1、Chapter 2 Polar covalent bonding , Acids and Bases,Chapter 3,2,Reactions and Their Mechanisms机理 There are four general types of organic reactions Substitutions取代Additions加成Eliminations消除,Chapter 3,3,Rearrangements重排Cleavage of Covalent Bonds Homolysis均裂Heterolysis异裂,Chapter 3,4,Heterolytic reactions
2、 almost always occur at polar bonds The reaction is often assisted by formation of a new bond to another molecule,Chapter 3,5,Introduction to Acid-Base Chemistry Brnsted-Lowry Definition of Acids and Bases Acid: a substance that can donate a proton质子 Base: a substance that can accept a proton Exampl
3、e Hydrogen chloride is a very strong acid and essentially all hydrogen chloride molecules transfer their proton to water,Chapter 3,6,Example Aqueous hydrogen chloride and aqueous sodium hydroxide are mixed The actual reaction is between hydronium and hydroxide ions,Chapter 3,7,Lewis Definition of Ac
4、ids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor Curved arrows show movement of electrons to form and break bonds,Chapter 3,8,Opposite Charges Attract and React BF3 and NH3 react based on their relative electron densities BF3 has substantial positive charge on the bor
5、on NH3 has substantial negative charge localized at the lone pair,Chapter 3,9,Heterolysis of Bonds to Carbons: Carbanions and Carbocations 负碳离子 与 正碳离子 Reaction can occur to give a carbocation or carbanion depending on the nature of ZCarbocations have only 6 valence electrons and a positive charge,Ch
6、apter 3,10,Carbanions have 8 valence electrons and a negative chargeOrganic chemistry terms for Lewis acids and bases Electrophiles亲电体 (“electron-loving” reagents ): seek electrons to obtain a stable valence shell of electrons Are electron-deficient themselves e.g. carbocations Nucleophiles亲核体 (“nuc
7、leus-loving” reagents): seek a proton or some other positively charged center Are electron-rich themselves e.g. carbanions,Chapter 3,11,The Use of Curved Arrows in Illustrating Reactions Curved arrows show the flow of electrons in a reaction An arrow starts at a site of higher electron density (a co
8、valent bond or unshared electron pair) and points to a site of electron deficiency Example: Mechanism of reaction of HCl and water,Chapter 3,12,Strengths of Acids and Bases Ka and pKa Acetic acid is a relatively weak acid and a 0.1M solution is only able to protonate water to the extent of about 1%T
9、he equilibrium equation for this reaction is:,Chapter 3,13,Dilute acids have a constant concentration of water (about 55.5 M) and so the concentration of water can be factored out to obtain the acidity constant (Ka) Ka for acetic acid is 1.76 X 10-5Any weak acid (HA) dissolved in water fits the gene
10、ral Ka expression The stronger the acid, the larger the Ka,Chapter 3,14,Acidity is usually expressed in terms of pKa pKa is the negative log of Ka The pKa for acetic acid is 4.75The larger the pKa, the weaker the acid,Chapter 3,15,Chapter 3,16,Predicting the Strengths of Bases The stronger the acid,
11、 the weaker its conjugate base will beAn acid with a low pKa will have a weak conjugate base Chloride is a very weak base because its conjugate acid HCl is a very strong acid,Chapter 3,17,Methylamine is a stronger base than ammonia The conjugate acid of methylamine is weaker than the conjugate acid
12、of ammonia,Chapter 3,18,Predicting the Outcome of Acid-Base Reactions Acid-base reaction always favor the formation of the weaker acid/weaker base pair The weaker acid/weaker base are always on the same side of the equation Example Acetic acid reacts with sodium hydroxide to greatly favor products,C
13、hapter 3,19,Water Solubility as a Result of Salt Formation 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 hydroxideWater insoluble amines can become soluble in aqueous hydrogen
14、 chloride,Chapter 3,20,The Relationship Between Structure and Acidity Acidity increases going down a row of the periodic table Bond strength to hydrogen decreases going down the row and therefore acidity increases,Chapter 3,21,Acidity increases from left to right in a row of the periodic table Incre
15、asingly electronegative atoms polarize the bond to hydrogen and also stabilize the conjugate base better,Chapter 3,22,Overview of Acidity Trends,Chapter 3,23,The Effect of Hybridization杂化 on Acidity Hydrogens connected to orbitals with more s character will be more acidic s orbitals are smaller and
16、closer to the nucleus than p orbitals Anions in hybrid orbitals with more s character will be held more closely to the nucleus and be more stabilized,Chapter 3,24,Inductive Effects Electronic effects that are transmitted through space and through the bonds of a molecule In ethyl fluoride the electro
17、negative fluorine is drawing electron density away from the carbons Fluorine is an electron withdrawing group (EWG) The effect gets weaker with increasing distance,Chapter 3,25,Energy Changes in Reactions Kinetic energy is the energy an object has because of its motion Potential energy is stored ene
18、rgy The higher the potential energy of an object the less stable it is Potential energy can be converted to kinetic energy (e.g. energy of motion),Chapter 3,26,Potential Energy and Covalent Bonds Potential energy in molecules is stored in the form of chemical bond energy Enthalpy DHo is a measure of
19、 the change in bond energies in a reaction Exothermic reactions DHo is negative and heat is evolved Potential energy in the bonds of reactants is more than that of products Endothermic reactions DHo is positive and heat is absorbed Potential energy in the bonds of reactants is less than that of prod
20、ucts,Chapter 3,27,Example : Formation of H2 from H atoms Formation of bonds from atoms is always exothermic The hydrogen molecule is more stable than hydrogen atoms,Chapter 3,28,The Relationship Between the Equilibrium Constant and DGo DGo is the standard free energy change in a reaction This is the
21、 overall energy change of a reaction It is directly related to the equilibrium constant of a reaction R is the gas constant (8.314 J K-1 mol-1) and T is measured in kelvin (K)If DGo is negative, products are favored at equilibrium (Keq 1) If DGo is positive, reactants are favored at equilibrium (Keq
22、1) If DGo is zero, products and reactants are equally favored (Keq = 0),Chapter 3,29,DGo encompasses both enthalpy changes (DHo) and entropy changes (DSo )DHo is associated with changes in bonding energy If DHo is negative (exothermic) this makes a negative contribution to DGo (products favored)DSo
23、is associated with the relative order of a system More disorder means greater entropy A positive DSo means a system which is going from more ordered to less ordered A positive DSo makes a negative contribution to DGo (products favored)In many cases DSo is small and DGo is approximately equal to DHo,
24、Chapter 3,30,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,Chapter 3,31,Explanation based on resonance effects Both acetic acid and acetate are stabilized by resonance Acetate is more
25、 stabilized by resonance than acetic acid This decreases DGo for the deprotonationNeither ethanol nor its anion is stabilized by resonance There is no decrease in DGo for the deprotonation,Chapter 3,32,Explanation based on inductive effect In acetic acid the highly polarized carbonyl group draws ele
26、ctron density away from the acidic hydrogenAlso the conjugate base of acetic acid is more stabilized by the carbonyl group,Chapter 3,33,Inductive Effects of Other Groups The electron withdrawing chloro group makes chloroacetic acid more acidic than acetic acid The hydroxyl proton is more polarized a
27、nd more acidic The conjugate base is more stabilized,Chapter 3,34,The Effect of Solvent on Acidity Acidity values in gas phase are generally very low It is difficult to separate the product ions without solvent molecules to stabilize them Acetic acid has pKa of 130 in the gas phaseA protic solvent i
28、s one in which hydrogen is attached to a highly electronegative atom such as oxygen or nitrogen e.g. water Solvation of both acetic acid and acetate ion occurs in water although the acetate is more stabilized by this solvation This solvation allows acetic acid to be much more acidic in water than in
29、 the gas phase,Chapter 3,35,Organic Compounds as Bases Any organic compound containing an atom with a lone pair (O,N) can act as a base,Chapter 3,36,p Electrons can also act as bases p Electrons are loosely held and available for reaction with strong acids,Chapter 3,37,A Mechanism for an Organic Rea
30、ction The Substitution Reaction of tert-Butyl AlcoholAll steps are acid-base reactions Step 1 is a Brnsted acid-base reaction Step 2 is a Lewis acid-base reaction in reverse with heterolytic cleavage of a bond Step 3 is a Lewis acid-base reaction with chloride acting as a Lewis base and the carbocat
31、ion acting as Lewis acid,Chapter 3,38,Chapter 3,39,Acids and Bases in Nonaqueous Solutions Water has a leveling拉平 effect on strong acids and bases Any base stronger than hydroxide will be converted to hydroxide in water Sodium amide can be used as a strong base in solvents such as liquid NH3,Chapter
32、 3,40,Alkyl lithium reagents in hexane are very strong bases The alkyl lithium is made from the alkyl bromide and lithium metal,Chapter 3,41,Synthesis of Deuterium- and Tritium-Labeled Compounds Deuterium (2H) and tritium (3H) are isotopes of hydrogen They are used for 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 T2O,
