3.5 Alcohols Flashcards
Key points about alcohols :
Physical properties of alcohols in water :
What are the boiling points of alcohols (key points)
- Higher melting and boiling points than alkanes of similar relative molecular mass due to hydrogen bonding
- -OH group can hydrogen bond to other alcohol molecules because it is polar
- The delta negative oxygen in alcohol and delta positive hydrogen in next alcohol are attracted to each other
What is the bonding shape of alcohols ?
Uses of ethanol
- Used as a solvent to make substanced like detergents and pharmaceuticals
- Can be used as a fuel, a biofuel if made by fermentation
Biofuel - A fuel that is made from biological material that has recently died
What are the two methods of production of ethanol ?
- Fermentation
- Hydration of Ethene
What is the process of fermentation ?
- Plants photosynthesise and produce glucose
- Carbohydrates (glucose) made from plants are converted into ethanol by the action of enzymes from yeast
It is an exothermic process called anaerobic respiration
What are the conditions for fermentation ?
- The rate of this chemical reaction is affected by temperature due to the yeast enzymes which denature if the temperature is too high or low and the reaction is too slow, a compromise temperature of about 35C is used
- It occurs at atmospheric pressure - 1 atmosphere
- Anaerobic - Air is kept out of the fermentation vessel to prevent oxidation of ethanol to ethanoic acid
What are the advantages of fermentation ?
- Fermentation is cheap
- Uses low tech equipment
- It uses renewable resources
What are the disadvantages of fermentation ?
- Batch process which is slow and gives high production costs
- Procuses impure ethanol which requires frac dist
- Fractional distillation occurs after to purify it which takes extra time and money
- Depletes land used for growing food crops
What is the process of the hydration of ethene ?
- Ethene is produced when crude oil fractions are obtained and cracked thermally (zeolite catalyst)
- Ethene is then hydrated where water is added over the double bond
- This reaction required the presence of an acid catalyst (phosphoric acid)
What are the conditions for the hydration of ethene ?
- The temperature is 250-300C, high so that the double bond can break
- The pressure needs to be high at 70 atmospheres so water can break the double bond
What are the advantages of hydration of ethene ?
- It is an continuous process
- Faster reaction
- It makes ethanol that is very pure
What are the disadvantages of hydration of ethene ?
- The raw material is crude oil which is non-renewable (will become more expensive when raw materials run out)
- High technology equipment needed (expensive initial costs)
- High energy costs for pumping to produce high pressures
What is the mechanism for ethene to ethanol ?
Write an equation for the reaction occurring during fermentation
C6H12O6 > 2C2H5OH + 2CO2
In industry, this fermentation reaction is carried out at 35 °C rather than 25 °C. Suggest one advantage and one disadvantage for industry of carrying out the fermentation at this higher temperature (2)
- Advantage – ethanol is produced at a faster rate
- Disadvantage – more energy is used / required in the reaction
The method used by the student in part (b) would result in the ethanol being contaminated by ethanoic acid. How does this contamination occur? (1)
Air gets in / oxidation occurs
(5)
Give the meaning of the term mean bond enthalpy (2)
- enthalpy / heat / energy change (at constant pressure) or enthalpy / heat / energy needed in breaking / dissociating (a) covalent bond(s)
- averaged for that type of bond over different / a range of molecules /
compounds
Give one reason why the ethanol produced by fermentation may not be carbon neutral (2)
- A specified process e.g. planting / harvesting / transport / extracting sugar / distilling ethanol solution / fertiliser production etc.
- The specified process uses / burns (fossil) fuel that releases CO2
Suggest one reason, other than safety, why this method is not used in industry to make ethanol (1)
- The yield is (very) low / not a high yield OR elimination occurs / ethene formed
- The rate of reaction slow
- Bromoethane has to be manufactured / made first
- Bromoethane is expensive
Use the equations for Processes 1, 2 and 3 to show why it can be claimed that there is no net emission of carbon-containing greenhouse gases (3)
- 6 (mol/molecules) CO2/carbon dioxide taken in/used/used
up (to form glucose or in photosynthesis) - 6 (mol/molecules) CO2/carbon dioxide given out due to
2 (mol/molecules) CO2/carbon dioxide from fermentation/
Process 2 and 4 (mol/molecules) CO2/carbon dioxide from
combustion/Process 3
The ethanol produced by fermentation of glucose may be regarded as a carbon-neutral fuel. Justify this statement. Include the relevant chemical equations in your answer (4)
Formation > Fermentation > Combustion ( 6 > 2 > 4 )
What are the different types of alcohols ?
Carbon-Neutral definition
an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere
Hydration definition
Hydration is the addition of water to a molecule
Comparing Ethanol production (Table)
What happens in the oxidation of primary alcohols ?
(Primary) alcohol →aldehyde →carboxylic acid
What happens in the oxidation of secondary alcohols ?
(Secondary) alcohol →ketone
What happens in the oxidation of Tertiary alcohols ?
Cannot occur (Requires removal of a C-C bond)
What happens in an alcohol dehydration reaction ?
(Dehydration) - alcohol →alkene
What is the oxidising agent used for alcohols ?
Potassium dichromate K2Cr2O7
To represent potassium dichromate / either Tollens or Fehling’s use [O]
What are the conditions and reagents for partial oxidation of primary alcohols ?
- Reagents - potassium dichromate (VI) solution (K2Cr2O7) and dilute sulfuric acid (H2SO4)
- Conditions - (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms which has a lower boiling point so boils off quickly
What are the observations for partial oxidation of primary alcohols ?
Orange Cr+6 ions are reduced to green Cr3+ ions as they oxidise the alcohol
What are the conditions and reagents for full oxidation of primary alcohols ?
Reagents - potassium dichromate (VI) solution (K2Cr2O7) and dilute sulfuric acid (H2SO4)
Conditions - use an excess of dichromate so there is excess oxygen to fully oxidise the alcohol , and heat under reflux: (distil off product after the reaction has finished)
What are the conditions and reagents for full oxidation of secondary alcohols ?
- Reagents - potassium dichromate (VI) solution (K2Cr2O7) and dilute sulfuric acid (H2SO4)
- Conditions - heat under reflux
How can we distinguish between aldehydes and ketones ?
Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised
What is Tollens Reagent ?
What is Felhing’s solutions
What are the reagents and conditions for the dehydration of alcohols ?
- Reagents - Concentrated sulfuric or phosphoric acids (H2SO4 or H3PO4)
- Conditions - heat under reflux, need a H on the adjacent carbon where the alcohol group is
- Reaction - gives major and minor products.
What is the mechanism for the dehydration of alcohols ?
Explain why propanone has a lower boiling point than propan-2-ol (3)
- propan-2-ol has stronger intermolecular forces
- propan-2-ol has hydrogen bonds between molecules
- propanone has dipole-dipole forces and/or van der Waals’ forces
Give two reasons why the apparatus shown in above figure is not safe (2)
Give one additional reagent that is needed to form any propanoic acid (1)
State two more mistakes in the way the apparatus is set up in above figure (2)
- flask not clamped
- sealed system / bung in condenser
- sulfuric acid needs adding
- direction of water flow through condenser + thermometer not needed
State the purpose of the small glass beads in the flask in above figure
- to prevent ‘bumping’
- allow prevent large bubbles / ensure small bubbles
State a simple chemical test that distinguishes the propanoic acid from the propan-1-ol. Give one observation for the test with each substance (3)
- add sodium carbonate/hydrogencarbonate
- effervescence / bubbles
- no (visible) change/reaction
- A mixture of liquids is heated to boiling point for a prolonged time
- Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture
- Any ethanal and ethanol that initially evaporates can then be oxidised
Write a half-equation for the overall oxidation of ethanol into ethanoic acid (1)
CH3CH2OH + H2O CH3COOH + 4H+ + 4e–
- Mixture heated in a suitable flask / container
- With still head containing a thermometer
- Water cooled condenser connected to the still head and suitable cooled collecting vessel
- Collect sample at the boiling point of ethanal
- Cooled collection vessel necessary to reduce evaporation of ethanal
Use your knowledge of structure and bonding to explain why it is possible to separate ethanal in this way (2)
- Hydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in ethanal
- Intermolecular forces / dipole-dipole are weaker than hydrogen bonding
Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did not contain ethanoic acid (3)
Reagent to confirm the presence of ethanal:
Suggest the identity of reagent P (1)
H2SO4
State the chemical change that causes the solution in the flask to appear green at the end of the reaction (1)
Dichromate / Cr(VI) reduced or Cr(III) formed
Allow Cr6+ and Cr3+
Give one reason why using a water bath is better than direct heating with a Bunsen burner (1)
The alcohol is flammable
Suggest a reagent that could be used to confirm the presence of an aldehyde in the distillate. State the observation you would expect to make if an aldehyde were present (2)
- Tollens
- Silver mirror
State what is meant by the term position isomers (2)
- Compounds with the same molecular formula
- but different structures due to different positions of the same functional group on the same carbon skeleton/chain
Identify a suitable combination of reagents for alcohol oxidation (1)
- K2Cr2O7 or potassium dichromate(VI)
- acidified or H2SO4 (or other identified strong acid)
State how you would ensure that compound A is converted into butanoic acid rather than into butanal (1)
- (heat under) reflux
- OR use excess oxidising agent
Give a reagent that can be used in a test to distinguish between a ketone and an aldehyde. State what you would observe in the test (4)
Butan-1-ol can be oxidised to form a carboxylic acid. Using [O] to represent the oxidising
agent, write an equation for this reaction and name the product (2)
CH3CH2CH2CH2OH (or C4H9OH) + 2[O] →
CH3CH2CH2COOH (or C3H7COOH) + H2O
butanoic acid (1)
Distillation Diagram :
Reflux Diagram :
State a simple chemical test that distinguishes the propanoic acid from the propan-1-ol.
Give one observation for the test with each substance (1)
- add sodium carbonate/hydrogencarbonate
- effervescence / bubbles
Describe a test-tube reaction, on the product, to show that the
cyclohexanol had been dehydrated (2)
- Add bromine (water)
- Would turn (from orange to) colourless / decolourise
Explain why it is important to open the tap of the separating funnel periodically (1)
avoid pressure build-up / release pressure
Describe how the students would use this apparatus to collect a sample of ethanol. Include in your answer the functions of the parts of the apparatus labelled
A, B and C (6)
Prop-2-en-1-ol can be prepared by reacting 3-chloroprop-1-ene with dilute aqueous sodium hydroxide.
Name the mechanism for this reaction (1)
nucleophilic substitution
- elimination
Prop-2-en-1-ol forms an addition polymer.
Draw the repeating unit of poly(prop-2-en-1-ol) (1)
This question is about pentan-2-ol and pent-1-ene.
(a) The boiling point of pentan-2-ol is 119 °C
The boiling point of pent-1-ene is 30 °C
Explain why pentan-2-ol has a higher boiling point than pent-1-ene (3)
- dea that pentan-2-ol has stronger intermolecular forces
- pent-1-ene has van der Waals’ forces (only)
- pentan-2-ol (also) has hydrogen bonds
- Reagent = conc sulfuric acid or conc phosphoric acid
- condition = hot / temperature in range 150-200°(C)
- more stable (carbocation formed)
- changes from secondary to tertiary (carbocation)
- M1 cyclohexene : van der Waals’ forces (between molecules)
- M2 cyclohexanol : hydrogen bonds (between molecules)
- M3 phosphoric acid: hydrogen bonds (between molecules)
- M4 idea that cyclohexene has weakest forces
- M5 separated by (simple / fractional) distillation
- M6 cyclohexene has lowest boiling point / boils off first
Give a reagent that can be used in a test to distinguish between a ketone and an aldehyde. State what you would observe in the test (4)
- Anti-bumping granules
- no anti-bumping granules / add anti-bumping granules
- to create smaller bubbles / to prevent large bubbles
- Open system with no thermometer
- system should be closed (above flask) to prevent gases escaping
- should be closed with (bung +) thermometer
- to allow collection of propanone (only) / to prevent distillation of other components / to stay in suitable temperature range
- The water direction in the condenser
- water flows in wrong direction through condenser / change water direction
- condenser not cool enough / not full of water
- product may not condense / comes through as gas
- propan-2-ol: tetrahedral and 109.5°
- propanone: trigonal planar and 120°
- A mixture of liquids is heated to boiling point for a prolonged time
- Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture
- Any ethanal and ethanol that initially evaporates can then be oxidised
- Mixture heated in a suitable flask / container
- With still head containing a thermometer
- Water cooled condenser connected to the still head and suitable cooled collecting vessel
- Collect sample at the boiling point of ethanal
- Cooled collection vessel necessary to reduce evaporation of ethanal
Why (in terms of structure and bonding) can we separate ethanal from ethanol and ethanoic acid using distilliation (2)
- Hydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in ethanal
- Intermolecular forces / dipole-dipole are weaker than hydrogen bonding
A student obtained a sample of a liquid using the apparatus in part (c).
Describe how the student could use chemical tests to confirm that the liquid contained
ethanal and did not contain ethanoic acid
- Tollens - Silver mirror
- Fehlings - orange / red precipitate
- Sodium carbonate - no effervesence
State the chemical change that causes the solution in the flask to appear green at the end of the reaction (1)
Dichromate / Cr(VI) reduced or Cr(III) formed.
State what is meant by the term position isomers (2)
- Compounds with the same molecular formula
- but different structures due to different positions of the same functional group on the same carbon skeleton/chain
Identify a reagent or combination of reagents that you would use in the
laboratory to oxidise ethanol to ethanoic acid.
State the colour change that you would expect to see (2)
- Acidified potassium or sodium dichromate
- Orange to green
State the meaning of the term carbon-neutral (1)
An activity which has no net / overall (annual) carbon emissions to the
atmosphere / air
Other than carbon-neutrality, state the main advantage of the use of glucose
from crops as the raw material for the production of ethanol (1)
Renewable
Give one disadvantage of the use of crops for the production of ethanol (1)
- Subject to weather / climate
- Depletes food supply OR the land use for (specified) food
In the production of bioethanol, glucose (C6H12O6) is converted into a dilute aqueous solution of ethanol and carbon dioxide.
Give the name of this process and state three essential conditions necessary to produce a good yield of ethanol (4)
- fermentation
- (enzymes from) yeast
- 25°C ≤ T ≤ 42°C (35)
- anaerobic
In terms of electrons, state the meaning of the term oxidising agent (1)
an electron acceptor
Give one advantage and one disadvantage of manufacturing ethanol by
fermentation rather than by hydration (2)
- low technology / renewable
- slow / low yield
- Acidified potassium dichromate
- Obs with 2-methylpropan-2-ol - No visible change
- Obs with butan-2-ol - Orange to green (both colours needed)
Explain, with reference to intermolecular forces, why distillation allows propanal to be separated from the other organic compounds in this reaction mixture (3)
- Aldehyde/propanal has dipole-dipole forces (between molecules)
- Alcohol/propan-1-ol AND Carboxylic acid/ propanoic acid have hydrogen bonding (between molecules)
- The forces between the molecules in aldehyde are weaker (than those in alcohol and acid so it will evaporate first.)
Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture (2)
- Cool the distillate / collecting vessel
- Keep the temperature of the reaction mixture below the boiling point of propan-1-ol/below 97 °C
Describe how you would carry out a simple test-tube reaction to confirm that the sample of propanal obtained by distillation does not contain any propanoic acid (2)
- Add named carbonate/hydrogencarbonate OR magnesium to a sample of the distillate
- Effervescence/fizz/bubbles would confirm presence of acid or converse
