Unit 2.9 - Mechanisms in organic chemistry

Chemical reactions can be broken down and studied in great detail.

By very careful experimentation we can tell exactly how a reaction

happens; in other words we can know its mechanism, right down to

the movement of individual electrons

1. Classifying reactions

There are an almost infinite number of different organic compounds

and, therefore, an even greater number of different organic

reactions. It is vital for chemists to be able to put reactions into

categories:

a) Addition

An addition reaction is…

Example of an addition reaction:

b) Elimination

An elimination reaction is…

Example of an elimination reaction:

c) Condensation

A condensation reaction is…

Example of a condensation reaction:

d) Substitution

A substitution reaction is…

Examples of substitution reactions:

e) Oxidation and reduction

Oxidation is…

Oxidation levels in organic chemistry

Alcohol Aldehyde / ketone Carboxylic acid

Examples of oxidation reactions:

f) Polymerisation

Polymerisation is…

Examples of polymerisation reactions:

[O]

2. Classifying reagents

Reagents are, simply put, the chemicals involved in making chemical reactions happen. Just like the

reactions themselves there are literally millions of different organic reagents. It is helpful to be able to

put them into categories:

a) Nucleophiles

Are attracted to partially _______________ atoms (look for polar bonds)

Are electron ‘rich’ with a _____________ charge or _______ pair of electrons

Can donate a pair of electrons to form a new ______________ bond

Nucleophilc substitution:

The hydrolysis of halogenoalkanes is an example of how nucleophiles ‘attack’, as this example with

1-iodopropane shows:

CH3CH2CH2I(l) + KOH(aq) CH3CH2CH2OH(l) + KI(aq)

Mechanism for nucleophilic substitution

b) Electrophiles

Are attracted to electron ‘rich’ atoms or groups

Either have a ____________ charge or are electron deficient (‘poor’)

Can accept a pair of _______________ to form a new _____________ bond

Electrophilic addition

The addition of hydrogen halides (HF, ___, ___ and ___) to alkenes is an example of how

eletrophiles can ‘attack’, as shown in this example with propene:

CH3CHCH2 + HBr CH3CHBrCH3

Mechanism for electrophilic addition

3. Breaking bonds

A single covalent bond contains a _________ of electrons. Bonds

can be made and broken by the movement of electrons. The way in

which the electrons move in a reaction is known as the reaction

_____________. Bonds can break in two different ways:

a) Homolytic fission

In homolytic fission the pair of electrons are shared evenly creating two _____ ___________

X Y X + Y e.g. formation of chlorine radicals:

The unpaired electrons of free radicals make them very ______________

A + B e.g. formation of chloroethane:

b) Heterolytic fission

In heterolytic fission the pair of electrons goes to one side of the bond creating one _________

and one ____________ ion:

X Y X + Y e.g. dissociation of hydrogen chloride:

Remember - Double and single-headed curly arrows show electron movement

Heterolytic or homolytic?

Homo

In this course homolytic fission appears twice: Firstly in free radical substitution reactions of

alkanes (p118) and secondly in the problems with CFCs section (p214).

Both cases have one thing in common - Light

If the question mentions ‘sunlight’ or ‘UV light’ think homolytic fission

Light is the initiator for the reaction, it causes the free radicals to form in the first place

Cl2 + h Cl + Cl Initiation

Cl + CH4 CH3 + HCl Propagation

CH3 + CH3 C2H6 _____________

Hetero

Hetero lytic fission is luckily also quite easy to spot if you know what to look for: Bonds which

are polar (have a highly ______________ atom at one end and a less ________________

atom at the other) or polarised tend to undergo homolytic fission. For example:

4. The ozone layer - Free radical chemistry

The ozone Earth’s ozone

layer can be found between

10-50 km above the surface.

It actually consists of quite

low ozone concentrations.

The ozone is naturally

formed: O2 + h O + O

O2 + O O3

And destroyed in a series of

free radical reactions.

The ozone layer is vital to life on Earth. It protects us

by absorbing harmful ultraviolet radiation from the

Sun:

O3 + h O2 + O

O + O3 O3

And replenishes itself naturally as the oxygen _____

____________ produced when ozone breaks down

are very ___________ and will rapidly react with

another oxygen molecule to form ___________

Without this process UV-B radiation can damage

cells and can lead to ____________

CFCs and destruction of the ozone layer

CFCs are organic compounds containing carbon ______________ and _______________

They were widely used as _______________, ________________ and _______________

until it was discovered that they catalyse the destruction of stratospheric ozone

The use of CFCs was banned in 1987 by the Montreal protocol. Since then ozone levels have

begun to recover

A typical mechanism for destruction of ozone by CFCs is shown below:

1. CCl2F2 + h CClF2 + Cl _______________

2. Cl + O3 ClO + O2 _______________

3. ClO + O3 Cl + 2O2 _______________

NB The chlorine radical is not removed in this process - one molecule of CFC can lead to the

destruction of thousands of molecules of ozone

The mechanism for part 1 involves the ________________ fission of the C-Cl bond and is shown

below:

Changes in Antarctic ozone levels (p228)

Rememberh represents energy from UV radiation

As shown in the graph (above right), stratospheric ozone layers drop dramatically in spring and are at

their lowest in October. The reason for this is…

Polar stratospheric clouds