College of Biotechnology
Organic Chemistry
Lecture 2
Dr. M. Abd-Elhakeem

Physical properties of organic compounds are very important to describe or even to identify the unknown compound.
Physical properties include melting, boiling points, solubility, color and optical rotation.

Solubility is defined as the maximum mass of a substance that can be dissolved in a fixed mass of a solvent at a given temperature.
A substance will have a different solubility in different solvents depending on polarity of both the substance and the solvent.
remember
” like dissolves like
”

Most of organic molecules are nonpolar and so usually soluble in organic solvents
(e.g.
diethyl ether, dichloromethane, chloroform, petroleum ether, hexanes etc)
H
3
CH
2
C O CH
2
CH
3 diethyl ether
H
Cl C H
Cl dichloromethane
CH
3
CH
2
CH
2
CH
2
CH
2
CH
3 n-hexane

However, some organic molecules are also soluble in water . This denotes a rather high ratio of polar group(s) to carbon chain, i.e., a low molecular weight compound containing an –OH, -NH
2
, or CO
2
H group,
(polar groups) that give to the organic molecule some polarity.

The solubility of an organic compound can provide evidence for the presence (or lack) of several important functional groups, as indicated in the following chart

Solvent water
5% NaHCO
3
5% NaOH
5% HCl
Some solubility or complete miscibility
Alcohols, amines, acids, ketones, aldehydes (typically only those with
C < 4)
Carboxylic acids (high acidic properties)
Carboxylic acids and phenols (weak acidic properties)
Amines

1. Melting point is the temperature range over which the solid melts to become a liquid
2. While boiling point is the temperature at which boiling occurs under a pressure of
1 bar

These properties affected by the forces that attract one molecule to another: ionic attraction, hydrogen bonding, van der Waals force and molecular weight.
OH
H
3
C C
H
H
2
C propylenglycol b.p. 188.2
o
C
OH H
3
C
H
2
C
H
2
C propanol b.p. 97.1
o
C
OH

In absence of hydrogen bonding the boiling point is mostly directly proportion with the molecular weight.
Note: Branching decrease the boiling point, where it causes decreasing of
Van der Wales force.

Also Melting and boiling point used to assess the purity of the organic compound
Pure crystalline solids have a sharp melting point whereas mixtures melt with a broad temperature range
Example: aspirin is synthesized by reacting salicylic acid & acetic acid anhydride. How can we ensure that aspirin has been yielded from the reaction?

By measuring of melting point the product
& the starting material we can sure that the product has already obtained.
Aspirin (m.p. 141-144 o C) but salicylic acid
(m.p. 158.5-161 o C)
O
C OH
+
OH salycilic acid m.p. 158.5-161 o
C
O
HO C
O
O C OH
O
C OH
O
O
+ CH
3
C OH
O aspirin m.p. 141-144 o
C
C CH
3

Measuring Melting Point
•
Using electrothermal Meltpoint tester

The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others (Complementary color).
Color of the molecule arise from a certain part called chromophore
In absence of chromophore the compound has no color and may be seen under UV light.

A chemical group capable of selective light absorption resulting in the coloration of certain organic compounds. Chromophores are generally groups of atoms having double bond.
For example;
C=N, C=S, NO
2
, N=N,

Conjugation is a sequence of alternating double and single bond along the molecule.
Previously we have considered conjugation in small molecules such as benzene.
H
H
H
H
H
H

Example: vitamin A
H
3
C CH
3
CH
3
CH
3
OH
CH
3
Vitamin A has a conjugated system that spreads over five carbon-to-carbon double bonds.
It appears yellow.

2. β-carotene
H
3
C CH
3
CH
3
CH
3
H
3
C
CH
3
β-carotene is found in carrots
CH
3
CH
3
H
3
C CH
3
It has a conjugated system that spreads over eleven carbon-to-carbon double bonds.
It appears orange.

3. Lycopene
Lycopene is found in watermelon, pink grapefruit and tomatoes.
It has a conjugated system that spreads over eleven carbon-to-carbon double bonds.
It appears red.

How can we explain these different colours?
The colours we observe are not absorbed by the molecule. If the chromophore absorbs light of one color, then the complementary colour is observed.
Colored compounds arise because visible light is absorbed by the electrons in the chromophore, which are then promoted to a higher energy molecular orbital.
By comparing chromophores, we can find out about the energy of light that is being absorbed.

As molecules with greater conjugation absorb lower energy light. Similarly, less conjugation results in compounds appearing yellow.
Compound
Vitamin A
Number of C=C in conjugated system
5
Main colour absorbed
Violet
Colour compound appears
Yellow
β-carotene 11 Blue Orange
Lycopene 11 Green Red

Since violet light has higher energy than blue or green, when it is absorbed we observe the yellow light that is transmitted.
Compound
Vitamin A
Number of C=C in conjugated system
5
Main colour absorbed
Violet
Colour compound appears
Yellow
β-carotene 11 Blue Orange
Lycopene 11 Green Red

An auxochrome is a functional group of atoms attached to the chromophore which modifies the ability of the chromophore to absorb light, altering the wavelength or intensity of the absorption.

The color of the molecule that absorb lower energy light shift to the red