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Chapter 3. An Introduction to Organic Compounds
Learning objectives:
1. Name alkanes, cycloalkanes, and alkyl halides.
2. Write the structures of constitutional (or structural) isomers of alkanes.
3. Draw Newman projections of alkanes in staggered and eclipsed conformations, and
arrange the order of stability for these conformations (conformational isomers).
4. Identify cis and trans relationship for the substituents on cycloalkanes.
5. Draw chair conformation of cyclohexane with unambiguous representation of axial and
equatorial substituents.
6. Complete the equilibrium of two chair conformational isomers for a substituted
cyclohexane, indicate the change for the relative positions of axial and equatorial
substituents, and reason the stability between these two isomers.
7. Know the classification of alkyl halides, alcohols, and amines.
Molecular model kit will be very helpful for learning the material in this chapter.
3.1
3.2
3.3
3.4
3.5
Nomenclature of Alkanes and Cycloalkanes
Nomenclature and Classification of alkyl halides
Alcohols, Amines and Ethers
Structures and Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and
Amines
Conformation of Alkanes and Cycloalkanes
3.1
Nomenclature of Alkanes and Cycloalkanes
A. Things to remember
 Know the names for alkanes from n=1 to 10.
 When viewed as substituents: alkanes become alkyl (R) groups.
 Know the names for alkyl groups from n=1 to 10.
Name
Formula
Name of
Substituent
Structure of
Substituent
Methane
CH4
Methyl
CH3-
Ethane
CH3CH3
Ethyl
CH3CH2-
Propyl
CH3CH2CH2-, npropyl
(CH3)2CH-, isopropyl
Propane
CH3CH2CH3
1
Butane
CH3(CH2)2CH3
Butyl
CH3CH2CH2CH2-, nbutyl
(CH3)2CHCH2-,
isobutyl
CH3CH2CH(CH3)-, sbutyl
(CH3)3C-, t-butyl
Pentane
CH3(CH2)3CH3
Pentyl
-
Hexane
CH3(CH2)4CH3
Hexyl
-
Heptane
CH3(CH2)5CH3
Heptyl
Octane
CH3(CH2)6CH3
Octyl
Nonane
CH3(CH2)7CH3
Nonyl
Decane
CH3(CH2)8CH3
Decyl
CH3(CH2)9CH3
-
CH3(CH2)10CH3
-
Undecane
Dodecane
B. Skeletal Structures
Know the meaning of dashed and wedged bonds
2
propyl (n-propyl)
isopropyl
butyl (n-butyl)
isobutyl
sec-butyl ((s-butyl)
tert-butyl (t-butyl)
C. The IUPAC System (Systematic name)
(i) Identify the longest carbon chain.
(ii) Locate the substituent with the lowest number on the parent (main) chain. If
equivalent positions are encountered, assign the substituent with higher alphabetical order
a higher priority (lower in numbering).
(iii) Place prefix of substituents according to their position on the main chain. Use di-, tri, tetra-, penta-, hexa- and so on for identical substituents.
(iv) Arrange the substituents, excluding di-, tri-, tetra-, penta-, hexa- and so on, in
alphabetical order.
Examples:
(i)
3
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
4
(ix)
(x)
(xi)
D. Structural isomers
Structural (constitutional) isomers: Compounds with the same molecular formula but
with different order of attachment (connectivity) of their atoms.
Examples:
(i) Butane (C4H10)
Normal butane (n-butane)
CH3CH2CH2CH3
H
H
H
H
H
C
C
C
C
H
H
H
H
5
H
2-Methylpropane or isobutane (iso-butane or i-butane)
H
CH3CH(CH3)CH3
H
H
H
C
H
C
C
C
H
H
H
H
H
(ii) Pentane (C5H12)
CH3CH2CH2CH2CH3
CH3CH(CH3)CH2CH3
E. The Nomenclature of Cycloalkanes
General formula: CnH2n
(i)
6
CH3C(CH3)2CH3
(ii)
(iii)
(iv)
(v)
(vi)
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F. Classification of Carbon and Hydrogen Atoms
This classification cannot be used for alkenes (C=C) or alkynes (CΞC).
G. Self-assessment Questions
 Can you provide the systematic names for alkanes and cycloalkanes?
 Can you identify primary, secondary, tertiary and quaternary carbons?
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3.2
Nomenclature and Classification of Alkyl Halides
A. Systematic name
Use the name of alkanes and treat halides as substituents.
B. Common Names
Use the name of alkyl groups.
Examples:
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C. Self-assessment Questions
 Can you provide the systematic and common names for alkyl halides?
 Can you identify primary, secondary and tertiary alkyl halides?
3.3
Alcohols, Amines and Ethers
A. Classification of Alcohols
10
B. Classification of Amines
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C. Ethers
D. Self-assessment Questions
 Can you identify primary, secondary and tertiary alkyl alcohols?
 Can you identify primary, secondary and tertiary amines, and quaternary
ammonium salts?
 Can you recognize an ether?
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3.4 Structures and Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and
Amines
A. Structures of Alkanes
non-polar, induced dipole-induced dipole interaction (van der Waals force)
B. Structures of Haloalkanes
Dipole–dipole interaction
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C. Structures of Ethers
Dipole–dipole interaction
D. Structures of Alcohols
Hydrogen bonding
14
E. Structures of Amines
Hydrogen bonding
F. Intermolecular Interactions
Know induced-dipole-induced-dipole interaction (van der Waals interaction)
Know hydrogen bond
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G. Comparison of Physical Properties
(i) Boiling point
Boiling points of alkanes increase with increasing molecule weight.
(ii) Solubility in water
The presence of hydrogen bond or polar groups
H. Self-assessment Questions
 Can you recognize molecules that have interactions through induced-dipoleinduced-dipole (van der Waals), dipole-dipole and hydrogen bond?
 Can you compare the b.p. and solubility of molecules based on their structures?
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3.5 Conformation of Alkanes and Cycloalkanes
A. Constitutional Isomers Have Different Physical Properties
B. Conformation: result of single bond rotation
Conformational isomer: same atomic connectivity but different spatial arrangements of
atoms.
H H
H
H
I
I
H H
H
H
C
II
H
C. Newman Projection
(i) ethane
H
H
HH
H
H
H
H
H
H
H
H
II
I
Staggered conformation
Eclipsed conformation
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H
C
H
II
C
H
C
(ii) butane
CH3
H
H
H
H
H3C
CH3
HCH3
H
CH3
H
H
H
H
H
H
III
II
I
CH3
H3CCH3
H
H
H
H
IV
Steric strain (steric hindrance)
If atoms within a molecule are brought too close together, there is an associated cost in
energy due to overlapping electron clouds
Anti-relationship
18
D. Conformers of Cycloalkanes
(i) Ring strain and stability
19
(ii) Conformation of cyclohexane
Know how to draw chair conformation of cyclohexane.
Know how to show axial and equatorial positions of cyclohexane.
twisted
Chair
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
H
H
H
H
H
H
H
H
H
H
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Equatorial position
Axial position
E. Conformers of monosubstituted cyclohexanes
H
H
H
H
C
H
H
H
H
H
C
H
H
H
H
H
H
H
1,3-diaxial interaction (diaxial interaction, axial-axial interaction)
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F. Cis and trans Isomers of Disubstituted Cycloalkanes
Know cis - trans isomerism in cycloalkanes
 Cis and trans isomers are stereoisomers.
 Stereoisomers: same molecular formula and atom connectivity but different in the
three-dimensional orientations of atoms in space
(i) cyclopentane
H
H
H
H
CH3
H
H
H
H3C
H
H
H
H
H
CH3
H3C
H
H
H
H
(ii) cyclohexane
CH3
H
H
H3C
H
H3C
H
H
H3C
H
H
H
ring flip
ring flip
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(iii) More examples:
H3C
CH3
Br
Cl
ring flip
H3C
ring flip
CH3
C(CH3)3
Br
ring flip
ring flip
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G. Fused Cyclohexane Rings
trans-fused rings
cis-fused rings
H. Self-assessment Questions
 Can you explain why cyclopropane has strong ring strain?
 Can you draw Newman projections of alkanes in staggered and eclipsed
conformations, and arrange the order of stability for these conformations
(conformational isomers)?
 Can you identify cis and trans isomers for disubstituted cycloalkanes?
 Can you draw chair conformation of cyclohexane with unambiguous
representation of axial and equatorial substituents?
 Can you complete the equilibrium of two chair conformational isomers for a
substituted cyclohexane, indicate the change for the relative positions of axial and
equatorial substituents, and reason the stability between these two isomers?
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Homework for Chapter 3 (optional)
Name:
A number:
25