Chemical Bonding Part II Adapted from: http://www.sciencegeek.net/APchemistry/Powerpoints.shtml

Resonance Resonance is used when more than one valid Lewis structure can be written for a particular molecule. (i.e. NO3-) The actual structure is an average of the resonance structures.

Resonance Example 2 – Benzene, C6H6 The bond lengths in the ring are identical, and between those of single and double bonds. Resonance bonds are shorter and stronger than single bonds. Resonance bonds are longer and weaker than double bonds.

Resonance Practice– Ozone, O3 Determine Resonance Structures Describe the bond length and strength in the Ozone resonance. **Note neither structure is “correct”. (Its an average)

Remember… Localized Electron Model (LEM) Lewis structures are an application of the LEM. Electron pairs can be thought of as “belonging” to pairs of atoms when bonding. Resonance points out a weakness in the LEM

Models in Science Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Models can be physical Models can be mathematical Models can be theoretical or philosophical A model does not equal reality. Models are oversimplifications, and are often wrong. Models become more complicated as they age. We must understand the underlying assumptions in a model so that we do not misuse it.

VSEPR (Valence Shell Electron Pair Repulsion) Recall ABE Geometry A= central atom B = atoms bonded to A E = nonbonding electron pairs on A (lone pairs) B + E Overall Structure Forms 2 Linear AB2 3 Trigonal Planar AB3, AB2E 4 Tetrahedral AB4, AB3E, AB2E2 5 Trigonal Bipyramidal AB5, AB4E, AB3E2, AB2E3 6 Octahedral AB6, AB5E, AB4E2

Model Kits and Sketches Grab a model kit. Locate a spot in notes for some labeled sketches. Be able to identify a dipole moment Resource (page 371 to 372)

Linear – AB2 CO2

Trigonal Planar AB3 BF3 AB2E SnCl2

Tetrahedral AB4 CCl4 AB3E PCl3 AB2E2 Cl2O

Trigonal Bipyramidal AB5 PCl5 AB4E SF4 AB3E2 ClF3 AB2E3 I3-

Octahedral AB6 SF6 AB5E BrF5 AB4E2 ICl4-

Individual Practice Page 385 # 73, 78, 79, 81, 87, 89, 91, 93, 97, 99, 101, 103

Hybridization – The Blending of Orbitals = + s orbital p orbital sp orbital Hybridization is a modification of the localized electron model to account for the observation that atoms often seem to use special atomic orbitals in forming molecules.

What proof exists for hybridization? We have studied electron configuration notation and the sharing of electrons in the formation of covalent bonds. Analyze methane, CH4 Methane is a simple natural gas. Its molecule has a carbon atom at the center with four hydrogen atoms covalently bonded around it. Expected orbital notation of carbon in ground state is… How many unpaired electrons does the carbon atom have available for bonding? (2p) What is taking place in order that carbon may form four bonds (and equal at that)?

Carbon’s Empty Orbital First attempt to explain what happens… a 2s electron jumps up to the empty 2p orbital leaving 3 2p sub-orbitals to fill and 1 2s sub-orbital. However, this serves to be a problem. The four C-H bonds in methane are all identical, so we cannot have 3 2p electron pairs of carbon matched with 1s electrons from hydrogen and the other with 2s and 1s (less energy than the 2p – 1s bond). The explanation proposed is that of HYBRIDIZATION. Hybridization is the combing of two or more orbitals of nearly equal energy within the same atom into orbitals of equal energy.

Methane’s Hybridization Hybridization is sp3, meaning that an s orbital is combined with three p orbitals to create four equal hybrid orbitals. These new orbitals have slightly MORE energy than the 2s orbital… and slightly LESS energy than the 2p orbitals. Shape like that on one previous slide.

Hybrid Orbitals sp3 sp2 sp Hybridization Tetrahedral Trigonal Planar Types of hybridization that atoms undergo… Note that a double bond acts as one effective electron pair Hybridization sp3 sp2 sp Arrangement of atomic orbtials Tetrahedral Trigonal Planar Linear Electron domains around atom 4 3 2 Forms (ABE Geometry) AB4,AB3E, AB2E2 AB3, AB2E AB

Note We will not be discussing “d” orbital hybridization in this course. Current evidence suggest that hybridization involving d orbitals does not exist.

Model Hybridization Describe the bonding in the water molecule using the LEM. Give the hybridization, and predict the geometry of each of the central atoms in the following molecules or ions. IF2+ AsH3

Practice Hybridization What geometry do the following hybrid bonds possess? sp sp2 sp3 Predict the geometry and type of hybrid orbital for the following: SiH4 H3O+ CH3+

Sigma and Pi Bonds Sigma (σ) bonds exist in the region directly between two bonded atoms. Pi (π) bonds exist in the region above and below a line drawn between two bonded atoms. Single bond 1 sigma bond Double bond 1 sigma bond, 1 pi bond Triple bond 1 sigma bond, 2 pi bonds

Sigma and Pi Bonds Methane (all sigma bonds) Carbon Dioxide (sigma and pi bond) Hydrogen cyanide (Sigma and two pi bonds)

Model Types of Bonds Answer the following questions about aspartame. How many sigma bonds are in the molecule? How many pi bonds? What is the hybridization on carbon 4? Carbon 1? Carbon 3? What is the hybridization on nitrogen 5? Oxygen 2?

Practice Types of Bonds The structure of urea is  How many sigma bonds are there? How many pi bonds? What is the hybridization at the carbon? How are the nitrogen atoms hybridized? How many lone pairs are there?

Individual Practice Page 418 # 17, 21, 23, 25, 27, 33 (omit f and g), 53 Begin working through Unit 4 Review Packet Read through Chromatography Lab Investigation 5 page 45 in the green guided inquiry manual.

Thin Layer Chromatography (TLC) Thin layer chromatography (TLC) is an important technique for identification and separation of mixtures of organic compounds. Components of the mixture are partitioned between an absorbent (stationary phase) and a solvent (mobile phase) which flows through the absorbent.

THIN LAYER CHROMATOGRAPHY In TLC, a plastic, glass or aluminum sheet is coated with a thin layer of silica gel. A very small amount of a solution of the substance to be analyzed is applied in a small spot with a capillary tube, ~1cm from the bottom of the TLC plate The TLC is developed in a chamber which contains the developing solvent (the mobile phase). A truncated filter paper placed in the chamber serves to saturate the chamber with mobile phase.

THIN LAYER CHROMATOGRAPHY As the mobile phase rises up the TLC plate by capillary action, the components dissolve in the solvent and move up the TLC plate. Individual components move up at different rates, depending on intermolecular forces between the component and the silica gel stationary phase and the component and the mobile phase. http://www.instructables.com/id/EW1YDCYF4REC0IU/ The stationary phase is SiO2 and is very “polar”. It is capable of strong dipole-dipole and H-bond donating and accepting interactions with the “analytes” (the components being analyzed). More polar analytes interact more strongly with the stationary phase in move very slowly up the TLC plate. By comparison, the mobile phase is relatively nonpolar and is capable of interacting with analytes by stronger London forces, as well as by dipole-dipole and H-bonding. More nonpolar analytes interact less strongly with the polar silica gel and more strongly with the less polar mobile phase and move higher up the TLC plate.

The solvent is allowed to evaporate from the TLC sheet in the hood. THIN LAYER CHROMATOGRAPHY Once the solvent is within ~1-2 cm of the top of the TLC sheet, the TLC is removed from the developing chamber and the farthest extent of the solvent (the solvent front) is marked with a pencil. The solvent is allowed to evaporate from the TLC sheet in the hood. http://orgchem.colorado.edu/hndbksupport/TLC/TLCprocedure.html

THIN LAYER CHROMATOGRAPHY - Visualization    As the chemicals being separated may be colorless, several methods exist to visualize the spots: Visualization of spots under a UV254 lamp. The adsorbent layer will thus fluoresce light green by itself, but spots of analyte quench this fluorescence. Iodine vapors are a general unspecific color. Specific color reagents exist into which the TLC plate is dipped or which are sprayed onto the plate. Once visible, the Rf value of each spot can be determined Chromatogram of 10 essential oils, Stained with vanillin reagent.

THIN LAYER CHROMATOGRAPHY Calculation of Rf’s The Rf is defined as the distance the center of the spot moved divided by the distance the solvent front moved (both measured from the origin)

THIN LAYER CHROMATOGRAPHY Calculation of Rf’s The Rf is defined as the distance the center of the spot moved divided by the distance the solvent front moved (both measured from the origin)

THIN LAYER CHROMATOGRAPHY – Rf’s Rf values can be used to aid in the identification of a substance by comparison to standards. The Rf value is not a physical constant, and comparison should be made only between spots on the same sheet, run at the same time. Two substances that have the same Rf value may be identical; those with different Rf values are not identical.