Class Summary and Notes from 9/5/00 and 9/7/00 - CHEM3312
Acidity/Basicity
Related Reading:
17.4, 20.3, 20.4, 20.5
Practice Problems:
17.4. 17.5, 20.4, 20.6, 20.7, 20.8, 20.9, 20.16, 20.17, 20.18, 20.37, 20.39
Summary:
Acidity can be considered the ability of a compound to donate a proton (H+), basicity can therefore be considered the ability of a compound to accept a proton. The following is a generic representation of an acid-base reaction:
H-acid + base <=====> H-base+ + acid- (note equilibrium arrows should be used here)
General features to be expected in acids: polar groups, must have H
attached to a group that can keep the lone pair
General features to be expected in bases: polar groups, must have lone
pair to accept H+
pKa (-log Ka) is a measure of acid strength, stronger acids will have lower
pKa values (and higher Ka values).
Problems Discussed In Class
Groups in class discussed the relative acidity of eight pairs of acids - specifically discussing the structural reasons why some compounds make better acids than others.
- Group 1: Ethanol (pKa=16) vs. 2,2,2-trifluoro-1-ethanol (pKa=12.43): The fluoro groups are electron-withdrawing and inductively stabilize the negative charge that is produced when 2,2,2-trifluoro-1-ethanol gives up a proton. Ethanol has no such stabilization for the the product produced when it acts as an acid.
- Group 2: Acetic acid (pKa=4.75) vs. ethanol (pKa=16): The negative charge produced when acetic acid loses a proton is delocalized (resonance stabilized) into the neighboring carbonyl group. Ethanol has no such stabilization for the product produced when it acts as an acid.
- Group 3: Trifluoroacetic acid (pKa=0.59) vs. acetic acid (pKa=4.75): same reason for difference as seen in group 1
- Group 4: 2-chlorobutanoic acid (pKa=2.86) vs. 3-chlorobutanoic acid (pKa=4.05): The halide has more impact on the stability of the anion produced upon loss of a proton when it is closer to the site of the reaction.
- Group 5: p-hydroxybenzoic acid (pKa=4.48) vs. p-nitrobenzoic acid (pKa=3.41): nitro groups are strongly electron-withdrawing (due to their formal + charge on nitrogen) and will stablize the anion formed by loss of a proton both through delocalization (resonance) and induction. Hydroxyl group are electron-releasing and do not stabilize the anion product.
- Group 6: Propane (pKa=~50) vs. ethanol (pKa=16): Ethanol loses a hydrogen from a polarized bond when it acts as an acid. Propane has no polarized bonds. Thus the type of atom losing the proton has a significant impact on the acidity.
- Group 7: Trifluoroacetic acid (pKa=0.59) vs. fluoroacetic acid (pKa=2.59): multiple electron-withdrawing groups have a greater ability to spread out the negative charge that appears in the product.
- Group 8: 2,2-dimethyl-2-propanol (pKa=18) vs. ethanol (pKa=16): alkyl groups are electron-releasing, this is good for reactions that form carbocations, but bad for reaction that create anions (such as the donation of a proton).
Last modified 9/5/2000
Dr. Abby Parrill
Department of Chemistry
University of Memphis
These pages may be downloaded and linked from other pages freely for academic and educational purposes. Questions, problems, and errors should be sent to
aparrill@memphis.edu.