Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328	Brief Organic Chemistry	Summer 2004
Lecture Notes: 26 January	© R. Paselk 2004
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Functional Groups

I want to very briefly look at a few important functional groups covered in your text. In each case these are groups of atoms that are commonly found in organic molecules and which have charectereistic properties. We will look at each of them later since each is given an entire chapter of discussion in the course.

• Alcohol: -OH
• Carbonyl: C=O
  • Aldehydes: -C(O)H
  • Ketones: -C(O)-
• Carboxylic acid: COOH
• Amine: three different forms:
  • Primary (1°) -NH₂
  • Secondary (2°) -NHR
  • Tertiary (3°) -NRR'

Acids and Bases

What are acids and bases? There are three major definitions. We will look at two (the third, the Arrhenius definition, is not needed for our study).

• Brønsted Definition:
  • Acids are proton donors.
  • Bases are proton acceptors.
  • Note that there is no restriction as to solvent, and many substances besides hydroxide ion can contribute basicity.

Although I will signify protons in water as H⁺, you should realize that naked protons do not exist in water - they are always hydrated. At a minimum we see the hydronium ion, H₃O⁺. But hydronium ion is in fact also generally though to t be hydrated, so you will sometimes see hydrogen ion represented as H₅O₂⁺, H₇O₃⁺, etc.

• Note that there is no restriction as to solvent, and many substances besides hydroxide ion can contribute basicity.
• A consequence of the Brønsted definition all acids and bases are related to one or more conjugate bases or acids. That is, when an acid dissociates to give a proton, it also generates a conjugate base which can react with (accept) a proton in the reverse reaction. For example, in the case of water:

H2O Æ  H+  + OH-           acid       conj. base                        H+ +  OH-  Æ  H2O           base    conj. acid                        H3O+   ¨  H+  +  H2O  Æ  OH- + H+    conj. acid         acid base   conj. base

Strong vs. Weak Acids & Bases: These terms have nothing to do with concentration, rather they refer to the degree of dissociation of an acid or base:

• A Strong acid is 100% dissociated at all concentrations up to 1M. Common strong acids include:
  • Nitric acid (HNO₃)
  • Hydrochloric acid (HCl)
  • Sulfuric acid (H₂SO₄) for the first dissociation only: H₂SO₄ ´ HSO₄^- + H⁺. The second dissociation is weak, that is it hardly dissociates at 1M.
• A Weak acid is only partly dissociated at 1M. The degree of dissociation varies widely, from a few percent to an infinitesimal degree. Common weak acids include:
  • Acetic acid (HC₂H₃O₂ or CH₃CO₂H, etc.)
  • Formic acid (HCO₂H)
  • Hydrofluoric acid (HF)
  • Most acids of biological origin such as amino acids, fatty acids, metabolites, nucleic acids etc.
• A Strong base is 100% dissociated at all concentrations up to 1M. Common strong bases include:
  • Sodium hydroxide (NaOH)
  • Potassium hydroxide (KOH)
• A Weak base is only partly dissociated at 1M. The degree of dissociation varies widely, from a few percent to an infinitesimal degree. Common weak acids include:
  • Ammonia (NH₃)
  • Aluminum hydroxide (Al(OH)₃)
  • Magnesium hydroxide (Mg(OH)₂)

Let's look at the equation for the dissociation of a weak organic acid, CH₃CO₂H (acetic acid, HOAc):

The values of K_a for most of the acids we will be interested are very small, and it is often convenient to express them in negative logrythmic form, analogous to pH, as pK_a values:

pK_a is a general measure of acidity, or the tendency of a substance to give up hydrogen ions, and has a tremendously broad range in this sense as seen in the table in your text and below:

Acidity vs. Molecular Structure

So why do some substances tend to dissociate readily, loosing a hydrogen ion, whereas others hold onto their hydrogens very tightly? A number of factors contribute, but we can look at it from the overall perspective of the stability of the resulting conjugate base. Factors then include:

• Electronegativity within a Period: Note the acidity of methane (pK_a = 51) vs. ammonia (pK_a = 38) vs. water (pK_a = 15.7) vs. hydrogen flouride (pK_a = 3.5). The more electronegative elements are better able to carry the excess negative charge in the anion form, so form more stable anions and thus stronger acids.
• Anion Size going Down a Row: HF (pK_a = 3.5) vs. HCl (pK_a = -7) vs. HBr (pK_a = -8) vs. HI (pK_a = -9). The larger elements are better able to carry the excess negative charge in the anion form because its spread over a greater volume. The proton counter ion also can't appraoch as closely to the positive nucleus, so is less strongly held. The larger elements thus form more stable anions and thus stronger acids.
• Electron Delocalization (Resonance Effect): CH₃OH (pK_a = 15.9) vs. CH₃CO₂H (pK_a = 4.76).

• Inductive Effect: CH₃CO₂H (pK_a = 4.76) vs. CF₃CO₂H (pK_a = 0.3). The fluorines, because of their high electronegativities pull electrons away from the carbon, again spreading the negative charge over a greater volume, stablizing the anion and thus making the acid stronger.

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Last modified 3 June 2004