CHEMISTRY 610A AND 618A: ORGANIC CHEMISTRY

EMPHASIS TOPICS FOR THE FIRST EXAM

Chapter 1:

Atomic Orbitals of the Hydrogen Atom

Shapes, sizes, and energy level diagram of the AOs of the hydrogen atom (1S,2S,etc.).Identify valence shell and inner shells. Maximum number of electrons in each shell. Ground state vs. excited states of the hydrogen atom.

Ground State Electronic Configurations of Other Atoms

Representations of the ground state electronic configurations of a variety of atoms up to and including the third row of the periodic table using both the linear format and the energy level diagram format. The Schrodinger equation and the origin of wave functions (also called AOs in the case of atoms and designated by a Greek letter phi).The relationship of the wave function to the electron density distribution (the square of the wave function describes the electron distribution or probability.Why are AO energies negative? Also, negative relative to what?

Rules for Predicting the Ground State Electronic Configurations of Atoms.

Be able to name the three rules and apply them to various atoms. What are degenerate orbitals? Give a familiar example. Explain how Hund's Rule applies to the ground state of atomic carbon. Why do electrons prefer to be in different AOs? Why, when they are in different AO's, do they prefer to have unpaired spins? Why do they have paired spins when in the same AO.

The Octet Rule:Its Basis and Limitations.

Why does the octet rule not apply to Row 1 of the periodic table and not rigorously beyond Row 2? Why is it still a useful rule for atoms beyond Group 2.Why are eight electrons especially stable in Group 2?

Ionic Bonding

Definition of ionic bonding.In what situations does bonding between two atoms occur prevalently by ionic bonding? Why in these specific situations (include both the octet rule and also electron affinity and ionization potential considerations)? Be able to depict, using the linear and energy level diagram formats, the ionic bonding which occurs between Gr 1 and Gr7 atoms, e.g., sodium and fluorine.

Covalent Bonding.

Definition of covalent bonding (sharing of an electron pair between two atoms).The energy level diagram depiction of MO (molecular orbital) formation from the interaction of AOs.Definition of MO. Definition of BMO (bonding molecular orbital) and ABMO (antibonding MO).What are the essential requirements for strong covalent bonding (net,strong orbital overlap and two, paired electrons). Explanation of why two electrons are the optimum. Why paired? Why is strong overlap needed to give strong bonding (Note:qualitatively, overlap of AOs means they share substantial common volumes of space;quantitatively, overlap is measured by the product of the mathematical functions representing the two AOs integrated over all space)? Why do two helium atoms not bond to each other? Why do two hydrogen atoms having electrons of the same spin not bond (construct energy level diagrams which illustrate your argument)? Can a hydrogen atom bond to a proton? Why or why not (illustrate with an MO energy level diagram)? Is there such a thing as three electron bonding(explain)?

The Shape of MO's and the Significance of Negative Overlap.

Show by depiction the combination of hydrogen 1S AOs to give the BMO and ABMO of the hydrogen molecule. Show the shape of these MOs, including the signs of the lobes.What is the significance of positive and negative overlap in relation to bonding? What is the significance of "net" overlap? In what situations can orbitals overlap without having "net" overlap?

Tetrahedral Hybridization State of Carbon.

What is the difference between tetrahedral and tetravalent? What is the angle of a regular tetrahedron? Write the ground state of atomic (neutral) carbon, and illustrate in a stepwise fashion how the tetrahedral orbitals are conceptually related to the ground state. Include the groundstate, excited state, and valence state in your illustration. Provide a brief discussion of the feasibility (energetically) of each step. What is the shape of an sp3 orbital? What is the %s and %p "character"? Give at least one reason for hybridization occurring? Is hybridization limited to carbon? How broadly applicable is the concept? Be able to discuss the hybridization of oxygen and nitrogen as well as silicon in the same manner as above.

Structure of Methane and Ethane.

What is the bond angle in methane? Explain why this angle is adopted.Be able to qualitatively depict the BMO and ABMO of the C-H bond of methane and the C-C bond of ethane.

SP2 Hybridization and the Pi Bond of Ethene.

Be able to provide a stepwise illustration of the development of the trigonal(sp2) orbitals of carbon in ethene, beginning with the ground state electronic configuration of atomic, neutral carbon. What is the bond angle between the three sp2 AOs? Explain. Why are they coplanar? Be able to depict the sigma bond framework of ethene (the bonds which involve the sp2 carbon AOs). Define the terms "sigma bond" and "pi" bond. Depict the pi bond of ethene using the 2p AOs of the two carbons. Be able to pictorially represent the BMO and the ABMO of the pi bond and how they arise from positive and negative combinations of the two 2p AOs. Explain why the geometry of ethene is planar. Describe the stepwise development of the sp2 hybrid AOs of neutral atomic boron and of positively charge trivalent carbon ( in carbocations). Also describe the sp3 hybridization of negatively charged boron and positively charged nitrogen.

Digonal (sp) Hybridization.

Starting from the ground state electronic configuration of neutral atomic carbon, show how the sp hybridization state of carbon is developed. What is the bond angle in ethyne ? Explain why.

Hybridization Effects.

Compare the relative bond lengths of the C-C bonds in ethane, ethene, and ethyne. Explain the trend. Compare also the relative bond lengths of the C-H bonds of ethane, ethene, and ethyne. Be able to provide an explanation of the trend here. You are not required to memorize the specific C-C and C-H bond lengths.

Polar Covalent Bonds.

Be familiar with the resonance theoretical description of polar covalent bonds in terms of ionic and covalent structures and the reason for the greater strength of polar covalent than pure covalent bonds (resonance stabilization). Dipole moment as a measure of the polarity of a bond. Dependence of the dipole moment upon the relative electronegativities of the atoms joined by the bond.Direction of the dipole moment (which atom is partial positive, and which partially negative).Use of delta to designate partial charges. Distinction between bond dipoles and molecular dipoles. Know that the bonds between carbon and hydrogen are essentially nonpolar (very small electronegativity difference), so that alkanes and alkenes are essentially nonpolar compounds.Be able to predict whether given molecules have dipole moments or not based upon their geometry (e.g. carbon dioxide and sulfur dioxide) and the direction of the dipole moment, if it exists.

Formal Charges.

Be able to calculate formal charges on various molecules and ions and to show the method of your calculation.Examples such as nitromethane, acetate ion, acetonitrile oxide, etc.

Resonance Theory

Be familiar with the language and the formalism of resonance theory, as well as the rules of resonance. Please refer to the tutorial on resonance theory under "Additional Information" at the top of the CH610 web page.Be able to draw resonance structures (e.g. acetate ion, benzene, nitromethane, carbonate ion) and to discuss the extent of the resonance stabilization in terms of the rules of resonance.Be certain to use the double-headed resonance arrow in your resonance treatments, and not the dual equilibrium arrows. Be able to discuss the structure of the real molecule or ion in terms of the canonical structures.

Chemical Structure Representations

.Be familiar with the Kekule , condensed, and skeletal structures.

Acidity/Basicity

You should know the definitions of Bronsted and Lewis Acidity and Basicity. You should be able to write the equation which defines pKa for a Bronsted acid. You should know a qualitative and a quantitative criterion for determining the position of an acid/base equilibrium. You should know why carboxylic acids are much more acidic than alcohols and be able to provide a resonance theoretical treatment of the carboxylate anion. You should know that there are two main factors which primarily determine the order of relative acidities of a series of Bronsted acids, namely bond strength of the H-A bond the the stability of the anion. You should know that greater anion stability causes increased acidity and that great bond strength decreases acidity. Know that anion stability is usually controlling, but also know when it is not controlling.You should know the different factors which affect anion stability (resonance, hybridization, electrostatic effects, electronegativity). You should know why ethyne is vastly more acidic than ethane or ethene.

Chapter Two

Isomers.

Definition of the term. Examples in the alkane family. Definition of hydrocarbons and of alkanes (saturated hydrocarbons or hydrocarbons in which all of the carbon atoms are tetrahedrally hybridized).General formula for alkanes. Branched vs normal (unbranched) alkanes.Definition of "constitutional isomers"(isomers which differ in connectivity). Constitutional isomerism in alkanes and in functional groups (e.g. ethanol and dimethyl ether).Names of the first ten alkanes

Alkyl Groups

Naming of alkyl groups (delete -ane of alkane name and add yl; e.g. -CH3 is methyl). Know the names of the simple alkyl groups (methyl, ethyl, propyl, isopropyl, butyl, isobutyl, and tertiary butyl).Be able to recognize primary, secondary, and tertiary carbon atoms.

Alkane and Cycloalkane Nomenclature

Be able to use the IUPAC Rules to name hydrocarbons. Also, be able to use IUPAC Rules for naming complex substituents.

Cis/Trans Cycloalkane Stereoisomers.

Be able to distinguish cis- and trans stereoisomers of cycloalkanes. Be able to define the term "stereoisomers" (isomers which have the same connectivity)

The Conformations of Ethane:

Staggered and Eclipsed. Definitions of conformations or conformers. Newman projection structures. Dihedral angles. Definition of torsional strain. Relative energies of the staggered and eclipsed conformations. The magnitude of the CH/CH torsional strain energy per eclipsing interaction.The basis for torsional strain and the distinction between this and steric strain.Conformational analysis of propane. Analysis of the 3.4 kcal/mol of torsional strain in the eclipsed conformer in terms of the two different sub-types of eclipsing interaction. Be able to provide Conformational Energy diagrams (energy vs. dihedral angle) diagrams for ethane and propane.

The Conformations of Butane:

Be able to draw Newman projection structures of all four energy extrema for butane, i.e., the two energy minima and the two energy maxima. Be able to provide names for the two energy minima, and explain what effect makes the gauche less stable. Know the relative energies of these four conformations, and be able to provide a conformational energy diagram for butane. Be able to state the definition of steric strain, and indicate which conformations have such strain and which have torsional strain and which have both.

Conformations of Cyclohexane and Substituted Cyclohexanes

You should be able to draw a conformational energy diagram for the chair-chair interconversion (ring flip) of cyclohexane. Be able to identify and properly draw axial and equatorial bonds.Explain why cyclohexane has neither angle nor torsional strain, but planar cyclohexane has at least 20 kcal/mol of strain.Be able to draw equatorial and axial methylcyclohexane and show the ring flip of each.Know the relative stability of each conformer and be able to explain and illustrate the basis for this energy difference. Be able to show how the energy difference correlates with the gauche butane strain energy. Be able to estimate energy differences between cis and trans isomers of 1,2; 1,3; and 1,4-dimethylcyclohexane as well as the total strain present in each isomer and each conformer of each isomer. Be familiar with the Equatoriality Principle.Know that tert-butylcyclohexane is essentially anchored as the equatorial conformer.

Ring Strain

You should know the approximate amount of ring strain in cyclopropane and cyclobutane and that the total ring strain is comprised of angle strain and torsional strain. You should be able to depict the bonding in cyclopropane in such a way as to illustrate why there is angle strain. In doing this you should be able to use the concepts of interorbital angle and internuclear angle, and you should know that the C-C bonds of cyclopropane are not pure sigma or pi but are intermediate in nature between sigma and pi. You should be familiar with the term "banana bond". You should also be able to explain why cyclohexane has no ring strain--either angle strain or torsional strain.
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