3.3.5 Alcohols Flashcards
Alcohols have _____ melting and boiling points than alkanes of similar relative molecular mass
higher
Why do alcohols have higher melting and boiling points than alkanes?
∵ -OH group in alcohol means hydrogen bonding occurs between molecules
-OH group of alcohols can…
hydrogen bond to water molecules
….cannot bond to water molecules
Non-polar hydrocarbon chain
Why are alcohols with short hydrocarbon chains are soluble in water?
∵ of hydrogen bonding predominates
Why are alcohols with longer hydrocarbon chains are insoluble in water?
Non-polar hydrocarbon chain dominates
What is the benefit of making alkenes from a dehydration reaction?
Allows you to make alkenes from renewable resources i.e. without crude oil
Elimination Reaction
Name the condition and product when an alcohol is used as a reactant
Product: Alkene
- Al2O3 Catalyst
- 450 K
- Concentrated, hot H2SO4
Ethanol + H2SO4
Name the mechanism
Elimination
Ethanol + H2SO4
Mechanism
H2SO4 - acts as a catalyst
How does distillation work?
Uses the fact that chemicals have different boiling points & separates them out to get a single purified substance
Name 2 ways ethanol can be produced
- Hydration
- Fermenation
Describe hydration (4)
- Hydration of ethene using steam to produce ethanol
- Acid catalyst
- High temperature + high pressure
- H2O added across double bond
Hydration
Name 3 conditions for producing ethanol
- 300°C
- Pressure of 60 atm
- Solid phosphoric(V) acid catalyst
- H3PO4
Describe fermenation
- Fermentation (exothermic reaction) is carried out by yeast in anaerobic respiration
- Yeast produces an enzyme that converts sugars into ethanol and carbon dioxide
- Ethanol formed is separated by fractional distillation
What is an advantage of using fermentation to make alcohol?
Low tech process (uses cheap equipment & renewable resources)
What is a disadvantage of using fermentation to make alcohol?
Fractional distillation is needed for purification = expensive & time consuming
State the equation for the hydration of ethene (include state symbols)
C2H4(g) + H2O(g) ⇌ C2H5OH(g)
State the equation for fermentation (include state symbols)
Name 2 ways alcohols can be oxidised
- Burn them
- Use oxidising agent acidified potassium dichromate(VI), K2Cr2O7 to mildly oxidise them
Define Oxidation
- Loss of electrons
- Gain of oxygen
- (Loss of hydrogen)
Define Reduction
- Gain of electrons
- Loss of oxygen
- (Gain of hydrogen)
Define oxidising agent
Chemical that accepts electrons and gets reduced
(supplies the oxygen or removes the hydrogen)
Define reducing agent
Chemical that donates electrons and gets oxidised
(removes the oxygen or supplies the hydrogen)
What are primary alcohols oxidised to?
Aldehydes and then to carboxylic acids
Draw the a primary alcohol oxidising into an aldehyde & then draw an aldehyde oxidising into a carboxylic acid
Draw the a primary alcohol oxidising into a carboxylic acid
+ H2O
Describe how you would oxidise an alcohol to just get an aldehyde
- Gently heating alcohol with potassium dichromate(VI) & sulfuric acid in test tube = aldehyde
- To get just aldehyde, distill it off immediately
Describe how you would oxidise an alcohol to just get a carboxylic acid
Alcohol has to be vigorously oxidised:
alcohol is mixed with excess oxidising agent & heated under reflux
Explain how heating under reflux ensures the complete oxidation of an alcohol to a carboxylic acid (2)
- Any escaped vapour is condensed back into liquid
- So any unreacted aldehyde can be oxidised/react further
What are secondary alcohols oxidised into?
Ketones
Describe how you would oxidise a secondary alcohol to a ketone
Reflux a secondary alcohol with acidified potassium dichromate(VI)
Draw the a secondary alcohol oxidising into a ketone
State what colour changes occur when potassium dichromate(VI) is added to a primary, secondary and tertiary alcohol
Primary and secondary alcohol:
Orange → Green
Tertiary alcohol:
No colour change - remains orange
Explain why potassium dichromate(VI) turns from orange to green
Orange dichromate(VI) ion, Cr2O72-, is reduced to green chromium(III) ion, Cr3+
Can tertiary alcohols be oxidised by reacting them with potassium dichromate(VI)?
NO
Why can’t tertiary alcohols be oxidised by reacting them with potassium dichromate(VI)?
∵ of lack of hydrogen atoms on carbon atom which the hydroxyl group is attached to
What is the only way to oxidise tertiary alcohols?
By burning them
List alcohol A, ketone B and alkene C in order of increasing boiling point
CBA
Alkene, ketone, alcohol
Explain why butanal has a lower boiling point than both butan-1-ol and butanoic acid (3)
- Dipole-dipole forces between butanal molecules
- Hydrogen bonds in both other molecules
- Dipole-dipole forces are weaker than hydrogen bonds, require less energy to break
Ethanol can be oxidised to ethanal. Describe this reaction. (3)
- Oxidising agent
- 2 hydrogen atoms removed
- Forming water
What is meant by a primary alcohol? (1)
Functional group (OH) at the end of the chain
Describe what happens when an alcohol is heated under reflux (3)
- Mixture of liquids heated to boiling point for prolonged time
- Vapour is formed which escapes from liquid mixture & is changed back into liquid + returned to liquid mixture
- Any alcohol and aldehyde that initally evaporates can then oxidised
