Organic and Biological Chemistry 4.4 Aldehydes and Ketones 1
Transcript
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4.4 Aldehydes and Ketones 1
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Aldehydes and Ketones 2 Carbonyl functional group Aldehyde:
terminal carbon Ketone: non terminal carbon
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Aldehydes and Ketones Aldehydes are prepared by a controlled
oxidation of the corresponding primary alcohol. The aldehyde is
distilled off as it forms to prevent further oxidation. 3
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Aldehydes and Ketones Aldehydes are oxidised by acidified
dichromate ions to the corresponding carboxylic acid. 4
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Aldehydes and Ketones Ketones are prepared by the oxidation of
secondary alcohols. No further oxidation occurs. 5
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Aldehydes and Ketones When heated with Tollens reagent
(ammoniacal silver nitrate), aldehydes are oxidised to carboxylate
ions. RCHO RCOO The silver diamine ion is reduced to metallic
silver which can be deposited on the test tube giving a silver
mirror. Ag(NH 3 ) 2 + Ag 6
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Aldehydes and Ketones Ketones (and alcohols) do not oxidise
with Tollens reagent. The silver mirror test is commonly used to
identify between aldehydes and ketones. It can also be used to
identify between an aldehyde and alcohol where dichromate ions
would react with both. 7
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8 Vanillin Tetracycline (antibiotic)
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4.5 Carboxylic Acids 9
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Carboxylic Acids 10 Carboxyl group Carboxylic acids can be
prepared by oxidation of primary alcohols or aldehydes with excess
acidified dichromate solution.
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Carboxylic Acids Carboxylic acids are weak acids If they are
soluble in water then they will partially ionise to form hydronium
and carboxylate ions. RCOOH + H 2 O H 3 O + + RCOO Equilibrium
position favours reactants. 11
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Carboxylic Acids Carboxylic acids react readily with bases such
as: Hydroxides and Oxides (Products: Salt & Water) RCOOH + OH
RCOO + H 2 O Carbonates (Products: Salt,Water and CO 2 ) 2RCOOH+CO
3 2 2RCOO +H 2 O+CO 2 Hydrogen carbonates (Products: Salt,Water and
CO 2 ) RCOOH + HCO 3 RCOO + H 2 O+CO 2 12
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Carboxylic Acids Carboxylate salts of sodium and potassium are
water soluble due to strong ion-dipole interactions which form
between the negative carboxylate ion and the water molecules
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Ion dipole interactions Strongest type of secondary interaction
Occurs between a full positive or negative charge (ion) and a polar
molecule Common between carboxylate ions and water or protonated
amines and water 14
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Carboxylic Acids Many drugs including (aspirin and other
painkillers) contain a carboxyl group. These drugs are often mixed
with solid sodium hydrogen carbonate which converts the acid to the
water soluble carboxylate on mixing with water. This makes the drug
easier to take and quicker acting that it would be in its insoluble
form. 15
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16 Salicylic Acid Citric Acid
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4.6 Amines 17
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Amines 18 Based on ammonia 1 o (primary) amine 2 o (secondary)
amine
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Amines 19 3 o (tertiary) amine Amines tend to have unpleasant
odours, often fishy.
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Amines Like ammonia, amines can act as a base accepting a
proton from an acid NH 3 + H + NH 4 + RNH 2 + H + RNH 3 + The
product of this reaction is referred to as the protonated form of
the amine The protonated form is soluble in water due to ion dipole
interactions between the protonated amine ion and water 20
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Amines Many drugs (eg. Anaesthetics) contain amine functional
groups These drugs are insoluble in water and so would not be
effective if administered as the amine By protonating these drugs
they are able to dissolve in water within the body and so can act
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Esters 24 Similar structure to carboxylic acids Hydrogen on
carboxyl group replaced by alkyl group Fruity odour Ester
functional group Alcohol Acid
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Naming Esters Identify the alcohol carbon chain and the carbon
chain from the acid. Name the alcohol part first followed by the
acid 25 Ethyl pentanoate
Production of Esters Esters are formed by the reaction of a
carboxylic acid and alcohol This reaction is called ESTERIFICATION
but can also be referred to as a CONDENSATION reaction because
water is one of the products. The reaction is slow at room temp and
so the reaction mixture must be refluxed for an extended period and
concentrated sulfuric acid is used as the catalyst This is an
equilibrium reaction 27
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28 Reflux
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Production of Esters 29 The yield of the ester can be increased
by using excess of one of the reagents. (Usually the alcohol
because it is cheaper) Large carbon chains (R groups) on the acid
and alcohol decrease the yield of ester
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Production of Esters The ester can be separated from the
mixture using a separating funnel The mixture is washed with sodium
bicarbonate which ensures the acid dissolves in the water layer
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Production of Esters The aqueous layer which contains any
unreacted alcohol and the sodium carboxylate salt can be tapped off
leaving the organic layer This layer is then distilled and the
ester is collected at the appropriate t b 31
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32 Distillation
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Reactions of Esters If an ester is refluxed with aqueous acid
or base, it undergoes hydrolysis In acidic conditions the products
formed are a carboxylic acid and an alcohol. The reaction mixture
is refluxed with sulfuric acid as the catalyst This is the reverse
of esterification 33 Acidic conditions
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Reactions of Esters In alkaline conditions the products formed
are the carboxylate ions and the alcohol The ester is refluxed with
sodium hydroxide solution and the alcohol product can be separated
from the sodium carboxylate salt by distillation The carboxylic
acid will reform if a solution of strong acid is added to the
sodium carboxylate salt. 34 Alkaline conditions