Carbonyls

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  • 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)

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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.

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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.

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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.)

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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.

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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.

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Benzene Structure

(http://images.tutorvista.com/cms/images/101/delocalization-of-benzene.png)

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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

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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

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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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