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All Textbook Solutions for Organic Chemistry: A Guided Inquiry

(E) What does the number (+Z) at the center of each atom in Figure 1.1 represent, and whatnumber would you expect at the center of a representation of a bromine atom (Br)?2CTQ 3CTQ 4CTQ 5CTQ 6CTQ 7CTQ You hear a student from a nearby group say that “Electron domains repel one another.” Citeevidence from Figure 1.2 to support or refute this statement.Use VSEPR to explain why the HBH bond angle of BH3 is 120°. (Hint: What is one-third of360°?)Both the HCH and HCO bond angles of H2CO (formaldehyde) are very close to 120°, but oneis slightly smaller than the other. Predict which is smaller, and explain your reasoning.11CTQ Consider the following flat drawing of methane (CH4) . a. What is HCH bond angle implied by this drawing if you assume it is flat? b. Are the electron domains of this flat CH4 spread out as much as possible? c. Use model materials to make a model of CH4 (methane). If you assembled it correctly, thefour bonds (bonding electron domains) of your model will be 109.5° apart. d. In which representation, the drawing above or the model in your hand (circle one) are theH’s of CH4 more spread out around the central carbon? e. Confirm that your model looks like the following drawing. The wedgebond represents a bond coming out of the page, and the dash bondrepresents a bond going into the page f. You will often see methane drawn as if it were flat (like on the previous page). Why is thismisleading, and what is left to the viewer’s imagination when looking at such a drawing?Use VSEPR to assign a value of (close to) 109.5, 180 or 120 to each marked bond angle.A student draws the picture of ammonia (NH3) in the box below, left, and predicts it will be a flatmolecule with HNH bond angles of exactly 120°. Unfortunately, the student left something out. a. What did the student omit from his drawing? b. What is the actual HNH bond angle of ammonia (based on the draw g above, right)? c. Explain why water, ammonia, and methane (shown below) all have about the same bondangles (close to 109.5°) even though they have different numbers of bonds.15CTQ How many central atoms does the molecule H2NCH3 have, and what is the shape about each?Indicate the bond angle and shape about each central atom.Explain how there can be two kinds of bent: “bent-109.5o” and bent-120o,” and give an exampleof each from the previous question. (Note that “bent-109.5°’ is more common than “bent-120°.”)A student makes the following statement: “The shape of water is tetrahedral because the fourelectron pairs about oxygen are approximately 109.5° apart and point to the corners of atetrahedron.” What misconception does this statement convey?A student who missed this class needs to know how to predict the bond angles and shape of amolecule from looking at its bond-line representation. Write a concise but complete explanationfor this student.1E 2E Consider the incomplete valence shell representation below. a. Assume the atom is neutral, and write the correct nuclear charge at the center of the atom. b. What is the identity of this neutral atom?How many valence electrons does a neutral a. K atom have? b. C atom? N atom? O atom?Consider the molecules AlCl3 (aluminum chloride) and CF4 (carbon tetrafluoride). a. Draw the valence shell representation of each. b. Predict the value of the XYX bond angle, and explain your reasoning.Draw an example of a bent molecule with a bond angle of near 109.5°; then draw a different bentmolecule with a bond angle of about 120°.Label each atom marked with an arrow with the appropriate shape name, and estimate the bond angles around it as being close to one of 180°, 120°, 109.5° or 90°. (circled charges indicate the charge on the molecule or fragment)a model of each of the following molecules: a. Based on your model, draw a bond-line representation with as many atoms as possible in the plane of the paper. Use wedge and dash bonds to represent any atoms that do not lie in the plane of the paper. b. Indicate each unique bond angle and the shape of each unique central atom.1CTQ The valence shell of an atom in a legitimate Lewis structure (see Figure 2.3) has what in commonwith the valence shell of a noble gas? (Noble gases are stable elements found in the last column ofthe periodic table, e.g., He, Ne, Ar, etc.)3CTQ 4CTQ 5CTQ It is impossible to draw a legitimate Lewis structure of a neutral NH4 molecule. Hypothetically,how many valence electrons would such a neutral NH4 molecule have ifit could exist? a. The +1 cation, NH4+ , does exist. How many valence electrons does one NH4+ ion have? b. Draw the Lewis structure for NH4+Describe how to calculate the total number of valence electrons in a +1 ion, in a 1 ion.8CTQ 9CTQ 10CTQ 11CTQ 12CTQ A complete Lewis structure must show all nonzero formal charges. Complete each of thefollowing Lewis structures by adding any missing formal charges.14CTQ 15CTQ 16CTQ 17CTQ 18CTQ Complete the rest of the table for N, O or X by drawing the number of Lewis structures specified.20CTQ 21CTQ Make a checklist that can be used to determine if a Lewis structure is correct and that it is the bestLewis structure.2E 3E Draw the Lewis structure of a neutral molecule that is a naturally occurring combination ofhydrogen atoms and one sulfur atom. What is the shape of this molecule?5E For each element, predict (and draw a Lewis structure of) the most commonly occurring ion (some of these have a charge greater than +/1 ) a. sulfur c. magnesium b. iodine d. oxygen Predict which of the following species is least likely to exist. CH2NO+HO The molecules BH3 and SF6 and the ion SO42 exist and are stable. Draw a Lewis structure of each,and comment on whether they are exceptions to Lewis’ octet rule.These are NOTlegitimate Lewisstructures (and aremissing formalcharges). Show (as inthe example) how apair of electrons canbe moved to make theLewis structurelegitimate.Fill in missing formal charges where needed (all lone pairs are shown).Below each structure in the previous question is a “condensed structure” that tells you somethingabout how the atoms are arranged. Draw complete Lewis structures for each of the followingcondensed structures. (The net charge, if any, on each molecule is given at the end.) a. CH3CH2 b. CH2CH2 c. CH2CCH2 d. C( CH3)3+ e. BF4 f. NCO g. CH2OH+ (two different acceptable answers) h. CH2CHCHCHCH2+ which may alsobe written as CH2(CH)3CH2+ (more than one acceptable answer)12E Carbon monoxide (CO) is an example of an overall neutral molecule (netcharge=0) that hasnon-zero formal charges. Draw a Lewis structure of carbon monoxide (CO).Explain why this Lewis structure for CO is not as valid as the Lewis structure you drew in theprevious question even though it has no “hot spots” of + or charge (formal charges).15E 16E 17E 18E 19E 1CTQ What neutral atom is represented by the electron configuration in Figure 3.2?3CTQ Consider any one of the four identical hybrid orbitals in the 109.5° set. a. What fraction of the clay in this hybrid orbital was originally red (carne from the 2s orbital)? b. What fraction of the clay in this hybrid orbital was originally green (carne from a 2porbital)? c. Explain the name “ s(1/4)p(3/4) hybrid orbital” for each of the four orbitals. d. In fact, each of the four hybrid orbitals in the 109.5° set is called an sp3 -hybrid orbital.Explain the name “ sp3 -hybrid orbital.”5CTQ 6CTQ 7CTQ 8CTQ 9CTQ 10CTQ On the left side of Figure 3.6, label the areas shown with a dotted line where... one bond can form. one bond can form.12CTQ 13CTQ 14CTQ 15CTQ Now consider the fully formed molecule on the right side of Figure 3.7. a. Draw a Lewis structure of this molecule. b. Identify orbital representations of the two bonds and three bonds in Figure 3.7, andmatch these with representations of bonds on the Lewis structure you just drew.1E Explain why the two molecules below cannot inter-convert without breaking the portion of thedouble bond.3E Consider the incomplete orbital representation of O2 , below right. a. Identify which lobes are hybrid orbitals (identify the type) and which lobes arep orbitals. b. Use dotted lines to show any bonds. c. Use up or down arrows to show electron occupation of each hybrid orbital or bond.Consider the following orbital representation of HCCH (ethyne). a. Answer the same three questions (a-c) from the previous exercise. b. Label each CH bond in the drawing above. c. What is the total number of a bonds found in ethyne?…. bonds? d. How many p orbitals are there on a single carbon of ethyne? e. How many hybrid orbitals are there on a single carbon of ethyne?Summarize how one determines the hybridization (also called hybridization state) of an atom ina molecule.Explain what is wrong with each of the following statements. a. “A bond is a double bond. b. “A bond consists of four electrons, one in each of the four p orbitals involved in the bond. c. “A bond is twice as strong as a bond because it consists of two orbital overlaps insteadof just one.”8E 9E Complete the following tables, and memorize their contents.Draw orbital representations of bonding in water and methane (CH4) similar to that of ammonia inFigure 3.4.Draw electron configuration diagrams for carbon in an unhybridized, sp3 -hybridized state, sp2 -hybridized state, and sp-hybridized state.13E 1CTQ Figure 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 anintermolecular force found in Figure 4.1. b. (E) Which are shorter and stronger: covalent bonds p intermolecular forces [circle one]?3CTQ 4CTQ In HF , neither H nor F holds a full formal charge of +1 or 1 . Organic chemists represent apartial charge using the Greek letter delta () . On the electron density map of the molecule HF above, add a + to one atom and a to the other to indicate which way the bond is polarized.6CTQ 7CTQ 8CTQ Within any one section of Table 4.2, boiling points trend with what physical property?10CTQ 11CTQ 12CTQ 13CTQ 14CTQ 15CTQ 16CTQ 17CTQ 18CTQ 19CTQ 20CTQ 21CTQ 22CTQ (E) Label each of the following as strong acid, strong base, or neither.24CTQ Draw the structure of the conjugate base of water. (Note that it does not appear in Figure 4.11).Does Cl have a conjugate acid? If so, what is it? . .. a conjugate base? If so, what is it?Draw the conjugate base of CH4 (methane).For the previous four questions, label each molecule that appears in the question or your answer asstrong acid, strong base, weak acid, or weak base.29CTQ According to the conventions above, what is the sign ( + or ) of the P.E. change (H) for Rxn 3?Draw an arrow on Figure 4.13 representing Hrxn4 . (Hint: study the Hrxn3 arrow in Figure 4.12)32CTQ Add a + or above each curved arrow in Figure 4.11 to show the sign of the energy change.34CTQ 35CTQ 36CTQ 37CTQ 38CTQ 39CTQ 40CTQ 41CTQ 42CTQ 43CTQ 44CTQ 45CTQ 46CTQ For NH3 (ammonia) and H2O (water)... a. Use curved arrows to show the most likely acid-base reaction, and draw the resultingproducts. (Hint: First 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) numberindicating the energy change associated with that arrow (in pKa units). c. Calculate Hrxn and write this number above a set of reaction arrows that indicate whichdirection 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-basereaction is endothermic?48CTQ 49CTQ 50CTQ 51CTQ 52CTQ 53CTQ 1E 2E 3E 4E 5E 6E 7E 8E Propanal (bp 48°C) and propanol (bp 97°C), both found on Table 4.2, have very similar surfaceareas and dipole moments. Construct an explanation for the large difference in boiling pointsbetween the two.Rank the following molecules from lowest to highest boiling point.12E For each molecule below, draw the conjugate acid or conjugate base or both if the molecule hasboth a conjugate acid and a conjugate base (e.g., water).For each structure you drew in the answer to the previous question, classify it as a strong acid,strong base, weak acid, or weak base.Mark each of the following statements True or False: 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.Organic chemistry is a bit like cooking. Later in this course we will study “recipes” for preparingnew molecules. A common instruction in such recipes is “add a strong acid” or “add a strongbase” (rarely are they added at the same time since a violent reaction ensues). You will see theseterms frequently from now on, and are expected to be able to recognize them or draw theirstructures from memory. Memorize them before next class.17E 18E Are endothermic reactions favorable or unfavorable? ... uphillor downhill?20E Is bond formation endothermic or exothermic? Write a + or sign above the arrow in the previousquestion to represent the sign of the energy change associated with the arrow.Summarize the relationship between pKa 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.Summarize the relationship between pKa and base strength by completing the followingsentences: a. For a given base, the higher the pKa of its conjugate acid, the stronger or weaker the base. b. For a given base, the lower the pKa of its conjugate acid, the stronger or weaker the base.25E Consider the following bases: a. For 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 H+ ) b. Rank the bases 1 (highest P.E./strongest base) to 7 (lowest PE/weakest base), and explainyour reasoning.27E The following are equivalent ways of asking about the acidity of an H atom: • What is the most acidic H on the molecule? • Which H is associated with the published pKa value? • Which H on the molecule is easiest to remove? • Which H on the molecule takes the least energy to remove? • Which bond to an H is most polarized? • For which H atom is removal least uphill in energy? • Which bond to an H 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 Hatoms on the molecule, and draw all possible conjugate bases.Only the lowest-energy one is the “real” conjugate base. Identify this structure, and you have found the most acidic H. Use this strategy to find the most acidic H on each of the following molecules. Note: Each structure hasat least three different kinds of H’s, so draw at least three unique conjugate bases for each.29E Which elements on the periodic table (other than H) are likely to form a+1 cation?You will not find “hydroxide” in the stockroom, but you will find sodium hydroxide (NaOH) andpotassium hydroxide (KOH). Lithium hydroxide (LiOH) is expensive and used in spacecraft airfilters since hydroxide reacts with carbon dioxide, and lithium is lighter than sodium or potassium.Cesium and francium hydroxides are very expensive and little used. Is this information consistentwith your answer to the previous question?3CTQ 4CTQ 5CTQ 6CTQ On which do you expect to have a more intense and concentrated “hotspot” of negative charge: methoxideion or bicarbonate ion?8CTQ 9CTQ 10CTQ 11CTQ 12CTQ 13CTQ 14CTQ 15CTQ 16CTQ For each proposed set of resonance structures: a. (E) Add curved arrows (starting from left) to show how each successive r.s. was generated. b. Cross out any resonance structures that are NOT important, and explain your reasoning.Consider the polarization of the C=O bond in the molecule acetone a. (E) Which atom (C or O) is expected to have a greater electron density? b. (E) Add a + and to this bond to emphasize its polarization.The C=O double bond is called a “carbonyl bond.” Acetone and othercarbonyl compounds are introduced in some texts along with this structure a. Is this an important resonance structure of acetone? Explain. b. Does this structure convey any useful information about acetone? If so, what?20CTQ 21CTQ 22CTQ 23CTQ 24CTQ 25CTQ 26CTQ 27CTQ 28CTQ 29CTQ 30CTQ 31CTQ Confirm that there is no legitimate Lewis structure of ozone with all zero formal charges.Draw all resonance structures of the molecule nitromethane (H3CNO2) .34CTQ 35CTQ 36CTQ Occasionally, we will see an ionic compound that has a 1 counterion. (Later we will find that 1 counterions are often more than spectators and take an active role in many reactions.) a. What elements (other than H) on the periodic table are most likely to form a 1 anion? b. Draw a Lewis structure for the ionic compound NH4Cl (Hint. One atom is a 1 counterion).2E 3E 4E Is it possible to draw a resonance structure of nitrate ion (NO3) that has one formal charge?6E 7E 8E Phenol (shown below) has a pKa10 . a. Based on pKa data, is phenol a stronger or weaker acid than an ordinaryalcohol (e.g.. CH3OH ) or water? b. Draw the conjugate base of phenol (called phenoxide) including allimportant resonance structures. c. Construct an explanation for why phenol is a stronger acid than anordinary alcohol. (You may want to consider first why phenoxide is lowerin PE than methoxide (RO) or hydroxide (HO) .Use curved arrows to show the most likely acid-base reaction between phenol and NaOH. a. Use pKa data to mark each curved arrow with a positive or negative energy change in pKa ,units. b. Calculate H for this reaction, and sketch an energy diagram showing H as an arrow onyour diagram.12E Complete each Lewis structure, draw all important resonance structures, predict a value for thebond angles requested, and explain your reasoning. a. Nitrous acid (HNO2)HONOHON=ONO= b. Enolate ion (C2H3O) HC1C2=HC2C1=Use curved arrows to show the most likely acid-base reaction for each pair of reactants below.Draw the resulting products including all important resonance structures. For each reaction, indicate if Hrxn is negative, positive, or close to zero, and explain your reasoning.Construct an explanation for why sulfuric acid is such a strong acid. (Note that sulfur is in thethird row of the periodic table and can have more than eight electrons.)16E 17E 18E 1CTQ (E) Write a correct name below each of the unbranched alkanes in CTQ 1.(E) What suffix do all the names in Model 1 have in common with each other?(E) What prefix stands for eight carbons?5CTQ 6CTQ 7CTQ 8CTQ 9CTQ Use Model 1 to propose names for three-, four-, five-, and six-carbon branches that follow the same pattern as “methyl” and “ethyl” for one- and two-carbon branches, respectively. (Note: The names of seven-, eight-, etc. carbon branches follow the same pattern, but branches of such length are rare since they are usually the parent chain.)11CTQ 12CTQ 13CTQ Name the following alkanes.(Check your work.) Explain what is wrong with each of the following names for the first moleculeabove: a. 4,4-dimethylpentane b. 1,1,1-trimethylbutane c. 2,2-methylpentane d. 2-dimethylpentane 16CTQ Draw the following alkanes a. 2,3,3-trimethylpentane b. 3-ethyl-2,5-dimethylhexane 18CTQ Draw structures that correspond to the following names: a. 3-methylhexane b. 1,1-diethylcyclobutane c. 1-t-butyl-4-methylcyclohexane For mono-substituted cycloalkanes the “1” is not included. That is, the name1-methylcyclohexane is not correct. The correct name is simply “methylcyclohexane”. a. Draw methylcyclohexane. b. Explain why adding a “1” to methylcyclohexanedoes not add any new information.21CTQ 22CTQ 23CTQ 24CTQ 25CTQ Write the name of the molecule on the left using alternate side chain names (where applicable).1E 2E Name each of the following structures.1CTQ 2CTQ 4CTQ 5CTQ Complete this graph of relative potential energy vs. rotation of the C1C2 bond of ethane. Thepoint drawn for you indicates the P.E. at 0° (eclipsed).7CTQ 8CTQ 9CTQ Consider the Newman projection below. a. Draw a full Lewis structure of this molecule with R1=Me,R2=Et , and R3=iPr . b. Given the sizes of these R groups (R3R2R1) , does the Newman projection above show thelowest potential energy conformation of this bond? If not, draw a Newman projectionshowing the lowest P.E. conformation (sighting down this same bond). c. To draw a Newman projection in the lowest P.E. conformation, the following rule of thumbusually applies: Place the largest group on the front carbon anti to the largest group on theback carbon. Is your answer to the previous question consistent with this rule of thumb?Draw a Newman projection showing the lowest P.E. conformation of the following moleculesighting down the C2C3 bond (as indicated below). Show methyl and ethyl groups as Me and Et.12CTQ 13CTQ In skeletal representations the hydrogens are not shown. Is it still possible to tell how manyhydrogens there are on a particular carbon? If so, explain how.15CTQ 16CTQ 17CTQ 19CTQ 20CTQ 21CTQ 22CTQ 23CTQ Draw a constitutional isomer of pentane, andexplain why branching does not change the ratioof C to H (the molecular formula remains C5H12 ).How many H’s are lost from the molecular formula of pentane if a double bond is introduced(changing the parent name of the molecule to pentene)?How many ifs are lost from the molecular formula of pentane if the ends are joined into a ring(changing the parent name of the molecule to cyclopentane)?27CTQ What is the degree of unsaturation for the example molecular formula C7H8NOBr ?Without counting hydrogens, determine which one of the following CANNOT be the unknownmolecule with molecular formula C7H8NOBr , and explain your reasoning.Determine the degree of unsaturation (and draw a possible structure) for a molecule withmolecular formula C6H4O2 a model of each molecule shown above: Is the molecule in the left box the same moleculeas the molecule in the right box? Use your models to answer the question, and recall that...32CTQ 33CTQ Label each double bond E, Z, or neither. (It may help to draw in some missing H’s.)35CTQ 36CTQ Indicate the relationship between each pair. Choose from: configurational stereoisomers,conformers, constitutional isomers, or different formulas (Each term is used at least twice.)38CTQ 1E 2E Using your model of butane (CH3CH2CH2CH3) , complete the following graph of the anglebetween the two Me groups vs. potential energy. a. Label each Newman projection of butane on the graph with the words staggered, eclipsed, gauche, and anti, as appropriate. (Note that some structures will have more than one label.) b. Draw a wedge and dash bond representation of butane in its lowest P.E. conformation.Consider the molecule 1-bromo-2-methylbutane. C3 and C4 should be drawn as Et as in theexample. This group is called an ethyl group and can be considered a sphere about twice the sizeof a methyl group. Draw the following Newman projections sighting down the C1C2 bond... a. The lowest potential energy conformation. b. The highest potential energy staggered conformation.5E

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