lecture 2 - CHEM 231 MORE ON RESONANCE, SHAPES CURVED ARROW...

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Unformatted text preview: CHEM 231 MORE ON RESONANCE, SHAPES CURVED ARROW NOTATION ACIDS AND BASES LECTURE 2 1 LECTURE 2 Lets Review Electronegativities Lecture 2 2 Lone pairs on neutral atoms 3 lone pairs 2 lone pairs F O Cl S Br I P C B Al Lecture 2 3 The two types of electron orbitals important to OCHEM s-orbitals p-orbitals The two types of true bonding in OCHEM Sigma bonding Pi bonding Lecture 2 4 Resonance Electrons in bonds can be in all locations at once, as far as our perspective goes This is why we call them electron clouds Under certain situations electrons can spread beyond just one bond To express this phenomenon we draw resonance structures Drawing resonance structures is a skill you will need for both organic courses Lecture 2 5 Curved arrows : practice makes the right move every time! This might look trivial to you You need to take it seriously and practice to perfection The difference between being able to correctly draw curved arrows and not is the difference between an A and a B grade Lecture 2 6 Rules for Curved Arrow Notation Any curved arrow is showing the path that an electron follows Curved arrows always start at a source of electrons . Curved arrows always point to where the electrons are going When a pair of electrons move (most cases are like this) A double headed arrow is used When movement of a single electron is to be indicated, a half head arrow is used. Lecture 2 7 Shapes of molecules dictated by orbitals We discuss molecules with a distinct central atom We use knowledge of hybridization and number of groups to determine overall shape Lecture 2 8 sp centers give: The molecule cannot have greater dimensionality than the central atom's orbitals Tetrahedral shape if 4 sigma bonds to central atom Pyramidal shape if there are 3 sigma bonds to the central atom Bent shape if there are 2 sigma bonds to central atom Linear shape if there is only one sigma bond to central atom. Write examples of these for simple 1 row element Lecture 2 9 sp centers give : The molecule cannot have greater dimensionality than the central atom's orbitals Trigonal planar shape if 3 sigma bonds to central atom Bent shape if 2 sigma bonds to the central atom Linear shape if 1 sigma bond to central atom. sp center gives linear molecules at all times Write examples of these for simple 1 row element Lecture 2 10 Molecular shape along with differences in atom electronegativities dictate MOLECULAR dipole moments Lecture 2 11 Some simple truths about acids Your book talks about Arrhenius, BronstedLowrey and Lewis acids; we Ochemists can consider a unified theory of acids When an acid does its job it takes on a negative charge When an acid does its job the conjugate base that is formed is more reduced than the parent acid Stronger acids are those that can take on a negative charge without becoming destabilized In a row we find better protic acids as the central atom becomes more and more electronegative (going from left to right in the row) Lecture 2 12 Some simple truths about acids Stronger acids are those that can take on a negative charge without becoming destabilized If protic acids whose central atoms are all in the same column are compared : the larger the central atom the stronger the acid For example: Hydrochloric acid is a stronger acid than hydrofluoric acid. But hydrochloric acid is weaker than hydrobromic acid. Lecture 2 13 Some simple truths about acids Stronger acids are those that can take on a negative charge without becoming destabilized Resonance provides excellent stabilization The more resonance structures that are accessible, the better stabilized the conjugate base Nitric is a much stronger acid than acetic acid Lecture 2 14 Some simple truths about acids When an acid does its job it takes on a negative charge Stronger acids are those that can take on a negative charge without becoming destabilized Lewis acids suffer from being electron deficient. The Lewis acids include most metals For us the most common Lewis acids are AlX BX FeX Lecture 2 15 Some simple truths about Bases We Ochemists can consider a unified theory of bases All bases behave in the same way. They all seek a positive charge. Bases that like protons are known as Arrhenius or BL bases. Lewis bases go after electron deficient atoms that are not hydrogen These Lewis bases have another moniker: Nucleophiles Because they like things that are nucleus like (the nucleus has positive charge) Lecture 2 16 Some simple truths about Bases Gauging the strength of bases can is a similar type of process as gauging acid strength A base's strenght is strongly influenced by the electronegativity of the central atom (the one with the pair of electrons) We often consider the strength of a base by talking about the pKa of the conjugate acid. Going across a row of the periodic table the less electronegative the central atom the strong a base it will be. For example Ammonia NH is much more basic than water, We often consider the strength of a base by talking about the pKa of the conjugate acid. Lecture 2 17 Some simple truths about Bases Pka is a value comparable to pH High pKa values indicate weak acid Low and negative pka values indicate strong acids We can estimate pka values given a broad list of data Try some examples Lecture 2 18 An introduction to pKa pK is calculated from the H2O + HA H3O+ + A[H3O+][A-] Ka = [H2O][HA] pKa = -log Ka pKa values of common compounds in O-Chem HI -10.4 methanol equilibriium of an acid 15.2 base reaction HCl -3.9 HF +3.1 acetic acid +4.7 Lecture 2 acetylene 26 ammonia 36 benzene 43 ethylene 45 methane 60 ethane 62 water 15.7 acetone 19 19 Things that affect how BL acidic a compound is Length (strength) of bond broken Electronegativity of anion center that is formed Nearby inductive influences (travel by sigma bond) Resonance influences (travel by pi bond) Lecture 2 20 Practice evaluating acid strength Lecture 2 21 Things that affect how Lewis acidic a compound is Electron deficiency of central atom Nearby inductive influences(travel by sigma bond) Nearby resonance influences (travel by pi bond) Lecture 2 22 Evaluation of Lewis acids Lecture 2 23 Expressing organic structure through drawings Lecture 2 24 ...
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This note was uploaded on 10/06/2008 for the course CHEM 231 taught by Professor Kissling during the Spring '08 term at Binghamton University.

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