Chemistry 350 Organic Chemistry I
Study Guide :: Unit 8
Alkenes: Reactions and Synthesis
Road-map Problems
Question
When optically active compound A (C5H9Br) is heated with an ethanolic solution of potassium hydroxide, the major product is the alkene B and the minor product is another alkene, C. Reduction of either B and C using hydrogen gas and a palladium catalyst yields compound D, which can also be prepared by reacting 1‑butene with diiodomethane treated with a zinc-copper alloy. Identify compounds A through D, clearly showing how you reached your conclusions.
The question above is a very simple example of a road-map problem, also known as a Sherlock Holmes question. The solution of such problems requires clarity of thought and a logical mind—as well as a solid understanding of organic chemistry. You will encounter many road-map problems in this course: use the following true road-map problem as an example of how to approach them.
The Jones family leaves City A, a provincial capital, and drives northwest for 259 km to City B, situated on a northeasterly flowing river, River C. The next day, the Jones family sets out in a westerly direction and, after crossing a provincial border, eventually arrives at City D. City D is situated on a river, River E, which flows into River C. After visiting Canada’s largest shopping mall in City D, the Jones family heads south for 286 km to City F, famous for its Stampede and the site of the 1988 Winter Olympics. On the final day of their trip, the Jones family leaves City F and heads for home along the Trans-Canada Highway. Identify the cities A, B, D and F, and the rivers C and E.
Your first step in attacking a problem such as this should be to read the question through carefully, perhaps making rough notes as you go. This particular problem is so easy that you could probably solve it immediately, but pretend that you are not particularly familiar with the geography of Western Canada, and try to solve the problem in a logical manner.
First you will note that City A is a provincial capital. This is a clue, but not a particularly useful one. You could make a list of the 10 provincial capitals and consider what would happen if you drove 259 km in a northwesterly direction from each of them, but this strategy would not be particularly helpful. It is far better to proceed through the problem in search of a more substantial clue. [Note: In chemical road maps, it is usually best to search through the problem looking for something familiar, rather than trying to start at the beginning.]
The information given on City B and River C does not look very promising, but it may provide a useful check later. You will note that a provincial border is crossed in travelling west from City B to City D, and so you know that B certainly was not in British Columbia, Prince Edward Island or Newfoundland. Next, you find a crucial piece of information: City D is the home of Canada’s largest shopping mall. Thus, City D must be Edmonton, and River E must be the North Saskatchewan River. Now everything else will fall into place. Driving 286 km south from Edmonton would take you to Calgary, indeed the site of the Calgary Stampede and 1988 Winter Olympics. Thus City F is Calgary. In order to identify A through C, you should return to Edmonton and work backwards. Recall that the Jones family arrived in Edmonton after travelling west and crossing a provincial border. Thus, City B must be in Saskatchewan. Saskatoon looks like a possibility: it is situated on a river, the South Saskatchewan, and this river does flow northeasterly and is joined by the North Saskatchewan River just east of Prince Albert. In addition, Saskatoon is 259 km northwest of Regina (City A?) and the latter can be reached from Calgary by driving along the Trans-Canada Highway.
City A = Regina, City B = Saskatoon, City D = Edmonton and City F = Calgary;
River C = the South Saskatchewan River and River E = the North Saskatchewan River.
You might wonder how anyone who did not know that Canada’s largest shopping mall is in Edmonton could possibly solve the above problem. The answer is that the problem could still be solved easily if the person could identify Calgary as the site of the 1988 Winter Olympics. However, to solve the problem readily, you must know one of these two facts, otherwise a much longer approach (possibly “trial and error”) would be required. A similar situation arises in chemical road-map problems. Generally you will need to recall some specific information about a compound or reaction to solve the problem.
Let us now return to our original chemical road-map problem. A careful reading of the problem reveals that there are four unknowns (A through D) that need to be identified. You would note that the formula of A is C5H9Br.
The fact that this compound reacts with a heated ethanolic KOH solution to form two alkenes suggests that A is an alkyl bromide undergoing a hydrodehalogenation. It is not practical to try to draw all the possible structures for an akyl bromide of formula C5H9Br, thus you would look to the second part of the problem for more information.
Both alkenes B and C react with hydrogen gas in the presence of a palladium catalyst to form the same compound D. Under these conditions you would know that alkenes reduce to their corresponding alkane. Because both B and C form the same compound (D) when reduced, the implication is that they have the identical carbon skeleton, but the double bond of the alkene is merely in a different position.
In the third part of the problem we are told that D can be directly prepared by reacting 1-butene with diiodomethane over Zn(Cu). Since we have already studied this particular reaction we know D can only be ethylcyclopropane. At this moment, we have the structure for D and know the carbon skeleton of A through C. However, we do not know exactly where the double bonds in B and C are located or where the bromine atom is located in A.
Here we go to the beginning of the problem where it notes that A is optically active. This is an important hint and could be easily missed upon a first reading. A bromine on either a ring carbon or on the terminal carbon of the ethyl group would not produce a stereogenic centre and therefore those compounds would not be optically active. However, (1-bromoethyl)cyclopropane is the only isomer that has a stereogenic centre and is optically active. Note that alkenes B and C could be in either order, but we have chosen the less sterically strained terminal alkene to be the most likely major product (B). We have now identified Compounds A through D as shown below in our summarized road-map graphic.
Note: On assignments and examinations, the policy will be to award half of the marks available on road-map problems for reasoning. Thus, you will be able to obtain a certain number of marks for recognizing what is happening in a particular reaction, and so on. You cannot obtain more than half marks by just identifying the unknowns, even if they are all correct. To obtain full marks, you must show the steps that led you to your conclusions.
When working out a road-map problem, do not guess. As Sherlock Holmes said to Dr. Watson in Sir Arthur Conan Doyle’s A Scandal in Bohemia, It is a capital mistake to theorize before one has data.
In many ways you are a detective as you try to solve road-map problems. Base your conclusions on the facts that are given in the question, not on some preconceived notion of your own.