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Chem 3A Final
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Topic F

Introduction to Acid-Base Reactions

the plot is : which bonds break and which bonds form
break an H-x bond
create a H-y bond
connect the two “leftovers” to form a salt from the oppositely charged ions
reactants: left side of reaction arrow
products: right side of reaction arrow
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lets “analyze” the last example
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4 roles in every A-B reaction:
acid: the reactant where the bond to H is breaking
base: the reactant where the bond to H is forming
conjugate base: the product formed when the bond to H is broken
conjugate acid: the product formed when the bond to H is formed
remember the plot:
acid→ conjugate base
base→ conjugate acid
NOTE: we could also reverse this reaction. This is how it would look.
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one way of combing both the forward and reverse reaction is by using the equilibrium arrow

Acid-Base Reactions: Introducing Ka and pKa

2.Examples Of Acid Dissociation Constants Ka For A Strong Acid (HI), A Weak Acid (CH3OH), And An Extremely Weak Acid (CH4)

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3.The Negative Logarithm Of Ka, “pKa” Is A Far More Convenient Measure Of Acidity Than Ka

pKa: the logarithm of the acitity constant Ka

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4.The pKa Scale Encompasses Over Sixty Orders Of Magnitude

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The Stronger The Acid, The Weaker The Conjugate Base

1.“Basicity” Is Just Another Word For “Stability Of A Lone Pair Of Electrons”

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Remember, the more the electronegative the atom, the better it will be able to handle or stabilize a negative charge, making it a better base
The best acids will have the best conjugate base because remember the plot( acid → conjugate base), so the reaction will favor what will be the most stable mechanism or result

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2. Which Of These Acid-Base Reactions Would Be The Most And Least Favorable?

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Since out of all the available bases, F is the most stable, it will be the weakest base, and since CH3 is the least stable, it will be the strongest base(most eager to grab a H to stabilize it)
the more stable a lone pair of electrons is, the less basic it will be
since a less basic acid translates to a unstable conjugate acid, the weakest bases will make the strongest acids after the reaction(if it occurs)
the less stable a lone pair of electrons is, the more basic it will be
since a more basic base translates to a more stable conjugate acid, the strongest bases will make the weakest acids after the reaction(when it occurs)

4. Beware This Misconception: “Weak Acids Are Strong Bases” <– Not True

“weak acids are strong bases”- NO!!
methane is a weak acid, but cannot act as a base because it does not have a lone pair to share a bond with hydrogen
“weak acids have strong conjugate bases”- YES!!

Five Key Factors That Influence Acidity

Factor #1 – Charge.

removal of a proton, H+, decreased the formal charge of a molecule by one unit. When a reactant or atom already bears this charge this becomes easiest and progressively becomes more challenging as the overall charge becomes more negative

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Factor #2 – The Role of the Atom

Acidity increases as we go across a row in the periodic table. Remember, that the more the electronegative an atom, the happier it will be carrying a negative charge, meaning that deprotanating it will be easier, making it a stronger acid.
NOTE: However the acidity trend on the periodic table does not correspond with the periodic electronegativity trend
Acidity increases with size of the “x” atom that the hydrogen is attached to. This is because of two reasons:
1) Bond length: The smaller the “x” atom H is attatched to, the shorter the bond of x-H will be, and the harder it will be to remove the H with a base
2)Stability of the conjugate base: the smaller the ionic radius of the “x” atom, the harder it is to distribute the negative charge of the conjugate base. The charge is spread over a smaller volume compared to larger atoms , which is energetically unfavorable

Factor #3 – Resonance

If a negative charge can be delocalized through resonance, the conjugate base will be more stable, increasing the strength of the acid.

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Factor #4 – Inductive effects

electronegative atoms can draw negative charge towards themselves which leads to the stabilization of the conjugate base, and make it easier for the H to deprotenate, making it the acid stronger

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this is related to two major factors:
1) the electronegativity of the atom
2)the distance between the electronegative element and the deprotenation site(acidic site/negative charge)

Factor #5 – Orbitals

Since the acidity relates to the stability of the conjugate base, and the stability of the conjugate base depends on how well it can hold negative charge, the more s character in an orbital, the closer the will be to the nucleus and the more stable with will be (=more favorable it will be)
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the first example will be the most acidic
the second example will be the most acidic
Helpful Tip: Don’t forget your CARDIO!!
C- charge
A- atom
R- resonance
D-dipole induction
O- orbitals

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