TAMU CHEM 237 - Hybridization and Geometry

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1. Hybridization and Geometry- All hybridized orbitals are of equal energy levels- Hybridization is determined by number of sigma bonds and lone pairs.- 3D geometry is dependent on both the number of sigma bonds and lone pairs. - Types of hybridization:sp3 – bond angles of 109.54 sigma bonds = tetrahedron3 sigma bonds, 1 lone pair = trigonal planar2 sigma bonds, 2 lone pairs = angular/bentsp2 – bond angles of 1203 sigma bonds = trigonal planar2 sigma bonds, 1 lone pair = angular/bent1 sigma bond, 2 lone pairs = no shapesp – bond angles of 1802 sigma bonds = linear1 sigma bond, 1 lone pair = no shape- Bulky groups and lone pairs causes the angle to deviate from the ideal- The more s-character the shorter and stronger the bond2. Who Has My CompoundMelting Point Determination (13-16)- Range from which first crystal melts until the last crystal melts, usually listings are of the upper point- Determination of Purity-melting points are sensitive to impurities, a range greater than 2C indicatesimpurity. - Determination of Unknown-obtaining a mixture melting point can allow identification of an unknown. Sharp m.p. is indicative of the identical compounds.- Melting point is a physical measure of forces holding the molecule in a lattice.- Impurities introduce entropy, lowering the energy needed to break the molecular attractions.- Eutectic mixture is the specific combination of two compounds that produces a sharp melting point.o Eutectic M.P. is lower than that of either compound or any other combination of the compounds- Some compounds decompose and others sublime, rather than becoming liquid.- Procedure:o 2-3mm of compound in the capillary tubeo Heat at 10C/min for approximate; 1/2C for more accurate m.p.Experiment- Determination of physical properties; melting point, TLC, and solubility- Solubility is tested in 10% aqueous sodium bicarbonate- TLC-adsorbent (silica gel or alumina) is bound to solid material (glass, aluminum, or plastic). The distance of elution up the column (Rf) is characteristic of compound.o Inert inorganic material allows for fluorescence under UV light; compounds block this fluorescence, appearing purple.o Spots need to be 1-2mm in diameter, allow them to dry before running TLC.o Rf (retention factor of ratio of fronts) = distance spot traveled/solvent distance.o Cap TLC jar, dispose of waste in organic container and do not wash with water or acetone. 3. RecrystallizationTechnique (33-41)- Reactions either do not go to completion or give a mixture of products.- Favored for compounds with high melting point solids, and thermally unstable solids.- Material is dissolved in solvent and allowed to form a more pure crystal from the solution; a pure material forms the best crystal lattice. - You should always recover less than what you started with.- Solvent Choice:o Cannot react with your compound.o Must dissolve the compound when hot, but not when it is cold. o Must dissolve impurities at all temperatures or not dissolve them at all. - Insoluble impurities are removed from solution with hot vacuum filtration.- Slow cooling produces the best crystals; warm flask into water then once at room temp it goes to ice bath. - Vacuum filtration rapidly separates solids from liquids.- Hot vacuum filtration removes solid impurities from a hot solution; other filtration isolates crystals. - For hot filtration Buchner funnel is heat on the steam bath so solution doesn’t crystallize during filtration.- Disconnect the water aspirator before turning it off.- Activated carbon is used to remove colored impurities, with polar sites. Experiment Purification of 1.5g acetanilide (m.p. 110-116C) using water as the solvent Addition of more hot water can get rid of acetanilide oil that may arise. If oil is produced while crystallizing, reheat the solution and add water.  Acetanilide is an irritant so avoid getting it on your hands.  Acetanilide is formed by the reaction of aniline and acetic acid and contains amide functional group.4. Molecular ModelingExperiment Conformations/Conformers-the various geometries a molecule can assume due to bond rotation; bonds are not broken or made. Energy of a molecule is raised by distorting bond lengths or angles, bring atoms or electron pairs into close contact, or eclipsing bonds.o Strain the deviation from the ideal measures. o Estrain = Estrech + Eangle + Etorsion + Esteric Global minimum is minimal energy level of all conformations, while local minimum is not.  Quantum Mechanical Modeling Problems:o All possible bonds can not be defined form a few exampleso Quantum mechanics is less understood than macroscopic Newtonian mechanics.o It is impossible to know the location and energy of an electron at the same time. Semi-empirical Method combines sound model of bonding with empirically determined parameters; determines electron’s energies and distributions.o We used it for dipole moments and multiple polar bonds.Theory Molecular polarity is the vector sum of bond polarities, its dipole moment Electron pairs contribute significantly to dipole moment as well. Steric strain increases with bulkiness of the groups Molecule is most stable/least strain when substituent is in the equatorial position.5. Infrared Spectrophy Infrared region is “heat waves”. IR is used to detect and identify functional groups.  Sample must be places on something that does not absorb between 4000 and 400 cm-1, many inorganic salts are the best choice. Dispersive IR All require a source of heat energy, beam splitter, and detector. Beam splitter corrects for the fact that the intensity of heat is not the same at all wavelengths.o one of the split beams is a reference the other is fed through the sample. o Dual beam splitter also eliminates peaks due to air.  Scans wave numbers (cm-1) 4000 to 600Fourier Transform Infrared Spectroscopy (FTIR) This is the form of IR used today.  Fourier transform is a math equation that separates and identifies components of the raw beam, no prism is needed. Allows for addition of spectras for samples that were previously too small, because of internal calibration and lack of prism. Michelson interferometer uses beam splitters and mirrors to tell energy differences due to absorbance.  FTIR spectrum is broader, wave numbers 4000 to 400. FTIR obtains a background spectrum that is subtracted from all runs to account of air activity. CO2 doublet peak is often seen


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TAMU CHEM 237 - Hybridization and Geometry

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