chapter 1- chm 261 organic chemistry

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Page 1: Chapter 1- CHM 261 organic chemistry

1-1

Organic Chemistry

Page 2: Chapter 1- CHM 261 organic chemistry

1-2

Review of Molecular Structure

Chapter 1

Page 3: Chapter 1- CHM 261 organic chemistry

1-3

Organic Chemistry

The study of the compounds of carbon Over 10 million compounds have been identified

• about 1000 new ones are identified each day! C is a small atom

• it forms single, double, and triple bonds• it is intermediate in electronegativity (2.5)• it forms strong bonds with C, H, O, N, and some metals

Page 4: Chapter 1- CHM 261 organic chemistry

1-4

Schematic View of an Atom

• a small dense nucleus, diameter 10-14 - 10-15 m, which contains positively charged protons and most of the mass of the atom

• an extranuclear space, diameter 10-10 m, which contains negatively charged electrons

Page 5: Chapter 1- CHM 261 organic chemistry

1-5

Electron Configuration of Atoms

Electrons are confined to regions of space called principle energy levels (shells)• each shell can hold 2n2 electrons (n = 1,2,3,4......)

Shell

Number of Electrons Shell

Can Hold

Relative Energiesof Electrons

in These Shells

3218 8 2

4321

higher

lower

Page 6: Chapter 1- CHM 261 organic chemistry

1-6

Electron Configuration of Atoms

Shells are divided into subshells called orbitals, which are designated by the letters s, p, d, f,........• s (one per shell) • p (set of three per shell 2 and higher) • d (set of five per shell 3 and higher) .....

Shell Orbitals Contained in That Shell

3

2

1 1s

2s, 2px, 2py, 2pz

3s, 3px, 3py, 3pz, plus five 3d orbitals

Page 7: Chapter 1- CHM 261 organic chemistry

1-7

Electron Configuration of Atoms

Aufbau Principle: • orbitals fill in order of increasing energy from lowest

energy to highest energy Pauli Exclusion Principle:

• only two electrons can occupy an orbital and their spins must be paired

Hund’s Rule: • when orbitals of equal energy are available but there

are not enough electrons to fill all of them, one electron is added to each orbital before a second electron is added to any one of them

Page 8: Chapter 1- CHM 261 organic chemistry

1-8

Electron Configuration of Atoms

The pairing of electron spins

Page 9: Chapter 1- CHM 261 organic chemistry

1-9

Electron Configuration of Atoms

Table 1.3 The Ground-State Electron Configuration of Elements 1-18

Page 10: Chapter 1- CHM 261 organic chemistry

1-10

Lewis Dot Structures

Gilbert N. Lewis Valence shell:

• the outermost occupied electron shell of an atom Valence electrons:

• electrons in the valence shell of an atom; these electrons are used to form chemical bonds and in chemical reactions

Lewis dot structure: • the symbol of an element represents the nucleus and

all inner shell electrons• dots represent valence electrons

Page 11: Chapter 1- CHM 261 organic chemistry

1-11

Lewis Dot Structures

Lewis Dot Structures for Elements 1-18

N OB

H

Li Be

Na

He

Cl

F

S

Ne

Ar

C

SiAl P

1A 2A 3A 4A 5A 6A 7A 8A

Mg ::

::

::

.

.

.

.

.

.

.

..

..

. .

.

.

.

:

:

:

::::::::

::::::.

:::

:

Page 12: Chapter 1- CHM 261 organic chemistry

1-12

Lewis Model of Bonding

Atoms bond together so that each atom acquires an electron configuration the same as that of the noble gas nearest it in atomic number• an atom that gains electrons becomes an anion• an atom that loses electrons becomes a cation• the attraction of anions and cations leads to the

formation of ionic solids• an atom may share electrons with one or more atoms

to complete its valence shell; a chemical bond formed by sharing electrons is called a covalent bond

• bonds may be partially ionic or partially covalent; these bonds are called polar covalent bonds

Page 13: Chapter 1- CHM 261 organic chemistry

1-13

Covalent Bonds

The simplest covalent bond is that in H2

• the single electrons from each atom combine to form an electron pair

• the shared pair functions in two ways simultaneously; it is shared by the two atoms and fills the valence shell of each atom

The number of shared pairs• one shared pair forms a single bond• two shared pairs form a double bond• three shared pairs form a triple bond

H H H-H+ • H0 = -435 kJ (-104 kcal)/mol•

Page 14: Chapter 1- CHM 261 organic chemistry

1-14

Lewis Structures

To write a Lewis structure• determine the number of valence electrons

• Atom by atom analysis• determine the arrangement of atoms

• Different arrangements are almost always possible - isomers• connect the atoms by single bonds• arrange the remaining electrons so that each atom has

a complete valence shell• Use common bonding patterns as your guide

• show a bonding pair of electrons as a single line• show a nonbonding pair of electrons as a pair of dots

Page 15: Chapter 1- CHM 261 organic chemistry

1-15

Stable Bonding in Neutral Molecules

In neutral molecules• hydrogen has one bond• carbon has 4 bonds and no lone pairs• nitrogen has 3 bonds and 1 lone pair• oxygen has 2 bonds and 2 lone pairs• halogens have 1 bond and 3 lone pairs• 3rd row and transition metal bonding patterns are more

complicated but we don’t encounter a lot of them here

Page 16: Chapter 1- CHM 261 organic chemistry

1-16

Multiple Bonding

Bonded atoms can share more than one pair of electrons to form multiple bonds

Molecular oxygen (O2) has a double bond between the oxygen atoms

+ givesO O O O

Page 17: Chapter 1- CHM 261 organic chemistry

1-17

Lewis Structures -

H2O (8) NH3 (8) CH4 (8) HCl (8)

C2H4 (12) C2H2 (10) CH2O (12) H2CO3 (24)

H-O-H H-N-HH

H-C-HH

HH-Cl

H-C C-HH

HC O

H

HC C

H

HO O

CH H

O

Ethylene

Hydrogen chlorideMethaneAmmoniaWater

Carbonic acidFormaldehydeAcetylene

Page 18: Chapter 1- CHM 261 organic chemistry

1-18

Electronegativity

Electronegativity: • a very crude measure of an atom’s attraction for the

electrons it shares with another atom in a chemical bond

Pauling scale• generally increases left to right in a row• generally increases bottom to top in a column

Page 19: Chapter 1- CHM 261 organic chemistry

1-19

Polar and Nonpolar Covalent Bonds

Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing

We divide covalent bonds into• nonpolar covalent bonds• polar covalent bonds

Difference in ElectronegativityBetween Bonded Atoms Type of Bond

Less than 0.5

0.5 to 1.9Greater than 1.9

Nonpolar covalentPolar covalent

Ions form

Page 20: Chapter 1- CHM 261 organic chemistry

1-20

Polar and Nonpolar Covalent Bonds

• an example of a polar covalent bond is that of H-Cl• the difference in electronegativity between Cl and H is

3.0 - 2.1 = 0.9• we show polarity by using the symbols d+ and d-, or

by using an arrow with the arrowhead pointing toward the negative end and a plus sign on the tail of the arrow at the positive end

H Cl+ -

H Cl

Page 21: Chapter 1- CHM 261 organic chemistry

1-21

Polar Covalent Bonds

Bond dipole moment (m): • a measure of the polarity of a covalent bond• the product of the charge on either atom of a polar

bond times the distance between the nuclei• average bond dipole moments of selected covalent

bonds

H-OH-NH-C

H-S C-I

C-FC-ClC-Br C-N

-

C-O

C=N

C=O

-

Bond Dipole (D) Bond

1.4

1.51.51.41.2

Bond

Bond Dipole (D)

1.30.3 0.7

0.20.7

2.3

3.5

Bond Dipole (D)Bond

Page 22: Chapter 1- CHM 261 organic chemistry

1-22

Polar and Nonpolar Molecules

To determine if a molecule is polar, we need to determine • if the molecule has polar bonds• the arrangement of these bonds in space

Molecular dipole moment (): the vector sum of the individual bond dipole moments in a molecule• reported in debyes (D)

Page 23: Chapter 1- CHM 261 organic chemistry

1-23

Polar and Nonpolar Molecules

these molecules have polar bonds, but each has a zero dipole moment

O C O

Carbon dioxide = 0 D

B

F

F

F

Boron trifluoride = 0 D

C

Cl

ClClCl

Carbon tetrachloride = 0 D

Page 24: Chapter 1- CHM 261 organic chemistry

1-24

Polar and Nonpolar Molecules

these molecules have polar bonds and are polar molecules

N

HH

H

O

H H

Water = 1.85D

Ammonia = 1.47D

directionof dipolemoment

directionof dipolemoment

Page 25: Chapter 1- CHM 261 organic chemistry

1-25

Polar and Nonpolar Molecules

• formaldehyde has polar bonds and is a polar molecule

Formaldehyde = 2.33 D

directionof dipolemoment H H

C

O

Page 26: Chapter 1- CHM 261 organic chemistry

1-26

Formation of Ions

A rough guideline: • ions will form if the difference in electronegativity

between interacting atoms is 1.9 or greater• example: sodium (EN 0.9) and fluorine (EN 4.0)• we use a single-headed (barbed) curved arrow to show

the transfer of one electron from Na to F

• in forming Na+F-, the single 3s electron from Na is transferred to the partially filled valence shell of F

Na + F Na+ F -

• •• •••

••

• •••

••

+ F-(1s22s22p6)Na+(1s22s22p6)F(1s22s22p5)+Na(1s22s22p63s1)

Page 27: Chapter 1- CHM 261 organic chemistry

1-27

Formal Charge

Formal charge: the charge on an atom in a molecule or a polyatomic ion

To derive formal charge1. write a correct Lewis structure for the molecule or ion

2. assign each atom all its unshared (nonbonding) electrons and one-half its shared (bonding) electrons

3. compare this number with the number of valence electrons in the neutral, unbonded atom

Number of valence electrons in the neutral, unbonded atom

All unsharedelectrons

One half of all shared electrons

+Formalcharge

=

Page 28: Chapter 1- CHM 261 organic chemistry

1-28

Formal Charge

Example: Draw Lewis structures, and show which atom in each bears the formal charge

NH2- HCO3

- CO32-

CH3NH3+ HCOO- CH3COO-

(b) (c)

(d) (e) (f)

(a)

C O

H

HC

H

H

H

C N

Page 29: Chapter 1- CHM 261 organic chemistry

1-29

Exceptions to the Octet Rule

Molecules containing atoms of Group 3A elements, particularly boron and aluminum

Aluminum chloride

:

:

:

F B

F

F

Cl Al

Cl

Cl

6 electrons in the valence shells of boron

and aluminum

Boron trifluoride

: :

: :

: :

::

::

:

::

::

Page 30: Chapter 1- CHM 261 organic chemistry

1-30

Exceptions to the Octet Rule

Atoms of third-period elements have 3d orbitals and may expand their valence shells to contain more than 8 electrons• Sulfur, phosphorus may have up to 10

Page 31: Chapter 1- CHM 261 organic chemistry

1-31

Resonance

For many molecules and ions, no single Lewis structure provides a truly accurate representation

Ethanoate ion(acetate ion)

C

O

O

H3CC

O

O

H3C

-

-

and

Page 32: Chapter 1- CHM 261 organic chemistry

1-32

Resonance

Linus Pauling - 1930s• many molecules and ions are best described by

writing two or more Lewis structures• individual Lewis structures are called contributing

structures• connect individual contributing structures by double-

headed (resonance) arrows• the molecule or ion is a hybrid of the various

contributing structures

Page 33: Chapter 1- CHM 261 organic chemistry

1-33

Resonance

Examples: equivalent contributing structures

Acetate ion(equivalent contributing

structures)

Nitrite ion(equivalent contributing

structures)

::

::

: : :

N

O -

O

: N

O

O -

:

: : :

:

:

: :

: :

C

O -

O

CH3 C

O

O -

CH3

: :

:

:

Page 34: Chapter 1- CHM 261 organic chemistry

1-34

Resonance

Curved arrow: a symbol used to show the redistribution of valence electrons

In using curved arrows, there are only two allowed types of electron redistribution:• from a bond to an adjacent atom• from an atom to an adjacent bond

Electron pushing is a survival skill in organic chemistry• learn it well!

Page 35: Chapter 1- CHM 261 organic chemistry

1-35

Resonance

All contributing structures must1. have the same number of valence electrons

2. obey the rules of covalent bonding• no more than 2 electrons in the valence shell of H • no more than 8 electrons in the valence shell of a 2nd

period element• 3rd period elements, such as P and S, may have up to

12 electrons in their valence shells

3. differ only in distribution of valence electrons; the position of all nuclei must be the same

4. have the same number of paired and unpaired electrons

Page 36: Chapter 1- CHM 261 organic chemistry

1-36

Resonance The carbonate ion, for example

• a hybrid of three equivalent contributing structures• the negative charge is distributed equally among the

three oxygens

Page 37: Chapter 1- CHM 261 organic chemistry

1-37

Resonance

Preference 1: filled valence shells• structures in which all atoms have filled valence shells

contribute more than those with one or more unfilled valence shells

••

••••

Greater contribution; both carbon and oxygen have complete valence shells

Lesser contribution;carbon has only 6 electrons in its valence shell

+ +CCH3 OCH3 O

H

HC

H

H

Page 38: Chapter 1- CHM 261 organic chemistry

1-38

Resonance

Preference 2: maximum number of covalent bonds• structures with a greater number of covalent bonds

contribute more than those with fewer covalent bonds

••

••••

Greater contribution(8 covalent bonds)

Lesser contribution(7 covalent bonds)

+ +CCH3 OCH3 O

H

HC

H

H

Page 39: Chapter 1- CHM 261 organic chemistry

1-39

Resonance

Preference 3: least separation of unlike charge• structures with separation of unlike charges contribute

less than those with no charge separation

Lesser contribution(separation of unlike

charges)

CH3-C-CH3 CH3-C-CH3

Greater contribution(no separation of unlike charges)

O -O

::

:::

Page 40: Chapter 1- CHM 261 organic chemistry

1-40

Resonance

Preference 4: negative charge on the more electronegative atom • structures that carry a negative charge on the more

electronegative atom contribute more than those with the negative charge on the less electronegative atom

CH3CH3H3CCH3H3CC

O

C

O

H3CC

O

(b)Greater

contribution

(c)Should notbe drawn

(a)Lesser

contribution

(1) (2)

Page 41: Chapter 1- CHM 261 organic chemistry

1-41

Structural Formulas

Lewis structures indicate atom connectivity• Do not show 3-D structure, but it is there

Drawing Lewis structures of large molecules can get cumbersome• Use shortcuts to simplify things• Rely on normal bonding properties of atoms

Types include• Condensed structural formulas• Line-angle formulas

All subsets of molecular formula

Page 42: Chapter 1- CHM 261 organic chemistry

1-42

Condensed Structural formulas

Based on typical bonding patterns• Carbon makes 4 bonds in stable molecules• Oxygen makes 2 bonds and has 2 lone pairs• Nitrogen makes 3 bonds and has 1 lone pair

Show molecule by “groups” centered on carbon

C C

H

H

H

H

H

H

C

H

HH

H

CH4

CH3CH3

C C

H

H

H

H

C

H

H

H

H

CH3CH2CH3

or

(CH3)2CH2

C

C

C C

H H

H

HH

H

H

H

HH

CH3CHCH3

CH3

CH3CH(CH3)CH3

(CH3)3CH

Page 43: Chapter 1- CHM 261 organic chemistry

1-43

Which is the correct Lewis structure?

(CH3)2CH(CH2)2CH(CH3)2

C

C CH

H

H

H H

C C

C

H

H

H

H

H

H

H H

H

CC

C

C

H

H

H

H

H

H

H

HH

C

C

H

H

C CH

H

H

HH

HH

CC C

C

C

H

H

H

H H

H

HH

H

H

C

C

H

H

H

HH

H

CC C

C

C

H

H

H

H H

H

HH

H

C

C

C

H

H

H

HH

H

HH

H

AB

CD

Page 44: Chapter 1- CHM 261 organic chemistry

1-44

Can add heteroatoms (N, O, halogens) and multiple bonds

CH3CH2OH

CH3CH2NHCH3

CH3CHClCH3

CH3CH=CHCH3

Page 45: Chapter 1- CHM 261 organic chemistry

1-45

Line-Angle structures

Lines to represent bonds • “skeletal structures”

Carbons are not drawn, but occur at• Intersections• Vertices (ends of lines)

Hydrogens are not usually shown• assumed to be as many as needed to fulfill carbon

valency

C

C CH

H

H

H H

C C

C

H

H

H

H

H

H

H H

H

CH3(CH2)4CH3

Page 46: Chapter 1- CHM 261 organic chemistry

1-46

A. 0 B. 1 C. 2 D. 3 E. 4

Heteroatoms are shown explicitly WITH Hydrogens

OH

O

NH2

How many hydrogens are present at marked position?

*

*

Page 47: Chapter 1- CHM 261 organic chemistry

1-47

Combined structures are allowed BUT…

…if you show C atoms, you MUST show the 4 bonds to it (including H)

H3C OH

H2CO

NH2

OK`

OH

O

NH2

O

CH(CH3)2

NOT OK

O

CCH3

CH3

C C C C

(C14H30)

Trivalent carbon

Page 48: Chapter 1- CHM 261 organic chemistry

1-48

Find the acceptable structure

C

Cl2C

NH2

Cl

C

ONH2

Cl

C

ONH2

Cl

C

ONH2

Cl

CH3

ONH2

Cl

CH3

A B

C D

E

Page 49: Chapter 1- CHM 261 organic chemistry

1-49

Trivalent carbon can be possible for ions and radicals

O

CCH3

CH3

O

CCH3

CH3

+

_

one missing electron around carbon(3 net electrons)

one extra electron around carbon(5 net electrons)

One for each bond (3) – no more (electron deficient)

3 from the bonds, must have a non-bonded lone pair (note that it has an octet even!)

Pentavalent carbon (5 bonds to C) is NEVER acceptable in this class

Page 50: Chapter 1- CHM 261 organic chemistry

1-50

Acids and Bases

Page 51: Chapter 1- CHM 261 organic chemistry

1-51

Arrhenius Acids and Bases

In 1884, Svante Arrhenius proposed these definitions • acid: a substance that produces H3O+ ions aqueous

solution• base: a substance that produces OH- ions in aqueous

solution• this definition of an acid is a slight modification of the

original Arrhenius definition, which was that an acid produces H+ in aqueous solution

• today we know that H+ reacts immediately with a water molecule to give a hydronium ion

H+(aq) + H2O(l) H3O+(aq)Hydronium ion

Page 52: Chapter 1- CHM 261 organic chemistry

1-52

Brønsted-Lowry Definitions

Acid: a proton donor Base: a proton acceptor

+

Proton donor

Protonacceptor

-O H

H

OH

H

O HH + +O H H

+

Protonacceptor

Protondonor

+O H

H

OH

H

N H

H

H

H

H

H + +N H H

: ::

: ::

::

: :

::

Page 53: Chapter 1- CHM 261 organic chemistry

1-53

Conjugate Acids & Bases

• conjugate base: the species formed from an acid when it donates a proton to a base

• conjugate acid: the species formed from a base when it accepts a proton from an acid

• acid-base reaction: a proton-transfer reaction• conjugate acid-base pair: any pair of molecules or ions

that can be interconverted by transfer of a proton

HCl(aq) H2O(l) Cl-(aq) H3O+(aq)+ +WaterHydrogen

chlorideHydronium

ionChloride

ion(base)(acid) (conjugate

acid of H2O)(conjugate

base of HCl)

conjugate acid-base pair

conjugate acid-base pair

Page 54: Chapter 1- CHM 261 organic chemistry

1-54

Conjugate Acids & Bases

• Brønsted-Lowry definitions do not require water as a reactant

• consider the following reaction between acetic acid and ammonia

NH4+CH3COOH CH3COO-

NH3+ +

Acetic acid Ammonia

(acid)

conjugate acid-base pair

Acetate ion

Ammoniumion

(base) (conjugate baseacetic acid)

(conjugate acidof ammonia)

conjugate acid-base pair

Page 55: Chapter 1- CHM 261 organic chemistry

1-55

Conjugate Acids & Bases

• we can use curved arrows to show the flow of electrons in an acid-base reaction

CH3-C-OO

H N HH

H CH3-C-O -

OH-N-H

H

H+ +

Acetic acid(proton donor)

Acetate ion

:

:: ::

:: :: :

Ammonia(proton acceptor)

Ammoniumion

Page 56: Chapter 1- CHM 261 organic chemistry

1-56

Conjugate Acids & Bases

Many organic molecules have two or more sites that can act as proton acceptors in these molecules, the favored site of protonation is

the one in which the charge is more delocalized question: which oxygen of a acetic acid is protonated?

CH3-C-O-H

O+ H2SO4 CH3-C-O-H

HO+

CH3-C-O-HH

O+

+ HSO4-or

A(protonation

on the carbonyl oxygen)

B(protonation

on thehydroxyl oxygen)

acetic acid

Page 57: Chapter 1- CHM 261 organic chemistry

1-57

Conjugate Acids & Bases

for protonation on the carbonyl oxygen, we can write three contributing structures

two place the positive charge on oxygen, one places it on carbon

A-1 and A-3 make the greater contribution because all atoms have complete octets

the positive charge is delocalized over three atoms with the greater share on the two equivalent oxygens

++

CH3-C-O-H

OH

CH3-C-O-H

OH

CH3-C

OH

++CH3-C-O-H

O

CH3-C=O-H

OH

CH3-C=O-H

OH+

A-1(C and O have

complete octets)

A-2(C has incomplete

octet)

A-3(C and O have

complete octets)

Page 58: Chapter 1- CHM 261 organic chemistry

1-58

Conjugate Acids & Bases

for protonation on the hydroxyl oxygen, we can write two contributing structures

B-2 makes only a minor contribution because of charge separation and adjacent positive charges

therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen

CH3-C-O-H

O

H

+

B-1

CH3-C-O-H

O

H+

+

B-2(charge separation and

adjacent positive charges)

Page 59: Chapter 1- CHM 261 organic chemistry

1-59

Conjugate Acids & Bases

Does proton transfer to acetamide occur preferentially on the oxygen or the nitrogen?

CH3-C-N-H

H

O+ HCl

+

CH3-C-N-H

OH

H

H

CH3-C-N-H

H

O+

+ Cl -or

A(protonation

on theamide oxygen)

B(protonation

on theamide nitrogen)

Acetamide(an amide)

Page 60: Chapter 1- CHM 261 organic chemistry

1-60

Acids & Base Strengths

The strength of an acid is expressed by an equilibrium constant the acid dissociation of acetic acid is given by the

following equation

H3O+

+CH3CO-

OH2O+CH3COH

O

Acetic acid Water Acetateion

Hydroniumion

Page 61: Chapter 1- CHM 261 organic chemistry

1-61

Weak Acids and Bases

We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as:

water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L

we can combine these constants to give a new constant, Ka, called an acid dissociation constant

HA + H2O

[H3O+][A

-]

[HA][H2O]Keq

A- + H3O

+

=

Keq[H2O]Ka[H3O+][A-]

[HA]= =

Page 62: Chapter 1- CHM 261 organic chemistry

1-62HI I-

HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6H5COOH C6H5COO-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6H5OH C6H5O-

HCO3-

CO32-

CH3NH2CH3NH3+

H2O HO-

CH3CH2OH CH3CH2O-

HC CH HC C-H2 H-NH3 NH2

-CH2=CH2 CH2=CH

-CH3CH3 CH3CH2

-Acid Formula pKa Conjugate BaseEthane

Ammonia

EthanolWater

Bicarbonate ionPhenol

Ammonium ion

Carbonic acidAcetic acid

3525

Benzoic acidPhosphoric acid

Sulfuric acidHydrogen chlorideHydrogen bromideHydrogen iodide

51

38

10.33

15.715.9

4.766.36

9.24

9.95

-5.2-7

-9-8

4.19

2.1-1.74Hydronium ion

Strongerconjugate

base

Weakerconjugate

base

Weaker acid

Stronger acid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

AcetyleneHydrogen

Ethylene 44

Page 63: Chapter 1- CHM 261 organic chemistry

1-63

Acid-Base Equilibria

Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

CH3COOH NH3 CH3COO-

NH4++ +

Ammonia(stronger base)

Acetate ion(weaker base)

Acetic acidpKa 4.76

(stronger acid)

Ammonium ionpKa 9.24

(weaker acid)

pKeq = 4.76 - 9.24 = -4.48

Keq = 3.0 x 104

Page 64: Chapter 1- CHM 261 organic chemistry

1-64

Acid-Base Equilibria

Consider the reaction between acetic acid and sodium bicarbonate we can write the equilibrium as a net ionic equation we omit Na+ because it does not undergo any

chemical change in the reaction

equilibrium lies to the right carbonic acid forms, which then decomposes to

carbon dioxide and water

CH3COH

OHCO3

- CH3CO-

O

H2CO3

Bicarbonate ion Acetate ionAcetic acidpKa 4.76

(stronger acid)

Carbonic acidpKa 6.36

(weaker acid)

+ +

Page 65: Chapter 1- CHM 261 organic chemistry

1-65

Lewis Acids and Bases

Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of electrons

Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of electrons

Lewis base

Lewis acid

+ BA new covalent bondformed in this Lewisacid-base reaction

:+-

A B

Page 66: Chapter 1- CHM 261 organic chemistry

1-66

Lewis Acids and Bases

• examples

:

: :

Br -CH3-C C-CH3

H H

H

CH3-C C-CH3

H H

H Br

Bromideion

sec-Butyl cation(a carbocation)

2-Bromobutane

++

: :

::

+ -

Diethyl ether(a Lewis base)

Boron trifluoride (a Lewis acid)

O

CH3CH2

CH3CH2

F

F

O

CH3CH2

CH3CH2

B-F

F

F

+ B

A BF3-ether complex

F:: :

Page 67: Chapter 1- CHM 261 organic chemistry

1-67

Lewis Acids include Bronsted Acids• A proton (H+) is a Lewis acid

• Proton donors therefore “accept a pair of electrons” to form a new covalent bond – between the base and proton

• A covalent bond is also broken but that doesn’t matter• The definition of Lewis acid doesn’t say anything about not

breaking bonds • All that matters is that it can ACCEPT a pair of electrons

• Hydroxide (OH) is clearly a Lewis base• Other things are bases in Lewis approach (ammonia, halide

ions)

• Most neutral molecules are acids and bases• However, they can be very different in acid/base strength NH2

+ H3O+ :NH3 + H2O NH4+ + OH

pKa(NH3) = 38 pKa(NH4+) = 9