ch 14 alcohols Flashcards
what are the three ways to make alcohols
- hydration (electrophilic addition)
- fermentation
- hydrolysis (nucleophilic subsitution)
alkene->alcohol
- hydration of alkene
- electrophilic addition
- H3PO4
- 300 degrees
- steam
glucose->alcohol
- 37 degrees
- anerobic respiration
- yeast catalyst
haloalkane->alcohol
- hydrolysis
- reflux
- nucleophilic subsitution
- NaOH aq
evaluate hydration vs fermentation to produce alcohols
Hydration of ethene:
ADVANTAGES
100% atom economy
95% yield
Continuous operation
DISADVANTAGES
Ethene is obtained from crude oil which is a finite
energy resource
Fermentation:
ADVANTAGES
Glucose is a renewabl resource
DISADVANTAGES
The concentration of ethanol produced is about
15% ethanol. Distillation increases the
concentration of the alcohol
Food prices may increase with the new market
for glucose.
Uses of methanol:
- clean burning fuel.
- feedstock for the chemical industry.
Uses of ethanol:
- Ethanol in alcoholic drinks:
- perfumes.
- Ethanol as a fuel:
The physical properties of alcohols are influenced by
their ability to form hydrogen bonds:
Definition: Volatility
is the
ease that a liquid turns
into a gas. Volatility
increases as boiling point
decreases.
Volatility and boiling points of alcohols
Alcohols have hydrogen bonds whereas their corresponding alkane (by Mr) has
London forces of attraction. As hydrogen bonds are stronger than London
forces they have higher boiling points. This means that their volatility is
much lower.
The physical properties of alcohols are influenced by their ability to form
hydrogen bonds.
- Alcohols are soluble in water (or other polar solvents) as they will form hydrogen bonds with water (whereas alkanes will not).
- The solubility of alcohols decreases as the carbon chain increases.
- Only the first 3 alcohols are soluble in water.
- The hydrocarbon chain does not form
hydrogen bonds and as it gets longer
it interferes with the ability for the OH
group to hydrogen bond with water.
Polyols
Polyols are molecules that contain more than one –OH
group. When alcohols contain more than 1 hydroxyl group
they are known as diols (2), triols (3) etc.
Classification of alcohols:
This is a Primary (1o) alcohol.
When the OH carbon (*) is
attached to only 1 other
carbon atom.
This is a Secondary (2o)
alcohol. When the OH
carbon (*) is attached to 2
other carbon atoms.
This is a Tertiary (3o) alcohol.
When the OH carbon (*) is
attached to 3 other carbon
state the Reactions of Alcohols
- Combustion of alcohols
- Oxidation of alcohols:
- Esterification (condensation reaction.)
- Dehydration
- SUBSTITUTION
Combustion of alcohols
- In a plentiful supply of oxygen, alcohols will burn to form carbon dioxide and water.
Oxidation of alcohols:
To oxidise alcohols we use acidified potassium or sodium dichromate solution.
This is orange in colour and is a mixture of sulfuric acid, H2SO4/K2Cr2O7 or
H2SO4/Na2Cr2O7.
The orange colour is due to the Cr6+ ions in Cr2O72-
The potassium dichromate is an oxidising agent. This means that it can cause a
species to become oxidised which in turn means that the dichromate becomes
reduced. When this occurs the dichromate becomes green. The green solution
is due to Cr3+ ions.
dichromate will remain
orange.
If the alcohol is resistant to oxidation
If a large excess of dichromate is used
why might it be difficult to see if an
oxidation of the alcohol has occurred?
Excess of orange colour may mask the
formation of the green solution.
Primary alcohol oxidise to form
Aldehyde or carboxylic acid
Primary alcohol to an Aldehyde
Gentle heating of primary alcohols with acidified potassium dichromate make
an aldehyde. The aldehyde must be distilled immediately as it forms.
Primary alcohol to a carboxylic acid
On stronger heating with excess acidified potassium dichromate the alcohol
will be completely oxidised passing through the aldehyde stage to form a
carboxylic acid.
Secondary alcohol to a ketone
- Kr2Cr2O7 / h2so4
- heat
- reflux
Tertiary alcohols oxidation?
Tertiary alcohols cannot be oxidised by acidified potassium dichromate. The
dichromate remains orange and no reaction occurs.
drawing
Apparatus for the oxidation of primary alcohols:
Common errors include:
The water being connected the wrong way to the condenser.
A sealed system - never cut off the condensers as this makes a closed system (increases
the pressure - explosion).
The bulb of the thermometer at the incorrect height - should be parallel to condenser.
Distinguishing between the products of oxidation:
Carboxylic acid:
If refluxed, a primary alcohol forms a carboxylic acid. The acid formed can be identified
by reaction with a base e.g. sodium carbonate. The reaction will produce bubbles as an
immediate observation.
Acid + metal carbonate → salt + water + carbon dioxide
Preparation of esters
Warming a carboxylic acid and alcohol in a
boiling tube with a few drops of concentrated
sulphuric acid catalyst will produce an ester.
Definition: Condensation is
the reaction
whereby two
molecules are joined with
the loss of a small molecule
e.g. water
naming esters
alky alcaonate
alkyl alchohol
alcanote from carboxylic acid
Esterification
An ester is formed when an alcohol is warmed with a
carboxylic acid in the presence of an acid catalyst.
Esterification is an example of a condensation reaction.
Uses of esters
Are used as good solvents and in adhesives.
They have a sweet smell so are more known for their uses in perfumes and
food flavour additives:
Definition: Dehydration
Definition: Dehydration is
an elimination reaction in
which water is removed
from a saturated molecule
to make an unsaturated
molecule.
alcohol->alkene
- dehydration
- conc h2so4
- heat under reflux
- The H and OH
atoms are lost from
adjacent carbon
atoms. The double
bond then forms
between these
carbon atoms.
