NEXT STEPS Organic Acids and Bases WHY STUDY

NEXT STEPS Organic Acids and Bases

WHY STUDY ACIDS AND BASES? Studying acids and bases allows us to predict reactivity. �Remember – all chemicals react to become more stable!! �“Strong” compounds react ‘quickly’ because they are unstable the way they are. The faster they can get rid of the thing making them unstable, the better! �“Weak” compounds react ‘slowly’ because they are already stable! They don’t want to change anything!

“STRONG” VS. “WEAK” – RELATIVE TERMS Strong acids are those that react (ionize) completely: Strong Acids include � HCl, HBr, HI, HNO 3, HCl. O 4, and H 2 SO 4 � Strong baes include Li. OH, Na. OH, KOH, Ca(OH)2, and Ba(OH)2 Most organic acids and bases are considered “weak” because they do not ionize completely.

STRONG ACIDS AND BASES Strong Acids and Bases react completely in one direction

WEAK ACIDS AND BASES ARE IN EQUILIBRIUM If Molecules Were People Our objective for this section is to determine whether a reaction in equilibrium will be favored to the right or the left direction. The next steps will show us the patterns we need to meet this objective!

WHAT DO WE ALREADY KNOW? Brønsted-Lowry Acids and Bases Acids: � Hydrogen Ion (H+) donor “Proton” is a synonym Bases: � Accept hydrogen ion/proton

CONJUGATE ACIDS AND BASES In all acid/base reactions, the product that results when the acid loses a proton is a conjugate base (Because it now has the ability to accept a proton) Likewise, the product that results when the base gains a proton is called the conjugate acid (Because it now has the ability to donate that proton) Relationship: � Strong acids always yield a weak CB; Weak acids yield strong CB � Strong bases always yield a weak CA; Weak bases yield strong CA

MEASURING STRENGTH OF ACIDS Different acids have different levels of reactivity that allows us to assess their strength. � This strength is expressed as an acidity constant, Ka For the reaction of any general acid, HA, with water, the acidity constant is: Remember [__] = molar concentration

WHAT’S IT MEAN? Think: If a lot of hydronium ion is produced, it’s because HA was a STRONG acid. So, a larger concentration of H 3 O+ will yield a larger Ka value. Relationship: Large Ka = Strong Acid Small Ka = Weak Acid

KA VS. PKA Because acid ionization constants for acids are numbers with negative exponents we often express them as p. Ka values instead. p. Ka = -log Ka For Example: If an acid has Ka = 1. 0 x 10 -4, it is much easier to say that the p. Ka = 4 For a stronger acid, the Ka value would be higher; perhaps Ka = 1. 0 x 10 -2, so p. Ka = 2

WHAT’S THE RELATIONSHIP The SMALLER the p. Ka, the STRONGER the acid! Do not forget this relationship!

TABLE 2. 2 PAGE 47 B(LACK BOOK)

PREDICTING THE REACTION DIRECTION Remember: Chemical reactions occur so that unstable chemicals (strong) can become more stable (weak) � Acid base reactions will always occur in the direction that produces the weaker acid

ORGANIC ACIDS Characterized by a positively polarized hydrogen atom

ORGANIC BASES Characterized by the presence of an atom with a lone pair of electrons that can bond to H+

YOU TRY IT: Acetone has a p. Ka = 19 while ammonia has a p. Ka = 36. Will this reaction happen in the forward direction?

ONE MORE… ? C 6 H 5 OH + HCO 3 ⇌ C 6 H 5 O- + H 2 CO 3 - Phenol p. Ka = 9. 95 Bicarbonate Ion Phenoxide Ion Carbonic Acid p. Ka = 6. 36

QUALITATIVE ASSESSMENT OF ACID STRENGTH Using p. Ka values is great, but we need to be able to assess the strength of organic acids and bases without always needing to refer to a p. Ka table. Major factors affecting acid strength � Atom: Electronegativity vs. Size � Resonance � Induction � Orbitals

THE METHOD TO THE MADNESS

ATOM: ELECTRONEGATIVITY Compare CH 4, NH 3, H 2 O, and HF: Which is the stronger acid? � Examine the conjugate bases: � The atom with the higher electronegativity is more stable when holding a negative charge. So… � HF > H 2 O > NH 3 > CH 4 Stronger Weaker Electronegativity may only be used to compare anions that are in the same period!

ATOM: SIZE For comparing anions that are in the same family, we evaluate the size of the anion. Generally, a negative charge is more stable when spread over a larger area. Compare HF, HCl, HBr, and HI � Conjugate Bases The Iodide ion is larger, therefore it is the most stable HI > HBr > HCl > HF

YOU TRY IT! Which Hydrogen is more acidic?

RESONANCE A negative charge is stabilized by resonance. Therefore, conjugate bases that can be stabilized by resonance come from stronger acids. Stronger Acid

YOU TRY IT! Which proton is more acidic?

INDUCTION Compare Acetic Acid to Trifluoroacetic Acid Negative charge is stabilized by the inductive effect of the electronegative fluorine atoms

NOTE: Compare Acetic Acid to Trimethylacetic Acid: Electronegative atoms will pull electron density away, but alkyl (carbon-containing) groups will donate electron density toward the negative charge. (The reasoning is due to hyperconjugation – if you care to know more, you can Google it!) So, which is more acidic?

YOU TRY IT! Which proton is more acidic?

ORBITALS Which type of orbital holds electrons closer to the nucleus? sp 3, sp 2, or sp Why? Do electrons closer to the nucleus have greater or less stability? Why?

ORDER OF IMPORTANCE: ARIO Always follow this order of importance: Atom, Resonance, Induction, Orbital Except when you are comparing a negative charge on an sp 3 hybrid Nitrogen to a negative charge on an sp hybrid carbon. See pages 66 -67 in Handout. Try problems 3. 19 – 3. 33 in the handout. Also, tell which factor allowed you to make your decision.

YOU TRY IT! Predict the position of equilibrium (left or right) for each of these reactions: