CHEM 191

MODULE 8

ORGANIC CHEMISTRY 2

Learning Objectives.

By the end of this module you should be able to:

• Identify the selected functional groups in organic molecules

• Write names and structural formulae for simple alcohols, haloalkanes, amines and carboxylic acids, and esters including constitutional isomers

• Identify substitution, elimination, oxidation and acid-base reactions occurring in reactions of the selected functional group

• Predict products of organic reactions and write equations for these using structural formulae

Reference: ESA Chapters 14, 15 and 16

INTRODUCTION

Most organic compounds contain at least one other element apart from carbon and hydrogen. These elements are usually non-metals, the common ones being halogens, oxygen, nitrogen, sulfur and phosphorus. The hydrocarbon part of these organic molecules is relatively inert and the other elements form the centre of interest for chemical reactions. The hydrocarbon part of the molecule is called a radical and can be represented by the symbol R-. It is essentially the hydrocarbon part of the molecule with one of the hydrogen atoms removed. The reactive portion of the molecule is termed the functional group.

Many biological molecules have large, relatively inert, carbon skeletons, often containing complicated ring systems. The reactive sites in these molecules usually contain a variety of functional groups which influence the reactivity of the molecules in biological systems. This can be seen in the structures of the molecules of some familiar compounds given below. The first three molecules have similar carbon frameworks - they belong to a class of compounds known as steroids. However, the first is the male hormone testosterone, the second the female hormone progesterone and the third is   norethynodrel, which has been used in contraceptive pills.

Some other interesting molecules include:

NOTE: that there seems to be a lot of 'memory' work in organic chemistry. However, the most important skills are being able to identify and use the patterns in structure and reactivity and the rules for naming organic molecules and for writing structures (isomers), including being able to determine the reactivity of a molecule from its functional groups. The classification of organic reactions on page 7.13 will be used throughout this module.

FUNCTIONAL GROUPS

A functional group is an atom or a specific group of atoms in an organic molecule that is responsible for the reactivity of the molecule. For example, in alkenes, the functional group is the double bond, in alkynes it is the triple bond. All the other functional groups contain atoms other than carbon such as O, S, N and halogens.

Table 8.1                          Organic Functional Groups

Note: R, R′ and R′′ are used to show that there can be different hydrocarbon chains in the molecule

Exercise 8.1

Identify the functional groups in the following molecules. Note that in these diagrams the carbon skeleton has been reduced to a line form like that used for cycloalkanes.

HALOALKANES

Haloalkanes are fluoride, chloride, bromide or iodide substituted hydrocarbons. The symbol X is often used to represent the halide, so that R⎯X is a general formula for an organic halide.  Haloalkanes are  made by substituting alkanes with the halogens or, more easily, by the addition of HX to the double bond in an alkene:

These molecules have found many uses in the past but the environmental impact of their use has been such that, in many cases, they are now being phased out. Freons (CFCs) e.g. CCl2F2, were once widely used as refrigerants, but the escape of these molecules into the atmosphere is a contributor to the partial destruction of the ozone layer in the stratosphere which, in turn, results in more ultraviolet light reaching the surface of the earth.

The haloalkane molecule commonly known as DDT has the systematic name 1,1-di(4-chlorophenyl)-2,2,2-trichoroethane.

DDT was widely and very successfully used as an insecticide from the 1940s to the 1960s and saved millions of human lives from death via malaria. However, it was not easily decomposed in the environment and found its way into the food chain of higher animals. One of its effects was to cause the thinning of the shells of birds’ eggs and so its use has been banned.

Naming Haloalkanes

Haloalkanes are named the same way as alkanes with the halogen included as a prefix similar to the branched chains. The position of the halogen must be indicated (unless its position is unambiguous) When there is more than one halogen present they are listed alphabetically.

Examples:

Reactions of Haloalkanes:

Substitution: It is possible to replace the halogen atom with either an –OH or an –NH2  functional group given the right conditions. These are examples of substitution reactions.

Note: When writing the equations for organic reactions it is sometimes more convenient to write only the structures of the reactants and products with the reaction conditions written over the arrow.

Examples:

1. Substitution with -OH using KOH(aq): When an aqueous solution of potassium hydroxide, KOH(aq), is reacted with haloalkanes the halogen is swapped for –OH and an alcohol is formed.

For example: chloroethane reacts with KOH(aq) to form. ethanol

The equation written below uses the expanded structure and so makes the change occurring more obvious.

2. Substitution with -NH2 using NH3(alc): When aconcentrated solution of ammonia in

ethanol, NH3(alc), is added to a haloalkane (under high pressure) the halogen atom is replaced by the  -NH2  group and an amine is formed.

For example: chloroethane reacts with NH3(alc) to form ethanamine.

Elimination: Under the right reaction conditions it is possible to remove the halogen atom and the hydrogen atom from a carbon atom adjacent to that to which the halogen is attached. This results in the formation of a double bond. The reagent used is a concentrated solution of potassium hydroxide in alcohol (ethanol), KOH(alc). Since atoms are removed and not replaced this is an example of an elimination reaction.

Example:

Exercise 8.2: Haloalkanes

(a) Draw structural formulae for the following molecules

(i)   1-bromo-2-methylpropane

(ii)   3-chlorocyclohexene

(iii)   2,2-dichloro- 4-methylhexane

(iv) cis – 4-iodoopent-2-ene

(b)  Write names for the following molecules.

(c)   Write the structural formula for the products A to F in the following reaction schemes and classify the type of reaction occurring.