MODULE – IV
- Aromatic Chemistry
- Carbonyl Compounds
- Carboxylic Acids and Derivatives
- Preparation and Properties of some Important Inorganic Compounds
P L A N N I N G
- In aromatic chemistry we are going to take up some important reactions like halogenation, oxidation of alkylbenzene, nitrocompounds and aromatic amines.
- Here we shall mainly focus on different methods of preparation of carboxyls and their reactions with special emphasis to condensation (Aldol, Claisen), oxidation and reduction (Cannizzaro) and other reduction (MPV, LAH) and oxidation reactions (Bayer Villiger oxidation) we shall also focus on the point of difference of aldehydes and ketones (in terms of reaction.)
- In carboxylic acid our focus should be on methods of preparation of acid , the relative order of reactivity of derivatives with special emphasis in esters.
- In amines our point focus is on the sythesis and reaction of amines (1°,2° and 3°)
- In this chapter we have mainly given emphasis to preparation and properties of certain important inorganic compounds
Alkylbenzene (Toluene) ⎯⎯→ Halogenation
Halogenation of toluene can be carried out in 2 ways
(i) side chain chlorination when the hydrogen of the side chain are replaced by chlorine
(ii) Nucleus chlorination where the hydrogens of the ring are replaced giving o & p products
The products of nucleus chlorination are resistant towards hydrolysis while the side chain products can be hydrolysed.
Side chain or benzylic halogenation taken place when the reaction is carried out in absence of Lewis acid and under conditions that favour formation of radical.
The greater stability of bezylic radicals accounts for the fact that when ethylbenzene is halogenated the major product is 1-halo-1-phenylethane.
Addition to double bond of Alkenyl benzenes
In absence of peroxides HBr adds to the double bond of 1-Phenyl propene to give 2- bromo 1 phenylpropane as the major product .
Strong oxidising agents oxidises toluene to benzoic acid eg. hot Alk. KMnO4. Alkyl benzenes with alkyl groups longer than methyl are ultimately degraded to benzoic acids.
Nitro compounds: Nitrobenzene can be reduced in different media to give different products.
In dilute acid solution phenyl hydroxylamine rearranges to p – aminophenol.
In aniline direct nitration cannot be carried out as NH2 being an activating group activates the ring ; so that any oxidising atmosphere will result in the charring of ring.
Illustration 1 : Aniline ⎯⎯→ O – Bromo Aniline
Mono and dialkylanilines ( and quaternary compounds) as their hydrochlorides undergo rearrangement on strong heating an alkyl group migrating from nitrogen atom and entering preferentially the p – position.
ALDEHYDE AND KETONES
Carbonyl compounds include both aldehydes and ketones having the formula where R and R′ may be different or same
Methods of Preparation
(a) A primary alcohol an oxidation gives an aldehyde and this on further oxidation gives an acid. Both aldehyde and acid contains the same number of carbon atoms as the original alcohol.
RCH2OH RCHO ⎯⎯→ RCOOH
It is better to use pyridinium chlorochromate (PCC) which does not oxidise the aldehyde further.
Secondary alcohols on oxidation gives ketones
R2CHOH R2C = O
There is however specific reagent for oxidising secondary alcohols to ketones by using aluminum t – butoxide [(CH3)3CO]3Al in acetone medium t – butyl alkoxide is used since t – butyl alcohol produced is not further oxidised under these condition.
By pyrolysis of Ca, Ba salts of fattyacids
Formic acid salts will always produce aldehydes.
The reaction first leads to the formation of β – keto acids which then eliminate CO2 to give the desired ketone.
Intramolecular chelation by hydrogen bonding through a 6 membered cyclic TS.
Rosenmund Reaction (From Acyl chloride)
Aldehydes are more easily reduced than acid chlorides and therefore we should expect to obtain the alcohol as the final product. It is BaSO4 that by acting as a poison to the palladium catalyst in this reaction prevents reduction of aldehydes further
RC ≡ N
R – C ≡ C – H + H2O
From 1,2 diols
illustration-1 : One mole of each of the following is treated with HIO4. What product will be formed and how many moles HIO4 will be consumed.
(c) cis – 1,2 cyclopentandiol
Solution: (a) CH3CHO and HCHO 1 mole HIO4
(b) HCHO and OHC CH2OCH3 1 mole HIO4
(c) 1 mole HIO4
As far reactivity is concerned ketones are less reactive than aldehydes because of (I) less electrophilicity of the carboxyl carbon and (ii) steric effect of R groups
- Addition of HCN
⎯⎯→ α – hydroxy acid
- Addition of NH2 – Z types of substances
The reaction is acid catalysed and fully reversible