[3.3.5] Alcohols

Question 1

How do you name alcohols?

Answer 1

1. These have the ending -ol and if necessary the position number for the OH group is added between the name stem and the -ol.
   e.g. butan-2-ol.
2. If the compound has an -OH group in addition to other functional groups that have a higher priority for taking the suffix ending (carboxylic acid, aldehyde, ketone) then the OH is named with the prefix hydroxy-.
   
   • 2-hydroxypropanoic acid.
3. If there are two or more -OH groups then di, tri are used. Add the ‘e’ onto the stem name though.
   
   • e.g. ethane-1,2-diol & propane-1,2,3-triol.

Question 2

Name these alcohols.

Answer 2

Question 3

Describe the bond angles in alcohols.

Answer 3

• All the H-C-H bonds and C-C-O are 109.5° (tetrahedral shape) because there are 4 bonding pairs of electrons repelling to a position of minimum repulsion.
• The H-O-C bond is 104.5° (bent line shape) because there 2 bonding pairs of electrons and 2 lone pairs repelling to a position of minimum repulsion.
  
  • Lone pairs repel more than bonding pairs so the bond angle is reduced.

Question 4

Describe the boiling points of alcohols.

Answer 4

• Alcohols have relatively low volatility and high boiling points due to their ability to form hydrogen bonds between alcohol molecules.

Question 5

Why can alcohol dissolve in water?

Answer 5

Smaller alcohols can dissolve in water because they can form hydrogen bonds to water molecules.

Question 6

Describe and explain the different types of alcohol structures.

Answer 6

PRIMARY

• Alcohols where 1 carbon is attached to the carbon adjoining the oxygen.

SECONDARY

• Alcohols where 2 carbons are attached to the carbon adjoining the oxygen.

TERTIARY

• Alcohols where 3 carbons are attached to the carbon adjoining the oxygen.

Question 7

Does methylpropan-2-ol have a primary, tertiary or secondary structure?

Answer 7

• Tertiary.
• There are 3 carbons attached to the carbon adjoining the oxygen.

Question 8

Does propan-1-ol have a primary, tertiary or secondary structure?

Answer 8

• Primary.
• There is 1 carbon attached to the carbon adjoining the oxygen.

Question 9

Does propan-2-ol have a primary, tertiary or secondary structure?

Answer 9

• Secondary.
• There are 2 carbons attached to the carbon adjoining the oxygen.

Question 10

What is the change in functional group, reagent and conditions in the partial oxidation of primary alcohols?

Answer 10

CHANGE IN FUNCTIONAL GROUP

• Primary alcohol -> aldehyde.

REAGENET

• Potassium dichromate (VI) solution and dilute sulfuric acid.

CONDITIONS

• (Use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms.

Question 11

Draw the displayed, skeletal and structural formula of the reaction between propan-1-ol and an oxidising agent (potassium dichromate) under distillation.

Name the product of this reaction.

What can you observe in this reaction?

Answer 11

OBSERVATION: the orange dichromate ion (Cr₂O₇²⁻) reduces to the green Cr³⁺ ion.

Question 12

What is the change in functional group, reagent and conditions in the full oxidation of primary alcohols?

Answer 12

CHANGE IN FUNCTIONAL GROUP

• Primary alcohol -> carboxylic acid.

REAGENET

• Potassium dichromate (VI) solution and dilute sulfuric acid.

CONDITIONS

• Use an excess of dichromate, and heat under reflux (distil off product after the reaction has finished).

Question 13

Draw the displayed, skeletal and structural formula of the reaction between propan-1-ol and an oxidising agent (potassium dichromate) under reflux.

Name the product of this reaction.

What can you observe in this reaction?

Answer 13

OBSERVATION: the orange dichromate ion (Cr₂O₇²⁻) reduces to the green Cr³⁺ ion.

Question 14

What is the change in functional group, reagent and conditions in the oxidation of secondary alcohols?

Answer 14

CHANGE IN FUNCTIONAL GROUP

• Secondary alcohol -> ketone.

REAGENET

• Potassium dichromate (VI) solution and dilute sulfuric acid.

CONDITIONS

• Heat under reflux.

Question 15

Draw the displayed formula of the reaction between propan-2-ol and an oxidising agent (acidified potassium dichromate).

Name the product of this reaction.

What can you observe in this reaction?

Answer 15

OBSERVATION: the orange dichromate ion (Cr₂O₇²⁻) reduces to the green Cr³⁺ ion.

Question 16

Can tertiary alcohols be oxidised by acidified potassium dichromate? Why, why not?

Answer 16

• Tertiary alcohols cannot be oxidised by potassium dichromate.
• This is because there is no hydrogen atom bonded to the carbon with the -OH group.

Question 17

What is the chemical basis for the tests used to distinguish between aldehydes and ketones?

Answer 17

Aldehydes can be further oxidised to carboxylic acids where as ketones cannot be further oxidised.

Question 18

Describe how you can distinguish between aldehydes and ketones.

Describe the reagents that can be used, the conditions for them, the reactions that take place and the observations.

Answer 18

TOLLENS REAGENT

1. REAGENT
   • Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate.
   • The active substance is the complex ion of [Ag(NH₃)₂]⁺.
2. CONDITIONS
   • Heat gently.
3. REACTION
   • Aldehydes only oxidised by Tollens’ reagent into a carboxylic acid.
   • The silver (I) ions are reduced to silver atoms.
4. OBSERVATION
   • With aldehydes, a silver mirror forms coating the inside of the test tube.
   • Ketones result in no visible change.

FEHLING’S SOLUTION

1. REAGENT
   • Fehling’s solution containing blue Cu²⁺ ions.
2. CONDITIONS
   • Heat gently.
3. REACTION
   • Aldehydes only oxidised by Fehling’s solution into a carboxylic acid.
   • The copper (II) ions are reduced to copper (I) oxide.
4. OBSERVATION
   • In the presence of aldehydes, blue Cu²⁺ ions in solution change to a red precipitate of Cu₂O.
   • Ketones do not react.

Question 19

How can we test for the presence of a carboxylic acid?

Answer 19

• Addition of sodium carbonate.
• It will fizz and produce carbon dioxide.

Question 20

What is the change in functional group, role of the reagent, conditions and mechanism when an alcohol reacts with concentrated sulfuric or phosphoric acid?

Answer 20

CHANGE IN FUNCTIONAL GROUP

• Alcohol -> alkene.

ROLE OF REAGENT

• Dehydrating agent/catalyst.
  
  • Dehydration reaction is the removal of a water molecule from a molecule.

CONDITIONS

• Warm (under reflux).

MECHANISM

• Acid catalysed elimination.

Question 21

Draw and state the mechanism for the reaction between propan-2-ol and concentrated sulfuric or phosphoric acid.

State the name of the product.

Answer 21

MECHANISM: Acid catalysed elimination.
PRODUCT: Propene.

Question 22

When propan-1-ol undergoes an acid-catalysed elimination reaction, what product is produced?

Draw the displayed and structural formula of the product.

Answer 22

Question 23

When butan-2-ol undergoes an acid-catalysed elimination reaction two products can be produced, what are these two products?

Draw the displayed and structural formula of the two products.

What product is the major product, what product is the minor product?

Answer 23

• MAJOR PRODUCT: But-2-ene.
  
  • There are 2 methyl groups surrounding the carbocation intermediate (secondary carbocation).
• MINOR PRODUCT: But-1-ene.
  
  • There is 1 methyl group surrounding the carbocation intermediate (primary carbocation).

(Secondary carbocations are more stable than primary carbocations and thus form the major product).

Question 24

Alkenes produced by acid-catalysed elimination of alcohols can be used to produce what?

Answer 24

Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from crude oil.

Question 25

What processes can we use to produce ethanol?

Answer 25

1. Ethanol can be produced industrially by **hydration of alkenes in the presence of an acid catalyst**. 2. Ethanol can be produced industrially by **fermentation of glucose**.

Question 26

What type of reaction and conditions are required to produce ethanol from ethene? What are the advantages and disadvantages of this process compared to producing ethanol from glucose?

Answer 26

_REAGENT_ - Ethene produced from the **cracking** of fractions from distilled crude oil. _TYPE OF REACTION_ - **Hydration/addition**. - Hydration is the addition of water to a molecule. _ESSENTIAL CONDITIONS_ - **High temperature** - 300°C - **High pressure** - 70 atm - Strong acid catalyst of concentrated **H₃PO₄**. _ADVANTAGES_ 1. **Faster** reaction. 2. **Purer** product. 3. Continuous process which means **cheaper** manpower. _DISADVANTAGES_ 1. High technology equipment needed (**expensive initial costs**). 2. Ethene is a **non-renewable** resource (will become more expensive when raw materials run out). 3. **High energy costs** for pumping to produce high pressures.

Question 27

Draw and state the mechanism for the reaction between ethene and phosphoric acid. Name the product of this reaction.

Answer 27

MECHANISM: **Acid catalysed addition/hydration**. PRODUCT: **Ethanol**.

Question 28

What type of reaction and conditions are required to produce ethanol from glucose? Justify the conditions used. What are the advantages and disadvantages of this process compared to producing ethanol from ethene?

Answer 28

_TYPE OF REACTION_ - **Fermentation**. _CONDITIONS REQUIRED & JUSTIFICATIONS_ - **Yeast**. - **No air**. - Fermentation is done in the absence of air because the presence of air can cause extra reactions to occur. - It oxidises the ethanol produced to ethanoic acid (vinegar). - **Temperatures 30 - 40°C**. - The **optimum temperature** for fermentation is around 38°C. - At **lower** temperatures, the reaction is **too slow**. - At **higher** temperatures, the **yeast dies** and the **enzymes denature**. _ADVANTAGES_ 1. Sugar is a **renewable resource**. 2. Production uses low-level technology/**cheap equipment**. _DISADVANTAGES_ 1. Batch process which is **slow** and gives **high production costs**. 2. Ethanol made is not pure and needs **purifying by fractional distillation**. 3. Depletes land used for growing crops.

Question 29

What is a biofuel? Give an example of biofuel we can use.

Answer 29

- Biofuel is a fuel produced by plants. - Ethanol produced industrially by fermentation and separated by fractional distillation can be used as biofuel.

Question 30

What does carbon neutral mean?

Answer 30

The term carbon neutral refers to an activity that has **no net annual** (greenhouse gas) emissions **to the atmosphere**.

Question 31

Ethanol produced industrially by fermentation is separated by fractional distillation and can be used as a biofuel. It can be argued that ethanol produced like this is carbon-neutral because any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant. Write equations to support this statement.

Answer 31

Question 32

Ethanol produced industrially by fermentation is separated by fractional distillation and can be used as a biofuel. It can be argued that ethanol produced from this method is classed as carbon–neutral because any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. There would be no net CO2 emission into the atmosphere. Give reasons why this statement is not valid.

Answer 32

1. Does not take into account the energy needed to irrigate the plants. 2. Does not take into account the energy needed to distil the ethanol from the reaction mixture. 3. Does not take into account the energy needed to process the fuel. 4. If the energy for these processes above comes from fossil fuels then the ethanol produced is not carbon neutral.