CHEM 3013 ORGANIC CHEMISTRY I LECTURE NOTES

Size: px

Start display at page:

Download "CHEM 3013 ORGANIC CHEMISTRY I LECTURE NOTES"

Charles Fisher
6 years ago
Views:

1 M 3013 OAI MIST I LTU OTS 1 APT 1 1. Atomic orbitals a. eisenberg Uncertainty Principle The exact position of an electron cannot be specified; only the probability that it occupies a certain position of space. b. The electron can only exist in certain regions of space called atomic orbitals. c. The energy of an electron in a particular orbital has a very precise vale. i. Orbitals are described by two quantum numbers: ii. The first descriptor is called the principal quantum numbers and is an interger value from one to seven. lectrons having higher numbers have higher energy. iii. The second quantum number is called the angular(azimuthal) quantum numbers. It describes the shape of the orbital. The four letter descriptors of these shapes are s,p,d,f. d. Orbitals of higher principal quantum number have more nodes. e. igher energy orbitals have the electron density located further from the nucleus. node orbital orbital three orbitals Orbitals 2. lectronic onfiguration a. Aufbau ules Building the complex atoms of the periodic table. 3s 3p nergy of different orbitals in a many electron atom i. lectrons are added to orbitals starting with those of lowest energy ii. Only two orbitals per atomic orbital and these must differ in their spin quantum number (Pauli xclusion Principal). iii. For orbitals of equal energy, single electrons with identical spin are added untill the orbital is half-full, then the electrons of opposite spin are added (unds ule).

2 2 3s 3p 3s 3p oxygen Filling the orbitals of and oxygen 3. Ionic Bonding a. Inert as onfiguration; Octet ule. ertain elements in the periodic table are especially stable. These are the inert gases, helium, neon, argon,krypton, xenon and radon found in group 8A. The common features of these elements is that they all have a completely filled valence shell of electrons. xcept for helium which has a filled shell of two electrons, all the remaining inert gases have 8 electrons in their outer shell. This special configuration can also be attained in other elements; it is the major driving force in the formation of ions (by gain or loss of electrons). lements or ions which have a filled shell of electrons are said to obey the octet rule. b. Formation of ions. Much of chemistry can be understood in terms of the various elements' wish to gain the filled shell configuration. In the reaction of lithium with fluorine gas, both elements can satisfy the octet rule by transfer of one electron from the lithium to the fluorine. During this process the fluroine becomes a fluoride anion, the lithium a lithium cation. Both ions have the same configuration as neon. The lithium half-reaction): Li ( 2 1 ) 1 e - + Li + ( 2 ) Same electronic configuration as e lemental lithium, a source of one electron available electron The fluorine half-reaction F ( ) + 1 e - F - (... 6 ) Same electronic configuration as e lemental fluorine, an acceptor of one electron electron needed Filled shell configuration by loss or gain of electrons lements at the left of the periodic table (like groups 1A and 2A) can most easily attain a filled shell configuration by donating electrons. onversely, elements on the right side of the periodic table are converted to the octet configuration by accepting electrons.

3 c. Isoelectronic hemical Species have the same electronic configuration. Thus the ions of O -2,F -, e, Li + and Mg +2 are isoelectronic, having the numbers of protons which makes them distinct chemical entities. d. lectrostatic Attraction The simple attraction of oppositely charged particles which holds the ions together in an ionic solid. 4. ovalent Bonding. 3 a. ot all compounds are comprised of ions. In particular, many compounds are not ionic. The reason for this difference can be found in the electronic configuration of ( 2, 2,x 1,y 1). To gain a filled shell configuration, would have to either gain or lose four electrons. This represents an energetically unfavorable situation. The solution to the bonding problem posed by is to share electrons in a covalent bond. This type of bonding was proposed by.. Lewis in The formalism depicts bonding resulting from havinga pair of nuclei being shared between two nuclei. The models resulting from this formalism are called Lewis Structures. This model is the cornerstone of understanding organic reaction mechanisms and is exceptionally useful for electronic bookkeeping. b. Writing Lewis Structures i. Start with a central atom (usually the heaviest, but make it if is present). ii. Place all the outer shell electrons on four imaginary sides. Don't pair electrons unless more than four are in the outer shell of an atom. iii. Bring the other atoms of the molecule in to the central atom so that single electrons on the central atom can be paired with single electrons on the other atoms. iv. If, after all atoms have been added, two adjacent atoms have unshared electrons, they share the unshared electrons to make multiple bonds. v. If the species is an ion, then add or subtract the appropriate number of electrons in order to obtain the charge of the ion. vi. The final Lewis-dot structure should have eight electrons around all atoms except hydrogen,which has two. (There are a few exceptions to the octet rule but these usually don't involve compounds containing.) ovalent Bonding in methane + 4 or ote that the hydrogen of methane have a helium closed shell and the has a neon closed shell LWIS STUTUS FO OVALT MOLULS c. Unshared pairs. ot all electrons in a covalent compound are necessarliy involved in bonding; some structures have orbitals that contain unshared pairs of electrons. While these electrons are not used in determining the primary bonding arrangement of the covalent compound, they are of basic importance in determining how the compoundwill react in later reactions. In drawing Lewis Structures, the lone pairs are often not shown, but we shall draw them explicitly at this stage of the course. Lewis Structures are also useful in understanding the bonding of covalent compounds containing multiple bonds (double and triple).

4 4 O O unshared pairs O 3 O dimethyl ether formaldehyde Lewis structures with unshared pairs and/or multiple bonds 5. VSP Theory a. The shape of a molecule can be predicted by allowing all the bonded and nonbonded electron pairs to move as far apart as possible. b. For the given number of different bonds and nonbonded pairs of electrons, the expected shapes would be as shown below. Tetrahedral. WXZ can be bonds or non-bonding electrons Z W X A O O = = = bent pyramidal tetrahedral Trigonal. XZ can be bonds or non-bondingelectrons. X A Z F B FBF = 120 planar F F Digonal (Linear). Z can bebonds or non-bonding electrons X A Bonds and geometry 6. Molecular Orbitals a. Quantum Mechanics tells us that combining two atomic orbitals leadsto two molecular orbitals. The positive combination is a bonding molecular orbital and the negative combination is an antibonding molecular orbital. The formation of a bond between two hydrogen atoms is a very favorable (104 Kcal/mole) process. This is because we put two electrons in a bonding MO downhill in energy. The combination of two atomic orbitals with spherical symmetry (e.g. ) generates a

5 MO with cylindrical symmetry (symmetric about the internuclear axis). This called a sigma (σ) bond. 5 b. An orbital is described as a probability function that defines the distribution of electron density in space, the overlap region of a MO, between the two nuclei has a considerably higher electron density. The covalent bond arises from the electrostatic attraction between the positive nuclei and the region of increased electron density. + + Antibonding ombination Bonding ombination egion of decreased electron density. sigma bond Anti-bonding MO nergy of isolated atoms Bonding MO Overlap region of increased electron density Molecular orbitals for the hydrogen molecule 7. ybrid Orbitals a. As a atom begins to form bonds with up to four other atoms, the ground state electronic configuration of is purturbed and begins to change. An electron in a orbital is promoted into a orbital and the new electronic configuration is 2 2 x 1 y 1 z 1. b. Two-electron bonds are like small magnets, the bonds tend to repel each other. Forming four covalent bonds to by using three p orbitals and one s orbital is not the best energetic situation. This situation would place the sigma bond formed using the s orbital quite close to the bonds formed from the three p orbitals. By undergoing hybridization, nature creates four new orbitals of lower energy called sp 3 hybrid orbitals which now have a tetrahedral structure. p 3 lemental lectron promoted to orbital ompletely hybridized in organic compounds ybridization in

6 8. xamples of sp 3 ompounds 6 The value of this bonding model is that it gives the chemist the ability to predict the geometry of chemical structures. This can be seen in the case of water. The oxygen in water has a ground state electronic configuration of 2, 2, x 2, y 1, x 1.This would suggest that oxygen would form covalent bonds with hydrogens using the half-filled orbitals of each hydrogen and the half-filled y and z orbitals of oxygen. Thus, only two sigma bonds from oxygen to hydrogen, which would seem to suggest that water has a bent structure, with a O bond angle of 90 degrees (the angle of the y and z orbitals with respect to each other). But does water have a 90 degree bent structure? O! ybridization results in the formation of four sp3 orbitals, two filled and two half-filled. The orientation of the four new orbitals is tetrahedral (109 degrees). When hydrogen covalently bonds to the two half-filled sp3 orbitals, the O bond angle is 109 degrees. mix and p 3 new hybrid orbitals all three lemental oxygen ompletely hybridized oxygen in water ybrid Orbitals of Oxygen in Water 9. Other ybridizations Other hybridization states exist for the atom (and others) when the number of orbitals mixed in with the is less than three. ybrid orbitals of sp 2 are formed when only two of the three available orbitals are mixed in. This now results in the formation of three "new" sp 2 orbitals, which are oriented in a trigonal planar orientation ( they look like a three-bladed propeller). There is one un-hybridized orbital left over, which is oriented perpendicular to the plane of the propeller.

7 7 mix and two orbitals p 2 new hybrid orbitals lemental lectron promoted to orbital sp 2 hybridized sp2 sp 2 sp 2 ybridization in sp 2 Two atoms of sp 2 hybridization will combine to form a (sigma) and (pi) bond. It should be noted that a double bond is comprised of two two-electron bonds ( and ); this is why double bonds are shorter and stronger than single bonds. The bond is formed by end-on overlap of the sp 2 orbitals. It has cylindrical symmetry about the - bond axis. The bond is formed by sideways overlap of the two orbitals. It is not cylindrically symmetric. A bond is about 1/2 as strong as a bond. 2 (sp 2 ) + 4 () ======> 2 = 2 π bond (two p orbitals) + σ bond (two sp2 orbitals from the two s). Bonding in sp2 ybridized arbons ybrid orbitals of sp hybridization are formed when one and one orbital are mixed together. A atom which has sp hybridization has two sp orbitals (oriented at 180 from each other) and two orthogonal orbitals.

8 8 mix and one orbital sp new hybrid orbitals lemental lectron promoted to orbital sp hybridized orbitals sp ybridization in arbon sp hybrid orbital sp hybrid orbital

Organic Chemistry Lecture I. Dr. John D. Spence

Organic Chemistry Lecture I. Dr. John D. Spence HEMISTRY 3 Organic hemistry Lecture I Dr. John D. Spence jdspence@scu.edu jspence@csus.eduedu http://www.csus.edu/indiv/s/spencej What is Organic hemistry? 780 s hemistry of compounds from living organisms