bond dipole moments for a structure gives the molecule s net dipole moment. If this net dipole moment is zero, the molecule is overall nonpolar. (We will not be doing fancy vector calculations in this course, but be on the lookout for symmetrical molecules whose bond dipole arrows cancel each other.) ircle the only overall nonpolar molecule shown in igure 4.5. opyright oughton Mifflin arcourt ompany. All rights reserved.

4 38 hemactivity 4: Polar Bonds, Polar eactions Model 4: Boiling Points ecall that intermolecular forces are the weak attractive forces that cause molecules to clump together into liquids or solids and keep them from drifting apart and becoming a gas. These forces often can be overcome by just a small amount of heat. The more heat required to overcome the intermolecular forces, the higher the boiling point of that compound. Table 4.2: Boiling Point (bp) Data for Selected rganic Liquids Name Structure bp Name Structure bp Alkanes: Alkyl hlorides: butane o 1-chloro propane l 46 o pentane chloro butane l 78 hexane chloro pentane l 108 octane chloro heptane Alcohols: Aldehydes: 1-propanol propanal butanol butanal pentanol pentanal heptanol heptanal ritical Thinking Questions 8. (E) Table 4.2 is blocked off into sections. Above the name of each section, briefly describe what molecules within that section have in common with other molecules in that same section. 9. Within any one section of Table 4.2, boiling points trend with what physical property? Information It turns out that surface area is more important than molecular weight for predicting boiling point. The larger the surface area of a molecule, the more chance it has of sticking to (or even wrapping around) neighboring molecules. or example: octane has a much higher bp than 2,2,3,3-tetramethylbutane (bp 107 o ), even though they have the same molecular formula ,2,3,3-tetramethylbutane opyright oughton Mifflin arcourt ompany. All rights reserved.

5 hemactivity 4: Polar Bonds, Polar eactions onsider the following three molecules with similar surface areas (from Table 4.2): (nonpolar) 2 3 l propanal 1-chloropropane butane a. Based on bp data from Table 4.2, which molecule makes the weakest intermolecular attachments to neighboring molecules in a liquid state? b. Does dipole moment (polarity) trend with boiling point for these three molecules? 11. onsider the following cartoons of a polar liquid and a nonpolar liquid. ( = ) δ+ δ l l δ+ δ+ δ δ l δ δ+ l l δ+ δ δ δ+ l δ+ l δ δ l δ igure 4.6: artoon of a Polar Liquid (1-chloropropane) igure 4.7: artoon of a Nonpolar Liquid (butane) a. or the polar liquid, draw a dotted line connecting each δ+ charge to a nearby δ charge. b. onstruct an explanation for why you must heat 1-chloropropane to 46 o to initiate boiling (boiling = rapid conversion from the liquid to the gas state) while butane boils at 0.5 o. Model 5: [Dipole-Dipole] vs. [Induced Dipole-Induced Dipole] orces The intermolecular attractive forces you added to igure 4.6 are called dipole-dipole forces. Nonpolar molecules like butane do not have dipoles and so do not experience dipole-dipole forces. To understand how a nonpolar chemical can be a liquid, consider that electron clouds are not fixed. andom fluctuations cause an electron cloud to slosh about like water in an unsteady tub. ritical Thinking Questions 12. In igure 4.8, the electron cloud on the left-most butane molecule is shown as having just sloshed to the right, generating momentary partial charges. Use a solid line to show what the new, disturbed electron clouds might look like on each successive butane molecule moving to the right across igure 4.8. igure 4.8: Attractive orces in Butane 13. The attractive forces that hold nonpolar molecules together in the liquid state are called induced dipole-induced dipole attractive forces. onstruct an explanation for this name, and compare them to the dipole-dipole attractive forces experienced by polar molecules in igure 4.6. opyright oughton Mifflin arcourt ompany. All rights reserved.

6 40 hemactivity 4: Polar Bonds, Polar eactions PAT B: TANSE EATINS (What happens when intermolecular forces between two molecules are super strong?) Model 6: ydrogen Bonding The dotted lines in igures 4.1 and 4.2 represent the strongest type of dipole-dipole attractive force called a hydrogen bond. A typical hydrogen bond is about 5% as strong as a typical covalent bond. ydrogen bonds will normally form only between a [lone pair on N,, or ] and an [ on N,, or X] or example: ritical Thinking Questions 2 N N (E) According to the rules in Model 6, is the dotted line marked with an X a hydrogen bond? X 15. onstruct an explanation for why an attached to an N or atom can participate in a hydrogen bond (-bond) while an attached to a carbon atom cannot. 16. ircle each molecule below that you expect to be capable of hydrogen bonding to itself. N 2 N l l An hydrogen-bond donor has an suitable for hydrogen bonding. An hydrogen-bond acceptor has a lone pair suitable for hydrogen bonding. nly structures in the top row of the previous question are both (and therefore -bond to themselves!). Label structures in the bottom row with -bond donor, -bond acceptor, or neither, as appropriate. (Note that only one structure is neither.) opyright oughton Mifflin arcourt ompany. All rights reserved.

7 hemactivity 4: Polar Bonds, Polar eactions 41 Model 7: rdinary vs. Extreme -Bond Donors and Acceptors ecall that an ordinary -bond donor will typically share an with an acceptor, but this will remain covalently bound to the donor, as shown in igure 4.9. Donor -bond Acceptor igure 4.9: rdinary -bond Donor and Acceptor An extreme -bond donor or acceptor will transfer an + ( an with no electrons) from donor to acceptor. n the next page you will be asked to memorize six common extreme -bond donors and a working definition of an extreme -bond acceptor. or now, some examples are provided below. 2 a ordinary -bond acceptor b l Extreme -bond donor eactants 2 N Extreme -bond acceptor ordinary -bond donor eaction 1 eaction 2 2 l Products 2 N igure 4.10: + Transfer eactions Involving an Extreme -bond Donor or Acceptor + Bond formation and bond breaking for the transfer reaction shown on the left side of igure 4.10 (eaction 1) can be represented using two curved arrows marked a and b: Arrow a says: ne lone pair of electrons on the (of 2 ) forms a new bond to the (of l). Arrow b says: The electron pair bonding to l breaks from and becomes a lone pair on l. ritical Thinking Questions 18. (E) or eactions 1 and 2 in igure 4.10, label any bond that is broken among the reactants and any bond that is new among the products. 19. (E) True or alse: An extreme -bond donor can react only with an extreme -bond acceptor. 20. Add curved arrows to eaction 2 in igure 4.10 showing bond formation and bond breakage. 21. By convention, organic chemists draw their curved arrows from the perspective of the electrons. Write a sentence describing what each of your curved arrows says about the electrons in xn 2. opyright oughton Mifflin arcourt ompany. All rights reserved.

8 42 hemactivity 4: Polar Bonds, Polar eactions Memorization Task 4.1: By the start of next class, memorize the contents of Tables 4.3 and 4.4 and the definitions of a strong acid and a strong base on this page. Strong Acid (e.g., l) easily donates an + (previously called an extreme -bond donor). Table 4.3: The Six Strong Acids Used in this Book ydroiodic Acid ydrobromic Acid ydrochloric Acid I Br l S Sulfuric Acid (when = ) Nitric Acid* N ydronium Ion (when = ) Strong Base (e.g., 2 N ) easily accepts an + (previously called an extreme -bond acceptor) Strong Base = molecule with a [lone pair] and [-1 formal charge] localized* on an,, N, or atom. Table 4.4: Examples of ommon Strong Bases Most strong bases are of the form of one of these generalized structures where = or alkyl group [ n m ] hydride ion N xygen strong bases are most common. Examples (and names) of bases hydroxide 3 methoxide 3 2 ethoxide tert-butoxide Examples of 2 N bases N 3 N 3 N Examples of 3 bases * A topic for next chapter: The -1 formal charge of nitric acid is not localized on, so it is not a strong base! ritical Thinking Questions 22. (E) or each strong acid in Table 4.3, circle the that can be donated. 23. (E) Label each of the following as strong acid, strong base, or neither. N l S 3 S 3 opyright oughton Mifflin arcourt ompany. All rights reserved.

9 hemactivity 4: Polar Bonds, Polar eactions 43 Model 8: onjugate Acid-onjugate Base Pairs Most any molecule can serve as an acid (it simply must have an ), and any molecule with a lone pair can be a base. Most (except those defined above as strong) are weak acids and weak bases. General Definitions: acid = any molecule with an (e.g., Z) generalized form, where Z can be any atom or group. Examples of weak acids: 2, (alcohols),, + N 3 (ammonium ions), 2 3, many others. base = any molecule with a lone pair (e.g., :Z or :Z ) weak base can be neutral or negative Examples of weak bases: 2, (alcohols), N 3 (amines), Na 3, (ethers), many others. (Later we will add or any molecule with π electrons to our definition of a base don t worry about this for now.) Z and :Z are a conjugate acid-conjugate base pair (as shown below). 3 and 2 are another conjugate acid-conjugate base pair. Z conjugate acid of :Z - conjugate base of 3 + Z conjugate base of Z conjugate acid of water igure 4.11: onjugate Acid-onjugate Base Pairs ritical Thinking Questions 24. (E) What is the molecular formula of the conjugate acid of water? 25. Draw the structure of the conjugate base of water. (Note that it does not appear in igure 4.11). 26. Does l have a conjugate acid? If so, what is it? a conjugate base? If so, what is it? 27. Draw the conjugate base of 4 (methane). 28. or the previous four questions, label each molecule that appears in the question or your answer as strong acid, strong base, weak acid, or weak base. opyright oughton Mifflin arcourt ompany. All rights reserved.

10 44 hemactivity 4: Polar Bonds, Polar eactions Model 9: Energy Diagrams hemists chart the total energy of a chemical reaction vs. time using a graph called an energy diagram. Think of an energy diagram as a side-view (cross-section) of a mountainous landscape. By analogy to gravitational potential energy (an object at high altitude has farther to fall), the tops of the peaks are the high Potential Energy (P.E.) points of the reaction. The valleys are the low P.E. points. B Potential Energy (PE) Starting Point A Ending Point PE of point A Δ rxm 3 PE of point Potential Energy (PE) Starting Point G Ending Point Progress of eaction 3 (time) igure 4.12: verall avorable (downhill) eaction Progress of eaction 4 (time) igure 4.13: verall Unfavorable (uphill) eaction. Memorization Task 4.2: Memorize the following energy diagram conventions a P.E. change* (Δ) from high to low is considered downhill or negative ( ) or exothermic a P.E. change* (Δ) from low to high is considered uphill or positive (+) or endothermic *rganic chemists often use change in heat (Δ) as an estimate for the change in total P.E. for a reaction (ΔG). Making a bond is downhill, negative ( ), exothermic energy is released by molecule Breaking a bond is uphill, positive (+), endothermic energy must be added to molecule ritical Thinking Questions 29. (E) Is eaction 3, taken as a whole from Point A to Point, endothermic or exothermic? What about eaction 4? [Label both igure 4.12 and igure 4.13 with one of these two terms.] 30. According to the conventions above, what is the sign (+ or ) of the P.E. change (Δ) for xn 3? 31. Draw an arrow on igure 4.13 representing Δ rxn 4. (int: study the Δ rxn 3 arrow in igure 4.12) 32. onsider the process shown at right using a curved arrow: a. Draw a + or above this curved arrow indicating the sign of the energy change associated with this process, and briefly explain your reasoning. b. Is breaking the bond uphill or downhill in energy? favorable or unfavorable? c. Is this arrow more likely associated with ig or ig. 4.13? 33. Add a + or above each curved arrow in igure 4.11 to show the sign of the energy change. opyright oughton Mifflin arcourt ompany. All rights reserved.

11 hemactivity 4: Polar Bonds, Polar eactions 45 PAT : AID-BASE EATINS AND pk a (What can a pk a value tell you about a molecule and its conjugate base?) Model 10: pk a as an Estimate of Z Bond Strength Each acid ( Z) has an experimentally determined pk a value. pk a is a measure of the energy difference between the conjugate acid ( Z) and the conjugate base (Z This means pk a is an estimate of ) in the solvent, water. amount of (+/uphill) energy needed to break the -Z bond Z Z +pk a of -Z -pk a of -Z amount of (-/downhill) energy released when the -Z bond re-forms Table 4.5: Energy Differences Between onjugate Acid-Base Pairs (in pka units) Potential Energy hange = or alkyl group N # = Energy Difference in pk a units N N N pk a 50 pk a 40 pk a 35 pk a 16 pk a 9 pk a 3 +3 ritical Thinking Questions 34. (E) In Table 4.5, review which structure is the conjugate acid and which structure is the conjugate base in each column. 35. (E) ow much energy is released when the lone pair on 2 N makes a bond to an? (Give the value and sign of the energy change in pk a units.) 36. A student answers -9 to the previous question. What mistake did this student make? opyright oughton Mifflin arcourt ompany. All rights reserved.

12 46 hemactivity 4: Polar Bonds, Polar eactions 37. ind the one curved arrow on Table 4.5. What is the energy change associated with this arrow? 38. Which will give up (donate) an more easily, a conjugate acid with a high pk a or a low pk a? 39. Is your answer to the previous question consistent with the fact that the six strong acids we will study in this course have pk a values close to zero? Explain. Memorization Task 4.3: Assume all six strong acids (see Table 4.3) have a pka 0. Strong acids actually have negative pk a values. This is the result of solvent effects that we will not discuss at this point. or now, it simplifies things to make the strong acid zero pk a assumption. 40. (E) According to the strong acid zero pk a assumption described above, approximately how much energy is required to break the Z bond of a strong acid? 41. Is your answer to the previous question consistent with the fact that strong acids give up an very, very easily? Explain why or why not. Table 4.6: pka Table onj. Acid approximate pk a onj. Base Definition of a Strong Base from Table 4.4: Molecule with a [lone pair] and [-1 charge] localized* on,, N, or. N N N N 42. (E) Identify each strong acid and strong base on Table A strong base releases a [large or small] amount of energy when it makes a bond to. (circle one) 44. Label the weakest acid (that is an acid) and weakest base (that is a base) on Table hoose the letter (a-e) that best completes the new definition of a strong base so that it is consistent with our old definition of a strong base above. New Definition of a Strong Base: Strong Base = molecule whose conjugate acid has a pk a a. ~ 0 b. > 5 c. < 5 d. >14 e. <14 opyright oughton Mifflin arcourt ompany. All rights reserved.

13 hemactivity 4: Polar Bonds, Polar eactions 47 Model 11: Using pk a to Estimate Δ rxn (heat of reaction) You can use pk a values to calculate the energy change (Δ in pk a units) for an acid-base reaction, e.g.: +3-9 N 3 Δ = -6 unequal rxn arrows can be used to indicate which direction is downhill N 3 igure 4.14: Use of pka to alculate Δ for the eaction of with 3N2 Potential Energy (PE) Starting Point A B Ending Point PE of point A Δ rxm = -6 pk a units PE of point xn Progress (time) igure 4.15: Energy Diagram of the eaction of with 3N2 ritical Thinking Questions 46. (E) Estimate the energy difference (to the closest pk a unit) between Point A and Point B on igure This value is called the activation energy (E act ), the energy required to start a reaction. a. Add a vertical arrow to igure 4.15 indicating the size and direction of E act for this reaction. b. Most students assume that E act should be +3 for this reaction. It turns out that pk a values are useful for calculating Δ, but not useful for calculating E act. Assume E act +1 pk a unit for all acid-base reactions. Is this assumption consistent with your answers above? 47. or N 3 (ammonia) and 2 (water) a. Use curved arrows to show the most likely acid-base reaction, and draw the resulting products. (int: irst decide which is the stronger acid, and which is the stronger base.) b. Mark each curved arrow with a positive (bond-breaking) or negative (bond-forming) number indicating the energy change associated with that arrow (in pk a units). c. alculate Δ rxn, and write this number above a set of reaction arrows that indicate which direction is downhill/favorable (in the example, the reaction is downhill to the right). d. Sketch an energy diagram for the reaction. e. Is your energy diagram consistent with the fact that, in this case, the most likely acid-base reaction is endothermic? opyright oughton Mifflin arcourt ompany. All rights reserved.

14 48 hemactivity 4: Polar Bonds, Polar eactions Model 12: Which is most acidic? The most acidic on a molecule is the that requires the least energy to remove. The pk a value next to a molecule on a pk a table (e.g., Table 4.5) refers to its most acidic. If there are multiple equivalent most acidic hydrogens the pk a tells you the energy required to remove the IST most acidic hydrogen. (Subsequent s are much harder to remove). ritical Thinking Questions 48. (E) Explain where the numbers +35, +50, and +16 (assigned to the curved arrows) came from N a. (E) ircle the on this molecule that requires the least energy to remove (the most acidic hydrogen). b. (E) Is your answer to the previous question consistent with the fact that, on a table of pk a values, this molecule would be assigned a pk a very close to 16? 49. ircle the most acidic on the following molecule. If there is a tie for most acidic, circle all s that tie for most acidic. N a. Estimate the amount of energy required to remove the most acidic. b. What pk a value would you expect to find next to this molecule on a pk a table? Model 13: The Inductive Effect Polarization caused by an electronegative atom (e.g., ) can induce minor polarization in neighboring bonds. This is called the inductive effect. Imagine a brick thrown into a still pond. The wave is intense at the impact site, but small even a short distance away. Warning: The inductive effect is weak. It cannot change that an on a positive atom is more acidic than an on a neutral atom or that an on N or is more acidic than an on. igure 4.16: Dipole Moments ritical Thinking Questions 50. ircle the most acidic / s on the structure in igure 4.16, and explain your reasoning. 51. onstruct an explanation for why no dipole moment arrows are shown on the bond on the left of the molecule in igure opyright oughton Mifflin arcourt ompany. All rights reserved.

15 hemactivity 4: Polar Bonds, Polar eactions ircle the most acidic on each structure and explain your reasoning. (If there is a tie, circle more than one ) N 2 l l What is the purpose of the previous question? (What misconception is it designed to dispel?) Exercises for Part A 1. When liquid water freezes into ice, the water molecules form up into an orderly repeating pattern and it ceases to flow. a. (E) According to Model 1 and your firsthand experiences, which takes up less space: 2 in the orderly repeating pattern found in ice or 2 in the fluid arrangement of molecules found in liquid water? b. Is your answer to the previous question consistent with the fact that ice floats in water? Explain. 2. Energy must be added to liquid water in order to make it boil (or evaporate) into gaseous water. In what ways does the energy that has been added change the system? 3. or each of the following molecules l 2 2 Br l All 2 Al 2 3 Li Na a. alculate the EN difference for each polar bond. (Assume and bonds are nonpolar.) b. Draw a dipole moment arrow for each polar bond. c. Add δ+ and δ to atoms you expect to have a lack or excess of electron density. d. Sketch the electron cloud for the molecule so as to indicate areas of high and low electron density. 4. arbon tetrachloride (l 4, boiling point = 77 o ) is a very potent ozone depleter. An unscrupulous technician decides to dispose of 100 ml (approximately 1 mole) of l 4 by boiling it off into the atmosphere. Estimate the percentage of -l bonds in the original ~6 x molecules of l 4 that are broken during this boiling process. opyright oughton Mifflin arcourt ompany. All rights reserved.

16 50 hemactivity 4: Polar Bonds, Polar eactions 5. ompare the strength of the following two effects to each other, and for each effect, draw the structure of a molecule whose boiling point is primarily determined by that effect. dipole-dipole attraction induced dipole-induced dipole attraction (London or dispersion forces) Note: Attractive forces called dipole-induced dipole forces occur when a fixed dipole interacts with a nonpolar molecule. 6. It seems to me that a molecule without a dipole moment should be a gas at any temperature. What information is the author of this erroneous statement missing? 7. A student decides to graph bp vs. molecular weight. This graph is shown below, on the left. a. What boiling point trends are highlighted by this graph? b. The alkyl bromide (-Br) line seems out of place to this student. She feels the alkane line (- 3 ) should be farthest to the right. onstruct an explanation for why she thinks this. c. She brings her graph to her professor. The professor suggests that she make a new graph with a different property plotted on the x axis. When the student does this, the graph comes out as she expected. What molecular property is plotted along the x axis of this new graph? (Shown below, right.) Br Boiling Point ( o ) Boiling Point ( o ) Br Molecular Weight (g/mol)?? 8. Learn the rules for naming alkanes. (Alkane = a molecule made up exclusively of and atoms connected via single bonds.) These rules are the first topic in Nomenclature Worksheet ead the assigned sections in your text, and do the assigned problems. opyright oughton Mifflin arcourt ompany. All rights reserved.

17 hemactivity 4: Polar Bonds, Polar eactions 51 Exercises for Part B 10. Propanal (bp 48 o ) and propanol (bp 97 o ), both found on Table 4.2, have very similar surface areas and dipole moments. onstruct an explanation for the large difference in boiling points between the two. 11. ank the following molecules from lowest to highest boiling point Summary of key intermolecular attractive forces: [strongest] hydrogen bond: polar molecules with [ on N,, or X] and [lone pair on N,, or ] [weak] dipole-dipole forces: polar molecules [weaker] dipole-induced dipole: when polar molecules or surfaces interact with nonpolar molecules [weakest] induced dipole-induced dipole: most important force for nonpolar molecules 12. Draw the conjugate base of each strong acid shown in Table 4.3 and the conjugate acid of each strong base shown in Table or each molecule below, draw the conjugate acid or conjugate base or both if the molecule has both a conjugate acid and a conjugate base (e.g., water). N l S 3 S or each structure you drew in the answer to the previous question, classify it as a strong acid, strong base, weak acid, or weak base. 15. Mark each of the following statements True or alse: a. The conjugate base of a strong acid is always a weak base. b. The conjugate acid of a strong base is always a weak acid. c. The stronger the acid, the weaker its conjugate base, and vice versa. 16. rganic chemistry is a bit like cooking. Later in this course we will study recipes for preparing new molecules. A common instruction in such recipes is add a strong acid or add a strong base (rarely are they added at the same time since a violent reaction ensues). You will see these terms frequently from now on, and are expected to be able to recognize them or draw their structures from memory. Memorize them before next class. opyright oughton Mifflin arcourt ompany. All rights reserved.

18 52 hemactivity 4: Polar Bonds, Polar eactions 17. Use curved arrows to show an acid-base reaction between each of the following pairs of reactants. In each case, the base is drawn on the right. 3 N 3 3 S 2 N or each curved arrow that you drew in the previous question, write a sentence that translates into words what that arrow is saying. 19. Are endothermic reactions favorable or unfavorable? uphill or downhill? 20. Is the arrow at right more likely associated with an energy change from the energy diagram in igure 4.12 or igure 4.13? l 21. Is bond formation endothermic or exothermic? Write a + or sign above the arrow in the previous question to represent the sign of the energy change associated with the arrow. 22. ead the assigned sections in your text, and do the assigned problems. Exercises for Part 23. Summarize the relationship between pk a and acid strength by completing the following sentences: a. The higher the pka of an acid, the stronger or weaker the acid. b. The lower the pka of an acid, the stronger or weaker the acid. 24. Summarize the relationship between pk a and base strength by completing the following sentences: a. or a given base, the higher the pk a of its conjugate acid, the stronger or weaker the base. b. or a given base, the lower the pk a of its conjugate acid, the stronger or weaker the base. opyright oughton Mifflin arcourt ompany. All rights reserved.

19 hemactivity 4: Polar Bonds, Polar eactions Perform a) through d) from TQ 47, and calculate K eq for each pair of reactants. 2 ( 3 ) 2 N and 26. onsider the following bases: N 2 N 2 a. or each base above, circle the atom/atoms with the highest PE (will release the most P.E. when a lone pair on this atom combines with an + ) b. ank the bases 1 (highest P.E./strongest base) to 7 (lowest PE/weakest base), and explain your reasoning. 27. onsider the pk a s listed below: pk a 25 pk a 45 pk a 50 a. Draw each conjugate base, and using the pk a data, rank the conjugate bases from strongest base/highest P.E. to weakest base/lowest P.E. b. or each conjugate base, indicate the type of orbital in which the lone pair of electrons resides. (int: What is the hybridization state of the atom holding the lone pair?) c. Indicate the percent s character ( red clay ) of each orbital holding a lone pair. d. Which is lower in PE, an s orbital or a p orbital? e. onstruct an explanation for the pattern in the pk a data above. opyright oughton Mifflin arcourt ompany. All rights reserved.

20 54 hemactivity 4: Polar Bonds, Polar eactions 28. The following are equivalent ways of asking about the acidity of an atom: What is the most acidic on the molecule? Which is associated with the published pk a value? Which on the molecule is easiest to remove? Which on the molecule takes the least energy to remove? Which bond to an is most polarized? or which atom is removal least uphill in energy? Which bond to an atom, when broken, results in the lowest PE conjugate base? We will often find the last of these questions is easiest to answer. To do this, find all the different atoms on the molecule, and draw all possible conjugate bases. nly the lowest-energy one is the real conjugate base. Identify this structure, and you have found the most acidic. Use this strategy to find the most acidic on each of the following molecules. Note: Each structure has at least three different kinds of s, so draw at least three unique conjugate bases for each N 2 2 Br N N The equilibrium constant for the reaction from TQ 47 is given below. [Square brackets] represent the concentration of that ion or molecule in moles/liter at equilibrium. K eq = [ + N 4 ] x [ - ] [N 3 ] x [ 2 ] = 10-7 a. Is K eq for this reaction a very large number or a very small number? or this reaction at equilibrium, will there be more reactants or products? b. hemical reactions, like water in a riverbed, tend to go downhill. Is your answer in part a consistent with the energy diagram you drew for this reaction in TQ 47? c. Which of the following best describes the mathematical relationship between Δ (in pk a units) and K eq? K eq = Δ K eq = Δ 10 K eq =10 Δ K eq =10 Δ 30. ead the assigned sections in your text and do the assigned problems. opyright oughton Mifflin arcourt ompany. All rights reserved.

21 hemactivity 4: Polar Bonds, Polar eactions 55 The Big Picture ow does this hemactivity fit with previous and upcoming activities? The Big Picture section is designed to help you answer this question. The title of the activity: Polar Bonds, Polar eactions, foreshadows the fact that almost all reactions we will study in this course are polar reactions. That is, they involve the breaking of bonds to yield partial (or full) + and charges. Naturally, a bond that is already polarized (e.g., l) is primed to participate in a polar reaction. Acid base reactions are just the first example, but you will see that all polar reactions contain elements of acid-base chemistry. Blindly memorizing everything in this workbook is not an effective (or enjoyable) strategy, but it is critical that you memorize certain key bits of information. Before next class, go back through this hemactivity and transcribe all information in the Memorization Tasks sections onto note cards (e.g., key pk a values). It is best to put the information to be memorized on the back and a hint on the front (e.g., 16 on back; pk a of on front). arry these note cards in your back pocket and flip through them whenever you have a spare minute. The key to memorization is repetition. The approximate pk a values and the energy conventions laid out in this activity are the keys to understanding acid-base reactions and all other polar reactions in this course. A moderate time investment at this point will save you vast amounts of frustration farther down the line. These memorized bits of information will serve as a crutch while you develop your deeper conceptual understanding of organic chemistry. As an aside, some students wonder why we memorize approximate pk a values (e.g., 16 for water, though the actual number is 15.7; zero for 3 +, though the actual number is -1.7, etc.). The answer is that our reason for memorizing pk a values is to know the relative acidities and basicities of the molecules we will encounter in this course. In other words, it is important to know that N 3 is a stronger base than 2, but not necessary to know exactly how much stronger it is. If you have memorized the pk a s of the conjugate acids as 9 and 0, respectively, this will tell you that N 3 is a much stronger base than 2. And though our approximations introduce some error in our calculations of Δ from pk a differences, it is well worth the trade-off in terms of the ease of memorizing round numbers. ommon Points of onfusion: An attached to a atom cannot participate in hydrogen bonding. Many students think of acid-base reactions as all or nothing. They erroneously assume that if the reaction is favorable (e.g., N 3 +, which is downhill by 12 pk a units of energy) then all N 3 and molecules will be consumed during the reaction. This is not the case with this reaction (and, in fact, no reaction goes etc. percent to completion. There is always an equilibrium point where the concentration of products causes the reaction to stop. At this point there are always some reactant molecules still around. The more the energy difference between reactants and products (the more downhill the reaction), the fewer reactant molecules remaining when the equilibrium point is reached. You may have used pk b values in a previous course, but they are not commonly used by organic chemists since the pk a of an acid tells you both its acidity and the basicity of its conjugate base. If you want to know the strength of a base, instead of looking for a pk b, find the pk a of its conjugate acid. The higher the pk a of the conjugate acid, the stronger the basicity of the conjugate base. opyright oughton Mifflin arcourt ompany. All rights reserved.

22 56 hemactivity 4: Polar Bonds, Polar eactions (water or an alcohol) can act as a weak acid or a weak base. But in each case different pk a values apply. When acting as an acid you say that it takes +16 pk a units of energy to break the bond. When it acts as a base you say that about -0 pk a units of energy are released when the lone pair on oxygen makes a bond to + to form 3 +. The exact same situation applies to amines (N 2 ). When acting as an acid you say that it takes +35 pk a units of energy to break the N bond. When it acts as a base you say that -9 pk a units of energy are released when the lone pair on nitrogen makes a bond to + to form 2 N 2 + Students love the inductive effect and so tend to overestimate its power. Do not assume that an near a halogen is acidic. Usually it is only slightly more acidic than s farther away from that halogen. In searching for the most acidic, be sure to first consider formal charges and atom electronegativities (an attached to an N or is much more acidic than an attached to a ). If neither of these gives you a clear answer, it may be that inductive effects serve as a tiebreaker, making an near an electronegative element slightly more acidic than one farther away. ur treatment of energy differs slightly from the key energy formulas found in most general chemistry textbooks: ΔG o = T lnk eq and ΔG o = Δ o T(ΔS o ) The following assumption and proof may help some of you reconcile our treatment of energy, Δ and pk a, with what you have learned in the past. Assumption: ΔS 0 for most organic reactions, therefore ΔG o Δ o Proof that pk a is a measure of energy proportional to ΔG: pk a = log K a, and under standard conditions K a K eq ( = proportional to ) ΔG o = T lnk eq, since (log x) (ln x) it follows that pk a ΔG o ΔG o is a measure of energy, therefore pk a is a measure of energy. opyright oughton Mifflin arcourt ompany. All rights reserved.