Answers To Chapter 1 Problems.

Transcription

1 Answers To Chapter Problems.. (a) Both and in amides have lone pairs that can react with electrophiles. When the reacts with an electrophile +, a product is obtained for which two good resonance structures can be drawn. When the reacts, only one good resonance structure can be drawn for the product. reaction on reaction on (b) sters are lower in energy than ketones because of resonance stabilization from the atom. Upon addition of a nucleophile to either an ester or a ketone, a tetrahedral intermediate is obtained for which resonance is not nearly as important, and therefore the tetrahedral product from the ester is nearly the same energy as the tetrahedral product from the ketone. As a result it costs more energy to add a nucleophile to an ester than it does to add one to a ketone. (c) xactly the same argument as in (b) can be applied to the acidity of acyl chlorides versus the acidity of esters. ote that Cl and have the same electronegativity, so the difference in acidity between acyl chlorides and esters cannot be due to inductive effects and must be due to resonance effects. (d) A resonance structure can be drawn for in which charge is separated. ormally a charge-separated structure would be a minor contributor, but in this case the two rings are made aromatic, so it is much more important than normal. (e) The difference between and is that the former is cyclic. Loss of an acidic from the γ C of gives a structure for which an aromatic resonance structure can be drawn. This is not true of..

2 . C C + C (f) Both imidazole and pyridine are aromatic compounds. The lone pair of the -bearing in imidazole is required to maintain aromaticity, so the other, which has its lone pair in an sp orbital that is perpendicular to the aromatic system, is the basic one. Protonation of this gives a compound for which two equally good aromatic resonance structures can be drawn. By contrast, protonation of pyridine gives an aromatic compound for which only one good resonance structure can be drawn. + (g) The C=C π bonds of simple hydrocarbons are usually nucleophilic, not electrophilic. owever, when a nucleophile attacks the exocyclic C atom of the nonaromatic compound fulvene, the electrons from the C=C π bond go to the endocyclic C and make the ring aromatic. u u non-aromatic aromatic (h) The tautomer of,-cyclohexadienone, a nonaromatic compound, is phenol, an aromatic compound. (i) Carbonyl groups C= have an important resonance contributor C +. In cyclopentadienone, this resonance contributor is antiaromatic. [Common error alert: Many cume points have been lost over the years when graduate students used cyclohexadienone or cyclopentadienone as a starting material in a synthesis problem!] (j) Ph is considerably more acidic than t (pk a = 0 vs. ) because of resonance stabilization of the conjugate base in the former. S is larger than, so the S(p) C(p) overlap in PhS is much smaller than

3 . the (p) C(p) overlap in Ph. The reduced overlap in PhS leads to reduced resonance stabilization, so the presence of a Ph ring makes less of a difference for the acidity of S than it does for the acidity of. (k) Attack of an electrophile + on C gives a carbocation for which three good resonance structures can be drawn. Attack of an electrophile + on C gives a carbocation for which only two good resonance structures can be drawn (a) F C Cl C inductive electron-withdrawing effect of F is greater than Cl (b) In general, A + is more acidic than A

4 (c). t C C C Ketones are more acidic than esters (d) Deprotonation of -membered ring gives aromatic anion; deprotonation of -membered ring gives anti-aromatic anion. (e) The (sp ) lone pair derived from deprotonation of pyridine is in lower energy orbital, hence more stable, than the (sp ) lone pair derived from deprotonation of piperidine. (f) P column in the periodic table due to Acidity increases as you move down a increasing atomic size and hence worse overlap in the A bond (g) C t C t The anion of phenylacetate is stabilized by resonance into the phenyl ring. (h) t C C t t C C t Anions of,-dicarbonyl compounds are stabilized by resonance into two carbonyl groups

5 (i). The anion of -nitrophenol is stabilized by resonance directly into the nitro group. The anion of -nitrophenol can't do this. Draw resonance structures to convince yourself of this. (j) C C More electronegative atoms are more acidic than less electronegative atoms in the same row of the periodic table (k) Ph C Ph C(sp) is more acidic than C(sp ), even when the anion of the latter can be delocalized into a Ph ring. (l) The anion of the latter cannot overlap with the C= π bond, hence cannot delocalize, hence is not made acidic by the carbonyl group. (m) this C atom this C atom The C(sp ) bond on the upper atom is the plane of the paper, orthogonal to the p orbitals of the C= bond, so the C= bond provides no acidifying influence. The C(sp ) bonds on the lower atom are in and out of the plane of the paper, so there is overlap with the C= orbitals.. (a) Free-radical. (Catalytic peroxide tips you off.)

6 . (b) tal-mediated. (s) (c) Polar, acidic. (itric acid.) (d) Polar, basic. (Fluoride ion is a good base. Clearly it s not acting as a nucleophile in this reaction.) (e) Free-radical. (Air.) Yes, an overall transformation can sometimes be achieved by more than one mechanism. (f) Pericyclic. (lectrons go around in circle. o nucleophile or electrophile, no metal.) (g) Polar, basic. (LDA is strong base; allyl bromide is electrophile.) (h) Free-radical. (AIB tips you off.) (i) Pericyclic. (lectrons go around in circle. o nucleophile or electrophile, no metal.) (j) tal-mediated. (k) Pericyclic. (lectrons go around in circle. o nucleophile or electrophile, no metal.) (l) Polar, basic. (thoxide base. Good nucleophile, good electrophile.) (m) Pericyclic. (lectrons go around in circle. o nucleophile or electrophile, no metal.). (a) The mechanism is free-radical (AIB). Sn and Br are missing from the product, so they re probably bound to one another in a by-product. Made: C C, Sn Br. Broken: C C, C Br. Br C Bu Sn cat. AIB C + Bu SnBr (b) Ag + is a good Lewis acid, especially where halides are concerned, so polar acidic mechanism is a reasonable guess, but mechanism is actually pericyclic (bonds forming to both C0 and C of the furan and C and C of the enamine). Cl is missing from the product; it must get together with Ag to make insoluble, very stable AgCl. An extra appears in the product; it must come from during workup. ne of the s in goes with the BF, while the other is attached to in the by-product. Made: C C0, C C, C (water), Ag Cl. Broken: C, C Cl.

7 . Cl + 0 AgBF 0 from water BF AgCl (c) This mechanism is also pericyclic. Use the carbonyl, Si, and C groups as anchors for numbering the atoms. Made: C C, C C. Broken: C C. 0 C Si Δ Si C 0 (d) Ph P is a Lewis base. The mechanism is polar under basic conditions. Made: C C, C, C. Broken: C. Ph C + C cat. Ph P C Ph C (e) The mechanism is polar under acidic conditions due to the strong acid S. Made: C C. Broken: C C. 0 cat. S C Cl, T 0 (f) The mechanism is polar under basic conditions (at). Two equivalents of cyanoacetate react with each equivalent of dibromoethane. ne of the C t groups from cyanoacetate is missing in the product

8 . and is replaced by. The can come from t or, so the C t is bound to t or. The two products differ only in the location of a atom and a π bond; their numbering is the same. Made: C C, C' C, C' C, C' t. Broken: C' C', C Br, C Br. t C C at, t; / BrC C Br t C ' ' ' ' t C t C + (g) Polar under acidic conditions. The enzyme serves to guide the reaction pathway toward one particular result, but the mechanism remains fundamentally unchanged from a solution phase mechanism. The groups provide clues as to the numbering. Made: C C, C C, C C. Broken: C enzyme P P Geranylgeranyl pyrophosphate A Taxane (h) Two types of mechanism are involved here: First polar under basic conditions, then pericyclic. At first the numbering might seem very difficult. There are two C groups in the starting material, C and C, and two in the product. Use these as anchors to decide the best numbering method. Made: C C, C C, C C. Broken: C C, Si. Si C 0 C C Li ; 0 warm to T; aq. ac C (i) The carboxylic acid suggests a polar acidic mechanism. Made: C C, C, C. Broken: C.

9 . Bn C Ph Bn Ph (j) Free-radical mechanism (AIB). Both Br and Sn are missing from the product, so they are probably connected to one another in a by-product. appears connected to C0 in the product, as C0 is the only C that has a different number of s attached in S.M. and product. Made: C C, C C, Br Sn. Broken: C Br. Br 0 Ph Bu Sn cat. AIB + Bu SnBr 0 Ph (k) o acid or base is present, and the reaction involves changes in π bonds. This is a pericyclic mechanism. Use C with its two groups as an anchor to start numbering. zone is a symmetrical molecule, but the middle is different from the end s; it s not clear which in ozone ends up attached to which atom in the product. owever, it is clear where ends up, as it remains attached to C. Made: C, C, C, C 0. Broken: C C, 0. C C C,0, 0 C C C (l) Polar mechanism under basic conditions. Again, use C with its two groups as an anchor to start numbering. C remains attached to C and in the product. C leaves as formate ion; the two s attached to C in the S.M. remain attached to it in the formate product. is still missing; it s probably lost as, with the two s in coming from C. Made: C C. Broken: C C,, C, C.

10 .0 C C 0 C aq. a C C 0, + C + (m) Bromine undergoes electrophilic (polar acidic) reactions in the absence of light. Use C as an anchor to begin numbering. In the S.M. there are two C groups, C and C. The one C group in the product must be either C or C. C is next to C in the S.M., while C is not; since the C group in the product is not next to C, it is probably C. Made: C C, C Br. Broken: Br Br. Br Br. = nucleophilic, = electrophilic, A= acidic. (a) (b) none* (c) (d) A (e) A** (f) (g) none (h) (i) none (j) (k) A (l) (m) none** (n) (o) A (p) (q) none (r) (s) A, (t) (u) (v) none (w) none (x) (y) A (z) (aa) A (bb) (cc), A (dd) (ee) none (ff) (gg), A (hh) (ii) none (jj) (kk) (ll) (mm) slightly A? *See text (Section B.) for an explanation. **The atom still has a lone pair, but if it were to use it in a nucleophilic reaction, it would acquire a very unfavorable + formal charge. The fact that an elimination reaction can occur upon removal of + from this atom (with loss of the leav-

11 . ing group next door) is irrelevant to the question of the acidity of this atom. Acidity is a measure of the difference in energy between an acid and its conjugate base. The conjugate base formed by removing + from this atom would be very high in energy.