Alcohols Flashcards
Naming 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 otherfunctional groups that need a suffix ending then the OH can be named with the prefix hydroxy-
If the re are two or more -OH groups then di, tri are used.
Add the ‘e’ on to the stem name though eg propane1,2,3triol
Bond angles in alcohols
All the H-C-H bonds and C-C-O are 109.5o (tetrahedral shape), because there are 4 bonding pairs of electrons
repelling to a position of minimum repulsion.
The H-O- C bond is 104.5o (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.
Boiling points of acohols
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.
Primary alcohols
Primary alcohols are alcohols
where 1 carbon is attached to
the carbon adjoining the
oxygen.
Secondary alcohols
Secondary alcohols are alcohols
where 2 carbon are attached to
the carbon adjoining the oxygen.
Tertiary alcohols
Tertiary alcohols are alcohols
where 3 carbon are attached to
the carbon adjoining the oxygen
What is the oxidising agent that causes alcohols to oxidise
Potassium dichromate K2Cr2O7
Partial oxidation of primary alcohols
Reaction: primary alcohol —> aldehyde
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it for
Naming aldehydes
An aldehyde’s name ends
in–al.
It always has the C=O bond on the first carbon of the chain so it does not need a number in its name
When writing the formulae of aldehydes in a condensed way write CHO and not COH e.g.CH3CH2CHO
Equation for oxidation of primary alcohol propan-1-ol
CH3CH2CH2OH + [O] —> CH3CH2CHO + H2O
[O] represents O from oxidising agent.
Observation for partial and full oxidation of primary alcohols
Observation: the orange dichromate ion
(Cr2O7 2-) reduces to the green Cr 3+ ion
Distilation
In general used as separation technique to separate an organic product from its reacting mixture.
In order to maximise yield collec e collection flask can be
cooled in ice to help improve
the yield of distillate ted, only collect the distillate at the approximate boiling point of the desired aldehyde and not high
Key points for setting up distilation method
The bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point
The water goes in to the bottom of the
condenser to go against gravity. This allows more efficient cooling and prevents back flow
of water.
Why are elctrical heaters often used to heat organic chemicals
Electric heaters are often used to heat organic chemicals. This is because organic chemicals are normally highly flammable and could set on fire with a naked flame.
Full oxidation of primary alcohols
Reaction: primary alcohol —> carboxylic acid
Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
Conditions: use an excess of dichromate, and heat under reflux: (distil off product after the reaction
has finished)
Equation of full oxidation of propan1ol
H3CH2CH2OH + 2[O] —> CH3CH2COOH + H2O
propan-1-ol —> propanoic acid
What is reflux
Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids.
Why is the reflux never sealed at the end of the condenser?
Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up.
Why are antibumping granules added to the flask in both distilation and reflux
To prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles.
Drawing and labelling reflux accurately
It’s important to be able to draw and label this apparatus accurately.
• Don’t draw lines between flask and condenser.
• Don’t have top of condenser sealed
• Condenser must have outer tube for water that is sealed at top and bottom
• Condenser must have two openings for water in and out
that are open
Oxidation of secondary alcohols
Reaction: secondary alcohol —> ketone
Reagent: potassium dichromate(VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
Naming ketones
Ketones end in-one
When ketones have 5C’s or more in a chain then it needs a number to show the position of the double
bond. E.g. pentan-2-one
Can ketones oxidise further
There is no further oxidation of the ketone under these conditions
Can tertiary alcohols be oxidised by potassium dichromate?
Tertiary alcohols cannot be oxidised at all by potassium dichromate: This is because there is no hydrogen atom bonded to the carbon with the -OH group
Distinguishing between Aldehydes and Ketones
Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised. This is the
chemical basis for two tests that are commonly used to distinguish between aldehydes and ketones.
Tollens’ Reagent - Reagent: and conditions
Tollens’ reagent formed by mixing
aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]+
Heat gently
Reaction of tollens reagent
aldehydes only are oxidised by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms
Observation witn tollens reagent
With aldehydes, a silver mirror forms coating the inside of the test tube.
Ketones result in no visible change
Example equation of a reaction with tollens reagent.
CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+
Fehlings solution reagent and conditions
Fehling’s solution
Reagent: Fehling’s solution containing blue Cu 2+ ions.
Conditions: heat gently
Fehlings solution reaction
Reaction: aldehydes only are oxidised by Fehling’s solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide
Fehlings solution observation
Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O.
Ketones do not react
Example equation of fehlings solution reaction
CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + Cu2O + 4H+
How can the presence of 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.
Reaction of Alcohols with Dehydrating Agents
Reaction: Alcohol —> Alkene
Reagents: Concentrated sulfuric or phosphoric acids
Conditions: warm (under reflux)
Role of reagent: dehydrating agent/catalyst
Type of reaction: acid catalysed elimina
What is a dehydration reaction
Dehydration Reaction: removal of a
water molecule from a molecule
Reaction of Alcohols with Dehydrating Agents eg propan1ol
Propan-1-ol —> propene
CH3CH2CH2OH —> CH2=CHCH3 + H2O
When is more than one product formed when alcohols react with dehydrating agents
Some 2o and 3o alcohols can give
more than one product, when the
double bond forms between different carbon atoms
Example of alcohols producing two products with dehydrating agents.
Butan-2-ol can form both alkenes
but-2-ene although more but-2-ene would be formed
But-2-ene could also exist as E and Z
isomers
What does producing alkenes from alcohols do
Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil.
What are the two ways ethanol can be formed
Comparing two methods for producing ethanol: Fermentation or industrial formation from ethene
Equation for fermentation
Glucose —> ethanol + carbon dioxide
C6H12O6 —> 2 CH3CH2OH + 2 CO2
Conditions for fermentation
The conditions needed are:
•Yeast
•No air
•temperatures 30 –40oC
What is the optimum temperature for fermentation and why
The optimum temperature for fermentation is around
38oC
At lower temperatures the reaction is too slow.
At higher temperatures the yeast dies and the enzymes
denature
Why is fermentation done in an absence of air?
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)
What are the advantages of fermentation
•sugar is a renewable resource
•production uses low level technology / cheap equipment
What are the disadvantages of fermentation
•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
Essential conditions for extrancting ethanol from ethene
high temperature 300°C
high pressure 70 atm
strong acidic catalyst of conc H3PO4
Reagent for extracting ethanol from ethene
Reagent: ETHENE - from cracking of
fractions from distilled crude oil
What type of reaction is extracting ethanol from ethene
Type of reaction: Hydration/addition
Definition: Hydration is the addition of water to a molecule
Equation for extracting ethanol from ethene
CH2=CH2 (g) + H2O (g) —> CH3CH2OH (l)
Advantages for extracting ethanol from ethene
faster reaction
•purer product
•continuous process (which means cheaper manpower)
Disadvantages for extracting ethanol from ethene
•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
pressure
What is a biofuel
A biofuel is a fuel produced from plants
What does carbon nuetral mean
The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”
When is ethanol reffered to as a biofuel
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 atmosphe
Equations to show no net contribution to CO2 (removal of co2)
Removal of CO2 by photosynthesis
6 CO2 + 6 H2O —> C6H12O6 + 6 O2
6 CO2 molecules are removed from the atmosphere when the plants grow
Equations to show no net contribution to CO2 (production of CO2 by fermentation and combustion)
C6H12O6 —> 2 CH3CH2OH + 2 CO2
2 CH3CH2OH + 6O2 —> 4 CO2 + 6 H2O
When 1 molecule of glucose is fermented 2 molecules of CO2 is
emitted. The two ethanol molecules produced will then produce 4 molecules of CO2 when they are combusted.
Overall for every 6 molecules of CO2 absorbed, 6 molecules of CO2 are emitted. There is no net
contribution of CO2 to the atmospher
Why is production of ethanol not carbon nuetral
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 neutra;
