Transcript ethanoic acid

Organic chemistry
Level 3 AQA Chemistry
Alcohols, carboxylic acids and esters
Alcohols 1 - Meet the family
Alcohols R OH
The first four members of the
homologous series are:
• Methanol
CH3OH
• Ethanol
C2H5OH
• Propanol
C3H7OH
• Butanol
C4H7OH
Think of the alkane,
then take off one
hydrogen atoms,
then replace it with
the –OH functional
group
Alcohols 2
Alcohols contain the functional
group –OH This –OH group is
not like the hydroxide ion in
alkalis.
The most important members of
the homologous series are
methanol and ethanol (which
we call ‘alcohol’).
Ethanol 1
Ethanol C2H5 OH
Think of ethane, C2H6
and then substitute one
of the hydrogen atoms
(C2H5) for the functional
group —OH
This gives ethanol.
C2H5OH
Ethanol 2
Ethanol C2H5 OH
Ethanol’s functional group gives it different
properties from ethane.
These are its main properties:
• it burns well with a clear blue flame
• it is an excellent solvent for perfume and scent
• it is completely miscible with water in all
proportions
• it is a neutral liquid
Ethanol 2 – Good and bad
Good points
Bad points
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the drinks industry is a
major employer
Scotch whisky is a major
export item
Alcohol is a social drug
Large tax revenues are
generated from sales
Alcohols are excellent
solvents
Alcohol gels are used to
clean hands in hospitals
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drinking alcohol can
change your behaviour
it is a depressant
it slows reactions and
makes accidents and
death more likely
Prolonged excessive
drinking causes damage
to the liver and brain
Cost of policing alcoholrelated crime is high
It is a drain on the NHS
Ethanol 3 A great mixer !
Water H-OH and ethanol C2H5-OH molecules are
very similar because of their –OH groups. The
interactions between these different molecules are
very similar to those in pure water and pure ethanol.
Ethanol and water are completely miscible in all
proportions. The solutions are neutral, pH 7.
Alcohol solutions include:
• beers (containing 4 to 5% ethanol)
• wines (containing 10-12% ethanol)
• spirits (containing 40% ethanol)
Fermentation
Ethanol is produced during the
fermentation of sugar.
This reaction is performed by enzymes
found in yeast.
Distilling the resulting solution changes
the alcohol content from about 10%
ethanol to a liquid called rectified
spirit. This has 96% ethanol and 4%
water.
Combustion 1
Alcohols, e.g. ethanol, burn well in air
in an exothermic reaction.
The clear blue flame is partly due to the
internal oxygen in the ethanol
molecule.
C2H5OH + 3O2 → 2CO2 + 3H2O
ΔH = -1370 kJ/mole
Combustion 2
Ethanol makes an excellent petrol
substitute. However …
Good points
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made from a renewable
resource (cane sugar/corn)
sugar and corn take in
CO2 one year when they
grow, and ethanol
burning releases this next
year, so it is carbon
neutral process
Bad points
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fuel crops grow on land
that could be used to feed
people
clearing the land,
transporting the fuel and
manufacturing the
fertilisers requires energy
from fuels … these are not
carbon neutral processes
Reaction with sodium
Alcohols react with sodium metal. The
reaction releases hydrogen gas and
forms sodium ethoxide.
2C2H5OH + 2Na → 2C2H5ONa + H2
In some respects this is similar to the reaction
between water (with its –OH group) and
sodium. During the reaction the O-H chemical
bond is broken by the sodium atom.
2H2O + 2Na → 2NaOH + H2
Oxidation
Ethanol can be oxidised by the oxygen
in the air to form an acid called
ethanoic acid. This is the main acid in
vinegar.
ethanol
ethanoic acid
Carboxylic acids
Homologous Series
Carboxylic acids R’COOH
The first four members of the
homologous series are:
• Methanoic acid
H COOH
• Ethanoic acid
CH3 COOH
• Propanoic acid
C2H5 COOH
• Butanoic acid
C3H7 COOH
Ethanoic acid
Carboxylic acids have
characteristic smells.
Vinegar gets its smell
from ethanoic acid.
All carboxylic acids
have the -COOH
functional group.
The name of the acid above
comes from the fact that it has:
• two carbon atoms
(therefore it will start eth-)
• the functional group –COOH
(so it will end
–anoic)
This is ethanoic acid.
We are family !
Methanoic acid
Ethanoic Acid
As you descend
the homologous
series
• the boiling
points increase
• the densities
decreases
Propanoic acid
Each carboxylic acid molecule has the same functional group,
-COOH. However, the length of the molecule chain affects
the boiling point and the density.
Acidic properties
Because all carboxylic
acids have the same
-COOH functional group
they have remarkably
similar chemical
properties.
One in every 1 000 acid
molecules dissociates into
ions, including hydrogen
ions, H+(aq) making these
compounds acidic.
The O–H bond
breaks leaving
both electrons on
the oxygen,
releasing a
hydrogen ion, H+
Typical acidic behaviour
Carboxylic acids dissolve in water to make
weakly acidic solutions and pH’s less than 7.
They react with metal carbonates to produce
carbon dioxide gas.
ethanoic + sodium = sodium + water + carbon
acid
carbonate ethanoate
dioxide
These acids also react with magnesium metal
to produce hydrogen gas. The reaction is
much slower than with hydrochloric acid the acid molecules are only partially ionised.
Explaining acidic behaviour
Ethanoic acid molecules may split up to form ions.
One in every thousand molecules does this. This is
partial ionisation.
For hydrochloric acid molecules, 100% split up to
form ions. Hydrochloric acid is fully ionised.
Comparing acidic behaviour
Acids like hydrochloric acid that are fully
ionised in solution are called strong acids.
Carboxylic acids are weak acids. Fewer than
1% of the molecules ionise when they are
dissolved in water.
For ethanoic acid, the position of equilibrium
lies well over to the left.
Weak & strong acids - pH
Acids have pH’s below 7. The lower the pH the
more strongly acidic the solution becomes.
The greater the extent of ionisation, the greater the
H+(aq) concentration becomes.
Ethanoic acid’s pH is nearly 3. Ethanoic acid:
• has a higher pH than hydrochloric acid
• is a weaker acid and contains a lower
concentration of hydrogen ions than HCl
Esters
Sweet smelly ester !
Esters R-COO-R’
Complex mixtures of different esters
give fruits their characteristic smells.
Ethyl ethanoate has a smell of pear
drops, the sort you buy in the ‘pound’
shop. Ghastly but somehow
addictive.
Esters form vapours readily. We say
they are volatile. Each ester has a
distinctive smell.
The ester
functional
group
-COO-
Useful esters
The different smells and tastes of esters make them
useful for:
• making sweet smelling perfumes, shampoos
and shower gels
• flavouring foods, sweets and chocolates
Ethyl ethanoate is a useful solvent.
It is cheap, non-toxic and has a pleasant smell.
It is used to clean circuit boards.
It is also used to ‘de-caffeinate’ coffee and tea.
It’s water and raw vegetables for me from now on.
Do ‘Apple’ use a unique ester packaging spray?
Making esters
The acid used is
ethanoic acid,
the alcohol is
ethanol.
The ester made
from this
reaction in ethyl
ethanoate.
The process
is called
esterification.
Making more ester
Sulphuric acid acts as a
catalyst in this reaction
– removing water (a
product of the forward
reaction) – driving the
equilibrium to the
right.
Name that ester in one!
The dotted red
line divides the
acid (on the left)
from the alcohol
(on the right)
Clue