15.3 Reactions of alcohols

Question 1

How do alcohols burn in enough oxygen

Answer 1

They completely burn to produce carbon dioxide and water

If it is incomplete combustion, carbon monoxide and carbon may be produced

Question 2

Give equation for complete combustion of ethanol

Answer 2

C2H5OH + 3O2 —> 2CO2 + 3H2O

Question 3

What is an elimination reaction

Answer 3

. A small molecule leaves the parent molecule

Question 4

Describe the elimination reaction in an alcohol

What is it called

Answer 4

. The small molecule leaving is water, which is made from the OH group, and a hydrogen on the carbon next to the carbon with the OH group

Called dehydration

Question 5

How does dehydration occur in alcohols
What are alcohols reacted with for it to react

What is the product

Answer 5

• Excess hot concentrated sulfuric acid
  or - Pass the alcohol vapours over heated aluminium oxide

Product is an alkene

Question 6

What is the overall reaction mechanism for dehydration of alcohols to produce alkenes and water

Eg in propan-1-ol

What is the specific mechanism for each stage of the reaction

Answer 6

. An OH on one carbon, and a hydrogen on an adjacent carbon will react together, so draw circle around them, and curved arrow pointing to a H2O which shows that they produce that

. Then draw a straight arrow to show the product, propene and then a H2O

The Stages:
. The OH on the propan-1-ol: the O has a lone pair which are attracted to H+ ions from the acid, so a curled arrow will come off it and point to a H+ ion, as a covalent bond is formed
. So this H joins the molecule by the oxygen, so the overall molecule has an O connected to two H at the end of it, so the O has a + charge
. So the bond between the oxygen and the carbon breaks as the lone pair on carbon goes to the oxygen with a curled arrow
. Then a hydrogen from an adjacent carbon sends the lone pair to the C-C bond making it a double bond

So overall propene is formed and a H+ and H2O

Question 7

What solution is used to oxidise alcohols

What colour is it

Answer 7

A solution of potassium dichromate, K2Cr2O7, is acidified with H2SO4 is often used to oxidise alcohols

The orange dichromate (IV) ions are reduced to green chromium (III) ions

Question 8

Describe process of oxidation of primary alcohols

Eg ethanol, so the OH is on the end of it

What is the equation

Answer 8

It is oxidised to make ethanal, and water is released
We lose a hydrogen from the carbon and the hydroxyl group (the OH group in alcohols) to make water
And then when it’s oxidised, an oxygen binds to the carbon in a double bond C=O.
This is now an aldehyde

Then, further oxidation occurs which turns the aldehyde into a carboxylic acid.
The carbon that is currently bonded to the O and an H: The oxidation adds an oxygen molecule to the H so it is now OH
So the carbon is bonded to an OH and =O

The equation is ethanol with an arrow pointing to ethanal and then an arrow pointing to ethanoic acid.
over each arrow is an [O] to show it is getting oxidised

Question 9

How do you end up with a double C=C bond when oxidising alcohols eg in ethene

Answer 9

Before, we have one bond of carbon to oxygen, but in the mechanism we lose a bond of carbon to hydrogen, and we’re going to gain another bond of carbon to oxygen

Because oxygen is being added

So a double C=O bond will form

Question 10

What would the oxidation state of the C connected to OH in the alcohol be before the oxidation

What about after the reaction when it is now an aldehyde

What about after further oxidation when it is a carboxylic acid

Answer 10

Before the reaction, the carbon is bonded to the OH group, but as oxygen is more electronegative than carbon, the oxygen would take both electrons in the bond.
The carbon is also bonded to two hydrogens, and in both bonds, carbon is more electronegative so would take both pairs of electrons.
The carbon bonded to another carbon will have an even split of the pair of electrons so one electron would go to each carbon.

This means that the carbon has 5 electrons now, despite it only usually having 4 valence electrons.
So oxidation state is 1-

After, when it is now an aldehyde,
In the double C=O bond, oxygen is more electronegative so will attract the two pairs of electrons away from the carbon.
Also, in the C-C bond they are equally electronegative so the electrons will be split evenly so one will go to the C.
In the C-H bond the C is more electronegative so will take the pair of electrons from the H.
So there are 3 valence electrons on the carbon when it usually has 4 so it has oxidation state +1

Carbon is bonded to the double oxygen, but oxygen is more electronegative so those two pairs
of electrons go to the oxygen
Carbon is bonded to another carbon so the electrons split equally and one goes to each carbon.
Carbon is bonded to oxygen again so pair of electrons go to oxygen.

This means there is one electron on the valence shell, compared to 4 usually so it has oxidation state +3.

As you can see, each time the molecule is oxidised, the oxidation state increases

Question 11

Describe oxidation of alcohols:

What are primary alcohols oxidised into, what can they be further oxidised into
What are secondary alcohols oxidised into, what can they be further oxidised into

What about tertiary alcohols

Answer 11

Primary alcohols are combusted into aldehydes , which can then be further combusted into carboxylic acids

Secondary alcohols can be oxidised into ketones , but can’t be further oxidised

Tertiary alcohols are not easily oxidised

Question 12

Why can primary alcohols be oxidised further but not secondary

Answer 12

• In order for it to be further oxidised, there has to be a carbon bonded to a H and the =O
• So in secondary alcohols, when they are first oxidised to ketones, the carbon is bonded to =O, and also two other carbons, so there are no hydrogens bonded to be oxidised

Question 13

Describe oxidation of secondary alcohols

Eg in propan-2-ol
Give equation

Answer 13

. Oxidised to ketones
. So eg in propan-2-ol, the middle carbon is bonded to an OH, an H and two other carbons
. So when it is oxidised, a double carbon oxygen bond will form, so that carbon will only be bonded to the oxygen, and to two other carbons

The reaction is
Propan-2-ol arrow to propanone with an [O] over the arrow to show it has been oxidised

Question 14

Why aren’t tertiary alcohols oxidised

Answer 14

Their oxidation would require a C-C bond to break, rather than a C-H bond like with primary and secondary alcohols

As there is no C-H bond to break

Question 15

Describe the conditions and reaction for oxidation of primary alcohol

Eg ethanol to ethanol

NOT TO ETHANOIC ACID

Answer 15

. Potassium dichromate acidified by sulfuric acid, which is the oxidising agent
. If you don’t want it to oxidise completely to ethanoic acid, use dilute acid with slightly less potassium dichromate than is needed for complete oxidation

Ethanal has a lower boiling point than ethanoic acid
. Heat the mixture gently in distillation apparatus, with the receiver cooled in ice so the ethanal evaporates as soon as it is formed and can distill off.
This stops it getting oxidised further to ethanoic acid

The remaining mixture is unreacted ethanol

Question 16

What is the equation for the reaction of ethanol to ethanal

Answer 16

CH3CH2OH(l) + [O] —> CH3CHO(g) + H2O(l)

Question 17

What are the conditions and method for complete oxidation of ethanol to ethanoic acid

Answer 17

. Concentrated sulfuric acid will oxidise plenty of potassium dichromate
. Carry it out under reflux, which means the vapour condenses and can then drip back to reaction flask

. This means any ethanol or ethanal vapour will condense and drip back to the flask until it is all oxidised to the acid

. Continue this for 30 minutes then distill off the ethanoic acid along with any water

Question 18

What is the equation for the reaction of ethanol to ethanoic acid

How does it work when oxidising ethanal to ethanoic acid

Answer 18

CH3CH2OH(l) + 2[O] —> CH3COOH + H2O
So there are two mol of oxygen in this reaction so it can be oxidised twice

Ethanal already has a C=O bond, so oxidising it more means we have to have 3 bonds of carbon to oxygen and the only way this can happen is in a carboxylic acid

Question 19

What do we use to oxidise secondary alcohols to ketones

Answer 19

Acidified dichromate, so same as for primary

Question 20

What functional group do both aldehydes and ketones have

What is the difference between them

Answer 20

C=O Carbonyl group

In aldehydes it is at the end of the chain whilst in ketones it is in the body of the chain

Ketones are named with the suffix -al whilst ketones are named with the suffix -one

Question 21

Why can we test them to tell them apart

What are the names of the two tests we can do

Answer 21

. Aldehydes can be oxidised to carboxylic acids whilst ketones can’t be oxidised

So there are two different tests we can do to tell them apart

• Tollens reagent sIlver mirror test
• Fehlings solution

In both tests, the positive result will be the aldehyde, and if there is no change in reaction it is a ketone

Question 22

Describe the tollens silver mirror test

Answer 22

. It is a gentle oxidising agent
. Solution of silver nitrate in aqueous ammonia
. It oxidises aldehydes but has no effect on ketones
. It contains colourless Ag+ ions which can be reduced to silver when they oxidise the aldehyde

So warm aldehyde with the solution, and a silver mirror will form on the inside of the test tube

Question 23

Describe Fehling’s solution test for aldehydes

Answer 23

. It is also a gentle oxidising agent
. Contains blue copper (II) complex ions which will oxidise aldehydes but not ketones

So during oxidation those ions turn from blue to brick red as they are reduced by the aldehyde to Cu+ ions

Cu2+ + e- —-> Cu+

So brick red precipitate forms

Think of it as the same colours as benedict’s solution for sugars

Question 24

In a condensing tube, why does water enter at the bottom and exit at the top

Answer 24

To ensure the whole tube fills with water so it is cooler so more water vapour condenses