Organic Chemistry I Honors
Organic Chemistry I Honors CHM 123
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This 7 page Class Notes was uploaded by Jordy Rice on Wednesday October 28, 2015. The Class Notes belongs to CHM 123 at Wake Forest University taught by Staff in Fall. Since its upload, it has received 10 views. For similar materials see /class/230752/chm-123-wake-forest-university in Chemistry at Wake Forest University.
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Chm 123 AcidBase review General Base Acid Conjugate Acid Conjugate Base Bronsted BAH r B H A Lewis B A A BA Bronsted Example n a H I N O I O L H N k e 9 Lewis Example CI CI I CI N CK IC AI CI 9 General Notes a each covalent bond solid straight line joining two atoms represents two electrons equivalent to a lone pair tW dots together but shared between two adjacent atoms b acidbase reactions always involve an equilibrium though occasionally this equilibrium strongly favors one side of the equation c The base is always an electron donor if it is a Bronsted base it is also a proton acceptor d The acid is always an electron acceptor if it is a Bronsted acid it is also a proton donor e the quotcurved arrowquot represents the movement of two electrons during the course of a chemical reaction the arrow is always drawn from electrons toward an electron acceptor ie from the base to the acid any two atoms connected in this mechanistic representation will be joined by a covalent bond in the product if an arrow moves from a bond to an atom that atom will have a new lone pair of electrons in the product Evaluating the equilibrium When a chemical reaction is in equilibrium the rate of the forward reaction equals the rate of the reverse reaction A reaction may be said to be quotcompletequot when equilibrium is reached and the compounds that make up one side of the chemical equation are undetectable The quotsidequot of the equation that is favored is the side that has the lowest overall energy Often it is possible to simply compare the relative energies of one reagent on each side This is the case with acidbase reactions A quick note a molecule39s quotenergyquot refers to its Gibbs free energy G A detailed discussion of G is beyond the scope ofthis course and will be revisited in Chm 280 341 and 361 G is a combination of enthalpy H and entropy S Enthalpic concerns are things like how strong the bonds are how much strain the molecule possesses due to favorable or unfavorable alignments of atoms bonds and electrons Entropic concerns are a re ection of the amount of order in a system Nature would like to see disorder increase a rise in entropy while some reactions make a more ordered system In every reaction enthalpy and entropy will both undergo some change and this is re ected in the Gibbs Free Energy calculation by the famous equation AG AH iTAS A negative change in G implies a quotspontaneousquot reaction All of this is the realm of thermodynamics which is the study of systems at equilibrium It is possible for a set of reagents to be stable on the disfavored side of a reaction because the movement from that state to the more stable system is slow That is a kinetic argument remember that in chemistry kinetics is the study of how fast chemical change occurs thermodynamics is the study of which chemical changes are favored To judge the position of the equilibrium in an acidbase reaction one must know the relative strengths of the two acids involved This is typically done with pKa values Consider the equilibrium of a dilute acid in water where water serves as both the solvent and the base H30A39l HA H2O H3O A H2OHA The equilibrium constant K is shown to the right of the chemical equation In this case because the reaction is carried out in water the concentration of water is at least is very very very close to being constant Thus it can be assumed and a new equilibrium expression termed quotK3quot is born H30A39 Ka H39A pKa is flog Ka it is exactly analogous to pH One result of this math is that an acid with a pKa of say 42 will be exactly 50 dissociated in an aqueous solution at pH 42 Do the math pH logH3O Another is that one can judge acid strength by knowing the pKas of the various acids one wishes to compare The lower the pKa the stronger the acid A list of acids and approximate pKa values that will be in common use in Chm 123 can be found on the next page The acids are listed in decreasing order of strength Therefore note that any acid in the table will undergo spontaneous reaction with the conjugate base of any acid below it Also note that all of the acids shown are Bronsted acids name sulfuric acid hydrochloric acid methanesulfonic acid trifluoroacetic acid hydronium ion benzoic acid acetic acid ammonium ion phenol triethylammonium ion ethylacetoacetate water ethanol acetone ammonia benzene methane pKa 10 42 48 49 conjugate base 9 9 o s OH N MOB H 6OH O9 0 A9 NH29 9 CH3e pKa values less than zero or greater than 14 are difficult to measure accurately name hydrogen sulfate chloride methane sulfonate trifluoroacetate water benzoate acetate ammonia phenolate triethylamine hydroxide ethoxide acetone enolate amide Any compound with protons can theoretically serve as a Bronsted acid Clearly some are stronger acids than others Remember that the pKa values shown in the table on the preceeding page are in logarithmic units the de nition of quotpquot Hence sulfuric acid is approximately 60 orders of magnitude 1060 stronger a Bronsted acid than is methane That is simply put a big difference If you could get a mole of sulfuric acid together with a mole of CH3 methyl anion the reaction would spontaneously proceed until there was no more methyl anion 1023 molecules but favored 60 orders of magnitude 7 you39d need approximately all the atoms in the universe to be only sulfuric acid and methyl anion until you might have one methyl anion hanging about at equilibrium You may not have any methyl anion left but you would have a fire I digress Another thing to note is that seemingly small changes in molecular structure make fairly large changes in reactivity This is actually the central theme of organic chemistry structure dictates function All the properties of an organic molecule including reactivity whether acidbase or other can be predicted on the basis of structure One last note before we get to some details stability requires a comparison Sulfuric acid in a bottle is perfectly stable Only once you39ve exposed it to a base does a reaction ensue It sounds simple but predicting the outcome of an acidbase reaction requires both an acid and a base Protons are never spontaneously lost Thus when considering acid strength you have to consider the base it is reacting with At equilibrium the reaction will be made up mostly of the acidbase pair with the weaker acid The magnitude of this di quoterence will depend on the magnitude of the difference in pKa values of the two acids in the reaction This is best illustrated with an example Let39s look at the first Bronsted acidbase reaction on the first page I I O O H O I o o N H A e 9 k 05 What is the acid and base on the left side of the equation The answer is triethylamine is the base and acetic acid is the acid helpful tip when the name of the compound has the word quotacidquot in it that compound is usually going to be the acid What is the acid and base on the right side of the equation The answer is acetate is the base and triethylammonium ion is the acid Okay which of these two is the stronger acid Acetic acid has a pKa of 48 Triethylammonium ion has a pKa of 108 Acetic acid is therefore the stronger acid So at equilibrium which side ofthe equation isfavored That is at equilibrium will there be more acetic acid and therefore triethylamine or acetate and therefore triethylammonium ion The weaker acid is favored Thus the right side of this equation is favored and there will be more acetate by about six orders of magnitude than acetic acid in the mixture Note that it doesn39t matter in which direction the equation is written If acetate and triethylammonium had appeared on the left and acetic acid and triethylamine on the right then the left would have been favored What matters are the molecules You may ask yourself why the bases need not be considered in that analysis But the bases ARE considered What are the two acids you considered Acetic acid and triethylammonium Those are one each of a pair Acetic acidacetate is one pair and triethylammoniumtriethylamine is the other By considering the two acids one on each side of the equation you consider one from each acidconj base pair In such a pair the strength of the conjugate base is inversely related to the strength of the acid So if you have a really strong acid say sulfuric acid the conjugate base is going to be very weak Conversely if you have a really weak acid say methane the conjugate base is going to be very strong Thus in the table above while the acids are listed down the page in decreasing order of strength the conjugate bases are listed down the page in increasing order of strength Things to consider when predicting acid strength 1 What atom gains or loses charge Simply put the more electronegative the atom that gains electrons in the reaction the more favored the reaction will be Imagine a general base B reacts with three different acids CH4B BHCH3e NH3 B BH NHa e H20B BHOH The reactions become more favorable whatever B is as you go down this series Why In the top reaction a covalent bond between carbon and hydrogen is broken resulting in protonation of the base B The electrons that had been in the covalent bond that was broken end up on the carbon which is now part of a methyl anion In the middle reaction the same events occur to break a N H covalent bond and put a negative charge on nitrogen Finally the same events in the bottom reaction break an OH bond and put a negative charge on the oxygen now part of hydroxide The order of electronegativity goes 0 gt N gt C Thus the oxygen is more stable bearing a negative charge than nitrogen which is in turn more stable bearing negative charge than carbon This is a general trend throughout chemistry 2 What is the starting charge on the acidic atom when the acidic atom is the same element That sentence is a bit confusing but it means only this when comparing two Bronsted acids in which the atom bearing the acidic proton is of the same element consider what the charge is on that element Consider these reactions H3o B BH H20 H20 B BH 0H9 eOHB BH029 In the top reaction a base B is accepting a proton from hydronium ion In the middle reaction the acid is water and in the bottom it is hydroxide In all three acids the atom bearing the acidic proton is of the element oxygen The reactions become LESS favorable down the series In the top reaction the oxygen is positively charged in the middle the oxygen is neutral nally in the bottom reaction the oxygen is negative Note that in the bottom reaction hydroxide a strong base is actually acting as an acid This can happen but usually doesn t But it is a good reason not to simply memorize Understand WHY hydroxide is usually a base Why is the top reaction more favorable than the middle ie the top acid is stronger than the middle Remember that an acid is an electron acceptor What will better accept electrons a cation or a neutral molecule If all else is equal the answer is clearly the cation The same argument holds in comparing the middle to the bottom reaction What accepts electrons better a neutral molecule or an anion Again all else being equal the neutral molecule 3 How quotspread ou quot is the charge or lone pair that results from loss of a proton The words in quote refer to delocalization 7 how many atoms are involved in the molecular orbital MO that the electrons in the AH covalent bond move into when the proton is taken by the base This is a more sophisticated argument than the previous two but as powerful Anytime an acid loses a proton the rest of that molecule will gain a lone pair of electrons and very often a negative charge Though the reasons behind it are beyond the scope of this class the more atoms that lone pair of electrons can be distributed over the more stable it will be This is the foundation of the argument you will often see in organic chemistry about counting resonance structures Resonance structures are a simple way to approximate the behavior of delocalized molecular orbitals we will talk more about this in class For now an example Consider these reactions 9 e I HO OH B O OH BH 0 o 0 son BH H B e Aquot 0 OH B 09 BH These reactions are arranged such that the right side of the equation becomes less favored as you move down the series Why In each case the acidic proton is on an atom of oxygen Thus negative charge accrues to the same element in each case The charge on the oxygen bearing the acidic proton is neutral in each acid Thus arguments l and 2 do not explain the difference in acidity of these three acids and yet that difference spans 26 pKa units As you will expect from reading above the charge is more delocalized in hydrogen sulfate than in acetate and more so in acetate than in ethoxide Consider the resonance structures of hydrogen sulfate 3 e l means Ozs OH i o e 9 os OH 393 O OH lt gt I lt gt oSOH oe bl resonance arrows are NOT equilibrium arrows Resonance arrows imply that the two structures are related by being resonance structures Resonance structures are generated by moving electrons about the atomic framework of a molecule in such a way that atomic connectivity is not changed nor are overall charge or valence rules changed See general review for more details This is a very important topicto understand The charge that appears to be on one oxygen atom is actually shared over four atoms three oxygen and on sulfur That is the lone pair gained by loss of a proton is in a MO spread out over four atoms Thus three resonance structures can be drawn for hydrogen sulfate Hydrogen sulfate is in fact best represented as a weighted sum of all the resonance structures pictured far right A similar operation can be done for acetate jig ze H 6 means In the case of acetate the lone pair gained from losing a proton from acetic acid is delocalized over three atoms two oxygen one carbon Thus the lone pair is more delocalized in hydrogen sulfate implying that sulfuric acid should be more stable than acetic acid and indeed it is by almost 15 orders of magnitude In the case of ethoxide the lone pair gained is not delocalized It is fully localized on a single oxygen atom Remember to delocalize electrons they must have access to a 11 bond network There is none in ethanol To put it another way to draw a resonance structure one must move electrons but not atoms and not violate any atoms valence rules 7 this can t be done in ethanol Thus we would predict that ethanol would be less acidic than acetic acid and it is by about 11 orders of magnitude Exercise why is phenol more acidic than ethanol but less acidic than acetic acid Why is H Cl more acidic than water