CHEMISTRY ORGANIC 307
LECTURE NOTES I R. Boikess
Welcome to Chemistry 307, Organic Chemistry: Be sure you understand the organizational and administrative aspects of the course. How to Succeed in Organic Chemistry 1. The first few weeks are critical. Learn to understand and write [active skill] the different representations of organic compounds [almost by reflex]. How? Practice 2. Develop a method for systematic memorization [flash cards] 3. Otherwise the approach is similar to Gen Chem: Read the assignment in the book before the lectures, come to lectures, reread the book and the notes, do the homework and do not use the solutions manual until you have given it your all. Make sure your knowledge is active; use problem solving sessions, help sessions, office hours, etc to get help and answers to your questions. Introduction A. What is Organic Chemistry and why do we spend at least a whole year studying it? Think about all the different substances you encounter every day. Almost every single one is an organic substance. 1. Organic chemistry is defined as the chemistry of compounds of carbon, [oversimplified all organic compounds contain carbon, but not all compounds that contain carbon are organic.] Organic compounds all have at least one C-H bond, except for a few that have only a C-X or a C-C bond. So compounds such as sodium bicarbonate or calcium carbonate are not true organic compounds. Most (but not all) of the organic compounds we will consider in this course consist of only nonmetals. There is, however, a whole important branch of organic called organometallic chemistry, in which the organic compounds also contain at least one metal atom.
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2. To some laypersons, organic chemistry has a different connotation, related to "organism" or life. This connotation comes from the fact that virtually every chemical of life is organic, one reason why Organic Chemistry is required for majors in the life sciences. 3. Historical: The Vital Force Theory was disproved in the early 19th century. Key experiment: urea prepared from inorganic sources, was indistinguishable from urea from animals. 4. Most known chemical compounds are organic and there are many, over 16 million and increasing every day. (40 years ago there were only 1 million) B. What's so special about C? Why are there so many compounds of C? ` 1 Review Hill, Petrucci, et al (H.P.) 7.8-7.9, 8.1-8.5, 9.1-9.2, 9.6-9.10, 10.3-10.5 (If you used a different Gen Chem text, review the corresponding sections.) 2. Position in the Periodic Table, In the middle of the first row, so it's small (can get close to other atoms and form multiple bonds) and in the middle of the EN scale. Thus C is good at forming covalent bonds. 3. Has four valence electrons, so it can form 4 covalent bonds by electron sharing with things that bring a total of 4 electrons. 4. But valence shell electron configuration is 2s22p2. Promotion is a convenient way to keep track of electrons, but you should not think of it as an actual physical process that takes place when carbon decides to form bonds. Just remember 4 valence electrons = 4 bonds, which completes the octet. Any cost of changes in orbitals is offset by formation of the 2 extra bonds. Remember bond formation is favorable. 5. Why so many compounds? a. Carbon can form chains and compounds that contain such chains of carbon atoms exist b. Two issues, stability and reactivity. c. Look at bond energies C-C 347 kJ/mol is a strong bond speaks to stability. The bond is strong because of good orbital overlap d. What could C-C bonds form by chemical reaction? (reactivity) C-O. But C-O is 358 kJ/mol not much different. e. Compare to Si, Si-Si 340, but Si-O 452 etc.
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f. We also observe that C-C bonds are not weakened when other atoms are attached to the C atoms. Review A. Valence Shell Electronic Configurations 1. Shells and Subshells, Atomic Orbitals a. Aufbau and electronic configuration b. Valence shell is the key. In nonmetals it is the orbitals with highest value of n. 2. Review electronic configurations of important elements: C, H, N, O, X, (less important P, and S,) they are all nonmetals. Organic cpds with other nonmetals are rare. B. Covalent Bonds 1. Electron pair sharing as a result of orbital overlap 2. Preferred number of bonds: C = 4, H = 1, N = 3 (and one lone pair), O = 2 (and two lone pairs), X = 1(and 3 lone pairs), P = 3, (like N), S =2 (like O). 3. Maximum number of bonds: [very important First Row Elements do not expand Octets under ordinary conditions] H = 1, C = 4, N = 4 (but formal charge +1), O = 3 (but formal charge +1), F = 1, X (in the 1 oxidation state) = 2 (but formal charge +1) except when bonded to O when X has higher oxidation numbers. P = 5 and S = 6 because they are not 1st row and can expand octets using d-orbitals. 4. Description of Covalent Bonds a. Orbital overlap, two main types, and , demonstrate. Which looks better?
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b. MO picture (H,P 10.6). Can be delocalized over many atoms. Never lose orbitals, 1 + 1 = 2, one higher (antibonding, * or *) and one lower (bonding or ). Each can hold 2 electrons.
c. Hybrid orbitals. (H,P 10.3-10.5) Bookkeeping we use it to give a simple picture of orbital overlap. Remember we never lose orbitals. Understand the notation (sp3 is one part s, 3 parts p. examples sp2 and sp. The number of hybrid orbitals is determined by the total number of bonds and lone pairs associated with the hybridized atom. Our choice of hybrid orbital descriptions is based on geometry and calculation of lowest energy states. Because hybridization is based on geometry we only talk about hybridization for central atoms (those bonded to two or more other atoms).
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d. Localized Bonding Model. Focus on the electron pairs, rather than the orbitals. Lewis structures (H,P 9.6-9.9) e. Resonance and Formal Charge (H,P 9.8). i. When you can draw more than one Lewis structure the actual compound is a blend of all the Lewis structures that can be drawn. But the structures must be reasonable, with preferred numbers of bonds and minimal formal charges. The more reasonable the structures, the more they contribute to the description of the actual structure. Equivalent structures are best (Consider the two structures for nitric acid. Remember that only the electrons and the nonbonding electrons move, not the atoms.)
O N+ HO O-
ON+
HO
O
ii Formal charges (which are sometimes also ionic charges) can be related to number of bonds, in the context of completed, but not expanded octets. It's all electron bookkeeping. Relation between number of bonds and formal charge for elements with completed octets and no expansion. Learn and understand this table. It will be very useful. Element C H N and P O and S F Cl, Br, I Preferred # of bonds 4 1 3 2 1 1 Maximum # Formal Fewer # of bonds Charge of bonds 4 3 1 no 4 +1 2 1 3 +1 1 1 no 2 +1 no Formal Charge -1 -1 -2 -1
5. Properties of Covalent Bonds a. Bond strength can be expressed as a bond energy: the heat (enthalpy) required to break the bond back to the atoms. What makes a strong bond?
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Good overlap, multiple bonds better, (although double is not twice single), other factors to be discussed. b. Think of bonds as lines connecting the centers of the two atoms to understand some other properties. c. Bond lengths. Indicates how good overlap is. Shorter bonds tend to be better, but limited by not getting the atoms so close that they repel. Thus smaller atoms tend to form stronger bonds [average bond energies: C-H 413 kJ/mol, C-C 347 kJ/mol] but other factors also operate. d. Bond angles (H,P 10.1) VSEPR and hybridization must be consistent. Review the ideals. Then, lone pairs and multiple pairs are bigger, different EN can be a factor. e. Bond moments; unequal sharing due to EN, (mostly) all the nonmetals xcpt H are more EN than C. Discuss polarity related to chemical behavior etc,, dipole moments, vector addition of bond moments, which have magnitude and direction. f. Let's apply some of these ideas to an actual molecule, chloromethane: CH3Cl. i. Connectivity comes from preferred number of bonds. Draw the one that looks the best, electroneutrality. There are really no other sensible choices here, but that will not always be the case. ii. Hybridization comes from 4 bonds around C. Geometry is tetrahedral. Discuss which orbitals overlap. Meaningless talk to about hybridization of Cl
iii But not ideal(as is methane) because of differences between Cl and H. iv Bond Length: C-Cl 178 pm. C-H 111 pm. Cl is more EN than H. Where are the electrons? Predict and explain the bond angles: answer 6
show in a projection: H-C-Cl 108.0 and H-C-H 110.9 . [Compare to CH4] v. Dipole moment is 1.86 D compared to C-Cl bond moment, which is 1.56 D because of the EN difference. The C-H bond moment is small, 0.30 D and in the direction of the C. Show vectors. Three equal magnitude vectors toward the corners of a regular tetrahedron add to one vector in the direction of the fourth corner of the magnitude of one of the original three. vi Bond Strength: C-Cl 350 kJ/mol compared to C-F 451 kJ/mol and CBr 294 kJ/mol. Two factors size and EN. Predict C-H should be strong because of size, It is 439 kJ/mol and C-O should be stronger than C-N because of EN. It is by over 50 kJ/mol g. Let's do another, C2H4, systematic name ethene, but in common usage ethylene. i. Connectivity comes from preferred number of bonds.
H C H C H H
ii. Hybridization comes from 3 bonds around C. Geometry is trigonal planar. Show orbital overlap. Discuss what we see and what the sigma pi description really is.
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iii Geometry is not ideal because two types of bonds and two types of atoms. iv Predict double bond is stronger, shorter, and bigger. v Bond angles are therefore not ideal HCH = 118 and HCC = 121.7 C. Representations of Compounds and their Structures Very Important What do we want to show? Increasing levels of information but we need shorthand to keep things manageable. The short hand is based on the fact that the common atoms in neutral organic molecules generally have a fixed number of bonds. (Carbon = 4, N = 3, O = 2, and X (halogen) = 1) and have completed octets. That tells us what is attached to what in our shorthand structures. Consider acetic acid. a. composition: C2H4O2 is not very useful in Orgo because of isomerism. Define b. Lewis Structure based on preferred number of bonds showing all the valence shell electrons bonding pairs (dashes) and lone pairs as dots [structure of acetic acid] is tedious, but tells us what is attached to what and how. But in most cases, we also need some information about the compound to draw the Lewis Structure. In this case the information is that it is an acid (actually, as we shall see, a carboxylic acid) c. Leave out the dots but leave in the dashes. We can add to 8 (Sometimes we will include some dots to explain chemistry) "Kekule"
O C H 3C OH
d. Leave out the dashes also. We can add to 4, 2, or 1 (You must be able to do this automatically) or accommodate for formal charges. CH3CO2H and we know the preferred number of bonds for the atoms. Note difference with a. While we can work out the structure of a simple molecule such as acetic acid from this kind of condensed formula, it will not be so easy for more complex compounds. It is for that reason that we tend to leave out dashes only for bonds to H and for carbon carbon and some other single bonds. Otherwise we usually include them, unless there is no ambiguity. (See top of page 40 in your text.) 8
e. We do even more condensation, consistent with unambiguity: CH3CH3 (or even C2H6), CH3CH2CH3, (or C3H8) or CH3CH2CH2CH3 (but not C4H10, why?).
CH3
CH CH3
CH3
or CH3CH(CH3)CH3
Note: You want to practice so that you can write and understand these kinds of condensed structures, reflexively and effortlessly.
f. There are some conventions we tend to follow in drawing these condensed structures. Most important is that the condensed structures correspond to a correct molecule and can be interpreted. The main (more later) carbon chain is written horizontally with the associated hydrogens to the right of each carbon or if they are attached to other atoms such as N or O, to the right of them as well. Other substituents in the middle of the main chain are indicated with vertical lines or parentheses. g. The point of the condensed structures is to save space. So we can go even further when things are unambiguous. Examples: CH3(CH2)5CH3 or (CH3)4C. h. Once we are comfortable adding up to 4, we can condense even further. In a bond line formula we do not show C and H atoms or even C-H bonds. A line is a C-C bond with the C at each end assumed to have the correct number of H's attached.
______
Represents ethane, C2H6. When two lines meet, there is only one C atom at the connection, as in propane A zig zag line is a chain of C atoms and the ends are CH3 groups. So the molecule CH3(CH2)3CH3 can be written as This method of drawing structures it the one we shall use most often because this is the way our software (Chem Draw) does it. 9
f. Geometry and 3-D. In 2-D use projection or perspective drawing. (Flat structures can be shown without projections.) The simplest one is the dotted line-wedge projection. We draw bonds in 3 ways. If the bond is in the plane it is a line, in back is a dotted line, and in front is a wedge. We draw the main carbon chain in the plane of the paper. We will learn other projections later that work well for specific situations.
g. In 3-D use a model. (learn to use the model kit that came with your textbook.) D. Acids and Bases Review (H.P. Chap 15), Remember the problems you had with understanding this topic. It is one of the more difficult in Gen Chem. Here we will make a "simplification." 1. In 162 we did KA and KB. Here we do only KA (and pKA) for all situations: neutral acids and the charged conjugate acids of neutral bases. For example KA of NH4+ and amines etc 2. Review of Lewis Acids and Bases Electron donor is a Lewis base, Electron acceptor is a Lewis acid. Very general. Includes most Bronsted acids and bases. Also nonhydroxylic ones such as NH3(because of lone pair) and BF3 (because of incomplete octet) and ions such as X- and some cations (subject to octet restrictions). So Ammonium is not a Lewis acid. It can't accept electrons, but CH3+ is a Lewis acid. It can accept electrons because it has an incomplete octet. 3. Electrophiles and Nucleophiles
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Another way of thinking about Lewis Acids and Bases. Very important because it allows us to easily characterize chemical properties and behavior. A Lewis Acid is an electron pair acceptor. It is a species that wants to get electrons. It is an electron lover or in Greek an electrophile. The more the species wants to get electrons, the stronger it is as a Lewis Acid or the more electrophilic. Electrophilicity is a measure of how strongly the given species wants to accept electrons. A Lewis Base is an electron donor. It is a species that has an electron pair that can be donated to something which can accept an electron pair. (an electrophile) It is a "nucleus" lover or a nucleophile. The more the species wants to donate the electron pair, that is the more it wants to find something to accept its pair, the more nucleophilic. Nucleophilicity is a measure of how strongly a species wants to donate electrons. E Kinetics and Thermodynamics. Review (H.P 13.2-13.4, 13.8-13.10, 6.4, 17.1-17.7) Both of these topics are very important in Orgo because they enable us to understand organic reactions and to predict the behavior of organic compounds in many situations. In general, Thermo tells us whether or not something can happen. Kinetics tells us how long it will take to happen if it can. Remember that time is not a thermodynamic variable. In particular, we will focus on and use 1. Enthalpy and refer to such measures of chemical behavior as heat of reaction, heat of combustion, heat of hydrogenation, bond energy, resonance energy, and strain energy, all of which are enthalpies. 2. Entropy, which can be loosely understood as a measure of disorganization, to help us understand and predict chemical behavior. 3. Free Energy as a qualitative and quantitative measure of a chemical system's tendency to undergo change. 4. Rate laws, the relationship between rate and concentration, because they give a great deal of information about how a reaction takes place. Reaction mechanisms (how a reaction takes place) will be one of the major themes in this course. 5. Activation energy, which comes from the relationship between rate and temperature, similarly can give us a great deal of information about reaction mechanism. 11