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6.2Organic Chemistry and Analysis

Carbonyl Compounds

5 min read 9 parts Lesson 6 of 33 Virtual lab

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Lesson summary

Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Carbonyl Compounds Contents Reactions of Carbonyl Compounds Testing for Carbonyl Compounds Page 2 Reactions of Carbonyl Compounds …

Part 1 of 7Aldehydes and ketones

Aldehydes and ketones
Carbonyl group
  • C=O, in both aldehydes and ketones, which differ in the groups bonded to the carbonyl carbon
Aldehydes
  • Aldehydes the carbonyl group is at the end of the chain
  • The carbonyl carbon is always carbon 1, so no locant is written.
  • The simplest is methanal, HCHO.
Ketones
  • Ketones the carbonyl group is in the middle of the chain, with an alkyl group on each side
  • The simplest is propan-2-one, CH3COCH3.

Part 2 of 7Oxidation of alcohols and carbonyls

Dnph carbonyl positive negative test
Dnph carbonyl positive negative test
Dnph carbonyl positive negative test
Dnph carbonyl positive negative test
Oxidation of alcohols and carbonyls
Primary alcohol to aldehyde
  • Primary alcohol to aldehyde set up to distil off the aldehyde as it forms
Primary alcohol to carboxylic acid
  • Primary alcohol to carboxylic acid heat under reflux
  • The aldehyde evaporates, condenses and drops back to be oxidised further.
Oxidising agent
  • Oxidising agent acidified potassium dichromate, K2Cr2O7 with H2SO4, for all these oxidations
Aldehydes
  • Aldehydes easily oxidised to carboxylic acids
Ketones
  • Ketones resistant to oxidation, so secondary alcohols give ketones only
  • Ketones have no readily available hydrogen on the carbonyl carbon.
  • A very strong oxidising agent would break a C–C bond destructively.

Part 3 of 7Nucleophilic addition: the carbonyl group

Carbonyl nucleophilic addition and protonation
Carbonyl nucleophilic addition and protonation
Carbonyl nucleophilic addition and protonation
Carbonyl nucleophilic addition and protonation
Nucleophilic addition: the carbonyl group
Polarity
  • Polarity oxygen is more electronegative than carbon, so the carbonyl carbon is δ+ and the oxygen δ−
Reactivity
  • Reactivity the δ+ carbon is attacked by nucleophiles such as the cyanide ion
General mechanism
(aldehydes and ketones)
  • The nucleophile attacks the carbonyl carbon, forming a negatively charged intermediate.
  • The oxygen then quickly picks up a proton.

Part 4 of 7Addition of HCN

Mechanism
  • Mechanism nucleophilic addition in two steps
  • Step 1: the cyanide ion attacks the carbonyl carbon, forming a negatively charged intermediate.
  • Step 2: the negative oxygen reacts with H+ (from HCN, water or dilute acid) to form a 2-hydroxynitrile such as 2-hydroxypropanenitrile.
Importance
  • Importance it adds a carbon to the chain, increasing chain length
Naming
  • Naming the nitrile is the priority group and is carbon 1
    • (suffix -nitrile)
  • The hydroxyl group is named with the prefix hydroxy-, not -ol.

Part 5 of 7Reduction of carbonyls

Products
  • Products aldehydes are reduced to primary alcohols and ketones to secondary alcohols
Reducing agent
  • Reducing agent sodium tetrahydridoborate, NaBH4 , in aqueous solution
    • (sodium borohydride)
  • It generates the hydride ion nucleophile, :H−, so the reaction is nucleophilic addition.
  • H− is attracted to the carbonyl carbon but repelled by the electron-rich C=C, so it does not reduce C=C bonds.
Carboxylic acids
  • Carboxylic acids reduced to primary alcohols
  • In theory this passes through the aldehyde, but in practice the acid goes straight to the primary alcohol;
    • check the wording of the question.

Part 6 of 7Testing with 2,4-DNPH

Propanone dnph hydrazone condensation
Propanone dnph hydrazone condensation
Propanone dnph hydrazone condensation
Propanone dnph hydrazone condensation
Test for carbonyls
  • 2,4-dinitrophenylhydrazine (2,4-DNPH) detects the C=O group in aldehydes and ketones
  • The carbonyl group undergoes a condensation reaction, in which two molecules join and a small molecule such as H2O is eliminated.
Positive result
  • Positive result a deep orange precipitate
  • It can be purified by recrystallisation.
  • Its melting point is compared with literature values to identify the specific aldehyde or ketone.
Specificity
  • Specificity carboxylic acids and esters do not give a positive result

Part 7 of 7Testing with Tollens reagent

Tollens silver mirror test
Tollens silver mirror test
Tollens silver mirror test
Tollens silver mirror test
Reagent
  • Tollens reagent contains the colourless silver(I) complex [Ag(NH3)2]+, formed by adding aqueous ammonia to silver nitrate
    • (ammoniacal silver nitrate)
Aldehyde
  • Aldehyde on gentle warming it is oxidised to a carboxylic acid
  • Ag+ acts as the oxidising agent and is reduced to silver metal, forming a silver mirror, the positive result.
Ketone
  • Ketone no reaction and no silver mirror, a negative result because ketones cannot be oxidised
Use together
  • Use together use 2,4-DNPH to show a carbonyl is present, then Tollens reagent to distinguish aldehyde from ketone

Key terms

TermMeaning
Carbonyl groupThe C=O functional group found in aldehydes and ketones
Nucleophilic additionAttack of a nucleophile on a δ+ carbon followed by addition of H+
2,4-DNPHA reagent giving an orange precipitate with aldehydes and ketones
Tollens reagentAmmoniacal silver nitrate that forms a silver mirror with aldehydes
Condensation reactionTwo molecules join with elimination of a small molecule
HydroxynitrileA compound with both –OH and –CN groups

Avoid these mistakes

1
Saying ketones are easily oxidised — they resist oxidation, unlike aldehydes
2
Saying NaBH4 reduces C=C bonds — the hydride is repelled by the C=C
3
Naming a hydroxynitrile with -ol — the hydroxy- prefix is used as nitrile has priority
4
Using 2,4-DNPH to distinguish an aldehyde from a ketone — use Tollens reagent
Try it in the virtual labIdentifying organic compounds \n