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Chemistry 230 Chapter Three Notes Chapter Three An Introduction to Organic Reactions: Acids and Bases Types of Organic Reactions Many different organic reactions will be taught in this course Most can be categorized into the following types Additions Substitutions Eliminations Rearrangements Oxidations Reductions An Overview of Organic Reaction Types: Additions An addition reaction involves the combination of an organic compound with some other element or compound In most cases, a reagent will add itself to a double or triple bond in a compound An Overview of Organic Reaction Types: Substitutions A substitution reaction involves the exchange of some part of an organic molecule for another group 3-1 Chemistry 230 Chapter Three Notes An Overview of Organic Reaction Types: Eliminations In an elimination reaction, an atom or groups of atoms is lost from each of two adjacent carbon atoms, forming a new π bond An Overview of Organic Reaction Types: Rearrangements As the name implies, a rearrangement reaction involves some fundamental change in the structure of a molecule without a net addition or loss of atoms We end up with the same atoms that we began with, only they are bonded in a different way An Overview of Organic Reaction Types: Oxidations In organic chemistry, an oxidation reaction causes one or more carbon atoms in the structure to Form new bonds with more electronegative atoms, especially oxygen Break bonds with hydrogren atoms A reagent which causes an organic molecule to become oxidized is called an oxidizing agent Common oxidizing agents include KMnO4 and CrO3 An Overview of Organic Reaction Types: Reductions A reduction is the opposite of an oxidation, and causes and organic compound to Break bonds with more electronegative atoms, especially oxygen Form new bonds with hydrogren atoms A reducing agent is a reagent used to effect this change Common oxidizing agents include NaBH4, LiAlH4, and H2 3-2 Chemistry 230 Chapter Three Notes Brønsted-Lowry Definition of Acids and Bases Of the ways in which we can define the terms acid and base, two are especially important in organic chemistry According to the Brønsted-Lowry definition, in a chemical reaction an acid donates a proton (H+), and a base accepts a proton Lewis Definition of Acids and Bases An alternative, more general, definition of acids and bases was given by G.N. Lewis. According to Lewis an acid is a substance which accepts an electron pair a base is a substance which donates an electron pair This definition does not contradict the Brønsted-Lowry definition, but simply expands the idea Example: H + NH3 H NH3 Fits the Bronsted-Lowry theory What About Cl Cl Al Cl Cl + N(CH3)3 Cl Al N(CH3)3 Cl Here the Lewis Theory must be used as there are no protons being accepted or donated 3-3 Chemistry 230 Chapter Three Notes Common Lewis Acids Metallic compounds which can accept electrons often act as Lewis Acids Common examples include AlCl3 (seen in previous slide) FeBr3 ZnCl2 HgCl2 The metal atoms in these compounds are capable of complexing with the electron pairs and/or π bonds of molecules Acids and Bases in Organic Reactions Acidity and basicity are important concepts in how organic molecules react + Many reactions involve the transfer of a proton (H ) The most common of the distinctly acidic organic compounds are carboxylic acids (i.e. Compounds which contain the group -COOH) O R C O H Other organic compounds have varying degrees of acidity Approximate pKa Values Examples of Organic Acid - Base Reactions 3-4 Chemistry 230 Chapter Three Notes Calculation of Equilibrium Constants Relationship of Bases and Conjugate Acid In comparing the strength of bases, we should first consider the acidity (pKa) of their conjugates acids A base derived from a strong mineral acid is very weak Consider HCl and Cl A base derived from a very weak acid is generally an extremely strong base Consider NH3 (as an acid) and NH2Example Rank the molecules below in order from 1 to 5 from weakest base to strongest base (strongest = 5). O Cl- NH2- NH3 OH- Effect of Structure on pKa The more electronegative the atom to which the hydrogen is bonded, the stronger the acid if the atoms have similar size (e.g. in the same period). 3-5 Chemistry 230 Chapter Three Notes When comparing acids which have hydrogen atoms bonded to atoms within the same group, we must consider the size of the atom hydrogen is bonded to Compare HCl and HBr Br is a larger atom and overlap with the 1s orbital of hydrogen is not as great as in the case of Cl The larger Br- ion is able to spread its charge out over a larger volume than Cl-, so Br- is a weaker base than ClHBr is a stronger acid than HCl despite the fact that Cl is more electronegative than Br! The effect of electronegative atoms also is felt when they are near the acidic hydrogen but not bonded directly to it. This is called the inductive effect. Note how the acidity decreases as the bromine is farther from the carboxylic acid group A Point of Confusion… Why is it that trifluoroacetic acid (CF3COOH) is more acidic than triiodoacetic acid (CI3COOH), but hydroiodic acid (HI) is more acidic than hydrofluoric acid (HF)? In the first pair of acids, the acidic proton of both acids is bonded to an oxygen atom. Trifluoroacetic acid is a stronger acid because it the acidic hydrogen experiences a greater inductive effect. In the second pair of acids, the acidic proton is bonded to different atoms, F and I. HI is more acidic because the iodide ion can disperse its charge over a larger volume of space. Iodide ion is therefore a weaker conjugate base, leading us to conclude that HI is the stronger acid. 3-6 Chemistry 230 Chapter Three Notes Hybridization and pKa Hydrogens bonded to carbon atoms are not usually very acidic The acidity of these hydrogen atoms depends somewhat on the amount of “s character” on the carbon 3 sp hybridized carbon atoms are generally the most electronegative, while sp carbon atoms are the least From this, we can conclude that hydrogens bonded to sp carbons are generally the most acidic, while those bonded to sp3 carbons are the least Electron Delocalization and Acids/Bases The pKa of a carboxylic acid is ~ 5 The pKa of an alcohol is ~ 16 There are two reasons for this difference: The carboxylic acid has a doubly bonded oxygen in place of the two hydrogens in the alcohol. The electronegative oxygen pulls the bonding electrons of the O-H bond away from the proton making it easier to ionize The conjugate base of the carboxylic acid is considerably more stable than the conjugate base of the alcohol When an alcohol loses a proton all the electron density resides on the single oxygen atom When the carboxylic acid loses a proton, the electron density is shared between the two oxygen atoms For acetate, the conjugate base of acetic acid, the electron density is shared by the two oxygens - the electrons are delocalized Delocalization of electrons stabilizes the base, making it less reactive The negative charge of the ethoxide ion is localized on the single oxygen atom Therefore, ethoxide ion is more basic than acetate ion, and Acetic acid is a stronger acid than ethanol Effect of pH on Structure Whether a given organic acid will lose a proton in aqueous solution depends on pH of solution 3-7 Chemistry 230 Chapter Three Notes Organic Compounds as Bases Organic molecules with available lone pairs may act as bases Ethers and alcohols may act as bases in the presence of strong mineral acids They are not significantly protonated by weaker acids, like acetic acid What about carbonyls? An Introduction to Curved Arrow Mechanisms Organic chemists often employ a combination of curved arrows and molecular drawings to illustrate the details of a reaction mechanism This technique is extremely useful in predicting the products of a given chemical reaction You will encounter many mechanisms in this course, and many more next semester A regular curved arrow is understood to represent the movement of a pair of electrons In general, electrons flow from a site which is “electron rich” to a site which is “electron poor” by comparison Curved arrows are always drawn away from the lone pair, towards the site which is electron deficient A proton transfer 3-8 Chemistry 230 Chapter Three Notes Draw curved-arrow mechanisms to illustrate proton transfer reactions between Hydronium ion and bisulfide ion (SH-) Acetylene (C2H2) and amide ion (NH2-) Phenol and methoxide ion (CH3O-) Acids & Bases in Nonaqueous Solutions Suppose that, for a given reaction, we must use a base stronger than OH Consider the deprotonation of an alkyne Water cannot be used as a solvent; it is more acidic than the alkyne Liquid ammonia (NH3) is an ideal solvent for the previous reaction Very strong bases (NH2-, H-, R3C-, etc.) are incompatible with water and other polar protic solvents, as they will attack the solvent Protic solvents are those which contain bonds between hydrogen and electronegative atoms (e.g. oxygen and hydrogen). Water and alcohols are the primary examples. Problem 3.13 (p. 126) addresses this issue 3-9
