McMurry Textbook Summaries (Lectures 1-12 Material)
McMurry Textbook Summaries (Lectures 1-12 Material)

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University of Melbourne

2.7 Acids and Bases: The BrØnsted-Lowry Definition  Acidity and bacidity are related to a molecules electronegativity and polarity.  Acids donate protons, and bases accept protons  Water can act either as an acid or a base 2.8 Acid and Base Strength  Acids differ in their ability to donate protons, some reacting almost completely and others only slightly. The strength is indicated by the acidity constant, which resembles the degree to which an acid is ionised in solution.  Stronger acids have their equilibria more towards the right and have larger acidity constants.  pKa=-logKa  A stronger acid has a smaller pKA and a weaker acid has a larger pKa.  There is an inverse relationship between the acid strength of an acid and base strength of its conjugate base. 2.9 Predicting Acid-Base Reactions from pK Values  An acid will donate a proton to the conjugate base of a weaker acid, and the conjugate base of a weaker acid will remove the proton from a stronger acid.  The product conjugate acid in an acid-base reaction must be weaker and less reactive than the starting base. 2.10 Organic Acids and Organic Bases  Organic acids are characterized by the presence of a positively polarized hydrogen atom, and are of two main kinds: (1) Contain a hydrogen tom bonded to an electronegative oxygen atom (i.e. methanol and acetic acid) (2) Contain a hydrogen atom bonded to a carbon atom next to a C=O bond (i.e. acetone)  In both cases acidity is due to the fact that the conjugate base resulting from loss of proton is stabilized by having its negative charge on a strongly electronegative oxygen atom. It is additionally stabilized by resonance.  The acidity of ketones is due to the conjugate base resulting from loss of a proton is stabilised by resonance. Additionally one of the resonance forms stabilises the negative charge by placing it on an electronegative oxygen atom.  Organic bases are characterized by the presence of an atom with a lone pair of electrons that can bond to protons. Nitrogen containing compounds are most common, followed by oxygen containing compounds. 5.2 How Organic Reaction Occur: Mechanisms  A reaction mechanism is an overall description of how a reaction occurs- i.e. exactly what takes place at each stage of a chemical transformation.  Considers which bonds are broken an in what order.  A bond can break in an electronically symmetrical way so that one electron remains with each product fragment. This is called homolytic.  A bond can also break in an electronically unsymmetrical way so that both bonding electrons remain with one product fragment, leaving the other with a vacant orbital. This is called heterolytic.  A bond can form in an electronically symmetrical way if one electron is donated to the new bond by each reactant or in an unsymmetrical way if both bonding electrons are donated by one reactant.  Radical (neutral chemical species that contains an odd number of electrons and has single unpaired electron) reactions involve symmetrical bond-breaking and bond-making.  Polar reactions involve unsymmetrical bond-breaking and bond-making and are the most common reaction. 5.4 Polar Reactions  Polar reactions occur because of the electrical attraction between positive and negative centres on functional groups in molecules.  Most organic compounds are electrically neutral, they have no net charge.  Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms.  Metals are less electronegative than carbons.  As the electric field around a given atom changes because of changing interactions with solvent or other polar molecules nearby, the electron distribution around that atom also changes. The measure of this response to an external electrical influence is called the Polarizability of the atom.  Large atoms with more, loosely held electrons are more polarisable, and smaller atoms less so.  The fundamental characteristic of all polar organic reactions is that electron rich sites react with electron poor sites.  A nucleophile is a substance that is “nucleus-loving”, has a negatively polarized, electron-rich atom and can form a bond by donating a pair of electrons to a positively charged; ammonia, water, hydroxide ion.  An electrophile has a positively polarized, electron poor atom and can form a bond by accepting a pair of electrons from a nucleophile. 5.6 Using Curved Arrows in Polar Reaction Mechanisms  An electron pair moves from the atom at the tail of the arrow to the atom at the head of the arrow.  Rules of arrows: (1) Electrons move from a nucleophilic (Nu: or Nu: ) source to an electrophilic sink + (E or E ) (2) The nucleophile can be either negatively charged or neutral (3) The electrophile can be either positively charged or neutral (4) The octet rule must be followed. 6.8 Orientation of Electrophilic Additions: Markovnikov’s Rule  Reactions in which an unsymmetrical substituted alkene has given a single addition product are regiospecific, when only one of the two possible orientations occur.  In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. 6.9 Carbocation Structure and Stability  Carbocations are planar, with the pi orbital being unoccupied.  The stability of carbocations increases with increasing substitution so that the stability order is tertiary > secondary > primary > methyl.  To determine carbocation stabilities one may measure the energy required to form the carbocation by dissociated of the corresponding alkyl halide.  Inductive effects result from the shifting of electrons in a sigma bond in response to the electronegativity of nearby atoms, thus the more alkyl groups there are attached to the positively charged carbon, the more electron density shifts toward the charge and the more inductive stabilisation of the cation occurs.  Hyperconjugation is the stabilising interaction between a vacant p orbital and properly oriented C-H bonds on neighbouring carbons. 6.10 The Hammond Postulate  Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation intermediate.  A more highly substituted carbocation is more stable than a less highly substituted one.  The more stable intermediate is forms faster than the less stable one.  Transition states are high-energy activated complexes that occur transiently during the course of a reaction and represent an energy maximum.  Hammond Postulate: The structure of a transition state resembles the structure of the nearest stable species. Transition states for endergonic steps structurally resemble products, and transition states for exergonic steps structurally resemble reactants.  The transition state for alkene protonation structurally resembles the carbocation intermediate  The transition state is stabilised by hyperconjugation and inductive effects in the same way as the product carbocation. 7.4 Addition of Water to Alkenes: Oxymercuration  Water adds to alkenes to yield alcohols, in the presence of a strong acid catalyst (HA).  Results in a carbon intermediate which reacts with water to yield a protonated alcohol product.  Biological hydration requires that the double bond be adjacent to a carbonyl group for reaction to proceed.  Alkenes are often hydrated by Oxymercuration  Nucleophilic addition of water as in halohydrin formation, followed by loss of a proton, then yields a stable organo-mercury product. 7.7 Reduction of Alkenes: Hydrogenation  In the presence of a metal catalyst alkenes react with H - th2 hydrogen bond has been hydrogenated or reduced.  In organic chemistry a reduction is a reaction that results in a gain of electron density by carbon, caused either by bond formation between carbon and a less electronegative atom or by bond-breaking between carbon and a more electronegative atom.  Catalytic hydrogenation is a heterogenic process rather than a homogenous one- takes place on the surface of insoluble catalyst particles.  Molecular hydrogen and the alkene adsorb to catalyst surface and dissociate, a hydrogen ion is transferred between them forming an artificially reduced intermediate with a C-H bond. A second hydrogen is transferred from the metal to the second carbon, giving the alkane product and regenerating the catalyst.  Aldehydes, ketones, esters and nitriles survive normal alkene hydrogenation conditions unchanged, although reaction with these groups does occur under more vigorous conditions. 11.1 The Discovery of Nucleophilic Substitution Reactions  Nucleophilic substitution reactions involve the substitution of one nucleophile or hydroxide ion with another. R-X + Nu:-  R-Nu + X:-  The inversion of stereo chemical configuration must therefore take place in the second step, the nucleophilic substitution of tosylate ion by acetate ion.  The nucleophilic substitution reaction of a primary or secondary alkyl halide or tosylate always proceeds with inversion of configuration. 11.2 The SN2 Reaction  There is a direct relationship between the rate at which the reaction occurs and the concentrations of the reactants. Measuring this is an indication of the kinetics of the reaction.  Substitutions occurs at different rates depending upon temperature, concentrations and pH.  Second order reaction: reaction rate is linearly dependent on the concentrations of two species.  Reaction rate = rate of disappearance of reactant = k X [RX] X [OH-]  SN2 short for, substitution, nucleophilic and bimolecular.  Takes place in a single step without intermediates when the incoming nucleophile reacts with the alkyl halide or tosylate (the substrate) from a direction opposite the group that is displaced (the leaving group)  Nucleophile must approach from the opposite end of the molecule to the leaving group.  Stereo chemical configuration is reversed compared the original molecule. 11.3 Characteristics of the N 2 Reaction  The rate of a chemical reaction is determined by the energy difference between reactant ground state and transition state.  The transition state for reaction of a sterically hindered (i.e. 3 methyl groups) alkyl halide, whose carbon atom is shielded from approach of the incoming nucleophile is higher in energy and forms more slowly than the corresponding transition state for a less hindered alkyl halide.  Nucleophile attacks carbon from the back  SN2 reactions occur only at relatively unhindered sites, usually only primary and a few secondary halides.  The exact nucleophilicity of a species in a given reaction depends on the substrate, the solvent, and even the reaction conditions.  Nucleophilicity roughly parallels basicity: strong bases often make strong nucleophiles.  Nucleophilicity usually increases going down a column of the periodic table as larger atoms hold their valence electrons less tightly are and consequentially more likely to react.  Negatively charged nucleophiles are usually more reactive than neutral ones (reactions generally carried out under basic conditions)  As the leaving group is expelled with a negative charge the best leaving groups are those - that best stabilize the negative charge in the transition state; i.e. Cl .  An alcohol can be treated with para-toluenesulfonyl chloride to form a tosylate to improve its reactivity towards nucleophilic substitution.  Protic solvents (containing an –OH or –NH group) are generally the worst for SN2 as they undergo salvation of the reactant nucleophile and polar aprotic solvents (polar but neither of above groups) are the best as they raise the ground state energy of the nucleophile.  Substrate characteristics: methyl and primary substrates.  Nucleophile: basic, negatively charged with a higher ground state energy.  Leaving group: more stable anions.  Polar aprotic solvents which surround the accompanying cation. 11.4 The S 1 Reaction N  Tertiary halide substrates are most effective in SN1 reactions, and is the only factor upon which the reaction rate is dependent.  Unimolecular rate limiting step. Reaction takes place by loss of the leaving group before the nucleophile approaches.  Reaction occurs through a carbocation intermediate, with products showing a 1:1 ratio of stereo chemical isomers of original asymmetric molecule. 11.5 Characteristics of the SN1 Reaction  Reaction is favoured whenever a stable carbocation is formed. As a result of resonance stabilization, tertiary carbocations are the most stable and thereby the most likely to react.  Most stable leaving groups are more reactive, i.e. tertiary alcohols.  Nucleophiles do not affect the rate of SN1 reactions.  Solvent effects in SN1 reaction are due largely to stabilization or destabilization of th
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