Reactions and Mechanisms. Chapter 3 An Introduction to Organic Reactions: Acids and Bases

Transcription

1 Reactions and Mechanisms Chapter 3 An Introduction to Organic Reactions: Acids and Bases Cl (aq) + NaO (aq) 2 O (aq) + NaCl (aq) In typical inorganic reactions involving ions, little focus on how reaction occurs energetics of the reaction equilibrium? precipitate? For Organic Chemistry Understanding Reaction Mechanisms is critical Classified by products, there are 4 types of organic reactions Substitutions { 2 } + Additions Understanding why, when, and how one reaction occurs allows predictability Classified by products, there are 4 types of organic reactions Eliminations Each can occur by a small number of different mechanisms Substitution reactions + NaO + Cl Rearrangements Cl Br 2 O Br Fe C l 2 Cl 1

2 Intermediates In Organic Reactions ow do bonds break two options Reaction mechanisms are detailed descriptions of changes at the molecular level as reactants become products. Often the reactions involve a sequence of steps with one or more chemical species called intermediates that are formed and consumed. omolysis Chemical intermediates typically are not stable structures that can be put in a bottle. Many exist for very short times ( seconds). We will explore how structural and electronic influences affect the stability of chemical intermediates and, thereby, control the path that reactions follow. eterolysis eterolytic reactions almost always occur at polar bonds Introduction to Acid- Base Chemistry The reaction is often assisted by formation of a new bond to another molecule Brønsted-Lowry Definition of Acids and Bases Acid: a substance that can donate a proton Base: a substance that can accept a proton Example: ydrogen chloride is a very strong acid Simple acid-base reaction Lewis Definition of Acids and Bases Aqueous hydrogen chloride and aqueous sodium hydroxide are mixed The actual reaction is between hydronium and hydroxide ions Lewis Acid = an electron pair acceptor Lewis Base = an electron pair donor 2

3 Lewis definition broadens acid beyond proton transfers Example: BF 3 and N 3 react based on their relative electron densities eterolysis of Bonds to Carbons Can give a carbocation or carbanion depending on the nature of leaving group Z BF 3 has substantial positive charge on the boron N 3 has substantial negative charge localized at the lone pair Carbocation has a formal charge on the carbon Carbocation is a reactive intermediate Recall process: how many valence electrons belong to C For each pair of bonding electrons, one "belongs" to carbon. All no nbo nding elect ro ns in the v a lence le ve l " belo ng" t o c ar bon. C+ a carbocation In a carbocation, only 3 of the 6 bonding electrons "belong" to the carbon. There are no nonbonding electrons. Reacts quickly with Lewis base to reform stable molecules Atomic carbon has 4 electrons in the valence level. Since in the carbocation, only 3 electrons belong to carbon, there is a deficiency of one electron (a formal charge imbalance at carbon). Therefore, there is a formal charge of +1 on carbon. Carbanion has 8 valence electrons and a negative charge Nucleophiles and Electrophiles The Organic chemistry terms for Lewis acids and bases Electrophiles ( electron-loving reagents ): seek electrons to obtain a stable valence shell of electrons Are electron-deficient themselves e.g. carbocations Nucleophiles ( nucleus-loving reagents): seek a proton or some other positively charged center examples Are electron-rich themselves e.g. carbanions Note formal charge on Carbon 3

4 Use of Curved Arrows in Illustrating Reactions Curved arrows show the movement of electrons in a reaction An arrow starts at site of higher electron density (a covalent bond or unshared electron pair) and points to site of electron deficiency Examples: Strengths of Acids and Bases K a and pk a : a measure of acid strength Acetic acid is a relatively weak acid and a 0.1M solution is only able to protonate water to the extent of about 1% C C + d+ d- - Cl C C + Cl The equilibrium equation for this reaction is: nucleophile elec trophile Electron pairs move from nucleophile to electrophile Strengths of Acids and Bases The concentration of water is usually constant (about 55.5 M) so the concentration of water is factored out to obtain an acidity cons tant (K a ) Acidity is usually expressed in terms of pk a pk a is the negative log of K a The pk a for acetic acid is 4.75 K a acetic acid = 1.76 X 10-5 Any weak acid (A) dissolved in water fits the general K a expression The larger the pk a, the weaker the acid Stronger acid = more dissociated = larger K a Acidities The Strong Acids 4

5 Weaker Acids Predicting the Strengths of Bases The stronger the acid, the weaker its conjugate base An acid with a low pk a will have a weak conjugate base Chloride is a very weak base because its conjugate acid Cl is a very strong acid Methylamine is a stronger base than ammonia Therefore the conjugate acid of methylamine is weaker than the conjugate acid of ammonia Predicting the Outcome of Acid-Base Reactions Acid-base reactions always favor the formation of the weaker acid/weaker base pair The weaker acid/weaker base must always be on the same side of the equation Example: Acetic acid reacts with sodium hydroxide to greatly favor products Water Solubility as a Result of Salt Formation Relationships Between Structure and Acidity Bond Strength Organic compounds which are water insoluble can sometimes be made soluble by turning them into salts Water insoluble carboxylic acids can become soluble in aqueous sodium hydroxide Water insoluble amines can become soluble in aqueous hydrogen chloride Acidity increases going down a row of the periodic table Bond strength to hydrogen decreases going down the row 5

6 Relationship Between Structure and Acidity: Electronegativity Increasing electronegativity of central atom increases acidity in two ways (1) The polarity of the covalent A- bond increases with increasing electronegativity of atom A, resulting in a more electropositive. As positive charge density on increases, its reactivity towards a base increases. Relationship Between Structure and Acidity: Electronegativity Acidity increases from left to right in a row of the periodic table Increasingly electronegative atoms polarize the bond to hydrogen and also stabilize the conjugate base better reactivity d- d+ 2 N- d-d+ O- towards B: - slower faster (2) As the electronegativity of A increases, the stability of the anion, A -, increases, resulting in a larger K a for the equilibrium: A- + 2O 3O + + A - Overview of Acidity Trends The Effect of ybridization on Acidity ydrogens connected to orbitals with more s character are more acidic s orbitals = smaller, closer to the nucleus than p orbitals anions held closer to the nucleus are more stabilized hydrocarbon C C C C C C ethane ethene ethyne pk a > hybrid orbital projected by C increasing acidity sp 3 sp 2 sp The Effect of ybridization on Acidity C C C sp 3 sp 2 sp Inductive Effects Electronic effects that are transmitted through space and through the bonds of a molecule In ethyl fluoride the electronegative fluorine is drawing electron density away from the carbons Fluorine is an electron withdrawing group The effect gets weaker with increasing distance increasing hybrid orbital electronegativity greater s-character in an alkyne makes the hybrid orbital less directional allows greater stabilization of the orbital electrons by positive nucleus 6

7 Energy Changes in Reactions -Thermodynamics Kinetic energyis the energy an object has because of its motion Potential energyis stored energy The higher the potential energy of an object the less stable it is Potential energy can be converted to kinetic energy Review the Nature of Energy Energy = the capacity to do work Energy is conserved in most processes (when not?) Kinetic energy = the energy of a body in motion K.E. = ½ mv 2 Potential energy = stored energy Including the energy of chemical bonds Potential and Kinetic Energy are interchangeable Classic example = swinging pendulum Potential Energy and Covalent Bonds Potential energy in molecules is stored in the form of chemical bond energy Maximum velocity = max Kinetic energy at position Enthalpy D o = the change in bond energies in a reaction Maximum Potential energy?? Exothermic reactions D o is negative and heat is evolved Potential energy in the bonds of reactants is more than that of products Endothermic reactions D o is positive and heat is absorbed Potential energy in the bonds of reactants is less than that of products Example : Formation of 2 from atoms The hydrogen molecule is more stable than hydrogen atoms Free Energy In , J. Willard Gibbs proposed the concept of free energy to explain why some reactions proceed The change in free en ergy, DG, for a chemical reaction indicates if it is spontaneous, if it proceeds without the input of work from the surrou ndings.? If DG is negative, the reaction in the forward direction proceeds spontaneously.? If DG is zero, the reaction is at equilibrium, there is no net driving force in the forward or reverse direction. Formation of bonds from atoms is always exothermic!? If DG is positive, the forward reaction will not proceed without input of work from the surroundings. 7

8 The Gibbs State Function Gibbs defined his new state function in terms of two existing thermodynamic state functions: enthalpy,, and entropy, S. G = - TS The Relationship Between K eq and?g o DG o is the standard free energy change of a reaction It is directly related to the equilibrium constant of a reaction and the change in free energy for a reaction is DG = D - TDS R is the gas constant (8.314 J K -1 mol -1 ) T is measured in kelvin (K) DG o negative = products are favored (K eq >1) DG o positive = reactants are favored (K eq <1) DG o = zero = products and reactants are equally favored (K eq = 0) K eq = e - DG o /RT The values of K eq in the table show that over a range of only 6 kcal/mol in free energy, reactions switch from highly favorable to highly unfavorable at ordinary temperatures. at 298 K DG o K eq cal cal/mole = 1 Kcal/mole = 4.2 kj/mole Two terms involved in Free Energy Changes DG o includes both enthalpy changes (D o ) and entropychanges (DS o ) D o is associated with changes in bonding energy If D o is negative (exothermic) this makes a negative contribution to DG o (products favored) DS o is associated with the relative order of a system More disorder means greater entropy A positive DS o means a system which is going from more ordered to less ordered A positive DS o makes a negative contribution to DG o (products favored) In many cases DS o is small and DG o is approximately equal to D o The Acidity of Carboxylic Acids Carboxylic acids are much more acidic than alcohols Deprotonation is unfavorable in both cases but much less favorable for ethanol Explanation based on resonance effects Both acetic acid and acetate are stabilized by resonance But much more important in the acetate anion This decreases D G o for the deprotonation No such effect in ethanol or its anion 8

9 Explanation based on inductive effect In acetic acid the highly polarized carbonyl group draws electron density away from the acidic hydrogen Inductive Effects of Other Groups The electron withdrawing chloro group makes chloroacetic acid more acidic than acetic acid The hydroxyl proton is more polarized and more acidic The conjugate base is more stabilized Also the conjugate base of acetic acid is more stabilized by the carbonyl group Which would be more acidic? Inductive effects in carboxylic acids Cl C 3 CCOO ClC 2 C 2 COO The Effect of Solvent on Acidity Acidity values in gas phase are very low It is difficult to separate the product ions without solvent molecules to stabilize them Acetic acid has pk a of 130 in the gas phase Importance of Solvation Stabilization of the ions through ion-dipole interactions (a charge dispersal mechanism) is worth hundreds of kilocalories per mole. When acetic acid ionizes in water, both the carboxylate anion and the hydronium ion are stabilized by solvation: Solvation is much more important for the acetate ion than for acetic acid itself This solvation allows acetic acid to be much more acidic in water than in the gas phase O C 3 -C-O- = + 2 O O O O O C C3 O O O + O O O O O O Solvation is a combination of hydrogen bonding and charge-dipole interactions. O 9

10 Organic Compounds as Bases Many organic compounds contain an atom with a lone pair (O,N) that can act as a base Alkenes, alkynes and aromatic compounds can act as bases Strong acids react with alkenes in an equilibrium : + C C + -A C-C + :A - In this reaction, the polarizable p-electrons are used to form the C- sigma bond: + d+ d- C C + -A C C + :A - a carbocation Many reaction s of alkenes are initiated by the p- electrons acting as a base. Our First Mechanism for an Organic Reaction Step 1: a Bronsted-Lowry proton transfer The Substitution Reaction of tert-butyl Alcohol Step 2: Lewis acid-base reaction in reverse Three steps : all are acid-base reactions Step 3: Lewis acid-base reaction Acids and Bases in Nonaqueous Solutions Water has a leveling effect on strong acids and bases Any base stronger than hydroxide will be converted to hydroxide in water We need to use non-protic solvents to have strong bases Alkyl lithium reagents in hexane are very strong bases The alkyl lithium is made from the alkyl bromide and lithium metal Sodium amide can be used as a strong base in solvents such as liquid N 3 10

11 Synthesis of Deuterium- and Tritium-Labeled Compounds Deuterium ( 2 ) and tritium ( 3 ) are isotopes of hydrogen Used to labeling organic compounds to be able to track where these compounds go (e.g. in biological systems) An alkyne can be labeled by deprotonating with a suitable base and then titrating with T 2 O 11