mol_str - Acids and Bases Molecular Structure and Acidity...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Organic Chemistry Tutorials: Acids and Bases - Molecular Structure and Acidity 1 Acids and Bases: Molecular Structure and Acidity Review the Acids and Bases Vocabulary List as needed. Tutorial Contents A. Introduction B. Resonance C. Atomic Radius D. Electronegativity E. Inductive Effect F. Exercises G. Exercise Solutions A. Introduction There are approximately twenty million known organic compounds, but only a few thousand are listed in even the most extensive tables of p K a values. How can we know the p K a of a compound that is not on such a table? There are certainly many situations in which it would be impractical to synthesize the compound just to measure its p K a (such as during an exam!). We have seen before that because similar functional groups react in similar ways that compounds with similar functional groups will have similar p K a values. We can use this fact as a basis to predict trends in acidity or basicity. Recall that the role of a base is to share an electron pair with a proton, resulting in a new bond to that proton. A molecule that can donate electrons more readily will therefore be a stronger base. (There are some additional factors that we will not consider at this level.) Because we know that stronger bases have weaker conjugate acids, we can apply this same analysis to determine acidity as well. Many factors can influence a molecule's ability to donate electrons. The most common factors are resonance, atomic radius, electronegativity and the inductive effect. We consider each of these below. (These same factors also influence nucleophilicity.) B. Resonance A molecule is said to have resonance when its structure cannot be adequately described by a single Lewis structure. How does resonance influence the ability of a base to share electrons with a proton? Resonance may delocalize this the electron pair that the base might use to form the new bond with the proton. This delocalization increases the stability of the base. Greater stability results in lower reactivity. A base that has resonance delocalization of the electron pair that is shared with the proton will therefore be less basic than a base without this feature. Since a weaker base has a stronger conjugate acid, a compound whose conjugate base enjoys resonance stabilization will be more acidic. Example 1 : Which O-H proton is more acidic, ethanol (CH 3 CH 2 OH) or acetic acid (CH 3 CO 2 H)?
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Organic Chemistry Tutorials: Acids and Bases - Molecular Structure and Acidity 2 Solution : Acidity can readily be analyzed by examining the corresponding conjugate bases: CH 3 O O H CH 3 O O CH 3 O O CH 3 CH 2 OH CH 3 CH 2 O Acetate ion: two contributing structures Ethoxide ion: no resonance Ethanol Acetic acid remove proton remove proton Deprotonation of ethanol affords ethoxide ion, which has no resonance (only one Lewis structure can be drawn). Deprotonation of acetic acid affords acetate ion that has resonance (two contributing Lewis structures can be drawn). Because acetate ion has resonance that delocalizes
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 10/15/2010 for the course CHEM 213 taught by Professor Johnson during the Spring '10 term at Lyndon State.

Page1 / 8

mol_str - Acids and Bases Molecular Structure and Acidity...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online