3.5 Alcohols

Question 1

What are some properties of alcohols?

Answer 1

They are a homologous series
Functional group OH
General formula: Cn H2n+1 OH
The prefix hydroxy is used if another functional group is present
V-shaped on the end
High boiling points

Question 2

Why do alcohols have high boiling points?

Answer 2

The have hydrogen bonding

Question 3

What are the types of alcohol?

Answer 3

The carbon with the OH functional group has

Primary - only 1 other carbon attached
Secondary - 2 other carbons attached
Tertiary - 3 other carbons attached

Question 4

What are some uses of alcohols?

Answer 4

Perfumes/ aftershave
Mouthwash

Used in manufacture of drugs, detergents and inks

Question 5

Does ethanol dissolve in water? Why?

Answer 5

Yes

They both have hydrogen bonding

Question 6

How can we produce alcohols?

Answer 6

Fermentation

Hydration of alkenes

Question 7

What is involved in fermentation of alcohols?

Answer 7

Yeast, sugar and water is heated between 30-40 degrees (37 optimum)

After several days the flask will contain around 15% ethanol = mixture of alcohol and water

As yeast is living this is a natural process - anaerobic respiration takes place

Question 8

What is the equation for fermentation?

Answer 8

Glucose ——> ethanol + CO2

Over the arrow: enzymes in yeast
Under the arrow: 30-40 degrees

Question 9

What is produced in fermentation?

Answer 9

A mixture of ethanol and water

The only product - alcoholic drinks

Question 10

Why is the temperature conditions as they are for fermentation?

Answer 10

Too low = prevents respiration of the yeast

Too high = denatures the enzymes

Question 11

What is used to acquire the pure alcohol from fermentation - in the lab?

Answer 11

Fractional distillation

Alcohol boils at 80 degrees
Water boils at 100 degrees

Question 12

What are some advantages and disadvantages for fermentation?

Answer 12

Renewable sugars used

Aqueous solution of ethanol - could be good if small quantities are needed

Batch process - labour intensive requiring people to change batches

Requires a lot of energy for fractional distillation

Question 13

How does hydration of alkenes take place?

Answer 13

Ethene is reacted with water(steam)
Using phosphoric acid as a catalyst - H3PO4

Requires high temperature and pressure
Done on a large industrial scale that the majority of alcohols are made from

Question 14

What is the mechanism for hydration of alkenes?

Answer 14

First arrow from the double bond of the alkene to the H+ ion

A carbocation is formed - second arrow from the lone pair of the oxygen in water to the positive carbon

The water is now attached to the chain with a + charge on the oxygen - the arrow goes from the middle of the O-H on to the oxygen

The alcohol and H+ ion are the final products

Question 15

What are some advantages and disadvantages for hydration?

Answer 15

Non renewable as the alkenes are from crude oil

Fast continuous process

Pure product - better for larger quantities

Question 16

Where did the H+ ion come from in hydration and dehydration? Significance?

Answer 16

The phosphoric acid

It takes part in the reaction but is still considered a catalyst as it is regenerated at the end

Question 17

What is carbon neutral?

Answer 17

No overall carbon dioxide emissions

Question 18

What is an example of carbon neutral?

Answer 18

Plants absorbing CO2 via photosynthesis

Fermented into ethanol (5-10%) used in biofuels in cars - as it burns nicely with less CO2 produced

Combustion - when the biofuel is used in the cars producing little CO2

The CO2 taken in balances that which is given out

Question 19

What is dehydration of alcohols?

Answer 19

Turns the alcohol back into an alkene by removing water from it

Question 20

How does dehydration take place?

Answer 20

Ceramic wool is soaked in an alcohol and heated with aluminium oxide granules

OR
Using phosphoric acid as a catalyst (not the aluminium oxide)

Question 21

What is the mechanism for dehydration?

Answer 21

First arrow from the lone pair on the oxygen in the alcohol to the H+ ion

On intermediate where the H is attached to the alcohol (like water attached) - first arrow from the middle of the C-O to the oxygen
Then from a the middle of a C-H bond to the middle of the C-C bond

Forming an alkene, water and a H+ ion

Question 22

What is the simplest oxidation of alcohols?

Answer 22

Combustion producing CO2 + H2O

Question 23

What is the usual oxidising agent? Written in equations as?

Answer 23

Acidified Potassium dichromate (VI)

• often acidified with dilute sulphuric acid

[O] - you use this as oxygen when balancing

Question 24

What is the colour change in oxidation of alcohols?

Answer 24

Orange to green

Dichromate (VI) ions are orange
Chromate (III) ions are green

Question 25

How do we define oxidation in organic chemistry?

Answer 25

Gaining oxygen or losing hydrogen

Question 26

How do we define reduction in organic chemistry?

Answer 26

Losing oxygen or gaining hydrogen

Question 27

What does a primary alcohol oxidise to? Using?

Answer 27

A primary alcohol —> aldehyde

Using simple distillation
Because we don’t want it to oxidise further to a carboxylic acid

It works well as aldehyde’s have low boiling points = 20.2 (this is due to no hydrogen bonding)

Question 28

What is the second stage of oxidising primary alcohols? Using?

Answer 28

Alcohol —> carboxylic acid

Using reflux
So the aldehyde can’t escape and will turn into a carboxylic acid

Question 29

What does a secondary alcohol oxidise to? Using?

Answer 29

Alcohol —> ketone

Using reflux and acidified Potassium dichromate

Question 30

What does a tertiary alcohol oxidise to? Why?

Answer 30

No reaction takes place

Can’t oxidise under these conditions

Question 31

What other oxidising agent could be used?

Answer 31

Acidified Potassium manganate (VII)

Question 32

What are the potential tests for aldehydes?

Answer 32

Tollen’s test

Fehling’s test

Question 33

What is Tollen’s test?

Answer 33

Warm the aldehyde with Tollen’s reagent (silver nitrate solution)

A silver mirror will form on the inside of the tube

If a ketone is tested there will be no visible change

Question 34

What is Fehling’s test?

Answer 34

Warm the aldehyde with Fehling’s solution and a brick red precipitate forms

If a ketone is tested there will be no visible change - remain deep blue

Question 35

What does Fehling’s solution contain?

Answer 35

A copper complex that is a oxidising agent

Made from Fehling’s A and B
Also called Benedict’s reagent - copper sulfate

Question 36

What is significant about Tollen’s and Fehling’s reagent?

Answer 36

The will not oxidise an alcohol to an aldehyde or ketone as they are not powerful enough oxidising agents

They only oxidise aldehydes

Question 37

What are the colours and changes of Tollen’s and Fehling’s for aldehydes?

Answer 37

Tollen’s - clear to silver mirror

Fehling’s - deep blue to brick red

Question 38

What both contain the carbonyl group?

Answer 38

Aldehydes
Ketones

Ie

C=O

Question 39

What are the properties of the carbonyl compound in aldehydes?

Answer 39

Polar bond
Always at the end of the carbon chain
No numbers are needed to locate the functional group

FG written as: RCHO

Question 40

What are the properties of the carbonyl compound in ketones?

Answer 40

In the body of the chain
Locating numbers are needed when 5 carbons are reached
Propanone = simplest ketone

FG written as: RCOR