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1
INTRODUCTION TO ORGANIC
CHEMISTRY
CHAPTER 4
2
Chapter 4 : INTRODUCTION TO ORGANIC CHEMISTRY
4.1 Molecular and
Structural Formulae
4.2 Functional Groups and
Homologous Series
4.3 Isomerism
Learning Outcomes:
a) Define structural formula.
b) Show structural formula in the form of expanded, condensed and skeletal structures based on molecular formula.
c) Classify carbons into primary, secondary, tertiary or quaternary (1°,2°,3°,4°) and hydrogens into primary, secondary or tertiary (1°,2°,3°).
3
4.1 MOLECULAR AND STRUCTURAL FORMULAE
•Structural formula shows how the atoms in a molecule are bonded to each other.
•3 types of structural formula:• expanded structure• skeletal structure• condensed structure
4
STRUCTURAL FORMULA
•Expanded structures indicate how atoms are attached to each other but are not representations of the actual shapes of the molecules.
C4H9Cl(Molecular
Formula)
(Expanded structure)
5
C C C CH H
H H HCl
H H H H
EXPANDED STRUCTURE
Examples:
Alcohol (C2H6O) C
H
H
H
C
H
H
OH
C
H
H
H
C
H
H
C
O
OH
66
Examples:
Carboxylic acid (C3H6O2 )
(Expanded structure)
(Expanded structure)
• Shows only the carbon skeleton.
• Hydrogen atoms are not written.
• Other atoms such as O, Cl, N etc. are shown.
Examples:
i) CH3CH(Cl)CH2CH3Cl
ii) H2C CH 2
H2C CH 2
77
SKELETAL STRUCTURE
• Does not show single bonds between carbon and hydrogen atoms, but double and triple bonds are shown.
• All atoms that are attached to a carbon are written immediately after that carbon.
C4H9Cl CH3CH(Cl)CH2CH3(Condensed structure)
8
Examples:
CONDENSED STRUCTURE
H2C
CH2
CH2
CH2
H2C
H2C
9
Cyclohexane, C6H12
Aldehyde, CH3CHO CH3CH
O
(Condensed structure)
(Condensed structure)
Examples:
A carbon atom can be classified as:
•primary carbon(1o) → bonded to 1 C
•secondary carbon(2o) → bonded to 2 C
•tertiary carbon(3o) → bonded to 3 C
•quarternary carbon(4o) → bonded to 4 C
10
CLASSIFICATION OF CARBON ATOMS
A hydrogen atom can be classified as:
•Primary hydrogen(1o) → bonded to 1o C
•Secondary hydrogen(2o) → bonded to 20 C
•Tertiary hydrogen(3o) → bonded to 30 C
11
CLASSIFICATION OF HYDROGEN ATOMS
THERE IS NO QUATERNARY(40) HYDROGEN
12
H C H
CH3
H
10 carbon
10 carbon
10 hydrogen
Examples:
13
H C CH3
CH3
CH3
30 carbon
30 hydrogen
Examples:
14
11
1
11
H C C C CH2 C CH3
H
H
H
H H
CH3 CH3
CH3
Identify the 1o carbon:
Exercise 1:
15
2
2
H C C C CH2 C CH3
H
H
H
H H
CH3 CH3
CH3
Identify the 2o carbon:
Exercise 2:
16
43
H C C C CH2 C CH3
H
H
H
H H
CH3 CH3
CH3
Identify the 3o and 4o carbon:
Exercise 3:
CH3(CH2)CCl(CH3)2
C
H
HH
C CH
H
H
CH3
CH3
17
Condensed
StructureExpanded
Structure
Skeletal
Structure
O
Exercise 4: Fill in the blank with the correct structure
Learning Outcomes:
a) Define functional group
b) State functional groups
c) Identify functional groups in a given compound
d) Define homologous series
e) Explain general characteristics of homologous series:
i. represented by a general formula
ii. Same functional group and chemical properties
iii.Gradual change in physical properties with increasing number of carbon atoms
iv. Successive member of a series differs by a –CH2-group
18
4.2 FUNCTIONAL GROUPS AND HOMOLOGOUS SERIES
•Is an atom or group of atoms that determine the chemical properties of that compound.
• Organic compounds are classified based on the functional group present in their molecules.
• Molecules with the same functional group are considered to be in the same homologous series
19
FUNCTIONAL GROUP
• Functional groups are important for three reasons:
1. A basic by which organic compounds are divided into different classes.
2. A basic for naming organic compounds
3. A particular functional group will
always undergo similar types of chemical reactions.
20
•Series of compounds where each member differs from the next member by a constant – CH2 unit
•Members of the same homologous series are called homologs
•Compounds in the same functional group
21
HOMOLOGOUS SERIES
1. Obey a general formula:
Examples:• Alkane: CnH2n+2• Alkene: CnH2n• Alcohol: CnH2n+1OH
2. Differ from the successive homolog by a CH2unit
3. Show a gradual change in the physical properties
4. Have same functional group
5. Have similar chemical properties
6. Can be prepared by similar general methods22
HOMOLOGS FEATURES
ALKANE (CnH2n+2)
Methane: CH4
Ethane: CH3CH3
Propane: CH3CH2CH3
Butane: CH3CH2CH2CH323
Examples:
CH2
CH2
CH2
24
CH3C CCH3
carbon-carbon
triple bond-C C-Alkyne
CH3CH=CH2
carbon-carbon
double bond-C=C-Alkene
CH3-CH3Alkane
Example
NameStructure
Class of
Compound
Functional Group
CLASSIFICATION OF ORGANIC COMPOUND
Aromatic Benzene ring -CH3
Haloalkane X (F, Cl, Br, I) Halogen CH3Cl
Alcohol -OH Hydroxyl CH3-OH
Phenol -OH Hydroxyl -OH
Ether -C-O-C- Alkoxyl CH3-O-CH3
25
Aldehyde
-C=O
H Carbonyl
CH3-C=O
H
Ketone
R-C=O
R Carbonyl
CH3-C=O
CH3
Carboxylic
acid
-C=O
OH Carboxyl
CH3-C=O
OH
Ester
-C-O-C-
O Carboalkoxyl
CH3-C=O
OCH3
Acyl chloride
-C=O
Cl
Acyl halide CH3-C=O
Cl
26
Anhydride
O O
-C-O-C-
Anhydride O O
CH3C-O-CCH3
Amide -C=O
N-
Carboxamide CH3-C=O
NH2
Amine -NH2 Amino CH3-NH2
Nitrile -C N Cyano group CH3C N
27
28
1. Identify the functional group in the following molecules
a)(CH3)3CCH2CH=CH2
b)(CH3)3CCH=CHCH2-OH
Exercises:
29
CH3 C
CH3
C C CH C NH2
O
NH2 CH
CH2
CH2
C
O
OH
OH
C
O
O CH3
CH2
C
O
O C
O
CH3
c)
Learning Outcomes:
a) Define isomerism, constitutional isomerism and stereoisomerism
b) Identify and construct constitutional isomerism;
i. chain isomers
ii. positional isomers
iii. functional group isomers
c) Describe cis-trans isomerism due to restricted rotation about:
i. C=C bond
ii. C-C bond in cyclic compounds
d) Identify cis-trans isomerism of a given structural formula
e) Define chirality centre and enantiomers
f) Identify chirality centre(s) in a molecule
g) Determine optical activity of a compound
h) Draw a pair of enantiomers using 3-dimensional formula30
4.3 ISOMERISM
Isomerism
Structural/
Constitutional IsomerismStereoisomerism
Chain
Isomerism
Positional
Isomerism
Functional Group
Isomerism
Optical
isomerism
Geometrical /
cis-trans/
diastreomer
isomerism
31
Active
(enantiomers)
Inactive
(racemate)
•is the existence of different compounds with the same molecular formula but different structural formulae.
•Different compound with different structural formula that have the same molecular formula are called isomers.
32
ISOMERISM
•Are isomers with the same molecular formula but differ in the order of attachment of atoms.
•Also known as structural isomer
33
Three types: a) Chain isomersb) Positional isomersc) Functional group isomers
CONSTITUTIONAL ISOMERISM
CH3CH2CH2CH2CH3
The isomers differ in the carbon skeleton (different carbon chain).
They possess the same functional group and belong to the same homologous series.
!Note : parent chain changed
Example: C5H12
CH3CHCH2CH3
CH3
CH3CCH3
CH3
CH3
34
CHAIN ISOMERS
35
These isomers have a substituent group/ functional group in different positions.
!Note: Parent chain unchanged.
Examples: C3H7Cl
CH3CH2CH2Cl
1-chloropropane
2-chloropropane
CH3CHCH3
Cl
POSITIONAL ISOMERS
36
• C4H8
• C8H10
CH3
CH3
CH3
CH3
1,2-dimethylbenzene
1,3-dimethylbenzene
1-butene 2-butene
CH2=CHCH2CH3 CH3CH=CHCH3
CH3
CH3
1,4-dimethylbenzene
37
• C6H13N
CH3
H2N
CH2NH2
CH3
NH2
CH3
NH2
These isomers have different functional groups and belong to different homologous series with the same general formula.
Different classes of compounds that exhibit functional group isomerism.
!Note: different functional group
General formula Classes of compounds
CnH2n+2O ; n > 1 alcohol and ether
CnH2nO ; n ≥ 3 aldehyde and ketone
CnH2nO2 ; n ≥ 2 carboxylic acid and ester
CnH2n ; n ≥ 3 alkene and cycloalkane
38
FUNCTIONAL GROUP ISOMERS
39
Examples:
C2H6O
C3H6O
C3H6O2
CH3CCH3
O
CH3CH2CH
O
CH3COCH3
O
CH3CH2COH
O
ethanol dimethyl ether
propanalpropanone
propanoic acid methyl ethanoate
CH3CH2OH CH3OCH3
1. State how many are isomers with the following
molecular formulae, identify the type of
isomerism and draw the structural formula of the
isomers.
a) C5H10
b) C5H10O2
c) CH3CH=C(Cl)CH3
d) C4H6Cl2
e) CH3CH2CH(OH)CH(Br)CH2CH3
40
Exercise:
Stereoisorism: Isomerism that resulting from different spatial arrangement of atoms in molecules.
Stereoisomers: Are isomers with the same molecular formula but different arrangement of atom in space.
41
STEREOISOMERISM
Two subdivisions of stereoisomers:
i) Geometrical/cis-trans/diastreomers isomers
ii) Optical isomers:
a) Enantiomers (optically active)
b) Racemate(optically inactive)
42
STEREOISOMERISM
The requirements for geometric isomerism :
i) Restricted rotation about a C=C double
bond in alkenes, or a C-C single bond
in cyclic compounds.
ii)Each carbon atom of a site of restricted
rotation has two different groups
attached to it.
43
GEOMETRICAL / CIS-TRANS / DIASTEREOMERS ISOMERS
44
Examples: 2-butane
C C
CH3
H
H
H3C
C C
CH3
H
H3C
H
trans-2-butene cis-2-butene
CH3CH=CHCH3
CH3-C=C-CH3
H H
45
Examples: 1,3-dimethylcyclohexane
cis-1,3-dimethylcyclohexane trans-1,3-dimethylcyclohexane
CH3
CH3
H
H
CH3
CH3
CH3
CH3
H
H
46
• If one of the doubly bonded carbons has 2 identical groups, geometric isomerism is not possible.
• Example
No cis – trans isomer
C C
CH3
H
H3C
H3C
47
• Are isomers which are optically active compounds and able to rotate plane-polarized light to the right (dextrorotary) and to the left (levorotary),and exist as mirror images(enantiomers)
• The angle of rotation can be measured with an instrument called polarimeter.
OPTICAL ISOMERISM
3-Dimensional formula ( wedge – dashed wedge – line formula )
•Describes how the atoms of a molecule are arranged in space.
48
Example : Bromoethane, CH3Br
Indication :
:bonds that lie in the plane
:bonds that lie behind the plane
:bonds that project out of the plane
C
Br
H
H
H
49
C
Br
HH
HC
H
BrH
H
C
H
HBr
H
50
Is a molecule which contains chiral carbon and not superimpose with its mirror images.
The requirements for optically active
isomerism/enantiomers :-
i) molecule contains a chiral carbon or chirality centre or stereogenic centre (a sp3-hybridized carbon atom with 4 different groups attached to it)
ii) molecule is not superimposable with its mirror image.
ENANTIOMERS / OPTICAL ISOMERS
51
Example: 2-butanol
C *
CH2CH3
H3C
O HH
C
CH2CH3
CH3HO H
CH3CHCH2CH3
OH
enantiomers
CH3 C C CH3
H H
OH H
*
*
52
Learning Outcomes:
a) Explain covalent bond cleavage:
i. homolytic cleavage
ii. heterolytic cleavage
b) Differentiate homolytic cleavage and heterolytic cleavage
c) State the relative stabilities of primary, secondary and tertiary free radicals, carbocations and carbanions.
d) Compare the stabilities of carbocations and carbanions by using the inductive effect of alkyl group.
e) Define
i. electrophile and nucleophile
ii. Lewis acid and Lewis base
f) State the main types of organic reactions
i. addition (nucleophilic and electrophilic)
ii. Substitution (free radical, nucleophilic, and electrophilic)
iii. Elimination
iv. Rearrangement
g) Identify the main type of organic reaction given a reaction equation.
4.4 REACTIONS OF ORGANIC COMPOUNDS
53
Types of Covalent Bond Cleavage/Fission
All chemical reactions involved bond breaking and bond making.
Two types of covalent bond cleavage :-
Homolytic cleavage
Heterolytic cleavage
54
• Occurs in a non-polar bond involving two atoms of similar electronegativity.
• A single bond breaks symmetrically into two equal parts, leaving each atom with one unpaired electron.
• Formed free radicals.
HOMOLYTIC CLEAVAGE
X : X X + X
free radicals
uv
Example:
55
• Occurs in a polar bond involving unequal sharing of electron pair between two atoms of different electronegativities.
• A single bond breaks unsymmetrically.
• Both the bonding electrons are transferred to the more electronegative atom.
• Formed cation and anion.
HETEROLYTIC CLEAVAGE
H Br H+ + :Br-
Example:
56
Reaction Intermediates
a) Carbocation
b) Carbanion
c) Free Radical formed from homolytic cleavage
They are unstable and highly reactive.
formed from heterolytic cleavage
57
•Also called carbonium ion.
•A very reactive species with a positive charge on a carbon atom.
•Carbocation is formed in heterolytic cleavage.
•Is an e- deficient/poor.
•Usually left with 6 valence e-
CARBOCATION
58
Example :
•(CH3)3C — Cl (CH3)3C+ + :Cl-
carbocation anion
•Chlorine is more electronegative than carbon and the C—Cl bond is polar.
•The C—Cl bond breaks heterolitically and both the bonding electrons are transferred to chlorine atom to form anion and carbocation.
59
Is an anion counterpart
A species with a negative charge on a carbon atom.
Carbanion is formed in heterolytic cleavage.
Is an e- rich.
Usually left with 8 valence e-
Example:
(CH3)3C — Li (CH3)3C- + Li+
- + carbanion cation
CARBANION
60
A very reactive species with an unpaired/single electron.
Formed from homolytic cleavage.
Is an e- deficient/poor.
Examples:
Cl – Cl Cl● + Cl ●
free radicals
FREE RADICAL
uv
61
C C C ●+ C●
H3C H H3C● H●+
ii)
iii)
Examples:
uv
uv
62
HOMOLYTIC CLEAVAGE HETEROLYTIC CLEAVAGE
Occur to a non-polar bond involving two atoms of similar electronegativity.
Occur to a polar bond involving unequal sharing of electron pair between two atoms of different electronegativity.
Bond breaks symmetricallyinto two equal parts leaving each atom with one unpaired/single electron.
Bond breaks unsymmetrically and both electron are transferred to the more electronegative atom.
Forms free radicals:
Cl-Cl Cl. + Cl.
Forms cation and anion:
H3C-Cl H3C+ + :Cl-
DIFFERENCES BETWEEN HOMOLYTIC AND HETEROLYTIC CLEAVAGE
uv
63
•Carbocation, carbanion and free radical can be classified into:
Primary
Secondary
Tertiary
RELATIVE STABILITIES OF CARBOCATIONS,CARBANIONS AND
FREE RADICALS
64
•The alkyl groups,R (e- releasing group @ ERG / e- donating group @ EDG) stabilisethe positive charge on the carbocation.
•As number of alkyl group present increases, the stability of carbocation increases.
CARBOCATION STABILITY
65
H
C HH+
R
C RH+
R
C RR+
H
C RH+
< < <
Methylcation
Primary10
Secondary20
Tertiary30
Increasing stability
CARBOCATION STABILITY
66
Alkyl group and other e- donating groups (EDG) destabilises carbanions.
As number of alkyl group present increases, the stability of carbanion decreases.
Electron withdrawing group / EWG (e.g: halogen) stabilises carbanions through the inductive withdrawal of electron density
CARBANION STABILITY
67
H
C HH-
R
C RH-
R
C RR-
H
C RH-> > >
Methylanion
Primary10
Secondary20
Tertiary30
Decreasing stability
CARBANION STABILITY
68
H R
C RH . C RH .
H
C HH . < < <
methylradical
Primary10
Secondary20
R
C RR .
Tertiary30
Increasing stability
As number of alkyl group present increases, the stability of free radical increases.
Radical carbon is an e- deficient/poor which stabilised by alkyl group (an e- donating group/EDG)
FREE RADICAL STABILITY
69
Means ‘electron loving’.
An electron-deficient species and accepting electron from an attacking nucleophile.
Can be either neutral or positively charged
ELECTROPHILE
70
Examples of electrophiles :
•cations such as H+, H3O+, NO2
+ etc.
•carbocations
•Alcohols (R-OH)
•Amines (R-NH2)
•Oxidizing agents (reduction, gain e-) such as Cl2, Br2 and etc.
•CO2•Lewis acids: a substance that can accept a pair of e- to form a coordinate covalent bond. Example: AlCl3, FeCl3, BF3etc.
71
Electrophilic sites are molecules with low electron density around a polar bond.
Examples:
-C = O (carbonyl) ; -C – X (haloalkanes)
+ - + -
-C – OH (hydroxy compounds)
+ -
i) ii)
iii)
72
means ‘nucleus loving’
An electron-rich species and donate electron to electrophile.
A nucleophile can be either neutral or negatively charged.
NUCLEOPHILE
73
Examples of nucleophiles :
•anions such as OH-, RO-, Cl-, CN- etc.
•carbanions (species with a negative charge on carbon atoms ).
•Alkenes (–C=C-)
•Alcohols (R-OH)
•Amines (R-NH2)
•Lewis Base: a substance that can donate a pair of e- to form a coordinate covalent bond. Example: NH3 & H2O
74
Addition
Substitution
Elimination
Rearrangement
4 TYPES OF REACTIONS
75
A reaction in which atoms or groups add to adjacent atoms of a multiple bond.
Two types of addition :-a) Electrophilic Additionb) Nucleophilic Addition
ADDITION REACTION
76
ELECTROPHILIC ADDTION
Initiated by an electrophile accepting electron from an attacking nucleophile.
Typical reaction of unsaturatedcompounds such as alkenes and alkynes.
Example :
CH3CH=CH2 + Br2 CH3CH-CH2‘Nu-’ ‘El+’ Br Br
Room temperature
CCl4
77
NUCLEOPHILIC ADDITION
Initiated by a nucleophile, which attacks an electrophilic site of a molecule.
Typical reaction of carbonyl compounds.
CCH3 CH3
O
+ HCN CH3 C CH3
O-H
CN‘El+’ ‘Nu-’
78
A reaction in which an atom or group in a molecule is replacedby another atom or group.
Three types of substitution :-a) free radical substitutionb) electrophilic substitutionc) nucleophilic substitution
SUBSTITUTION REACTION
79
Substitution which involves free
radicals as intermediate species.
Typical reaction of ALKANES.
Example :
CH3CH3 + Cl2 CH3CH2Cl + HCl
FREE RADICAL SUBSTITUTION
uv
80
The aromatic nucleus has high electron density, thus it is nucleophilic and is prone to electrophilic attack.
Typical reaction of aromatic compounds(benzene).
+ Br2 Br + HBrFe
catalyst
Example:
ELECTROPHILIC SUBSTITUTION
81
Typical reaction of saturated organic compounds bearing polar bond as functional group, such as haloalkane and alcohol.
Example :
CH3CH2Br + OH- CH3CH2OH + Br
-
Δ
NUCLEOPHILIC SUBSTITUTION
82
An atoms or groups are removed from adjacent carbon atoms of a molecule to form a multiple bond (double or triple bond).
Results in the formation of unsaturated molecules.
Example :
CH3CH2OH CH2=CH2 + H2O
ELIMINATION REACTION
conc. H2SO4
83
A reaction in which atoms or groups in a molecule change position.
Occurs when a single reactant reorganizes the bonds and atoms.
Example :
C CC C
OHH
H R R
H
H
H
O
REARRANGEMENT REACTION
Exercises
1. Explain how the free radicals are formed in homolytic cleavage.
2. Write an equation for the bromine-bromine bond cleavage in the bromination of methane. State the type of bond cleavage.
3. Which would you expect to be the most stable free radical?
CH2CH3 , (CH3)2 CH , CH3 ,
CH3
84