Transcript Carboxylic Acids - BSAK Chemistry weebly

Carboxylic Acids
Naming: carbon chain stem + oic
General Information
• Functional group
carboxyl
Examples
Ethanoic
Benzoic
Physical Properties
The boiling point of propane is -42oC
Q: Will the b.p. of propan-1-ol be lower or higher
than that of propane? Explain.
A: Significantly higher, due to hydrogen bonding.
propan-1-ol CH3CH2CH2OH 97.2°C
Q: What about the b.p. of ethanoic acid?
A: Higher still due to the formation of dimers.
ethanoic acid CH3COOH
118°C
Q: Are carboxylic acids soluble in
water? Explain.
A: The polar C=O and the ionic O- group mean
that short chain (up to C4) carboxylic acids are
readily water soluble due to H bond formation
with water molecules.
Q: With larger chain acids solubility decreases
proportionally with size. Why?
A: Hydrocarbon chains are hydrophobic (water
fearing).
Chemical Properties
Weak acids - partially dissociate in water:
Are carboxylic acids more or less acidic than
alcohols? Explain
Acidity
The OH group in the acids is more acidic than
the OH group in the alcohols
The carboxylate ion (COO-) is stabilised by
delocalisation of electrons (pi orbitals)
Q: How are they made?
A: Oxidation of primary alcohols and aldehydes
OR : Hydrolysis of a nitrile by boiling with dilute acid
Aromatic Carboxylic Acids
• Benzoic acid is produced by oxidation of the
side chain
Q: What test would you use for the
carboxylic acid functional group?
A: Add PCl5 - misty white fumes of HCl are
produced and this confirms the presence of
the OH group.
It also produces a second product called an acyl
chloride.
CH3COOH + PCl5 CH3COCl + HCl + POCl3
Reactions of Carboxylic Acids
Looking at the structures of the carboxylic acids,
what type of reactions will they undergo?
In your groups write your ideas on a piece of
paper. Don’t forget to justify them!
3 main sites of reaction
• -OH bond gives rise to acidity
• -C=O bond undergoes nucleophilic addition
reactions
• -C-O bond undergoes substitution reactions
Formation of Esters
• General reaction from acid and alcohol as
learnt at AS:
• At A2 we need to see how esters can be
formed from acyl chlorides
Formation of Acyl Chlorides
• PCl5 generates the nucleophile Cld-which
replaces the OH group in a substitution
reaction:
CH3COOH + PCl5 CH3COCl + HCl + POCl3
What other reagents can be used to generate an
acyl chloride from a carboxylic acid?
Write equations for ethanoic acid:
Acyl Chlorides
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Derivatives of carboxylic acids
Contain an acyl group
Reactive substances
Intermediates in organic synthesis
Also called acid chlorides
Naming Acyl Chlorides
carboxylic acid
name
ethanoic acid
acyl chloride name
ethanoyl chloride
acyl chloride
formula
CH3COCl
propanoic acid
propanoyl chloride
CH3CH2COCl
butanoic acid
butanoyl chloride
CH3CH2CH2COCl
Properties of Acyl Chlorides
• Colourless fuming liquids
• Strong smell - a mixture of the smell of
vinegar (ethanoic acid) and the acrid smell of
hydrogen chloride gas.
• The smell and the fumes are because ethanoyl
chloride reacts with water vapour in the air.
Reactions of Acyl Chlorides
Ethanoyl chloride reacts vigorously with water
(Hydrolysis)
Predict what the products of this reaction might be:
Mechanism
Q: What type of reaction is this?
In your groups predict a mechanism for the
reaction.
A: Nucleophilic addition/elimination
Q: In your groups draw the mechanism for this
reaction. Clue: There are three stages
• The first stage (the addition stage of the
reaction) involves a nucleophilic attack on the
fairly positive carbon atom by one of the lone
pairs on the oxygen of a water molecule.
• The second stage (the elimination stage)
happens in two steps. In the first, the carbonoxygen double bond reforms and a chloride
ion is pushed off.
• This is followed by removal of a hydrogen ion
by the chloride ion to give ethanoic acid and
hydrogen chloride
Ease of hydrolysis
Predict and explain the order of relative ease of
hydrolysis of the following:
• Acyl chlorides
• Alkyl chlorides
• Aryl chlorides
You should include diagrams and equations
where appropriate
• Acyl chlorides: These contain a -COCl group,
e.g. ethanoyl chloride, CH3COCl, or benzoyl
chloride, C6H5COCl
• Alkyl chlorides: These have a chlorine
attached to a carbon chain, e.g. chloroethane,
C2H5Cl
• Aryl chlorides: These have a chlorine attached
directly to a benzene ring, e.g. chlorobenzene,
C6H5Cl
Answer
Acyl chlorides are much more reactive towards
hydrolysis than alkyl chlorides.
Aryl chlorides are resistant to hydrolysis.
Why? Notice that the carbon atom being attacked by a
nucleophile is made more positive by the fact that
there are two electronegative atoms attached to it. In
an alkyl chloride, all you have attached is one chlorine
atom which is fairly, but not very, electronegative.
Esters from Acyl Chlorides
• If you add an acyl chloride to an alcohol, you
get a vigorous (even violent) reaction at room
temperature producing an ester and misty
fumes of hydrogen chloride.
• Add the liquid ethanoyl chloride to ethanol,
you get a burst of hydrogen chloride produced
together with the liquid ester ethyl ethanoate.
In your groups
• What reactants are required to make phenyl
ethanoate?
• Write an equation to show this reaction:
Benzoyl Chloride
• Benzoyl chloride has the formula C6H5COCl.
• How does the reactivity of benzoyl chloride
compare to that of ethanoyl chloride? Explain.
• The -COCl group is attached directly to a
benzene ring. It is much less reactive than
simple acyl chlorides like ethanoyl chloride.
Acyl chlorides with ammonia
• Ethanoyl chloride reacts violently with a cold
concentrated solution of ammonia.
• A white product is formed which is a mixture
of two solid products
Predict what the products will be and write an
equation for this reaction.
• The name of the major product is ethanamide
• Unlike the reactions between ethanoyl
chloride and water or ethanol, hydrogen
chloride isn't produced – why not?
Formation of ethanamide
• In your groups draw out the mechanism for
this reaction
• Explain why 2 moles of ammonia are needed
for this reaction
• The first stage (the addition stage of the
reaction) involves a nucleophilic attack on the
fairly positive carbon atom by the lone pair on
the nitrogen atom in the ammonia.
• The second stage (the elimination stage)
happens in two steps. In the first, the carbonoxygen double bond reforms and a chloride
ion is pushed off.
• That is followed by removal of a hydrogen ion
from the nitrogen. This might happen in one
of two ways:
• It might be removed by a chloride ion,
producing HCl (which would immediately
react with excess ammonia to give ammonium
chloride as above) . . .
• And…
• . . or it might be removed directly by an
ammonia molecule
• The ammonium ion, together with the
chloride ion already there, makes up the
ammonium chloride formed in the reaction.
Reactions with amines
• Ethanoyl chloride reacts violently with a cold
concentrated solution of ethylamine. A white
solid product is formed which is a mixture of
N-ethylethanamide (an N-substituted amide)
and ethylammonium chloride.
N means that the methyl group is attached to
the nitrogen atom of the amide.