Transcript carboxylic acidsx

CARBOXYLIC ACIDS AND
THEIR DERIVATIVES
Alkanoic (Carboxylic) Acids
•
•
•
•
General Formula: R-COOH
Naming:
Suffix -anoic acid
CH3COOH ethanoic acid
CH2ClCH2COOH 3-chloropropanoic acid
General Properties
• Organic acids contain a carbonyl group (C=O)
and a hydroxyl group (-OH) together on the end
carbon.
• Low mass carboxylic acids are oily liquids with a
strong smell and are miscible with water.
• Higher mass carboxylic acids are waxy solids
with no smell and are not miscible with water.
• Carboxylic acids have significantly higher boiling
points than their corresponding alcohols or
alkanes.
Preparation
• can be made by the oxidation of primary
alcohols (those alcohols with the -OH group on
the end carbon). An oxidising agent such as
acidified potassium dichromate or acidified
potassium permanganate may be used.
• can also be prepared using certain bacteria.
Vinegar is made by fermenting sugar (from
barley, apple juice or whey) to make ethanol,
then oxidising the ethanol to ethanoic (acetic)
acid, using a special bacteria.
Reactions
•
Organic acids are weak acids. They react with bases, carbonates and
reactive metals like inorganic acids, only more slowly. The H from the -OH
group is lost forming RCOO - and H+. The name -oic acid changes to anoate ion. e.g. the ion formed from methanoic acid (HCOOH) is the
methanoate ion (HCOO -).
e.g. CH3COOH(aq) + NaOH(aq)
•
CH3COONa(aq) + H2O(l)
carboxylic acids also react with alcohols to form esters and water:
HCOOH
+ HOCH2CH3
CH3CH2OCHO + H2O
methanoic acid ethanol
ethyl methanoate water
The reaction requires an acid catalyst. Usually a drop of concentrated
sulfuric acid is used (it also acts as a dehydrating agent, uses up the water
& speeds up the reaction).
Derivatives of Carboxylic Acids include acid (acyl) chlorides, esters, amides
and amino acids.
Esters
• Esters contain the functional group COO.
O
R
C
O
R'
R and R’ are alkyl chains. E.g. CH3COOCH2CH2CH3
• Naming: Name the part of the molecule that comes
from the alcohol first, like a side chain (e.g. ethyl). Then
name the part that comes from the acid, like an acid ion
(e.g. methanoate)
CH3CH2CH2COOCH2CH3
ethyl butanoate
General
• Esters have lower melting and boiling points than the
carboxylic acid and alcohol from which they were
formed.
• Esters are not very soluble in water except for methyl
methanoate.
• Low mass esters are volatile with distinctive, often fruity
odours.
• Used as solvents, perfumes and in flavourings. The
scents of many flowers are due to esters.
• Higher mass esters are waxy solids with little odour.
Complex esters are contained in fats and oils.
Preparation of esters
•
Esters are produced by esterification reactions. These are
condensation reactions involving an alcohol and a carboxylic acid
(with conc, H2SO4 as catalyst) or an alcohol and an acid chloride.
•
carboxylic acid and alcohol – heated under reflux with conc.
Sulfuric acid as catalyst. Excess acid is then neutralised with
sodium carbonate, and anhydrous magnesium sulfate is added to
remove excess water. Because the esters are more volatile than
the rest of the mixture they can be removed by fractional
distillation.
•
acid chloride and alcohol – acid chloride dropped into pure
alcohol in a fume cupboard. Reaction is fast and yield is high. No
heat or catalyst required. Most efficient way to prepare esters.
Hydrolysis of esters
• This is the reverse of esterification – the ester is split into two
smaller molecules. Under acidic conditions the product is an alcohol
and a carboxylic acid.
CH3COOC2H5 + H2O
H+
C2H5OH + CH3COOH
• Under basic (alkaline) conditions the products are an alcohol and a
carboxylate ion/salt.
--
CH3COOC2H5 + OH
--
C2H5OH + CH3COO
• More product is favoured under alkaline conditions since the alkali
shifts the equilibrium in favour of the more unreactive carboxylate
salt.
• The alkaline hydrolysis of esters is known as saponification. Soap
making is the saponification of the triester of glycerol.
The saponification of triester
of glycerol
CH2COO(CH2)16CH3
CH2OH
CHCOO(CH2)16CH3 + 3NaOH
CHOH + 3CH3(CH2)16CH2COONa
CH2COO(CH2)16CH3
CH2OH
• The carboxylate salt, sodium stearate is what we
know as soap. Other long chain carboxylate
salts of sodium are also used as soap.
Acid (Acyl) Chlorides
• Derived from carboxylic acids with a chlorine atom
replacing the hydroxyl group (-OH) of the carboxylic acid
COOH group.
COOH
COCl
• the IUPAC name for acid chloride is alkanoyl chloride.
Acid chloride functional group
R
Acid chloride functional
group
C
Cl
O
• naming follows the usual rules: e.g. CH3COCl is ethanoyl
chloride, CH3CH2COCl is propanoyl chloride.
Properties
• Acid chlorides have low melting and
boiling points as there is no hydrogen
bonding between molecules.
• They exist as pungent smelling, fuming
liquids.
Reactions
• They are highly reactive because the carbon of the acid chloride
group is attached to two very electronegative atoms, Cl and O. This
means there is a relatively large + charge on the carbon atom, and
it is therefore open to attack by nucleophiles.
• The reactions that acid chlorides undergo are nucleophilic
substitutions.
• With water: acid chlorides react violently with water to form a
carboxylic acid and hydrogen chloride. The hydrogen chloride
dissolves in moist air to form droplets of hydrochloric acid. This is
the fumes that are given off by acid chlorides.
CH3COCl + HOH
CH3COOH + HCl
Reactions (Continued)
• With ammonia: acid chlorides react readily with ammonia to form
amides and ammonium chloride:
CH3COCl + HNH2 + NH3
Ethanoyl
chloride
2NH3
ammonia
CH3CONH2 + NH4Cl
ethanamide ammonium
chloride
• with aminoalkanes: acid chlorides react with primary aminoalkanes
forming secondary amides and hydrogen chloride:
CH3COCl + 2CH3CH2NH2
CH3CONHCH2CH3 + C2H5NH3Cl
• with alcohols: acid chlorides react readily with alcohols to form an
ester and hydrogen chloride. This is often a preferred way to
prepare an ester as it needs no catalyst.
CH3COCl + C2H5OH
CH3COOC2H5 + HCl
Reaction Summary for Ethanoyl
Chloride
Ethyl ethanoate
Ethanoic acid
Ethanoic acid
PCl5 or
SOCl2
Ethanoyl
chloride
Ethanamide
N-ethyl ethanamide