ALCOHOLS Flashcards
what are the bond angles in alcohols?
- all the H-C-H bonds and C-O are 109.5 (tetrahedral shape)
- the H-O-C bond is 104.5 (bent line shape),
what is the reasoning behinds the bond angle in alcohols?
H-C-H there are 4 bonding pairs of electrons repelling to a position of minimum repulsion
H-O-C 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
is the boiling point of alcohols low or high and why?
The alcohols have relatively low volatility and
high boiling points due to their ability to form
hydrogen bond between alcohol molecules
can alcohols dissolve in water?
Smaller alcohols can dissolve in
water because they can form
hydrogen bonds to water molecules.
what is the difference between a primary, secondary and tertiary alcochol?
-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 is the oxidising agent that causes alcohols to oxidise?
Potassium dichromate K2Cr2O7
is an oxidising agent
that causes alcohols to oxidise.
what does the exact reaction of the oxidation of alcohols depend on?
The exact reaction, however, depends on the
type of alcohol, i.e. whether it is primary,
secondary, or tertiary, and on the conditions.
what is the reaction for the partial oxidation of alcohols?
primary alcohol -> aldehyde
what is the reagent for the partial oxidation of alcohols?
potassium dichromate (VI) solution and dilute sulfuric acid.
what are the conditions for the partial oxidation of alcohols?
(use a limited amount of dichromate)
warm gently
distil out the aldehyde as it forms:
what is the difference between an aldehyde and a ketone?
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
WHEREAS
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
what is distillation used for?
In general used as separation technique to separate
an organic product from its reacting mixture.
how do we maximise yield in distillation?
In order to maximise yield collected, only collect the distillate
at the approximate boiling point of the desired
aldehyde and not higher.
where in the distillation apparatus should the bulb of the thermometer be and why?
The bulb of the thermometer should be at
the T junction connecting to the
condenser to measure the correct boiling
point.
why does the water in the distillation apparatus go in to the bottom of the condenser and backwards?
- to go against gravity.
This allows
more efficient cooling and prevents back flow
of water
what is used to heat organic chemicals and why?
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
what can be done to the collection flask to improve the yield of the distillate?
The collection flask can be
cooled in ice to help improve
the yield of distillate
what is the reaction for the full oxidation of primary alcohols?
primary alcohol->carboxylic acid
what is the reagent for the full oxidation of primary alcohols?
potassium dichromate(VI) solution and dilute sulfuric acid
what are the conditions for the full oxidation of primary alcohols?
use an excess of dichromate
heat under reflux:
(distil off product after the reaction
has finished)
what is the observation when a primary alcohol is partially oxidised?
Observation: the
orange dichromate ion
(Cr2O7 2-) reduces to
the green Cr 3+ ion
what is the observation when a primary alcohol is fully oxidised?
Observation: the orange dichromate ion (Cr2O7 2-)
reduces to the green Cr 3+ ion
when is reflux used?
Reflux is used when heating organic reaction mixtures for long
periods.
what does the condenser in the reflux apparatus do?
The condenser prevents organic vapours from escaping
by condensing them back to liquids.
why should you never seal the end of the condenser in reflux apparatus?
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.
where and why are anti bumping granules added?
Anti-bumping granules are added to the flask in both distillation and
reflux to prevent vigorous, uneven boiling by making small bubbles
form instead of large bubbles.
what is the reaction for the oxidation of secondary alcohols?
secondary alcohol-> ketone
what is the reagent for the oxidation of secondary alcohols?
potassium dichromate(VI) solution and dilute sulfuric acid
what is the condition for the oxidation of secondary alcohols?
heat under reflux
what is reaction for the oxidation of tertiary alcohols?
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
how can we distinguish between aldehydes and ketones?
Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised.
we can test using: tollens’ reagent and fehling’s reagent
what is the tollens’ reagent?
Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]+ .
what are the conditions needed or the tollens’ reagent?
heat gently
what is the reaction that happens when aldehydes and ketones are reacted with tollens’ reagent?
aldehydes only are oxidised by
Tollens’ reagent into a carboxylic acid. The
silver(I) ions are reduced to silver atoms
what is the observation that occurs when aldehydes and ketones are reacted with tollens’ reagent?
with aldehydes, a silver mirror
forms coating the inside of the test tube.
Ketones result in no visible change
write the equation for ethanal reacting with tollens reagent?
CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+
what is the reagent for the chemical test for fehlings solution?
Fehling’s solution containing blue Cu 2+
ions
what is the condition for fehlings solution?
heat gently
what is the reaction that aldehydes and ketones have with fehling’s solution?
aldehydes only are oxidised by
Fehling’s solution into a carboxylic acid. The
copper (II) ions are reduced to copper(I)
oxide .
what is the observation for when aldehydes and ketones react with fehling’s solution?
Aldehydes :Blue Cu 2+ ions in
solution change to a red precipitate of Cu2O.
Ketones do not react
write the equation for ethanal reacting with fehling’s solution?
CH3CHO + 2Cu2+ + 2H2O -> CH3COOH + Cu2O + 4H+
how can we test for the presence of carboxylic acids?
The presence of a carboxylic acid can be tested by addition of sodium carbonate. It will fizz and produce
carbon dioxide
what is the reaction of alcohols with dehydrating agents?
Alcohol -> Alkene
what are the reagents for the reaction of alcohols with dehydrating agents?
Concentrated sulfuric or phosphoric acids
what is the condition for the reaction of alcohols with dehydrating agents?
warm (under reflux)
what is the role of the reagent in the reaction of alcohols with dehydrating agents?
dehydrating agent/catalyst
what type of reaction is the reaction of alcohols with dehydrating agents?
acid catalysed elimination
what are the 2 different products that can be formed when butan-2-ol reacts with a dehydrating agent?
-but-1-ene
-but-2-ene
more but-2-ene would be formed
but-2-ene could exist as E and Z isomers
what is the benefit of being able to produce alkenes form alcohols?
Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil.
what are the 2 methods of producing ethanol?
-fermentation
industrial formation from ethene
what is the equation for the fermentation of glucose?
glucose-> ethanol + carbon dioxide
C6H12O6 –> 2CH3CH2OH + 2CO2
what are the conditions needed for fermentation?
- Yeast
- No air
- temperatures 30 –40oC
what is the optimum temperature for fermentation?
around 38 degrees celcius
what happens to the fermentation reaction when the temp is away from the optimum?
At lower temperatures the reaction is too slow.
At higher temperatures the yeast dies and the enzymes
denature.
why is fermentation done in the 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
what is the reagent used to form ethanol from ethene?
ETHENE - from cracking of
fractions from distilled crude oil
what is the equation for forming ethanol from ethene?
CH2=CH2 (g) + H2O (g) -> CH3CH2OH (l)
what type of reaction is the formation of ethanol from ethene?
Hydration/addition
define hydration
Hydration is the addition of water to a molecule
what are the conditions required to form ethanol from ethene?
- high temperature 300 °C
- high pressure 70 atm
- strong acidic catalyst of conc H3PO4
what are the advantages of forming ethanol from ethene?
•faster reaction
•purer product
•continuous process (which means cheaper
manpower)
what are the disadvantages of forming 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
pressures
what is a biofuel?
A biofuel is a fuel produced from plants
what is meant by the term carbon neutral?
The term carbon neutral refers to “an activity that
has no net annual carbon (greenhouse gas)
emissions to the atmosphere
what method of producing ethanol forms a biofuel?
Ethanol produced from fermentation is a biofuel.
why can it be argued that producing ethanol by fermentation is a carbon neutral process?
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
what are the equations to show to net contribution to CO2?
Removal of CO2 by photosynthesis
6 CO2 + 6 H2O ->C6H12O6 + 6 O2
Production of CO2 by fermentation and combustion
C6H12O6 -> 2 CH3CH2OH + 2 CO2
2 CH3CH2OH + 6O2 -> 4 CO2 + 6 H2O
Overall for every 6 molecules of CO2 absorbed, 6 molecules of CO2 are emitted. There is no net
contribution of CO2
to the atmosphere.
why can it be argued that producing ethanol by fermentation is NOT a carbon neutral process?
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.
