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Understanding Reactions: Acids, Bases, and Stereoisomers, Study Guides, Projects, Research of Chemistry

An in-depth exploration of various types of reactions, focusing on acids and bases. It covers Lewis acids and bases, reaction types such as addition, elimination, substitution, and rearrangement, and reaction mechanisms. Additionally, it discusses the concepts of nucleophiles, electrophiles, and radicals. The document also touches upon isomers, including constitutional, stereo, and conformational isomers, and their impact on the properties of organic molecules.

Typology: Study Guides, Projects, Research

2021/2022

Uploaded on 09/12/2022

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Acids and Bases
Acids and Bases
Acidity Constant (K
Acidity Constant (Ka
a)
)
¾Acids differ in their H+donating ability
¾Measured based on their ability to
donate H+to water
H-A + H2OA
-+ H 3O+
¾Position of the eq. relates to acid strength
equilibrium favors the rhs strong acid
equilibrium favors the lhs weak acid
2
Acids and Bases
Acids and Bases
¾quantified by measuring the eq. constant
¾often expressed as pKa(= -log Ka)
H-A + H2OA
-+ H3O+
Keq =[H3O+][A
-]
[HA] [H2O]
Ka=[H3O+][A
-]
[HA]
3
Acids and Bases
Acids and Bases
Relative Acid Strengths
Relative Acid Strengths
CH3CH2OH
H2O
CH3COOH
HNO3
HCl
Acid pKa
16.0
15.7
4.8
-1.3
-7.0
increasing
acid
strength
Ka
10-16
10-15.7
10-4.8
101.3
107
Ka= [H3O+][A-]
[HA]
4
Acids and Bases
Acids and Bases
The Lewis Definition
The Lewis Definition
¾a Lewis acid
Lewis acid is an electron pair acceptor
acceptor.
¾a Lewis base
Lewis base is an electron pair donor
donor.
Example
H++NH3HNH
3
lewis
a
ci
d
lewis
b
a
se
5
Acids and Bases
Acids and Bases
¾
¾Lewis acids
Lewis acids require a unfilled low energy
orbital:
O
H
CH3
N
HH
H
S
H
CH3
¾
¾Lewis bases
Lewis bases require a lone pair of
electrons
Only six electrons,
empty 2p orbital
B
F
F
F
H+No electrons,
empty s orbital
6
Reaction Types
What kind of Reactions can we
carry out!
Reaction Types
What kind of Reactions can we
carry out!
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe

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Acids and BasesAcids and Bases

Acidity Constant (K Acidity Constant (K aa ))

¾ Acids differ in their H+^ donating ability

¾ Measured based on their ability to

donate H +^ to water

H-A + H 2 O A-^ + H 3 O+

¾Position of the eq. relates to acid strength

  • equilibrium favors the rhs ⇒ strong acid
  • equilibrium favors the lhs ⇒ weak acid

Acids and Bases Acids and Bases

¾ quantified by measuring the eq. constant

¾ often expressed as pKa (= -log Ka )

H-A + H 2 O A -^ + H 3 O +
Keq =
[H 3 O
] [A
]
[HA] [H 2 O]
Ka =
[H 3 O
] [A
]
[HA]

3

Acids and BasesAcids and Bases

Relative Acid Strengths Relative Acid Strengths

CH 3 CH 2 OH

H 2 O

CH 3 COOH

HNO 3

HCl

Acid pKa

increasing acid strength

K a

K (^) a = [H^3 O

  • (^) ][A - ] [HA]

4

Acids and Bases Acids and Bases

The Lewis DefinitionThe Lewis Definition

¾ a Lewis acidLewis acid is an electron pair acceptoracceptor.

¾ a Lewis baseLewis base is an electron pair donordonor.

Example

H

+ NH 3 H NH 3

lewis acid lewis base

5

Acids and BasesAcids and Bases

¾¾^ Lewis acidsLewis acids require a unfilled low energy

orbital:

O
H
CH 3
N
H H
H
S
H
CH 3

¾ ¾Lewis basesLewis bases require a lone pair of

electrons

Only six electrons, B ∴ empty 2p orbital

F

F

H F

+ No electrons,

∴ empty s orbital

6

Reaction Types

What kind of Reactions can we

carry out!

Reaction Types

What kind of Reactions can we

carry out!

Reaction TypesReaction Types

1. Addition Reactions 1. Addition Reactions

¾ Two reactants add to form one product

A + B → C

Example

C C
H
H
H
H
+ HBr C C
Br
H
H
H
H
H

Starting materials (^) Product

Reaction TypesReaction Types

2. Elimination Reactions2. Elimination Reactions

¾ One reactant splits apart into two products

A → B + C

Example

C C

H

H

H

H

C C + HBr

Br

H H

H

H H

NaOH

9

Reaction TypesReaction Types

3. Substitution Reactions 3. Substitution Reactions

¾ Two reactants exchange parts

A-B + C-DA-C + B-D

Example

H H C H

H Cl^ Cl

H H C H

Cl H Cl

hv

10

Reaction TypesReaction Types

4. Rearrangement Reactions4. Rearrangement Reactions

¾ One reactant ‘rearranges’ its bonds to

yield a different product

AB

Example

C C
CH 3 CH 2
H
H
H
C C
CH 3
CH 3
H
H
acid

11

ReactionsReactions

ReactantsReactants

¾ Nucleophiles (nucleus loving)

  • electron rich species (eg. – OH, :NH 3 )

C

H

H H I

δ−

δ+

¾ Electrophiles (electron loving)

  • electron deficient species (eg. H+, )

¾ Radicals

  • species with an unpaired electron (eg. Cl•)

12

Reactions Reactions

Reaction MechanismsReaction Mechanisms

¾ Step by step description of a reaction

¾ Shows which bonds are broken and

formed

¾ Shows the order in which bonds are

breaking and forming

¾ Accounts for all reactants and products

¾ ¾ Draw using curly arrows to showDraw using curly arrows to show

direction of electron flowdirection of electron flow

HydrocarbonsHydrocarbons

Introduction Introduction

¾ Alkanes

¾ Alkynes

¾ Alkenes

¾ Aromatic

Hydrocarbons

C C

H

H

H

H

HH

C C

H H

H H

H C C H

AlkanesAlkanes

General FormulaGeneral Formula

  • CnH (^) 2n+

Industrial SourcesIndustrial Sources

Petroleum

Natural Gas C 1 -C 4

Gasoline C 5 -C 11

Kerosene C 11 -C (^14)

Gas Oil C 14 -C 25

Grease Wax

Asphalt

21

AlkanesAlkanes

Structure Structure

Geometry:

  • bond angle = 109.5 o
  • sp 3
  • tetrahedral

eg. Methane

Look for the 3D structure!

22

Importance Importance

¾ ¾ Crude OilCrude Oil

  • Petroleum, tar, paraffin etc.

¾ ¾ Medicinal ChemistryMedicinal Chemistry

  • Hydrophobic pockets

No VDW Interactions: moderate binding Many VDW Interactions: Strong Binding

N CO 2

O 2 C N N O CO^2

O 2 C N

Hydrophobic pocket

ACE (Angiotensin Converting Enzyme) inhibitors

  • treat hypertension
  • bind to enzyme active site captopril Enalapril

23

PreparationPreparation

Hydrogenation of Alkenes Hydrogenation of Alkenes

¾ Alkene is treated with hydrogen gas

and a metal catalyst

C C

H H

H H

H (^2)

Pd/C

C (^) nH (^) 2n C (^) nH (^) 2n+

C C

H

H

H H

H H

Catalyst Reagent

24

ReactionsReactions

1. Combustion

CH 4 + 2O 2 → CO 2 + 2H 2 O

2. Substitution

CH 4 + Cl 2 → CH 3 Cl + HCl

3. Pyrolysis

Cracking

ReactionsReactions

2. Substitution (Halogenation 2. Substitution (Halogenation))

¾ Free radical halogenation

Example

¾ Methane + chlorine

H C Cl (^2) or hν H—Cl

H

H

H

H C

Cl

H

H

Light

HalogenationHalogenation

Free Radicals Free Radicals

¾ Very reactive species with an odd (unpaired)

electron (a total of only 7)

¾ sp 2 hybridised

¾ Trigonal planar

H C

H

H

27

Halogenation Halogenation

Mechanism Mechanism

1. Initiation

¾ Production of free radicals

Cl Cl Cl Cl

28

HalogenationHalogenation

2. Propagation

¾ Major process

H 3 C H Cl H^3 C^ +^ H^ Cl
H 3 C Cl Cl H 3 C Cl + Cl

29

Halogenation Halogenation

3. Termination

¾ Destruction / removal of free radicals

Cl Cl Cl Cl
H 3 C CH 3 H 3 C CH 3
H 3 C Cl H 3 C Cl

30

HalogenationHalogenation

ProblemProblem

¾ Substitution by >1 halogen (difficult to

control!)

CH 4 → CH 3 X → CH 2 X 2 → CHX 3 → CX 4

Reason:

[CH 4 ]↓ and the [CH 3 X]↑

AlkanesAlkanes

Conformational Isomers Conformational Isomers

¾ Ethane (CH 3 -CH 3 )

  • view along C-C bond

observer

Rotations around single bonds

AlkanesAlkanes

Conformational IsomersConformational Isomers

¾ Ethane

Eclipsed (unfavoured)

Staggered (favoured)

H

H H

H

H H H
H
H H
H
H

NewmanNewman

ProjectionsProjections

39

AlkanesAlkanes

Conformational Isomers Conformational Isomers

¾ Butane (CH 3 CH 2 CH 2 CH 3 )

  • view along C 2 -C 3 bond

observer

40

AlkanesAlkanes

Butane

¾ Large groups near one another cause stericsteric

strainstrain

¾ Arises when such groups try to occupy the

same space

¾ Apparent in the gauchegauche and eclipsedeclipsed

conformations

41

AlkanesAlkanes

Conformational Isomers Conformational Isomers

anti (^) gauche (^) eclipsed

42

Conformational IsomersConformational Isomers

Cycloalkanes Cycloalkanes

¾ Most cyclic organic molecules have large

strain energies

¾ Cyclohexane is nearly strain free

¾ Three types of strain:

  • angle strain
    • steric strain
  • torsional strain

Conformational Isomers Conformational Isomers

Angle strain

¾ Strain due to expansion or compression of

bond angles

Steric Strain

¾ Strain due to repulsive interactions when

atoms approach each other too closely

Torsional Strain

¾ Strain due to eclipsing of bonds.

  • Bond = electron pair
  • Electron pairs repel!

Conformational IsomersConformational Isomers

Cyclopropane

¾ C-C bond angle is only

60 o^ (normally 109.5o)

  • high angle strain

¾ Cyclopropane must be

planar

  • high torsional strain

45

Conformational Isomers Conformational Isomers

Cyclohexane

¾ Exists mainly in the

‘chair’ conformation

¾ All angles are 109.5o

(no angle strain)

¾ Fully staggered

(no torsional strain)

H H

H

H

H H

H

H

46

Conformational IsomersConformational Isomers

Chair Cyclohexane

¾ Axial and equatorial substituents

H

H

H H

H H H

H

H H H

H

H

H

H H

H H H

H

H H H

H

Axial (^) Equatorial

47

Conformational Isomers Conformational Isomers

Chair Cyclohexane

¾ Undergoes ring flips

H
H
H H
H
H
H
H
H
H
H
H

48

Conformational IsomersConformational Isomers

Boat Cyclohexane

¾ All C-C angles 109.5o

  • no angle strain

¾ Fully eclipsed

  • high torsional strain

H

H

H

H H H

H

H H

H

H H H

H

H H

H H H

H

H H H

H

H H

Limonene

oranges lemons

N NO O N

O O O

H H

O H N

OH O

Thalidomide

foetal abnormalities tranquiliser

Vs.

P. Guiry UCD

StereoisomersStereoisomers

Types of chiral moleculesTypes of chiral molecules

¾ Enantiomers

¾ Diastereomers

  • configurational
  • cis/trans or E/Z (alkene)

¾ Meso Compounds

57

3D Structure 3D Structure

Enantiomers Enantiomers

¾ Mirror images (just like our hands!):

Cl

C Br

F H

Cl

C Br

H F

¾ One enantiomer active, the other potentially harmful

  • Eg. Thalidimide

58

59

3D Structure 3D Structure

Enantiomers Enantiomers

¾ Non-superimposable, mirror images

eg.

Cl

C

Br

F H

Cl

C

H

F Br

60

3D Structure3D Structure

Enantiomers:Enantiomers:

¾ only differ in:

  • the way they react or interact with other
chiral molecules
  • the shape of the crystals they form
(sometimes)
  • the way they interact with plane polarised
light (optical acivity)

¾ All other physical properties are

identical

StereoisomersStereoisomers

Optical ActivityOptical Activity

¾ Chiral compounds rotate plane polarised light

¾ Clockwise rotation ⇒ dextrorotatory ( D / +)

¾ Anticlockwise rotation ⇒ levorotatory ( L / -)

¾ Called optical activity

StereoisomersStereoisomers

The PolarimeterThe Polarimeter

¾ schematic

α

light source

polariser sample tube analyser observer containing organic molecules

63

Stereoisomers Stereoisomers

The Polarimeter The Polarimeter

Observed Rotation α

= the angle through which the plane of

light is rotated

¾ Depends on the nature of the sample

  • the enantiomeric purity of the material
  • concentration
  • size of the polarimeter cell
  • wavelength of light

64

StereoisomersStereoisomers

Specific Rotation [α]D

¾ Physical constant

¾ Use standard path length and conc.

[α]D = α / l x C

α = observed rotation (in degrees) l = path length (in decimeters) C = concentration (in g/mL)

65

enantiomersenantiomers

Enantiomeric excess Enantiomericexcess

  • More of one enantiomer than the other
  • Quoted as %ee

% ee =

moles of one − moles of the other total moles of both

x 100%

observed specific rotation known specific rotation of pure enantiomer

% ee = x 100%

  • equal amounts of each enantiomer = 0%ee = racemic mixture

66

StereoisomersStereoisomers

To specify ConfigurationTo specify Configuration

Cahn-Ingold-Prelog Rules

¾ Used to describe the arrangement of

groups around a chiral centre

¾ Chiral centres are described as R or S

  • Independent of D(+) and L(–)

StereoisomersStereoisomers

>More then one chiral centre >More then one chiral centre

¾ Consider the following molecules with twotwo

chiral centres:

  • Structure with n chiral centres has total of 2 n^ stereoisomers:

H 2 N H

COOH

HO H CH 3

H NH 2

COOH

H OH CH 3

H NH 2

COOH

HO H CH 3

H 2 N H

COOH

H OH CH 3

EnantiomersEnantiomers

StereoisomersStereoisomers

>1 chiral centre>1 chiral centre

¾ Consider the following molecules:

H 2 N H
COOH
HO H
CH 3
H NH 2
COOH
H OH
CH 3
H NH 2
COOH
HO H
CH 3
H 2 N H
COOH
H OH
CH 3

EnantiomersEnantiomers

75

StereoisomersStereoisomers

>1 chiral centre >1 chiral centre

¾ Consider the following molecules

H 2 N H
COOH
HO H
CH 3
H NH 2
COOH
H OH
CH 3
H NH 2
COOH
HO H
CH 3
H 2 N H
COOH
H OH
CH 3

Diastereomers Diastereomers

76

StereoisomersStereoisomers

>1 chiral centre>1 chiral centre

¾ consider the following molecules

H 2 N H

COOH

HO H CH (^3)

H NH 2

COOH

H OH CH (^3)

H NH 2

COOH

HO H CH (^3)

H 2 N H

COOH

H OH CH (^3)

Diastereomers Diastereomers

2 pairs of enantiomers 2 pairs of diastereomers

Summary:

77

StereoisomersStereoisomers

Diastereomers Diastereomers

¾ Stereoisomers that are notnot mirror images

( i.e. not enantiomers)

¾¾ SomeSome (one or more) of the chiral centres

have opposite configuration

¾ Diastereomers have differentdifferent physical

properties.

¾ Can be separated by crystallisation,

distillation, etc.

78

StereoisomersStereoisomers

MesoMeso CompoundsCompounds

H OH

COOH

H OH COOH

HO H

COOH

HO H COOH

Plane of symmetry

  • Not optically active
  • Different groups attached but the

compound has got a plane of symmetry

(∴not chiral)

StereoisomersStereoisomers

Cis/Trans Cis/Trans DiastereomersDiastereomers

¾ Differ in their stereochemistry about a

double bond or ring

¾ Also called geometric isomers

H 3 C
H
H
CH 3
H 3 C
H
H
CH 3

Assign priority to each group attached to double bond or ring Highest priority groups on same side = cis ( Z for zusammen) opposite side = trans ( E for entgegen)

TransTrans EE

TransTrans EE

H 3 C
H
H
CH 3
H 3 C
H
H
CH 3

StereoisomersStereoisomers

Fischer ProjectionsFischer Projections

¾ Allow representation of 3D structures

on a 2D page

A B
C D
A B
D
C

Up/down groups into page Left/right groups out from page

81

3D Structure 3D Structure

Isomerism (Summary) Isomerism (Summary)

isomers

constitutional stereoisomers

enantiomers diastereomers cis/trans

conformational

82

PrioritiesPriorities

Straight from IUPACStraight from IUPAC

H C CH 2

H C CH 2 (C) (C)

(C) = atom with nothing else attached
C^ C^ C
H
Br
H
H 2 C
CH
C C
C
Br
H 2 C
CH
(C)
(C)
(C)
(C)
(C)(C)
H
H

1

4

3 2

*C *C