A2 Chemistry

1
Redox and addition of Carbonyls

• The oxidation and reduction reaction of Carbonyl
  Compounds including nucleophilic addition

2
Oxidation

• Aldehydes can be easily oxidised to carboxylic
  acids
• This is the end of a two stage process that
  started with a primary alcohol

3
Oxidation

• This is more simply represented as
• Ketones cannot be oxidised without the molecule
  being broken up.
• Notice the reagent potassium dichromate

4
Oxidation

• Aldehydes are powerful reducing agents.
• Reduce copper 2 to copper (I)
• Silver (I) to metallic silver
• There are used as a basis to distinguish
  aldehydes from ketones.

5
Reduction

• It is possible to make primary and secondary
  alcohols by reduction of aldehydes and ketones.
• Aldehydes give 1 alcohols.

6
Reduction

• Ketones give 2 alcohols.

7
Reduction

• Notice the reducing agent is in both cases
• NaBH4
• aka sodium tetrahydridoborate (III)

8
Reduction

• Other possible reducing agents include
• Lithium tetrahydridoaluminate (III) LiAlH4 needs
  to be in ethoxyethane water eliminated.
• Sodium in ethanol
• Hydrogen gas under pressure with platinum catalyst

9
Reduction

• Remember.
• It is okay to use O to represent an oxidation
• It is also okay to use H to represent a
  reduction
• You must balance these though!

10
Nucleophilic addition to the carbonyl bond

• Alkenes undergo electrophilic addition
• So why dont carbonyls?
• Its all down to the electronegativity of the
  Oxygen atom
• Pulls electron density away from the carbon so
  that the bond is polar.

11
Nucleophilic addition to the carbonyl bond
12
Nucleophilic addition to the carbonyl bond

• This partial withdrawal of electron density from
  the carbon exposes it to attack by negatively
  charged nucleophiles.
• Nucleophiles are often negatively charged (CN-),
  but sometimes are neutral molecules with a lone
  pair of electrons e.g. NH3.

13
Nucleophilic addition to the carbonyl bond

• The basic mechanism for nucleophilic attack on a
  carbonyl is

14
Nucleophilic addition to the carbonyl bond

• Need to know the reaction between aldehydes and
  ketones with hydrogen cyanide
• Hydrogen cyanide (HCN) is too toxic to use, so
  potassium or sodium cyanide is used with a
  mixture of sulphuric or hydrochloric acid.

15
Nucleophilic addition to the carbonyl bond

• The Mechanism

16
Nucleophilic addition to the carbonyl bond

• Nitriles can be hydrolysed to carboxylic acids by
  refluxing with dilute hydrochloric acid. This
  gives a way of synthesising 2-hydroxycarboxylic
  acids.
• Perhaps the most important is 2-hydroxypropanoic
  acid lactic acid. A product of anaerobic
  respiration.

17
Condensation reactions of the carbonyl bonds

• Ammonia or better still hydrazine (NH2NH2) can
  add across the double bond
• Water is eliminated and a substituted hydrazone
  is formed.
• The most important of these is the reaction of
  carbonyls with 2,4-dinitrophenylhydrazine
  (2,4-DNPH).

18
Condensation reactions of the carbonyl bonds

• 2,4-dinitrophenylhydrazine has the following
  structure.

19
Condensation reactions of the carbonyl bonds

• This reaction occurs as follows

20
Condensation reactions of the carbonyl bonds

• This substituted 2,4-dinitrophenylhydrazone
  produces a precipitate of orange crystals.
• These are essential in identifying unknown
  carbonyl.

21
Identifying an unknown Carbonyl

• React unknown with 2,4-DNPH, orange ppte formed
• Filter and recrystalise the orange crystals, dry
  thoroughly.
• Find melting point of crystals, this will be
  sharp if crystals are pure
• Look up in melting point tables to identify the
  unknown carbonyl.

22
Testing for aldehydes and ketones

• Distinguishing between carbonyls and other
  organic compounds is quite easy.
• Simply add 2,4-DNPH as above.
• The trick is to distinguish between aldehydes and
  ketones themselves.

23
Testing for aldehydes and ketones
Is it a carbonyl?
Add 2,4-DNPH
Orange Precipitate Carbonyl
Tollens Silver Mirror
Aldehyde or Ketone?
Fehlings test
No result Ketone
Positive Aldehyde
Red/brown ppte aldehyde
24
Tollens Test

• Reduction of Silver (I) to silver (0) using
  aldehydes
• This is seen as a silver mirror on the inside of
  the test tube.

25
Tollens Test

• The creation of the reagent in this case is quite
  complex
• Ag has to be in alkaline solution for this
  reaction to occur
• Addition of hydroxide ions (OH-) precipitates
  silver oxide (Ag2O)
• Ammonia is the complexing agent

26
Tollens Test

• CH3CH2CHO O ? CH3CH2COOH
• 2Ag(NH3)2(aq) H2O(l) ? 2Ag(s) O
  2NH4(aq) 2NH3(aq)

27
Tollens Test

• Add ammonia carefully to about 3cm3 of silver
  nitrate in a clean test tube
• Stop adding drops when the precipitate is just
  redissolved
• Careful when adding ammonia XS is not good!
• This clear solution now contains the complex ion
  Ag(NH3)2.
• A couple of drops of the aldehyde are added to
  the clear solution
• The solution is warmed gently in warm water
• A positive result silver mirror. A dirty test
  tube will give a black or grey ppte.