Chemistry Final Examinations Summer 2006 answers

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1 Chemistry Final Examinations Summer 2006 answers A GEERAL CEMISTRY answers are given from 1-20: no reaction C 3 CC 3 Ph C C C C C 3 Et 2 2 2

2 B. REACTIS AD REAGETS [32 MARKS] 2. A single substance can often be converted into more than one closely related synthetic intermediate by appropriate selection of experimental conditions. Provide experimental conditions to carry out each of the indicated conversions [the answer requires the starting material to be converted independently to each of the products]. Some of the conversions may require more than one step. Mg Et 2 Mg C 3 C and: C 2 C 3 C 3 BS 2 hν Fe 3. Answer the following questions about the Diels Alder reaction. Remember, stereochemistry is important. [8 marks] a) Each of the following pairs of starting materials provides a single major Diels Alder product. Give the formula of each one.

3 + 3 CC CC 3 CC 3 CC 3 + C C b) Give the starting materials which would yield the Diels Alder products shown here: C C C 2 C 3 C 4. Provide a structure for each of the lettered intermediates [A - D] in the reaction sequence shown below. [8 marks] Et + selective for ketone; no reaction with ester A Et DIBAL- (one mole) toluene 2 selective reduction ester to aldehyde B C 3 MgI dry Et remember - the hydrolysis cleaves off the ketal group C ab C 3 D

4 5. Consider the compound shown here: a) Draw the structure of all possible enols of this compound b) Give the product of its reaction with excess D 2 and a catalytic amount of base ( - ) c) Give the product of its reaction with one mole of 2 and a catalytic amount of acid ( + ) d) Give the product which results when this compound is treated with sodium ethoxide (aet) d) Give the product of the crossed aldol reaction which takes place when the following two compounds are reacted (for example with a/): + a) b) D D D D D D D D D D c) d) e) 6. There are fatal flaws in each step in the following sequence of reactions. early identify what is wrong with each reaction.

5 2 S 3 S 3 S 3 S 3 ab Al 3 Et 2 Fe S 3 S 3 Mg C 3 + dry Et 2 2 KMn 4 heat Li 3 + dry Et 2 step 1: must be 2 S 4 step 2: Friedel Crafts reactions cannot be carried out when only meta- substituents are present step 3: ab 4 does not reduce ketones to methylenes but to alcohol step 4: reaction will not occur between the two substituents step 5: cannot perform a Grignard reaction in the presence of the sulfonic acid step 6: S 3 is removed by dilute acid, not dilute base step 7: oxidation would cause both substituents to go to acid step 8: reaction of organolithium with aldehyde does not yield ketone, but alcohol C. MECAISMS AD CCEPTS [32MARKS] D FUR F QUESTIS Aromaticity Use the ückel rule to estimate whether each of the following compounds will exhibit aromaticity or not. Give your reasons. The correct answer for the wrong reason will not get any marks. asssume it can be planar anion is 2 electrons sp 3 centres each triple bond is 2 each oxygen is 2 total 10; n=2 not conjugatged total 18; n=4 total 10; n=2 ARMATIC T ARMATIC ARMATIC ARMATIC

6 8. a) Provide an explanation for the following observations: p-,-dimethylaminotoluene [below] undergoes nitration in mildly acidic solution to give 4-dimethylamino-3-nitrotoluene and in strongly acidic solution to give 4-dimethylamino-2-nitrotoluene. 2 mild + strong + 2 In mild acid, no protonation of the nitrogen occurs, and thus the 2 substituent acts normally: δ The nitrogen is able to stabilize the ortho- or para- intermediate in the usual fashion, leading to the normal substitution pattern. In strong acid solution, the nitrogen protonates, and becomes a very deactivating, metadirecting group: 2 Both the methyl and the ammonium now direct to the other position. 2 b) 3-thoxy-1,2,3-trimethylcyclopropene [shown below] has been found to react with tetrafluoroboric acid to yield methanol and a compound C 6 9+ BF 4-. What is the structure of this product? Why is it formed so easily?. C 3 C 3 BF 4 C 6 9 BF 4 + C 3 3 C C 3 Ionization is particularly easy since the product is aromatic; (4n+2) where n=0

7 - BF 4 BF 4 C 3 C 3 C 3 C 3 -C 3 C 3 C 3 BF 4-3 C C 3 3 C C 3 3 C C 3 3 C C 3 9. Rank the following compounds in order of ease of addition of a nucleophile [for example methanol]; clearly indicating for which the reaction is easiest [fastest], and give a brief explanation as to your choice of order. F 3 C 3 1 (best) 4 (worst) 2 Reaction depends upon the relative positive charge located on the carbonyl carbon, and upon steric influences. Considering both of these criteria, aldehydes normally react faster than ketones the alkyl group is inductively electron donating, decreasing the *+ charge on carbonyl carbon, making attack slower replacing a (small) with an alkyl (larger) increases the steric problems in both addition and product crowding, making aldehydes faster reacting. Between the two aldehydes, CF 3 will react much faster than C 3. Since F is strongly electron withdrawing, *+ will be very large and addition will occur very easily. Between the two ketones, we must compare isopropyl (2-propyl) and phenyl as substituents. Both are large, but phenyl has the added feature of being able to delocalize the + charge through resonance with the ring: This makes it much less *+ on the carbon, and reaction the slowest of these four. 10. The acidity of carboxylic acids is dependent upon the nature and position of substituents. The pk a values of the four acids given below each differ from the other by one pk unit or more. Rank the four acids in order of acidity, from most acidic to least, and give a brief explanation for this ranking. The actual pk values are 1.7, 2.9, 4.1, and 5.0.

8 (most acidic) Acidity depends upon the relative stability of the carboxylate ions. The substituents here can only affect this stability through inductive effects; since the charge on the carboxylate is negative, electron withdrawing groups will help stabilize the anion and electron donors will not. 2 is a strong electron withdrawing group, and as such will stabilize the anion most effectively and make the corresponding acid the most acidic (1.7). is a weaker electron withdrawer, but will still stabilize and increase acidity; 2- will be more acidic (2.9) than 3- (4.1) since the inductive effect of a substituent decreases rapidly with distance. thyls are electron donors, and as such increases electron density on the carboxylate anion, and thus this acid is the least acidic (5.0). 11. Give the mechanism of either of the reactions shown here. Remember to include all intermediate structures and use proper arrows. [8 marks] C 3 + or C 3 Li 3 + Et 2

9 + + 3 C 3 C C C 3 B 3 3 C 2 3 C C 3 3 C C3 3 C C3 C 3 Li + Li ,4-Dinitrochlorobenzene undergoes an easy nucleophilic aromatic substitution (as shown below) to yield the methoxy compound. 3,5-Dinitrochlorobenzene (or, for that matter, chlorobenzene itself) does not react under the same conditions. Using proper mechanistic diagrams, clearly explain why this is so. 2 a ucleophilic aromatic substitution occurs by an addition-elimination mechanism. In the initial step, the nucleophile, methoxide ion, attacks the benzene ring adding to it and forming an intermediate which is negatively charged:

10 In the absence of substituents (in chlorobenzene itself), this anion is not sufficiently stable, even with resonance delocalization, for the process to occur. Strong electron-withdrawing substituents must be present at appropriate positions i.e. where they can further delocalize the charge. This happens readily for the compound above: 2 2 and Each of the nitro groups can become involved and the anionic intermediate is greatly stabilized to the point where addition can occur. ote that if the position of the nitros is incorrect (as in the 3,5- isomer), no reaction occurs. The process then completes by loss of the leaving group: D SYTESIS 13. Complete TREE of the four syntheses shown below, using only the indicated starting materials and any other necessary reagents, solvents, etc., remembering that you must make any organometallic or Wittig reagents you with to use [this refers to RMgX or RLi, not to LiAl 4 ]. Each synthesis may be carried out in five or less steps.

11 a) from any organic starting materials of 4 carbons or less P 3 Li Et 2 C 3 + P 3 PPh 3 TF n-buli PPh 3 PCC PPh 3 C 2 2 TF b) or Et or Et Et and any other necessary reagents Et aet Et Bu Et aet Et I Et 3 + Li dry Et and I + 2 Li dry Et 2 Li

12 c) C from benzene and any necessary reagents 2 S 3 2 S 4 S 3 K dil 2 S 4 S 3 Fe 2 S 4 S 3 d) from benzene and any necessary reagents C 3 C 2 Al 3 Fe BF 3 Mg C C dry Et 2