alcohols Flashcards
what is the general formula for the alcohol
the alcohol homologous series has a general formula of CnH2n+1OH
when is an alcohol primary, secondary or tertiary
an alcohol is primary depending on which carbon atom the hydroxyl group -OH is bonded to
Primary alchols- the -OH it attached to a carbon with one alkyl group attached
Secondary alcohol - the -OH group is attached to a carbon with two alkyl groups attached
tertiary alcohol - the -OH group is attached to a carbon with three alkyl groups attached
how do we make alkenes from an alcohol
you can make alkenes by eliminating water from alcohols in a dehydration reaction (i..e. elimination of water)
what does a dehydration reaction allow you to produce
this reaction allows you to produce alkenes from renewable resources
you can produce ethanol from the fermentation of glucose, which you can get from plants
we can the dehydrate this ethanol to make ethene which can be used to make polymers e.g. poly(ethene)
this whole process does not require oil
outline the mechanism for the elimination of water from an alcohol
The OH acts as a base
a lone pair of electrons from the oxygen bonds to an H+ from the acid
- The alcohol is protonated giving the oxygen a positive charge
- the positively charged oxygen pulls electrons away from the carbon. An H2O molecule leaves, creating an unstable carbonation intermediate
- the carbonation loses an H+ and the alkene is formed
what are the conditions for acid catalysed elimination
REAGENT: concentrated sulfuric or phosphoric acid
CONDITIONS: warm (under reflux)
ROLE OF REAGENT: dehydrating agent/ catalyst
what happens when you dehydrate longer unsymmetrical alcohols
the result is more than one product
this is because the double bond can go either side of the carbon that had the OH group on it
why must we purify the products of an organic reaction
the products are often impure so you need to get rid of any unwanted by-products or leftover reactants from the reaction mixture
for example, in the dehydration of alcohols to form alkenes, the mixture at the end contains:
- the reactant
- acid
- water
- other impurities
distillation is a technique which used the fact that different chemicals have different boiling points to separate them
example
how do you produce cyclohexane from cyclohexanol and separate and purify the product
reaction and first distillation
1) add concentrated H2SO4 and H3PO4 to a round bottom flask containing cyclohexanone
Mix the solution by swirling the flask and add 2-3carborundum boiling chips (these make the mixture boil more calmly)
2) connect the flask to the rest of the distillation apparatus, including a thermometer, condenser and a cooled collection flask
3) Gently heat the mixture to around 83C ( the boiling point of cyclohexane) using a water bath or electric heater
4) chemicals with boiling points up to 83C will evaporate. The war, gas will rise out of the flask and into the condenser
5) the condenser has cold water running through the outside. The cooler temps turn the gas back into a liquid
6) the liquid can the be collected in a cooled flask
stage 2 in separating and purifying a product: separation
1) the product collected after the first distillation will still contain some impurities which need to be removed
2) transfer the product mixture to a separating funnel and add water to dissolve water-soluble impurities and create an aqueous solution
3) allow the mixture to settle in LAYERS. Drain off the aqueous layer at the bottom leaving behind the impure cyclohexene
the impure cyclohexane is at the top of this layer and the aqueous layer containing water-soluble impurities is at the bottom
what is likely to be the water-soluble impurities
most of the unreacted cyclohexanol
stage three: purification
1) drain the impure cyclohexene into a round bottomed flask
2) add anhydrous CaCl2 (a drying agent- it removes any traces of water) and stopper the flask. Let the mixture dry for at least 20 minutes with occasional swirling
3) the cyclohexene may still have small amounts of impurities so distil the mixture one last time. This time, only collet the product that is released when the product is around 83C - this will be the pure cyclohexane
what are the two methods to make methanol/ ana alcohol
- hydrating alkenes
- fermentation of glucose
what is the standard industrial method for producing alcohol
the standard industrial method for producing alcohols is to hydrate an alkene using steam in the presence of an acid catalyst:
CnH2n + H2O ⮀ CnH2n+1OH
Steam hydration of then is used industrially to produce ethanol
Ethene can be hydrated by steam at 300C and pressure of 60atm. It needs a solid phosphoric acid catalyst
the reaction is reversible and the reaction yield is low -only 5%
however, you can recycle the unreacted ethene gas so the overall yield is a more profitable 95%
what is the mechanism for the hydration of ethene
what are the conditions:
high temps of 300C
high pressures of 60atm-70atm
strong acid catalyst of H3PO4
what are the advantages of using hydration to form an alkene
- faster reaction
- purer product
- continuous process (which means cheaper manpower)
what are the disadvantages of using hydration
- high technology equipment is needed (so the process is expensive)
- ethene is a non renewable resource
3, high energy costs for pumping to high pressures
what is fermentation
it is an exothermic process carried out by yeast in anaerobic conditions (without oxygen)
The equation is:
C6H12O6 🡢 2C2H5OH + CO2
What does yeast do
yeast produces enzymes that convert glucose (C6H12O6) into ethanol and carbon dioxide
The enzyme works at an optimum (ideal) temperature of 30-40 C.
If it is too cold, the reaction is slow - if it’s to hot, the enzymes denatured (damaged).
what happens when the solution reaches about 15% ethanol
the yeast dies
Fractional distillation is used to increase the concentration of the ethanol.
what are the advantages of fermenting glucose to get ethanol
glucose is a renewable source
the production uses low-level technology so the equipment is cheap
what are the disadvantages of fermenting glucose to get ethanol
batch process which is slow and gives high production costs
the ethanol made is not pure and so it needs to undergo fractional distillation which takes time and money
depletes lands that can be used for food crops
what is a biofuel
a biofuel is a fuel that is made from biological material that has recently died
e.g. sugar from sugar canes can be fermented to produce ethanol which can be added to petrol
Ethanol produced in this way is a biofuel (bioethanol)
