Introductory Organic and Biochemistry Quiz Alkanes and cycloalkanes When a question asks for an explanation you will normally receive zero credit if you give an answer without an explanation The point is to emphasize reasons Answers result from reasons answer rs alone are not interesting 1 Draw the structure of 3 4 diethylheptane 2 Draw the structure of 2 bromo 2 3 3 trichloro 4 ethyl 1 iodohexane 3 Is cyclopropane an isomer of propane Explain There is only one relevant consideration Be clear and specific 4 Draw the structure of cis 1 3 dimethylcyclopentane For practice try to draw this structure two ways one using stereo bonds e g wedges and one not using them Be sure that key features are very clear 5 Which has the higher boiling point pentane or hexane Why 6 Which C4H10 isomer n butane or methylpropane has the higher boiling point Why Are there any other compounds with the given molecular formula 7 a Draw the structure of propane Show all the atoms b There are 8 hydrogen atoms in your structure in part a You could replace each one of them individually by a chlorine atom In each case you get a chloropropane However you don t get 8 distinct compounds How many distinct mono chloropropanes are there Draw them and name them The following parts reexamine the issues from part b in a somewhat different way Before doing this part try to see that you have the basics from part b c Consider replacing each one of those 8 H from the original compound propane with a Cl atom In how many cases do you get 1 chloropropane d Now take the structure of propane that you drew in part a Mark all of the H that you referred to in part c For example you might mark these as Ha That is use Ha to mark all the H which when replaced with a Cl give 1 chloropropane e Now use Hb to mark all the H which when replaced with Cl give 2 chloropropane Parts c e introduce the idea of equivalent hydrogens It is hard to explain in words without reference to a specific example The purpose here is for you to discover equivalent hydrogens by working through an example Hydrogens are equivalent if they behave exactly the same way In this case you see that substituting equivalent H s gives the same compound Ouellette develops the idea of equivalent atoms in a structure in the Explorations with Molecular Models 8 Which would you expect to be more soluble in water propane or any mono chloropropane Explain 9 What is the simplest compound that contains a quaternary C atom 10 Draw the structure of diiodobutadiyne Show all the atoms including all the hydrogens Give the molecular formula Answer key 1 2 Strategy Draw heptane the root or parent compound as the name indicates Number the C atoms 1 to 7 The circled numbers show the chain numbering Then attach the indicated groups Finally add enough H so that each C has four bonds 3 No Isomers have the same molecular formula but different structures These two compounds do not have the same molecular formula Be sure you have the correct chemicals before trying to answer the question Don t be misled by the names 4 Both methyl groups are shown on the same side up but down would be fine the given compound is cis It is very important that your structure show clearly that the two methyl groups are on the same side of the ring both up or both down Look at some of the structures in the book for ways to show this I will not give you the benefit of the doubt on a critical issue such as this For those who really have trouble drawing it clearly I suggest you add a little note to say what you mean See my web page Rings Showing cis trans and axial equatorial relationships for guidance on making such drawings 5 Hexane The two chemicals are of the same type i e both are alkanes and hexane is bigger heavier Therefore there are greater London forces between the molecules 6 n butane has the higher boiling point BP The two chemicals are of the same type alkanes and they have the same mass So what gives them different BP Their shapes The more tightly two molecules can stick together the harder it is to break them apart thus the higher the BP Linear molecules can stick together better than branched ones which tend to look like balls Simply quoting the boiling points of these compounds misses the purpose of the question and would receive no credit on a test The question discusses an issue and asks for a reason Of course if you do happen to know which has the higher BP that can help guide you but per se makes no intellectual contribution The two compounds stated are the only two compounds with the molecular formula C4H10 b There are only two 1 chloropropane with a Cl on a terminal C and 2 chloropropane with a Cl on the middle C If you think you have more than 2 I suggest that you make a model of each See if you can rotate one model so that it looks identical to another If so the two structures you thought were different are actually the same c 6 Replacing any one of the H on the CH3 group at either end gives the same result 7 a d e 8 The chloropropane is somewhat more polar than propane so should be somewhat more soluble In this case neither is very soluble But it is true that the chloro compound is more soluble 9 2 2 dimethylpropane common name neopentane A quaternary C has 4 other C attached to it 10 It is C4I2 No hydrogens So how did we get that Why are there no numbers in the name to tell us where the triple bonds or I atoms are The name is diiodobutadiyne Let s break that down di iodo buta di yne Four C two triple bonds two I So lay out 4 C atoms C C C C Now two triple bonds Where Triple bonds cannot be next to each other that is one C cannot have two triple bonds Therefore the only possible place for the triple bonds is at the ends i e 1 3 butadiyne Now look for places to put I or H Each C forms 4 bonds the only places left are at the ends so put the two I atoms there That s it There are no more bonds available on the C atoms so there are no H This compound is an example where numbers locants are not needed to specify the location of groups because there is only one possibility If you were asked to name this compound you might well include numbers without noticing that they are not needed in this special case That s ok But here you are given the name and should be able to figure out where things go Remember …