Page 1 of 10 CHM!2201! Fall!2008 Department!of!ChemistryOrganic!Chemistry Lab I Villanova University Week 4 – September 16 – 22, 2008 Learning to Use the Molecular Model Kit and the Basics of Isomerism If you haven't already done so, buy a "Molecular Visions" molecular model kit from the stockroom. It is the kit that comes in the green plastic box. Here is what we'll cover in today's lab: 1) The Common 2nd Row Element Presentations (pp 10-11 of the model kit manual) With the help of your professor and TA's, together we are going to go through some of the contents of your kit. We will follow what is on pages 10-11 of the manual that came with your kit. Note: try to learn which pieces are needed to make organic compounds; also note how to put all the pieces back in the box as it is a tight fit and every piece has a particular place! 2) Assembly and Disassembly of Atoms "With Bonds" (pp 15-17) Carefully follow the descriptions on making an sp3 hybridized atom, and making atoms joined by a double bond. It is important to learn how to put these pieces together properly and securely, and learn how to take them apart efficientlyPage 2 of 10 3) Constructing a variety of molecules We'll begin by keeping things simple: putting together molecules containing only carbon and hydrogen, frequently called !"#$%&'$(%)*+ And we'll start off with *'-.$'-/# acyclic hydrocarbons. What does this mean? Saturated means all the carbons are *03 hybridized (or maybe a better way to think of it is that the molecule is saturated with hydrogens, thus all carbon- carbon bonds present are single bonds). Acyclic means "non-cyclic", meaning there are no rings of carbon atoms present, which says that from any carbon atom in the molecule, you cannot traverse along the carbon chain and get back to the same carbon you started from. Exercise #1 Try putting together these molecules with your models, given the molecular formula (name in parentheses): CH4 (methane) C2H6 (ethane) C3H8 (propane) Note how three-dimensional these systems are, and how many different shapes (what organic chemists call &%)1%$2'-3%)* or &%)1%$2/$*4 the latter is short for 'conformational isomers') that ethane and propane can have. Various conformers of a hydrocarbon are obtained by rotating about the C-C single bonds. You will learn from lecture that there is usually considered to be "free rotation" about C-C single bonds. Note that 1$// $%-'-3%) is in quotes. That is because there are barriers to the rotation about C-C single bonds, but these barriers are usually very small and the thermal energy present at ambient conditions (i.e. ~20 °C) is sufficient that the barriers are easily traversed.Page 3 of 10 Question #1: What is the general formula for a saturated acyclic hydrocarbon? Exercise #2 Now put together a molecule with molecular formula C4H10. You probably put together this one, didn't you? CH3CH2CH2CH3 The molecule above is known as n-butane (where the 5)5 stands for normal, meaning somehow it is 'normal' in organic chemistry when all the carbons in a molecule are bonded together one after the other, in one continuous chain). Note that n-butane is also commonly written on paper as the following &'$(%) *6/7/-'7 #3'8$'2 (also called a (%)#973)/ 1%$2.7'*:; Note that in this notation a carbon atom is implied at the end of each line (unless some other atom is explicitly written), and that none of the hydrogens are drawn. In this type of drawing, the only atoms explicitly shown are those that are not carbon or a hydrogen bonded to carbons (but hydrogens bonded to other atoms like N or O are shown).Page 4 of 10 Maybe some of you put together this one for C4H10: This structure is also a C4H10, but it is not the same as n-butane. This C4H10 is called isobutane (or 2-methylpropane using the IUPAC systematic nomenclature). Since it has the same molecular formula as n-butane, but it is a different compound, they are isomers. <-butane and isobutane are more accurately called constitutional isomers because the atoms in each are bonded in a different order. Exercise #3 As the molecular formula becomes larger, the number of isomers rises rapidly. As an exercise, put together all the isomers of formula C5H12. You should come up with three total. Now that you know a bit about bond-line drawings and what they mean, put together a model corresponding to the following bond-line formula: This five-carbon molecule is cyclic and is known as cyclopentane.Page 5 of 10 Question #2: What is the molecular formula of cyclopentane? Does it fit the general formula you derived for saturated acyclic hydrocarbons? If not, derive a general formula for a saturated monocyclic hydrocarbon. Note: although it is easiest to write cyclopentane on paper as a simple pentagon, this drawing doesn't represent well the three-dimensional shape of the molecule. Play around with the cyclopentane molecule to see if you can guess what might be the preferred shape (i.e. conformation). Exercise #4 Now put together the one carbon larger saturated cyclic hydrocarbon, known as cyclohexane: Notice how difficult it is to get this structure in the shape of a regular hexagon with all carbons in the same plane. Here we again demonstrate the three- dimensional nature of organic molecules and why they can be difficult to represent on a two-dimensional piece of paper! After playing around with cyclohexane you may have discovered that the preferred conformation for cyclohexane is commonly called the "chair" form, as it somewhat resembles the shape of a reclining chair. An attempt to draw this on paper using a bond-line formula looks like the following:Page 6 of 10 Note that the twelve hydrogens in the planar conformation of cyclohexane are equivalent, but in the preferred chair conformation they are not, with there being two sets of 6 equivalent hydrogens. These two types of hydrogens are called axial (Ha) and equatorial (He): Exercise #5 Let's now put together the simplest example of a hydrocarbon containing one carbon-carbon double bond, ethene, or more commonly called ethylene (C2H4, or CH2CH2). Hydrocarbons with C=C double bonds are usually called '76/)/* (sometimes you'll hear them called %7/13)* as well). They are considered to be unsaturated