1

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 Nomenclature of Alkanes and Cycloalkanes

3.2 Nomenclature and Classification of alkyl halides

3.3 Alcohols, Amines and Ethers

3.4 Structures and Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and

Amines

3.5 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-

Propane CH3CH2CH3 Propyl

CH3CH2CH2-, n-

propyl

(CH3)2CH-, isopropyl

2

Butane CH3(CH2)2CH3 Butyl

CH3CH2CH2CH2-, n-

butyl

(CH3)2CHCH2-,

isobutyl

CH3CH2CH(CH3)-, s-

butyl

(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

Undecane CH3(CH2)9CH3 -

Dodecane CH3(CH2)10CH3 -

B. Skeletal Structures

Know the meaning of dashed and wedged bonds

3

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)

4

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

5

(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)

C

H

C

H

H

H

HH C

H

C

H

H

H

CH3CH2CH2CH3

6

2-Methylpropane or isobutane (iso-butane or i-butane)

C

H

H

H

H C

H

C

H

C

H

CH3CH(CH3)CH3

H H

H

(ii) Pentane (C5H12)

CH3CH2CH2CH2CH3 CH3CH(CH3)CH2CH3CH3C(CH3)2CH3

E. The Nomenclature of Cycloalkanes

General formula: CnH2n

(i)

7

(ii)

(iii)

(iv)

(v)

(vi)

8

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?

9

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:

10

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

11

B. Classification of Amines

12

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?

13

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

14

C. Structures of Ethers

Dipole–dipole interaction

D. Structures of Alcohols

Hydrogen bonding

15

E. Structures of Amines

Hydrogen bonding

F. Intermolecular Interactions

Know induced-dipole-induced-dipole interaction (van der Waals interaction)

Know hydrogen bond

16

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-dipole-

induced-dipole (van der Waals), dipole-dipole and hydrogen bond?

Can you compare the b.p. and solubility of molecules based on their structures?

17

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

CC H

H

H

H

H

H C

C

HH

H

H H

I II

C. Newman Projection

(i) ethane

H

H

H

H

HH

H

HH

H

H

H

I II

Staggered conformation

Eclipsed conformation

I

II

18

(ii) butane

H

H3C

H

CH3

HH

CH3

HH

H

H

CH3

CH3

HH

H3C

H

H

CH3

HH

H

H

CH3

I II III 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

19

D. Conformers of Cycloalkanes

(i) Ring strain and stability

20

(ii) Conformation of cyclohexane

Know how to draw chair conformation of cyclohexane.

Know how to show axial and equatorial positions of cyclohexane.

twisted Chair

HH

H

H

H

H

H

H

H

H

H

H

HH

H

H

H

H

H

H

H

H

H

H

H

HH

H

H

H

H

H

H

H

H

H

21

Equatorial position

Axial position

E. Conformers of monosubstituted cyclohexanes

H

HH

H

H

C

H

H

H

C

H

HHHH

H

H

H

1,3-diaxial interaction (diaxial interaction, axial-axial interaction)

22

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

CH3

H

H

H3CH

H

HH

HH

H

CH3

H

H3CH

H

HH

HH

(ii) cyclohexane

H

HH

H

H3C

H3C

H

HH

HCH3

H3C

ring flip

ring flip

23

(iii) More examples:

ClH3C CH3Br

ring flip

ring flip

C(CH3)3H3C CH3

Br

ring flip

ring flip

24

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?

25

Homework for Chapter 3 (optional)

Name: A number: