Carbonyls
- Created by: ava.scott
- Created on: 04-10-14 15:23
Tollens reagent
Ammoniacal Silver Nitrate
Used to distinguish between aldehydes and ketones.
Aldehydes react because they are oxidised further into carboxylic acids, but ketones cannot react further.
Aldehydes gives a positive test, showing the silver mirror effect.
CH3COH + [O] >> CHOOH +H2O
Ag+(aq) + e- >>> Ag(s)
Brady's reagent
2,4-dinitrophenylhydrazine
Detects a carbonyl group in a organic compund.
When mixed, an orange precipitate forms called 2,4-dinitrophenylhydrazone.
This is derivatie is filtered and recrystalised.
Then its melting point is measured, and compared to a database to find the carbonyl derivative's identity.
Oxidation of acohols
Oxidation uses H+/Cr2o7(2-) Potassium dichromate
Primary alcohols are oxidised once to form aldehydes. They must be isolated by distillation.
CH3CH2OH + [O]>> CH3CHO +H2O
Primary alcohols can be oxidised twice to make carboxylic acids. This is usually done by reflux, in acidic conditions.
CH3CH2OH +2[O] >> CH3COOH + H2O
In alkaline conditions, carboxylate ions are produced from aldehydes instead.
CH3CHO + [O] >>> CH3CHO- and H+
Secondary alcohols can only be oxidised once, to make ketones.
CH3CHOHCH3 + {O} >>> CH3COCH3 +H2O
Tertiary alcohols cannot be oxidised.
Reduction of alcohols
Uses sodium tetrahydridoborate/ NaBH4, by nucleophilic addition.
Carboxylic acids cannot be reduced.
Aldhdyes are reduced back to primary alcohols.
CH3CHO + 2[H] >> CH3CH2OH
Ketones are reduced to secondary alcohols
CH3COCH3 + 2[H] >>. CH3CHOHCH3
Each carbonyl must be reduced twice, once by a hydride ion (H-) which donates an electron pair to a positively charge carbon atom. The double oxygen bond is broken, and then the postive carbon atom pulls a hydrogen from a neighbouring water molecule (which has a dipole.)
Phenol
The additional OH group means that an additional lone pair (form a p-orbital) is donated to the delocalised pi system of the benzen ring. This increases it selectron density, and allows it to induce dipoles easily e.g. in halogens.
It can act as a very weak acid, as the H+ dissociates.
Phenol + NaOH >>> Phenol-Na+ (Sodium Phenoxide) and H2O
2Phenol + 2Na >>> 2Phenol-Na+ (Sodium Phenoxide) and H2
it also brominates very eaily.
Phenol + 3Br2 >> 2,4,6-tribromophenol + 3HBr
Phenol was first used as a disinfectant after surgery, but caused major skin irritation.
It is now used in production of plastics, antiseptics, disinfectants (hard surfaces) and resins for paints.
Benzene Structure
Kekule structure showed that between each carbon there was alternating double and single bonds.
However:
- Benzene has a higher enthalpy of hrodgenation than expected- this is alled resonance energy.
- After analysis of crystals, t was found that all bonds in benzene are of the same length, and no shorter and longer like double and single bonds.
- Benzene is also relatively resistant to reactions that cyclohexene, and other doubler-carbon bond contaning compounds undergo.
We now know that benzene actually jas 6 delocalised electrons from hyrbrid sp-orbitals, that overlap above and below the carbon chain, to make a delocalised pi system. The bond angles sit at 120 degrees from eachother.
Benzene Structure
Bromination of benzene
Needs AlBr3 as a catalyst.
Step 1- making the electrophile
AlBr3 +Cl2 > AlBr4- and Br+
Step 2- addition of halogen
Electrons jump from benzene ring to Br+. Benzen ring is now positively charged, and benzene is bonded to hyrdogen and haloge. Electrons from hydrogen bond, jump to the benzne ring, and release H+ ion.
Step 3- catalyst is rejuevnated
AlBr4- +H+ >> AlBr3 + HBr
OVERALL:
Benzene + Br2 >>>> Bromobenzene + HBr
Nitration of Benzene
Needs H2SO4 and 50 degrees centigrade
Step 1
H2SO4 + HNO3 > HSO4- + H2O + NO2+ (nitronium ion)
Step 2
Benzen delcoalised pi system donates electron pair to the nitonium ion, and bonds to it. The ring is now positive, and benzen is bonded to hydrogen and nitronium. The Hydrogen bond donates a electron pair to the ring, and its released as a H+ ion.
Step 3
H+ + HSO4- >> H2SO4
Bromination: Benzene v Phenol
Benzene needs a catalyst. Phenol doesnt, as it reacts via ELECTROPHILIC ADDITION with bromine water.
Benzene results in one bonded halogen/bromine. Phenol is brominated at 2, 4 and 6 carbons, making 2,4,6- tribromophenol which is a white solid.
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