10.2 Functional Group Chemistry

Syllabus

What does this mean?

Alkanes

The lack of a functional group means that alkanes are unreactive.

But burning them is the basis of the world economy.

  • Be able to balance equations for the complete combustion of any Alkane (remember the IB doesn’t usually like you to use ½ in balancing even though it uses them itself occasionally).

C3H8 + O2 -->

C5H12 + O2 -->

C2H6 + O2 -->

C8H18 + O2 -->

  • You may be asked to balance an equation for incomplete combustion. This may make Carbon Monoxide, Carbon and Carbon Dioxide as well as water.

Balance an equation for the incomplete combustion of

C3H8 + O2 -->

C2H6 + O2 -->

  • List the disadvantages of incomplete combustion:

Curly Arrows.

Curly arrows are important in Organic Chemistry.

They show us how the reaction happens – reaction mechanisms.

A full curly arrow is a movement of a pair of electrons. This is what you’ll see in most reaction mechanisms

Curly half arrows indicate a movement of single electrons. You’ll only see this in free-radical reactions.

Bond Breaking

A full curly arrow can show a bond pair moving & the bond breaking.

In the case below one Br atom loses its share of both electrons (becoming +) and the other gains complete control of the pair (becoming -).

This is heterolytic fission: –

Fission = bond-breaking Hetero = different outcome for both atoms.

Half curly arrows can show a bond pair moving & the bond breaking too.

In the case above both Br atoms gain one of the electrons in the bond pair.

This is homolytic fission: – Homo = same outcome for both atoms.

Halogenating Alkanes

If you mix alkanes and halogens together in the dark nothing happens.

Initiation

It takes Ultraviolet light to make the reaction start by causing homolytic fission of a Chlorine or Bromine molecule.

The two Chlorine atoms made are called radicals (because they react with almost anything). They’re usually represented with a single dot to represent the unpaired electron

Propagation

In the first propagation stage the halogen radical collides with a reacts with the alkane molecule.

As the single electron on the Cl radical pairs with one of the electrons in the bond pair an H-Cl molecule is made.

But the CH3 left behind now has a single electron – it is a methyl radical.

The methyl radical may then collide with a full Chlorine molecule, forming Chloromethane and re-forming a chlorine molecule.

Termination

Termination means to end.

The only way for radicals to stop reacting is if the unpaired electrons of two radicals pair-up.

There are 3 ways this can happen

But, even then the Chloromethane made may be further substituted, so radical substitution is a poor, uncontrollable way to make anything

QUESTION.

1. Write the initiation, two propagation and three termination steps for the reaction between ethane and bromine.

Halogenation of alkenes.

Unlike Alkanes, Alkenes have a functional group (C=C). So the reaction happens there.

When a Bromine molecule nears an Alkene the electron density in the C=C double bond repels electrons around the nearest Bromine atom. This induces a dipole

And the ∂+ Carbon then attracts the bond pair, breaking one of the bonds in the C=C, and forming a C-Br bond

At the same time, the Br-Br bond breaks, forming a Br- ion.

One of the lone pairs on the Br- ion is then attracted to the carbocation, forming a dibromo-compound. The Bromine’s orange colour has been destroyed.

So adding Bromine to Alkenes is the test for unsaturation.

Question.

1. Why is this reaction referred to as an ADDITION reaction?

2. Why is this reaction referred to as an ELECTROPHILIC addition reaction?

3. Why is the intermediate referred to as a Carbocation?

4. Draw a mechanism for the formation of a di-chloro compound from the reaction between chlorine and But-2-ene. What is the name of the final compound?

Hydrogenation of alkenes

Hydrogen can be added to alkenes. But unlike halogens it requires specific conditions.

  • LEARN - Conditions for Hydrogenation of Alkenes -150oC, Nickel catalyst

QUESTION

Complete the equation for the hydrogenations of i) Propene ii) cyclopentene

i)

ii)

Polymerisation

Polymers are long molecules made up of repeating units.

Addition polymerisation joins small alkene molecules (monomers) together.

Question

Look at the example below and use it as a model to complete the following equations for addition polymerisation.

Alcohols.

Alcohols are much more reactive than alkanes but are also used as fuels.

  • Complete the equations for the complete combustion of....

1. C2H5OH(l) + O2(g) --> CO2(g) + H2O(g)

2. C4H9OH(l) + O2(g) --> CO2(g) + H2O(g)

3. C6H13OH(l) + O2(g) --> CO2(g) + H2O(g)

Oxidation of Alcohols

  • LEARN: The most common Oxidising Agent for IB is Acidified Potassium Dichromate (if you miss acidified you may be marked incorrect).

  • LEARN: That dichromate is orange and turns green.

  • LEARN: The next most common Oxidising Agent for IB is Acidified Potassium Manganate (VII) (or Potassium permanaganate).

  • LEARN: That Manganate (VII) is Purple and turns pink.

Note: After Topic 9 you’ll be able to write complete redox equations for how these oxidising agents work but for now (and most other times) we will use [O} to represent any oxidising agent in an equation.


Tertiary Alcohols

Cannot be oxidised – if an exam question asks you to distinguish between different alcohols this fact is important

Secondary Alcohols

Are oxidised to Ketones.

The best way to do this reaction is to reflux the reaction mixture.

Heating makes the reaction faster but might evaporate the ketone product, which has a low boiling point.

Setting up the condenser as a reflux stops this and also condenses any alcohol that might evaporate, giving it another chance to be oxidised.

We would usually heat the mixture in a water bath.

QUESTIONS.

1. Why use a water bath?

2. Draw the displayed formula of the product you would expect from the following alcohols. Name both the alcohol provided and the ketone you draw.

Oxidation Equations.

This equation is “balanced” – the [O] has taken 2 Hydrogen atoms from the propan-2-ol to form water.

Question

Write balanced equations, using [O] to represent an oxidising agent, for the oxidation of:

1)

2)

3)

Primary Alcohols


Are oxidised to Aldehydes...

....and then further oxidised to Carboxylic Acids.

If we wish to make the carboxylic acid we reflux – giving the reactants multiple opportunities to react together.

But if we wish to stop the oxidation at the aldehyde stage we must set up the apparatus to distil.

This works because aldehydes have much lower boiling points than alcohols or acids

We also have to chill the product.

QUESTION


1. Why should an Aldehyde have a lower boiling point than the alcohol it is made from? (HINT: think about their bonding and structure)

2. Why chill the product?

The equations.


We can think of the oxidation of ethanol happening in 2 stages.

Or all in one go.

Question

Write equations, using [O], for the oxidation of:

1) Propan-1-ol to Propanoic acid

2) Butan-1-ol to Butanal

3) Pentan-1-ol to Pentanoic acid

4) Pentane-1,5-diol

Esterification

Esterification is a reaction that makes an ester.

In general: Carboxylic Acid + Alcohol --> Water + Ester

Because water is also made we call this a condensation reaction (although some condensation reactions make small molecules that aren’t actually water).

This reaction is very, very slow without the presence of a concentrated acid catalyst

In the diagram below you can see how the water forms. The name of the ester starts with the prefix from the alcohol and ends with the prefix of the acid

So,

Methanol + Propanoic acid --> Methyl Propanoate + water

Ethanol + Butanoic acid --> Ethyl Butanoate + water

Pentanoic acid + Hexan-1-ol --> Hexyl Pentanoate + water etc

Questions

1. Complete the table

2. Draw the reactants and products for the following combinations of reactants. Name the product.

a) Methanol & pentanoic acid

b) Propan-1-ol & ethanoic acid

c) Propan-2-ol & methanoic acid

Nucleophilic Substitution.

Substitution means to replace one thing with another.

So any reaction in which an atom/ group in a molecule is replaced must be a substitution reaction.

We already looked at radical substitution.

Nucleophilic substitution doesn’t use radicals – it uses nucleophiles, obviously.

The word nucleophile menas “positive-loving” – but if they ask for a definition “a nucleophile is an electron pair donor” – anything with a lone pair will be attracted to a ∂+ Carbon atom on another molecule.

There are two ways this can happen shown below (you won’t need to learn teh mechanisms unless studying Higher Level)