CHEM 2211 1nd Edition Lecture 5 Outline of Last Lecture I Chapter two acids and bases intro Outline of Current Lecture I Organic acids and bases II Predicting acid base reactions III Factors affecting pKa of an acid Current Lecture I Organic Acids and bases Note that the high the pKa the less acidic a compound becomes O Carboxylic acids Acidic acid o pKa 4 76 C OH CH3 Formic acid o pKa 3 75 O C H OH The functional group of carboxylic acid is the O C OH The general pKa range for carboxylic acids is 3 5 Alcohols These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Methyl alcohol o pKa 15 5 o CH3OH Ethyl alcohol o pKa 15 9 o CH3CH2OH The functional group of alcohol is OH The general pKa range for alcohols is close to 16 Making them weaker acids than carboxylic acids Protonated alcohols Protonation o The adding of a proton to an atom molecule or ion Protonated methyl alcohol o CH3O H H o pKa 2 5 Protonated ethyl alcohol o CH2CH3O H H o pKa 2 4 Protonated alcohols are very strong acids with low pKas Amines II Are the result of replacing one or more of the hydrogens on NH3 ammonia with a carbon substitute Ex o Methyl amine CH3NH2 o pKa 40 amines pKa are so high that they rarely act as acids Predicting acid base reactions An acid base reaction can be predicted using the pKa of the reactants CH3 O H CH3C O OH pKa 15 5 pKa 4 76 The lower the pKa the stronger the acid so these pKas show that the second reactant acetic acid with a pKa of 4 76 is the acid in this reaction The high pKa 15 5 in this reaction is the base methyl alcohol now that the acid and base are identified the reaction can be carried out CH3 O H CH3C O OH CH3O H2 CH3C O O The full reaction shows the conjugate acid and conjugate base 1 The conjugate acid is on the product side o It is the base after it accepts the H from the acid o In this case CH3O H2 2 The conjugate base is on the product side o It is the acid after it has given up its H o In this case CH3C O OIt is sometimes easiest to identify the conjugate acid and bases by the charges on the product side for conjugate acid for conjugate base III 1 2 3 4 5 Factors affecting pKa Electronegativity Hybridization Size Inductive substitutes Resonance Electronegativity The greater the difference in two atoms electronegativity the more polar the bond is The more polar the bonds the more acidic the pKa Hybridization SP SP2 SP3 Ex o HC CH triple bond pKa 25 SP o H2C CH2 double bond pKa 44 SP2 o H3C CH3 single bond pKa 60 SP3 More bonds lower pKa Size As size goes up acidity goes up Atomic size On periodic table o Moving to the right size increasing o And going down size increases F Cl Br IHI strongest acid Inductive substituent Replacing a hydrogen with a higher electronegative atom causes electron withdrawal Pulling bonding electrons toward the electronegative atom stabilizes the compound o Increases acidity EX o CH3C OOH pKa 4 76 o BrCH2 OOH pKa 2 86 o ClCH2 OOH pKa 2 81 o FCH2 OOH pKa 2 66 The substituent with the highest electronegativity has the greatest affect on the acidity of the compound as above Resonance delocalized electrons Localized electrons Occur when a compound loses a proton an the negative charge resides on a single atom Delocalized electrons Occur when a compound loses a proton an the negative charge resides between two atoms shared Best example CH3 O O C C Carboxylic acid acetic acid o CH3C OOH o When acetic acid loses a proton its resonance structures can look like O CH3 O Or But neither of these are the true structure they are resonance contributors The charge is most stable if it delocalizes between the two oxygen atoms creating a resonance hybrid This causes the acidity of the compound too increase And it looks like this The lines show the charge is not just on O the O atom but between the two O atoms C CH3 O
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