OCR A 6.1.2 Carbonyl Compounds

aldehyde

carbonyl group on end

-al

ketone

carbonyl grp not on end

-one

oxidation of aldehydes

-oxidised to carboxylic acids using oxidising agent

oxidation of ketones

not oxidised

tollens reagent

-used to distinguish between aldehyde and ketone

-contains [Ag(NH₃)₂]⁺

make tollens reagent

-put silver nitrate sol in test tube

-add few drops of NaOH: pale brown ppt forms

-add few drops of dilute of ammonia until ppt dissolves

test using tollens reagent

-add unknown to it in test tube

-place in hot water bath

-if aldehyde present, silver mirror forms: silver reduced and coats test tube and oxidises into carboxylic acid

-if ketone present nothing forms

Brady’s reagent

-2,4-dinitrophenylhyrazine

-2,4DNP

-used to distinguish between aldehydes and ketones

-dissolved in conc sulfuric acid and methanol

testing with brady’s reagent

-when added if carbonyl grp present, bright orange ppt forms

-ppt is derivative of a carbonyl compound 2,4 dinitrophenylhyrazone

-fitlter and recrystalize to find pure

If melting point is sharp then pure if not sharp then recrastalizs

-find melting point and compare to table of known melting points

reduction agent for carbonyls

NaBH₄ (sodium tetrahyrdridoborate)(III))

dissolved in methanol and water

represented by [H]

reduction of carbonyls

-reduced to alcohols

-nucleophilic addition

aldehydes reduction

-to primary alcohols

ketones reduction

-to secondary alcohols

carbonyl reduction mechanism explanation

-lone pair of e^- from H^-(nucleophile) attracted and donated to S⁺ C atom in C=O bond

-dative covalent bond formed between nucleophile and C

-pi bond in C=O breaks by heterolytic fission; forming negative intermediate

-O atom of intermediate donates lone pair to H atom in H₂O molecule

-alcohol forms

carbonyl reduction mechanism

KCN reactions with carbonyl

-produces hydroxynitriles

-nucleophilic addition

-nucleophile attacks C=O and adds on

-reagent is HCN

HCN

-colourless, poisonous,

-made by reacting NaCN and H₂SO₄

-reaction is useful as it increases chain length

HCN + carbonyl mechanism

HCN + carbonyl mechanism

-lone pair from cyanide ion attracted and donated to S⁺ C atom in C=O bond

-dative covalent bond formed between nucleophile and C

-pi bond in C=O breaks by heterolytic fission; forming negative intermediate

-intermediate protonated by donating lone pair of electrons H ion

-hydroxynitrile forms

KCN risks

-irritant; v dangerous if ingested or inhaled

-if reacts w/ moisture can form toxic gas HCN

KCN usage precautions

wear:

-gloves

-lab coat

-safety goggles

use fume cupboard to prevent exposure to toxic fumes