3.3.5 Alcohols Flashcards
How do you name alcohols?
These have the ending -ol and if necessary the position number for the OH group is added between the name stem and the -ol
If the compound has an -OH group in addition to other functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-
What are the bond angles in alcohols?
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 are 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.
What are the properties of alcohols?
The alcohols have relatively low volatility and high boiling points due to their ability to form hydrogen bond between alcohol molecules.
Smaller alcohols can dissolve in water because they can form hydrogen bonds to water molecules.
What are primary, secondary and tertiary alcohols?
Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen.
Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen.
Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen.
What oxidising agent causes alcohols to oxidise?
K2Cr2O7 + H2SO4 (potassium dichromate + dilute sulfuric acid).
Orange Cr2O7^2- ions are reduced to green Cr^3+ ions as they oxidise the alcohol.
What oxidation reactions can occur with primary alcohols? What are the conditions and reagents required?
Oxidising agent [O] = K2Cr2O7 (potassium dichromate) + H2SO4 (dilute sulphuric acid)
Partially oxidising primary alcohol:
Primary alcohol + [O] → aldehyde
(warm gently and distil out the aldehyde as it forms - prevents oxidation of the aldehyde to a carboxylic acid)
Fully oxidising primary alcohol:
Primary alcohol + 2[O] → carboxylic acid
(use an excess of dichromate, and heat under reflux: (distil off product after the reaction has finished))
What oxidation reactions can occur with secondary alcohols? What are the conditions and reagents required?
Oxidising agent [O] = K2Cr2O7 (potassium dichromate) + H2SO4 (dilute sulphuric acid)
Secondary alcohol + [O] → ketone
(heat or reflux)
What oxidation reactions can occur with tertiary alcohols? What are the conditions and reagents required?
No reactions.
Not easily oxidised as you would have to break C-C bonds - all the other oxidations involve the breaking of C-H bonds.
(Which is also why ketones aren’t easily oxidised)
What is an aldehyde? What is a ketone? What is a carboxylic acid?
An aldehyde’s name ends in -al. E.g. ethanal.
Written in structural formula as CHO.
It always has the C=O bond on the first carbon of the chain.
A ketones name ends in -one. E.g. propanone.
Written in structural formula as CO.
When ketones have 5C’s or more in a chain then it needs a number to show the position of the C=O double bond. E.g. pentan-2-one
A carboxylic acids name ends in -oic acid. E.g. ethanoic acid.
Written in structural formula as COOH.
It always has the C=O and C-O-H bond on the first carbon of the chain.
How can you distinguish between aldehydes and ketones?
Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised.
What is Tollens’ Reagent?
Reagent: Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]^+
Conditions: Heat gently
Reaction: Aldehydes only are oxidised by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms.
Observation: With aldehydes - a silver mirror forms coating the inside of the test tube. With ketones - no visible change.
What is Fehling’s solution?
Reagent: Fehling’s solution containing blue Cu^2+ ions.
Conditions: Heat gently
Reaction: Aldehydes only are oxidised by Fehling’s solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide.
Observation: With aldehydes - Blue Cu^2+ ions in solution change to a red precipitate of Cu2O. With ketones - no visible change.
How can the presence of a carboxylic acid be tested?
The presence of a carboxylic acid can be tested by addition of sodium carbonate. It will fizz and produce carbon dioxide.
What dehydration reaction occurs with alcohols?
Dehydration agent = conc H2SO4
Alcohols that have an H atom on the C next to the OH group can be dehydrated.
Dehydration = removal of a water molecule from a molecule.
Alcohol + conc H2SO4 (at 180°C) → Alkene + H2O
Mechanism = elimination
What are the 2 methods of producing ethanol?
Fermentation or industrial formation from ethene.
Describe the formation of ethanol by fermentation.
glucose → ethanol + carbon dioxide
C6H12O6 → 2CH3CH2OH + 2CO2
The conditions needed are:
• Yeast
• No air
• Temperatures 30-40°C
Type of reaction: Fermentation
- 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.
- Fermentation is done in an absence of air because the presence of air can cause extra reactions to occur.
- It oxidises the ethanol produced to ethanoic acid (vinegar).
Advantages
• sugar is a renewable resource
• production uses low level technology / cheap equipment
Disadvantages
• batch process which is slow and gives high production costs
• ethanol made is not pure and needs purifying by fractional distillation
• depletes land used for growing food crops
Describe the formation of ethanol from ethene.
Reagent: ETHENE - from cracking of fractions from distilled crude oil
CH2=CH2(g) + H2O(g) → CH3CH2OH(l)
Type of reaction: Hydration/addition
Definition: Hydration is the addition of water to a molecule
Essential Conditions
- high temperature 300°C
- high pressure 70atm
- strong acidic catalyst of conc H3PO4
Advantages:
• faster reaction
• purer product
• continuous process (which means cheaper manpower)
Disadvantages:
• high technology equipment needed (expensive initial costs)
• ethene is non-renewable resource (will become more expensive when raw materials run out)
• high energy costs for pumping to produce high pressures
Evaluate ethanol as a biofuel and it being ‘carbon neutral’.
Biofuel = a fuel produced from plants.
Carbon neutral = an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere.
Ethanol produced from fermentation is 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.
Equations to show no net contribution to CO2:
- Removal of CO2 by photosynthesis
6CO2 + 6H2O → C6H12O6 + 6O2
- Production of CO2 by fermentation and combustion
C6H12O6 → 2CH3CH2OH + 2CO2
2CH3CH2OH + 6O2 → 4CO2 + 6H2O
Overall for every 6 molecules of CO2 absorbed, 6 molecules of CO2 are emitted. There is no net contribution of CO2 to the atmosphere.
This does not take into account any energy needed to irrigate plants, fractionally distil the ethanol from the reaction mixture or process the fuel. If the energy for these processes comes from fossil fuels then the ethanol produced is not carbon neutral.
