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Chemistry > AS - Alcohols > Flashcards

AS - Alcohols Flashcards

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1
Q

How are alcohols produced industrially?

A

By hydration of alkenes using steam in the presence if an acid catalyst.

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2
Q

How is ethanol produced industrially?

A

By the fermentation of glucose.

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3
Q

What is the general formula of an alcohol? What is this often shortened to?

A

CnH2n+1OH.

ROH.

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4
Q

What is the functional group in an alcohol? Name this group.

A

-OH, the hydroxyl group.

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5
Q

How are alcohols classified?

A

Either as primary, secondary or tertiary depending on how many other groups (R) are bond to the carbon that had the hydroxyl group.

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6
Q

Define a primary alcohol.

A

The carbon with the -OH group had one R group and two hydrogen atoms attached.

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7
Q

What classification is methanol considered as?

A

Primary.

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8
Q

Define a secondary alcohol.

A

The -OH group is attached to a carbon with two R groups and one hydrogen atom.

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9
Q

Define a tertiary alcohol.

A

Three R groups are attached to the carbon bonded to the -OH group.

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10
Q

Outline the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst.

A
  1. A pair of electrons from the double bond in the alkene bonds to an H+ ion if the acid catalyst.
  2. A lone pair of electrons from a water molecule bonds to the resulting carbocation.
  3. The water loses an H+ ion forming an alcohol and H+.
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11
Q

What conditions are used in the production of ethanol from the hydration of ethene?

A

300 degrees Celsius, pressure of 60 atm and the catalyst is solid phosphoric (V) acid.

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12
Q

What is a biofuel?

A

A fuel thats made from renewable biological material that’s recently died.

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13
Q

Is fermentation an exothermic or endothermic process?

A

Exothermic.

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14
Q

Outline the conditions required for the fermentation of glucose to form ethanol.

A
  1. Carried out by yeast.
  2. Anaerobic conditions (without oxygen).
  3. Temperature of 30-40 degrees C (the optimum temperature for the enzyme produced by yeast.
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15
Q

Give the equation for the fermentation of glucose to form ethanol.

A

C6H12O6 (aq) —> 2C2H5OH (aq) + 2CO2 (g)

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16
Q

How does the process of fermentation of glucose to form ethanol work?

A

Yeast is used in anaerobic conditions and produces an enzyme which converts sugars, such as glucose, into ethanol and carbon dioxide.

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17
Q

Why is a temperature of 30-40 degrees C used in the fermentation of glucose?

A

The enzyme produced by yeast functions at an optimum temperature. If the temperature is too cold, the reaction is slow, if it’s too hot, the enzyme is denatured.

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18
Q

What happens once ethanol is formed by the fermentation of glucose?

A

Ethanol is separated from the rest of the mixture using fractional distillation.

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19
Q

What are the advantages of fermentation?

A
  1. It’s low-tech so uses cheap equipment.

2. Uses renewable resources.

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20
Q

What are the disadvantages of fermentation?

A

Fractional distillation is required which takes extra time and money.

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21
Q

Why is air kept out of a fermentation vessel?

A

To prevent the oxidation of an alcohol.

22
Q

What type of fuel can ethanol be used as?

A

A biofuel - bioethanol.

23
Q

Why can bioethanol be thought of as carbon neutral? Give equations to support this statement.

A

Bioethanol does produce CO2 when burnt but the plants used to make the bioethanol take in the same amount of CO2 when growing. This is seen as carbon neutral.

Equations:

  1. Plants take in CO2 from the atmosphere to produce glucose by photosynthesis where 6 moles of CO2 are taken in by the plant

6CO2 + 6H2O —> C6H12O6 + 6O2

  1. Glucose is converted into ethanol in fermentation where 2 moles of CO2 are produced

C6H12O6 —> 2C2H5OH + 2CO2

  1. When ethanol is burned, 4 moles of CO2 and water are produced

2C2H5OH + 6O2 —> 4CO2 + 6H2O

So 6 moles of CO2 are taken in and 6 moles are given out.

24
Q

Why, in reality, are biofuels not carbon neutral?

A

Fossil fuels will need to be burned to power the machinery used to make fertilisers for the crops, harvest the crops and refine and transport the bioethanol. Burning the fuel to power this machinery produces carbon dioxide.

25
Q

What are the advantages of using biofuels?

A
  1. Renewable energy sources do they’re more sustainable.
  2. Biofuels are classed as carbon neutral and so don’t produce as much carbon dioxide as fossil fuels which is beneficial to the environment.
26
Q

What are the disadvantages of using biofuels?

A
  1. Petrol car engines would need to be modified to use fuels with high ethanol concentrations. This incurs a cost.
  2. When land is used to grow crops for fuel, the land cannot be used to grow food which could lead to a shortage of food supply.
27
Q

How much an alcohol can be oxidised depends on it’s what?

A

Structure.

28
Q

What are primary alcohols oxidised to?

A

Aldehydes which can then be oxidised further to carboxylic acids.

29
Q

What can secondary alcohols be oxidised to?

A

Ketones only.

30
Q

Can tertiary alcohols be oxidised?

A

Not easily, only by burning them, they do not react with potassium dichromate(VI).

31
Q

What is a suitable oxidising agent for oxidising alcohols?

A

Acidified potassium dichromate (VI).

K2Cr2O7

32
Q

What colour is the dichromate(VI) ion (Cr2O7 2-)? What is it reduced to, including the colour?

A

The dichromate(VI) ion is orange and is reduced to the green chromium(III) ion, Cr3+.

33
Q

What type of compounds are aldehydes and ketones? What is their functional group?

A

Carbonyl compounds with the functional group C=O.

34
Q

What is the general formula for aldehydes and ketones?

A

CnH2nO

35
Q

What functional group do carboxylic acids have? What is their general formula?

A

-COOH

CnH2n+1COOH

36
Q

Describe the structure of an aldehyde.

What is the suffix? Do you have to say which carbon the functional group is on?

A

They have a hydrogen and one alkyl group attached to the carbonyl carbon atom.

The suffix is -al. You don’t need to say which carbon the functional group is on because it’s always in carbon-1.

37
Q

Describe the structure of ketones.

What is the suffix? When do you have to say which carbon atom the functional group is on?

A

They have two alkyl groups attached to the carbonyl carbon atom.

The suffix is -one. For ketones with five or more carbons, you need to say which carbon the functional group is on.

38
Q

Describe the structure of carboxylic acids.

What is the suffix?

A

They have a COOH group at the end of their carbon chain.

The suffix is -oic acid.

39
Q

What symbol can be used in an equation to represent an oxidising agent?

A

[O]

40
Q

Give the general equation for the reaction of a primary alcohol with an oxidising agent.

A

RCH2OH + [O] —> RCHO +H2O

Then

RCHO + [O] —> RCOOH (conditions need to be under reflux for this reaction)

41
Q

Give the general equation for the reaction of a secondary alcohol with an oxidising agent.

A

R1R2CHOH + [O] —> R1R2CO +H2O (conditions need to be under reflux).

42
Q

How do you oxidise a primary alcohol to an aldehyde only?

A

Gently heat ethanol with potassium dichromate(VI) and sulfuric acid in a test tube to produce ethanal (an aldehyde).

To make sure the oxidation doesn’t continue to produce a carboxylic acid, the aldehyde needs to be removed as soon as it is formed.

This is possible using distillation apparatus so that the aldehyde, which boils at a lower temperature than the alcohol, is distilled off immediately.

43
Q

How is a carboxylic acid produced from the oxidation of a primary alcohol?

A

The alcohol had to be oxidised vigorously so the alcohol is mixed with excess oxidising agent and heated under reflux.

44
Q

What does heating under reflux mean?

A

This refers to heating a solution with an attached condenser to prevent reagents from escaping. Any vapor will condense on the cool surface of the attached condenser and flow back into the flask.

This means you can increase the temperature of an organic reaction without losing volatile solvents, reactants or products.

45
Q

What does volatile mean?

A

A volatile substance is a substance that evaporates readily at room temperature.

46
Q

What are secondary alcohols oxidised to and how?

A

Refluxing a secondary alcohol with acidified dichromate (VI) will produce a ketone.

47
Q

What are the two methods for distinguishing aldehydes from ketones?

A
  1. Fehling’s solution and Benedict’s solution are deep blue Cu2+ complexes which reduce to brick red Cu2O when warmed with an aldehyde, but stay blue with a ketone.
  2. Tollen’s reagent is [Ag(NH3)2]+ - it’s reduced to a silver which coats the inside of the apparatus to form a silver mirror when warmed with an aldehyde, but not with a ketone.
48
Q

How can you form alkenes from alcohols? What is the advantage of this?

A

You can make alkenes by eliminating water from alcohols in a dehydration reaction.

It’s advantageous as it means you can produce alkenes from renewable resources, you can then produce polymers from alkenes.

49
Q

What’s the general overall equation for the dehydration reaction of an alcohol?

A

CnH2n+1OH —> CnH2n + H2O

50
Q

Acid-catalysts are needed for elimination reactions of alcohols to form alkenes. What two acid-catalysts could you use?

A
  1. Concentrated sulfuric acid.

2. Phosphoric acid.

51
Q

Outline the mechanism for the elimination of water from alcohols.

A
  1. A lone pair of electrons from the oxygen atom in the alcohol bonds to an H+ ion from the acid catalyst.
  2. The alcohol is protonated which gives the oxygen a positive charge.
  3. The positively charged oxygen atom pulls electrons away from the carbon atom. An H2O molecules leaves, creating an unstable carbocation intermediate.
  4. The carbocations loses an H+ and the alkene is formed with the double bond. H2O and H+ are also products.

Chemistry (34 decks)

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