Acids and Bases: Functional Groups (Part 2) Organic Chemistry, 8th Edition L. G. Wade, Jr. Chapter 2 Lecture Acids and Bases: Functional Groups (Part 2)

Lecture 6: Acids & Bases (Part II): Lewis System, Hydrocarbons, Functional Groups Just in Time (A First Look) Quantifying Acidity without a Table The Steps to the Acronym “ARIO” Lewis Acids and Bases Nucleophiles and Electrophiles Curved Arrow Notation Hydrocarbons & Functional Groups

JIT Conjugate Base?

Quantifying Acidity without a Table The more effectively a conjugate base can stabilize its negative charge, the stronger the acid. What factors affect the stability of a negative formal charge? The type of Atom that carries the charge Resonance Induction The type of Orbital where charge resides These factors can be remembered by the acronym ARIO

Quantifying Acidity without a Table ARIO – The type of atom that carries the charge More electronegative atoms are better at stabilizing negative charge. Compare the acidity of the two compounds below.

Effect of Electronegativity on pKa As the bond to H becomes more polarized, H becomes more positive and the bond is easier to break. Across a row, look at the electronegativity

Effect of Size on pKa When looking at atoms in same column, compare the sizes! As size increases, the H is more loosely held and the bond is easier to break. A larger size also stabilizes the anion.

Quantifying Acidity without a Table ARIO – Resonance can greatly stabilize a formal negative charge by spreading it out into partial charges Compare the acidity of the two compounds below by comparing the stabilities of their conjugate bases. How does resonance play a role? Which acid is stronger? To answer question, draw the conjugate bases for each along with any resonance forms that may exist.

Resonance Delocalization of the negative charge on the conjugate base will stabilize the anion, so the substance is a stronger acid. More resonance structures usually mean greater stabilization. Let’s compare acetic acid with methanesulfonic acid (HOSO2CH3) in terms of acid strength. Which one is stronger? Explain.

Effect of Resonance on pKa If the negative charge on an atom can be delocalized over two or more atoms, the acidity of that compound will be greater than when the negative charge cannot be delocalized. The ethoxide anion is less acidic than the acetate ion simply because the acetate ion can delocalize the negative charge. Methanesulfonic acid can delocalize the charge in three different resonance forms, making it more acidic than the acetate ion. File Name: AAAKPBW0 Figure: 01-07-053un.jpg Chapter 1

Quantifying Acidity without a Table ARIO – Induction can also stabilize a formal negative charge by spreading it out. How is induction different from resonance? Compare the acidity of the two compounds below by comparing the stabilities of their conjugate bases. How does induction play a role? Acetic Acid Trichloroacetic acid

Quantifying Acidity without a Table Explain the pKa differences below

Quantifying Acidity without a Table ARIO – The type of Orbital also can affect the stability of a formal negative charge. A negative charge is more stable if it is held closely to an atom’s nucleus.

Quantifying Acidity without a Table Compare the acidity of the compounds below by comparing the stabilities of their conjugate bases. How does the type of orbital play a role? pKa = 50 44 25

Lewis Acids and Bases Lewis acid/base definition A Lewis acid accepts and shares a pair of electrons A Lewis base donates and shares a pair of electrons Acids and Bases in Bronsted-Lowry world are also the same in Lewis system.

Lewis Acids and Bases Lewis Acids are electron pair acceptors Lewis Bases are electron pair donors In Lewis acid-base theory, the attraction of oppositely charged species is fundamental to reactivity.

Nucleophiles and Electrophiles File Name: AAAKPBY0 Figure: 01-07-055un.jpg Chapter 1

Nucleophiles and Electrophiles Nucleophile: Donates electrons to a nucleus with an empty orbital. Electrophile: Accepts a pair of electrons.

Classes of Compounds Classifications are based on functional group. Part of the molecule where reactions usually occur Three broad classes: Hydrocarbons: Compounds composed of only carbon and hydrogen. Compounds containing Oxygen. Compounds containing Nitrogen. Chapter 2 19

Hydrocarbons Alkanes: single bonds, sp3 carbons. Cycloalkanes: sp3 carbons form a ring. Alkanes and cycloalkanes are not considered functional groups! Alkenes: double bond; sp2 carbons. Cycloalkenes: double bond in a ring. Alkynes: triple bond; sp carbons. Aromatic: (Arenes): contains a benzene ring. Chapter 2 20

Compounds Containing Oxygen Alcohols: R-OH Ethers: R-O-R´ Aldehydes: Ketones: Chapter 2 21

Compounds Containing Oxygen: Carboxylic Acids & Their Derivatives Acid Chloride: Ester:

Compounds Containing Nitrogen Amines: RNH2, RNHR´, or R3N Amides: RCONH2, RCONHR´, RCONR2 Nitriles: RCN Chapter 2 23

Skill Building: Practice Problems Problem 2-12 thru 2-27

Chapter Problems Problem Set 1: 21, 23, 25-27, 31, 33 (a,b)