Pre-lecture exercises

First 4 letters of last (family) name Name ______________________________________ Pre-lecture exercises. Remember that you may be asked to turn this page in at the beginning of Wednesday's lecture. To make grading easier, please print and work directly on this page. 1 How many valence electrons does each of these atoms have? Write the number below the atom. (review the text if necessary) C F Na H S N 2 One of the most important rules for structure drawing is the octet rule. You learned this in general chem; it's also covered in your organic chem text — review that if necessary. Make sure you're clear about which elements the octet rule applies to. More generally, the rule is a prohibition of "valence shell overflows". So for example, the corresponding restriction for H is – the "duet rule" since H's valence shell can only accommodate 2 e s. Here's a bunch of structures. Draw a big fat X through the ones that are not valid because of an octet violation (or similar issue). H O H B O CH3 O O C N H C C H H H H H OH N H2C C H2C CH2 H O S H O N O H CH3 3 On p 40 of the text, skeletal structures are introduced. At right is the skeletal structure of oseltamivir (Tamiflu™) (2 lone pairs on each O and one on each N are assumed.) What is the molecular formula of this compound? Just fill in the numbers of Cs and Hs below. C H N2O4 O O O O O N H NH2 Lecture outline Structure drawing and the octet rule — Here are some common structural units that you will see in organic compounds — note that each atom drawn below has a complete octet C C H C F N N C O Cl N O Never ever draw stuff like this... (be absolutely sure you know what's wrong with each one) N C N O H Some different ways of representing structures — Molecular formula — provides no structural information Lewis structures (sometimes called Kekulé structures) Condensed structures — these show the bonding between “heavy atoms” explicitly (a "heavy atom" is anything bigger than H) Skeletal structures — Formal charge (FC) — "e– - ownership" Each e– is assigned to nearest atom to give an approximation to the actual charge distribution (and for "e– bookkeeping") — do not torture yourself by "calculating" FC by the method in the text. Instead... 1. Circle the atom of interest so that all lone pairs and half (i.e. 1 e–) of each bond is inside. 2. Count e–s the atom "owns" (i.e. the ones inside the circle) and compare with # of valence e–s the atom "wants" (i.e. the number for the isolated, neutral atom) Use this method to determine charges for the heavy atoms in the following molecules H H C H N C H H C H H H H C H H H H H O H H N H H Warning: Do not confuse octet rule counting with formal charge counting. Be absolutely certain that you know the difference between the two e–-counting schemes. Skeletal structures for ions — How do we interpret the following structures? Write the "partially condensed" structure corresponding to each charged skeletal structure below. Note that the charged Cs cannot have 4 bonds (if they did, they couldn't be charged, eh?), so we're going to have to modify that "4-bond rule" we used earlier. What are the corresponding "rules" for # of bonds to charged Cs? Hmmm.... Guidelines for drawing structures — Ordinary neutral organic molecules usually have no charged atoms and an octet on every atom. (As we'll see later, this isn't always the case — in some compounds, the only structure that can be drawn has + and – charges or an atom with an incomplete octets. But these cases are exceptions. For now, you need to concentrate on drawing ONLY "perfect" structures — structures with complete octets and no charges.) Organic ions normally have one charged atom and octets on as many atoms as possible. Now, what's the relationship between the charges on the individual atoms and the overall charge on the molecule? If you can't easily answer this, think about it — this is important! Condensed formula —> structure (don't waste time adding up all the valence electrons! — this may have been fine for the small molecules you worked with in general chem, but for most organic molecules it's a pointless, time consuming exercise.) 1. Connect atoms with single bonds 2. Add electrons to make each atom neutral (That doesn't say give them octets — it says give them each a formal charge of zero. Be sure you know the difference!) 3. For ions, add or remove 1 e–. To create a cation (+), should you add or remove an electron? What if you're creating an anion (–)? Steps #1 - 3 automatically give correct total number of valence e–s! This must absolutely be true, every time. WHY? 4. Finally, pair or move e–s to maximize bonding. This will also maximize the number of atoms that have octets — but be careful you don't overflow the octets and create nonsense! Once you're comfortable with this method, you can take a shortcut — for many structures, it's not hard to write in the double and triple bonds right away if you keep in mind that all-octet, neutral Cs make 4 bonds, Ns 3, and Os 2 Draw these: CH3CCCHO CH3CHCH3O (CH3)3NCHCBr2 + – CH3CH2CH(CH3)OCH3 CH3(CH2)5NH2 Draw CH3CO2H (often written as "CH3COOH") I call this the "embedded carbonyl" issue — the one structural feature that organic chemists often don't make clear in condensed notation is the >C=O unit. Why? Because this structural unit is so common, that it's obvious when there must be a >C=O group — well... obvious to chemists. It's not so obvious to students just starting the course, so you need to be on the lookout for these. For example, these also have embedded carbonyls... how do we recognize this? HCOOCH2CH3 CH3COCH3 ... but these don't! What's the difference??? CH3OCHCH2 (CH3)3COOCH3