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3.3 A level Organic Chemistry > 3.3.5 Alcohols > Flashcards

3.3.5 Alcohols Flashcards

1
Q

What is the functional group in an alcohol homologous series called?

A

Hydroxyl group (OH-)

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2
Q

What is a primary alcohol?

1o

A

When a hydroxyl group is bonded to a carbon with only one alkyl group bonded to it

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3
Q

What is a secondary alcohol?

2o

A

When a hydroxyl group is bonded to a carbon with two alkyl groups bonded to it

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4
Q

What is a tertiary alcohol?

3o

A

When a hydroxyl group is bonded to a carbon with three alkyl groups bonded to it

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5
Q

Why is being able to eliminate alcohols into alkenes useful?

A
  • This reaction allows alkenes to be produced from a renewable source (ethanol from fermentation)
  • This mean polymers can be produced without oil
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6
Q

Give the equation for dehydration (eliminating) ethanol to form ethene

A

Ethanol is heated with a sulfuric acid catalyst

C2H5OH -H2SO4-> C2H4 + H2O

The product formed is usually a mixture of water acid and ethanol so the alkene must be separated out

Phosphoric acid can also be used as a catalyst

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7
Q

Breifly describe the mechanism for elimination of water from alcohol

A
  1. A lone pair of electrons from the oxygen bonds to an H+ from the acid. The alcohol is protonated giving the positve charge
  2. The positively charged oxygen pulls electrons away from the carbon. An H2O molecule leaves, creating an unstable carbocation intermediate
  3. The carboncation loses an H+, the bond from which creates a C=C double bond, and an alkene is formed
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8
Q

Why may dehydration of alcohols have more than one product?

A

If the carbon the hydroxyl group has two carbons attached to it the double bond can form either side of the carbon forming two products

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9
Q

Describe how to produce cyclohexene from cyclohexanol using distilation

A

Stage 1 distilation:
1. Add conc. sulfuric acid and phosphoric acid to a round bottomed flask containing cyclohexanol. Mix the solution by swirling the flask and add antibumbing granules
2. The mixture should be gently heated around 83oC using a water bath or electric heater
3. Chemical with boiling points >83oC will evaporate. Warm gas will rise out of the flask and condense
4. The product can be collected in a cooled flask

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10
Q

Describe how to produce cyclohexene from cyclohexanol using seperation after distillation

A

Step 2 seperation:
1. The product collected after distillation will still contain impurities
2. Transfer the product mixture into a seperating funnel and add water to dissolve water soluble impurites and create an aqueous solution
3. Allow the mixture to settle into layers. Drain the aqueous lower layer, leaving the impure cyclohexene

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11
Q

Describe how to produce cyclohexene from cyclohexanol using purification after seperation and distillation

A

Step 3 purification:
1. Drain the cyclohexene into a round-bottomed flask
2. Add anyhdrous CaCl2 (a drying agent) and stopper the flask. Let the mixture dry for at least 20 min with occasional swirling
3. The cyclohexene will still have small amount of impurities so distil the mixture one last time

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12
Q

Explain how alcohols can be produced by hydration of an alkene

A
  1. A pair of electrons from the double bond bonds to an H+ from the acid
  2. A lone pair of electrons from a water molecule bonds to the carbocation
  3. The water loses an H+
  4. The alcohol is now formed
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13
Q

What conditions are required for an alkene to be hydrated into an alcohol

A

Ethanol can be produced by the hydration of ethene by steam at a temperature of 300oC and a preasure of 60 atm. It needs a solid phosphoric(V) acid catalyst

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14
Q

Why is production of ethanol from fermentation more sustainable than from hydration of alkenes

A

When crude oil supplies start running out, petrochemical like ethene will be expensive, so being able to produce ethanol from glucose which can be grown on plants means ethanol can be produced sustainably as its a renewable resource

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15
Q

Describe how ethanol is produced via fermentation of glucose

A
  1. Fermentation is an exothermic process, carried about by yeast in anaerobic conditions
  2. The equation is C6H12O6(aq) -30-40oC / yeast-> 2C2H5OHaq + 2CO2(g)
  3. Yeast produces an enzyme which converts sugers, such as glucose, into ethanol and carbon dioxide
  4. The enzyme works at an optimum temperature of 30-40oC. If its too cold, the reaction is slow, if its too hot the enzyme is denatured
  5. Once formed, ethanol is seperated from the rest of the mixture by fractional distillation
  6. Fermentation is low-tech. It uses cheap equipment and renewable resources. Buth the fractional distillation that is neeed to purify the ethanol produced takes extra time an money
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16
Q

What is a biofuel?

A

A fuel that is made from biological material that’s recently died

17
Q

What are the advantages of biofuels?

A
  • Biofuels are renewable sources of energy, unlike fossil fuels they won’t run out so they are sustainable
  • Biofuels do produce CO2 when they are burnt, but it’s CO2 that the plants absorbed while growing, so biofuels are normally classed as carbon neutral
18
Q

What are the disadvantages of biofuels?

A
  • Petrol cars still require modification to be able to use fuels with high ethanol concentrations
  • Land used to grow crops for fuel cannot be used to grow food, which may cause food shortages
19
Q

Give the equations that can be used to support the argument that biofuels are carbon neutral

A
  • Plants take in carbon dioxide from the atmoshere to produce glucose by photosynthesis - 6CO2 + 6H2O -> C6H12O6 + and 6O2 - 6 moles of carbon dioxide are removed to produce 1 mole of glucose
  • In the fermentation process, glucose is converted into ethanol - C6H12O6 -> 2C2H5OH + 2CO2 - 2 moles of carbon dioxide are released into the atmosphere when 1 mole of glucose is converted to 2 moles of ethanol
  • When ethanol is burned, CO2 and water are produced - 2C2H5OH + 6O2 -> 4CO2 + 6H2O - 4 moles of carbon dioxide are released into the atmosphere when 2 moles of ethanol are burned completely

If you combine all three of these equations, you’ll find 6 moles of carbon dioxide are abosrbed and 6 moles are emited, therefore biofuels are theoretically carbon neutral

20
Q

Why aren’t biofuels completely carbon neutral

A

Fossil fuels still need to be burned to power the machinery used to make fertilisers for the crops, harvest the crops and transport the bioethanol. Burning the fossil fuels to power this machinery produces CO2, therefore its not completely carbon neutral

21
Q

How can an alcohol be oxidised other than burning?

What colour change occurs?

A

The oxidising agent acidified potassium dichromate(VI), K2Cr2O7, to mildly oxidise alcohols.

The orange dichromate(VI) ion, Cr2O72-, is reduced to the green chromium(III) ion, Cr3+.

22
Q

What can primary alcohols be oxidised into?

A

Aldehydes then carboxylic acids

23
Q

What are secondary alcohols oxidised into?

A

Ketones only

24
Q

What are tertiary alcohols oxidised into?

A

Tertiary alcohols cannot be oxidised

25
Q

What is an aldehyde?

A
  • Have a hydrogen and one alkyl group attached to the carbonyl carbon atom
  • Thier suffix is -al.
  • You don’t need to say the carbon the functional group is attached to as it’s always on carbon-1
26
Q

What is a ketone?

A
  • Have two alkyl groups attached to the carbonyl carbon atom
  • Thier suffix is -one
  • For kentones with five or more carbons, you have to always say which carbon the functional group is on
27
Q

What is a carboxyllic acid?

A
  • Have a COOH group at the end of the carbon chain
  • Their suffix is -oic acid
28
Q

Explain how you an control reaction conditions to produce either an aldehyde or carboxylic acid from a primary alcohol

A
  1. Gently heating ethanol with potassium dichromate(VI) and sulfuric acid in a test tube produces ethanal. The aldehyde will be oxidised into a carboxylic acid if the heat isn’t controlled.
  2. To get just an aldehyde, it needs to be removed from the solution as soon as it forms. This can be done by distillation as aldehydes boil at a lower temperature than alcohols.
  3. To produce the carboxylic acid, the alcohol needs to be vigourously oxidised. The alcohol is mixed with excess oxidising agent and heated under reflux.
  4. By heating under reflux, the temperature of an organic reaction can be increased without loosing volitile solvents, reactants or products. Any vapourised compounds cool, condense and return to the reaction mixture. So the aldehyde stays in the reaction mixture and is oxidised to carboxyllic acid
29
Q

Explain how secondary alcohol can be oxidised into ketones

A
  1. Refluxing a secondary alcohol with acidified potassium dichromate(VI) will produce a ketone
  2. Ketones can’t be oxidised easily, so even prolonged refluxing won’t produce anything more
30
Q

What is the only way to oxidise a tertiary alcohol?

A

Burning

3.3 A level Organic Chemistry (16 decks)

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