FHSC1124Organic Chemistry

Introduction to Organic Chemistry

Chapter 1

Chapter Scopes

• Introduction• Homologous series

• Functional groups• Naming/Nomenclature• Isomerisms

Objectives

After this chapter, you will:

• Learn and differentiate functional groups

• Learn to name organic compounds according to IUPAC nomenclature and draw correct structure

• Learn about types of isomerism

Introduction to Organic Chemistry

Introduction to Organic Chemistry

• Organic compounds are classified into different types, such as alkanes, alkenes, alcohols, amines and etc.

• Each type of organic compound contains the same reactive group of atoms, which is called functional group.

E.g. alcohols contain the –OH functional group

Functional GroupsFunctional Group Structure

Alkane

Alkene (Olefin) C=C (ethylenic bond)

Alkyne CC (acetylenic bond)

Alcohol (hydroxyl)

Arenes (aromatic hydrocarbon)

Ar (Aryl)

Aldehyde

R – H

O

R – C – H

R – O – H

Functional Group Structure

Ketone

Carboxylic Acid (carboxyl)

Ester

O

R – C – R’

O

R – C – OH

O

R – C – O – R’

Functional Group Structure

Anhydride

Amide

Amine

Nitrile

O O

R – C – O – C – R’

O

R – C – NH2

R – NH2

IUPAC Nomenclature

• IUPAC International Union of Pure & Applied Chemistry

• The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to name the organic compounds.

IUPAC Rules

1. Select the longest continuous C chain as parent chain (use root word for the no. of C)

2. Name each of the branch/substituents as an alkyl / aryl group

3. Number the C chain begin from the end nearest to the branch

branch/substituents appear at the lowest no. possible

4. Name each substituent according to its chemical identity & the no. of the C atom to which it is attached

For identical substituent, use the prefix di, tri… & write appropriate C no. for each substituent

5. Separate no. from no. by commas (,) & no. from letters by hyphens (-)

6. List the substituents alphabetically by name di, tri…. don’t count

Functional Group Nomenclature

Alkane End with -ane

Alkene End with –ene

Arene End with –benzene

Alcohol End with –ol

Aldehyde End with –al

Ketone End with –one

IUPAC Nomenclature

Functional Group Nomenclature

Carboxylic acid End with –oic acid

Ester End with –oat

Anhydride End with –oic anhydride

Amine End with –amine

Amide End with –amide

Nitrile End with –nitrile

Prefix No. of Carbons (n)

Prefix No. of Carbons (n)

Meth 1 Hex 6

Eth 2 Hept 7

Prop 3 Oct 8

But 4 Non 9

Pent 5 Dec 10

Base Names

Straight-Chain Alkyl Groups, R

Alkyl group Name (abbreviation)

CH3 Methyl (Me)

CH2CH3 Ethyl (Et)

CH2CH2CH3 Propyl (Pr)

CH2CH2CH2CH3 Butyl (Bu)

CH2CH2CH2CH2CH3 Pentyl• Alkyl groups are named by replacing the

–ane ending of the parent alkane with an –yl ending

Naming of Identical Branch Substituents

No. of substituents Prefix

2 Di

3 Tri

4 Tetra

Isomerism

Definition of isomerism:

• A phenomena where 2 or more compounds have the same molecular formulae but with different arrangements of their constituent atoms

• Such molecules are known as isomers

IsomerismIsomerism

Structural Isomerism

Stereoisomerism

Chain Isomerism

Position Isomerism

Functional Group Isomerism

Optical Isomerism

Geometric Isomerism

Structural Isomerism• Isomers have the same molecular formulae

but different structural formulae

1. Chain Isomerism• The isomers are from the same homologous

series & have the same functional groups but different type of carbon chain.

• Example: C4H10

2. Position Isomerism• The isomers are from the same

homologous series & have the same functional groups but the position / location of the functional group is different

• Same C skeleton

• Example: C3H8O

3. Functional Group Isomerism• The isomers are from different homologous

series & have different functional groups• The chemical & physical properties are

different

• Example: C3H6O

Geometric Isomerism (Cis-trans Isomerism)

• The atoms making up the isomers are joined up in the same order, but manage to have a different spatial arrangement.

• Due to restricted rotation of groups in double bonds & in cyclic compounds.

Example: Geometric Isomerism

C C

Cl

Cl

H

H

trans-1,2-dichloroethene

C C

Cl

H

Cl

H

cis-1,2-dichloroethene

• trans isomer 2 chlorine atoms are locked on opposite sides of the double bond (trans : Latin meaning "across“)

• cis isomer 2 chlorine atoms are locked on the same side of the double bond

(cis : Latin meaning "on this side")

The Effect of Geometric Isomerism on Physical Properties

1. cis isomer has higher boiling point.• Bp depends on the polarity of the molecules• cis isomers are > polar, stronger attractive

intermolecular forces exist between cis isomer

2. trans isomer has higher melting point.• Mp depends on the arrangement & packing

of molecules in the crystal lattice• trans isomer with > symmetrical structure,

can be > closely packed in the crystal lattice

?? Cis-isomer has higher bp

• e.g. 1,2-dichloroethene• 1 side will be more positive charge & the

other side more negative polar• Van der Waals + dipole-dipole interaction • Need extra energy bp increases

?? Trans-isomer has lower bp

• the slight charge on the top of the molecule is exactly balanced by an equivalent charge on the bottom

• No dipole-dipole force as it is non-polar molecule.

• Thus, only held by weak Van der Waals • Less energy needed lower bp

?? Trans-isomer has lower bp

• Trans-isomer• No dipole-dipole force as it is non-polar

molecule.• Thus, only held by weak Van der Waals • Less energy needed lower bp

?? Trans-isomer has higher mp

• trans isomer has the higher melting pointMelting point = solid solution

• In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid.

• Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.

?? Cis-isomer has lower mp

• The poorer packing in the cis isomers means that the intermolecular forces aren't as effective be

• Less energy is needed mp lower

Optical Isomerism• Optical isomers are 2 compounds with the

same structural formulae, but one isomer is the mirror image of the other & cannot be superimposed on one another in any orientation

• It occurs when 4 different groups of atoms are joined to a C atom by 4 single covalent bonds.

Optical Isomerism• Occurs because of the tetrahedral bonding

around a C atom• Structures that can exist as 2 optical

isomers are said to be optically active & possess a chiral centre

• Simple substances which show optical isomerism exist as two isomers known as enantiomers

• Chiral centre = atom bond with the 4 different groups, which is normally marked with an asterisk ()

Example: Optical Isomerism

Chiral centre

Example: Optical Isomerism

• It is important this time to draw the COOH group backwards in the mirror image.

Incorrect

Exercise For each of the following molecules, draw the possible stereoisomers and state the type of stereoisomers shown.

(a) C6H5CH=CHCOOH

(b) C6H5CH(Cl)CH=CH2

(c) CH3CH=CHCH3

(d) CH3CH2CH(NH2)COOH

Summary

use IUPAC nomenclature to name and draw correct structure of simple organic compounds with different functional groups.

Differentiate types of isomerism and draw isomers.