Chapter 3 Acids and Bases. The Curved-Arrow Notation

Chapter 3Acids and Bases. The Curved-Arrow Notation

Arrhenius Acids and Bases

• Acid: a substance that, when dissolved in water, increases the concentration of H+ (protons)• HCl H+ + Cl-

• HCl + H2O H3O+ + Cl-

H2O

• Base: a substance that, when put in water, increases the concentration of OH- ions or a substance that accepts H+ ions• NaOH(aq) Na+(aq) + OH-(aq)

Bronsted-Lowry Acids and Bases

• Acid: proton (H+) donor• Base: proton (H+) acceptor

Lewis Acids and Bases• Lewis Acid

– Electron deficient/poor– Electron acceptor – Electrophile– Tend to have less than an

octet• Lewis Base

– Electron rich– Electron donor– Nucleophile– Must have a lone pair of

electrons• product called an adduct

Fluorine is electron rich

• Lewis acids tend to react so as to fulfill their valence-shell octet

• Note the conservation of charge• Recall: FC = # valence e-’s – ( # LP e-’s + ½ # of bonding e-’s)

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ProblemsComplete the following Lewis acid-base reactions. Draw in any missing electrons, label the nucleophiles and electrophiles, identify the adduct, and calculate any formal charges needed for the adduct

OH

H C H

+ OH-1

Curved-Arrow Notation

• A tool for tracking electrons in a chemical reaction

• Electrons flow from the electron donor (Lewis base) to the electron acceptor (Lewis acid)

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ProblemsUse the curved arrow notation to derive a structure for your product in each of the following reactions

CH3NH2 + H+

Electron Pair Displacement Reactions

• Not all acceptors are electron-deficient• When an atom is NOT electron deficient, an

electron pair must depart from the atom before it receives another electron pair

• This preserves the octet rule

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Curved-Arrow Notation for Displacement• Displacement reactions require two arrows• Watch for conservation of total charge!

• Donated electron pairs can also originate from a lone pair or a bond

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The Wrong Way

• Curved-arrows show the movement of electron pairs not nuclei

• Electrons are responsible for chemistry!

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ProblemsProvide a curved arrow notation for each of the following reactions

H2O + HCl H3O+ + Cl-

+CH2CH2 + Br2

Problems

For each of the following reactions, give the product that results

Two Reactions Represented by Curved Arrows

• Most reactions in O-chem involve moving electrons

– Every reaction involving electron pairs fits into one of these two categories:

1) Lewis base + Lewis acid2) Electron-pair displacement reactions

• Reactions may be a combination of the two types above

153.3 Review of the Curved-Arrow Notation

Problems

• For the following reactions, indicate whether you have a Lewis acid-base reaction or an electron pair displacement reaction

Curved-Arrow Notation for Resonance

• Resonance structures differ only by movement of electrons (and usually electron pairs)

• Curved-arrow notation is ideal to help derive resonance contributors

• Note: the interconversion of resonance structure by movement of electron is NOT a reaction

173.3 Review of the Curved-Arrow Notation

Problems• Using the curved arrow

notation, derive resonance structures for the following compounds:

1) Benzene2) Aniline3) Diazomethane

BrØnsted Acid-Bases Reactions

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• A Bronsted acid-base reaction involves an electron-pair displacement on a proton

• Bronsted Acid: A species that donates a H+

– Keeps the electrons that were bonding to H• Bronsted Bases: A Lewis base that donates its electron pair to

a proton (in order to grab it)

Conjugate Acids and Bases

• When a BrØnsted acid loses a proton, its conjugate base is formed

• When a BrØnsted base gains a proton, its conjugate acid is formed

203.4 BrØnsted-Lowry Acids and Bases

Amphoteric Compounds

• Compounds that can act as either an acid or a base are called amphoteric

• Observe the behavior of a compound in a reaction to classify it as an acid or base

• Water is amphoteric

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Problems

• H2O

• F-• HCO3

-

• SO42-

• H2O

• HCO3-

• HPO42-

• H2S

Identify the conjugate acids for the compounds on the left and the conjugate bases for the compounds on the right. Also, identify all amphoteric compounds

Organic Reactions

• The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry

• For example:

• …looks similar to:


Nucleophiles and Electrophiles

• Nucleophile = Lewis base (“nucleus loving”)


Nucleophiles and Electrophiles

• Electrophile = Lewis acid (“electron loving”)• The atom that receives the electron pair


Leaving Groups

• The group or atom that receives electrons from the breaking bond is a leaving group


Leaving Groups

• Can also be applied to Lewis acid-base dissociation reactions


Problems• Classify each of the following reactions as a Bronsted acid-base reaction or

a Lewis acid-base association/dissociation. Identify each species in the following reactions as a Bronsted acid, Bronsted base, Lewis acid, Lewis base, nucleophile, electrophile, and/or leaving group. Draw in the appropriate curved arrow notation where appropriate.

H3O+

HCl

Strengths of BrØnsted Acids

• A measure of the extent of proton release to a BrØnsted base

• The standard base traditionally used is water

• The equilibrium constant is:


The Dissociation Constant

• As [H2O] effectively remains constant:

• Each acid has its own dissociation constant• A large Ka = many H+ transferred

– Strong acid– Weak conjugate base


The pKa Scale and pH

• pKa = -log Ka

• pKa values are more manageable than Ka values

• Stronger acids have smaller pKa values

• pH is a measure of [H+], a property of a solution (recall: pH = - log[H3O+])

• pKa is a measure of acid strength, a fixed property


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Strengths of BrØnsted Bases

• Directly related to Ka/pKa of the conjugate acid

• If a base is weak, its conjugate acid is strong• If a base is strong, its conjugate acid is weak


Problems

1) Write out the dissociation constant expression for formic acid, HCO2H, in water

2) Identify the conjugate acid-base pairs in the equation for problem #1

3) Using the Ka for formic acid, calculate the pKa

4) What is the Ka for acetic acid if its pKa = 4.74? Is acetic acid’s conjugate base weaker or stronger than the conjugate base of HF? HF’s Ka = 7.2 x 10-4

Relationship of Structure to Acidity• Which of the following molecules is the

weakest acid? Which is the strongest?• HF• HCl• HBr• HI

• What about these:

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The Element Effect

• Evaluate the atom attached to the proton• The acidities of Bronsted acids (H-A) increase

down a group• Acidities increase as the atomic # of A

increases• Due to decrease in bond strength

• The acidities of Bronsted acids (H-A) increase across a period from left to right• Due to increasing electronegativity of A

433.6 Relationship of Structure to Acidity

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The Charge Effect

• Who is more acidic, H2O or H3O+?

• Positively charged compounds attract electrons better than neutral ones

• pKa of H2O = 15.7

• pKa of H3O+ = -1.7

463.6 Relationship of Structure to Acidity

Problems

• Which of the following is the stronger acid?1) PH3 or SH2

2) H2O or SH2

3) NH3, NH4+, or NH2

-

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The Polar Effect

• Which of the following is more acidic?

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The Polar Effect

• The presence of electronegative substituents has an acid strengthening effect = polar effect or inductive effect– Such substituents are said to be electron

withdrawing• Acids with stable conjugate bases tend to be more

acidic– Resonance = stabilization

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• Consider the following series:

• Which molecule is the most acidic and why?

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Problems

• Rank the following molecules, in each series according to order of increasing acidity and explain your reasoning

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Homework Problems

• 3.1 – 3.13, 3.19 – 3.21, 3.24 – 3.37, 3.39, 3.44, 3.49, 3.50

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Chapter 3 Acids and Bases. The Curved-Arrow Notation