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Carbonyls Flashcards

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1
Q

What are aldehydes?

A

The carbonyl C=O group is at the end of the carbon chain.
The carbon atom is attached to one or two hydrogen atoms.
Structural formula is CHO.

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2
Q

What are ketones?

A

The carbonyl C=O functional group is joined to two carbon atoms in the carbon chain (middle).
Structural formula is CO.

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3
Q

How are carbonyl compounds named?

A

-al for an aldehyde.
-one for a ketone, ketones need numbering.

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4
Q

What is the oxidation of aldehydes?

A

Aldehydes are oxidised to carboxylic acids when refluxed with acidified potassium dichromate (VI) ions, K2Cr2O7, and dilute H2SO4.
Ketones do not undergo oxidation, so this lack of reactivity is used to distinguish between this and aldehydes.

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5
Q

What is the C=O bond of carbonyls?

A

This is responsible for the reactivity of the ketones and aldehydes.
It is made up of a σ-bond and π-bond.
(Draw diagram)
The π bond is above and below the plane of the carbon and oxygen atoms, formed by the sideways overlap of p oribtals.

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6
Q

Why do carbonyls not react in the same way to alkenes?

A

The C=C bond in alkenes is non-polar.
The C=O bond in carbonyls is polar.
Oxygen is more electronegative than carbon, so the electron density is closer to the oxygen than carbon atom.
This makes the carbon slightly positive and oxygen slightly negative.

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7
Q

What reactions do carbonyls and alkenes undergo?

A

Due to the polarity of the C=O bond, aldehydes and ketones react with some nucleophiles.
A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the C=O bond.
Carbonyls - nucleophilic addition, alkenes - electrophilic addition.

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8
Q

What is the reduction of aldehydes?

A

Aldehydes are reduced to primary alcohols by NaBH4.
Usually warmed in aqueous solution.
CH3CH2CHO + 2[H] —> CH3CH2CHOH
Make sure to include 2 of the reducing agent [H].

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9
Q

What is the reduction of ketones?

A

Ketones are reduced to secondary alcohols by NaBH4.
Usually warmed in aqueous solution.
CH3COCH3 + 2[H] —> CH3CHOHCH3
Make sure to include 2 of the reducing agent [H].

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10
Q

What is the mechanism for reduction of a carbonyl by nucleophilic addition?

A

(Draw)
NaBH4 contains the hydride ion :H-, which acts as the nucleophile.
The lone pair of electrons from :H- is attracted and donated to the S+ C atom in the C=O bond.
The π-bond in the C=O bond breaks by heterolytic fission, forming a negative intermediate.
The oxygen atom of the intermediate donates a lone pair to a hydrogen atom in a water molecule.
The intermediate has been protonated to form an alcohol.

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11
Q

What is hydrogen cyanide?

A

Hydrogen cyanide (HCN) adds across the C=O bond of aldehydes and ketones.
HCN is a colourless, poisonous liquid that boils at room temperature, so cannot be used safely.
Sodium cyanide (NaCN) and sulfuric acid are used to provide the hydrogen cyanide, but are still hazardous.

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12
Q

What is the reaction of carbonyl compounds with HCN?

A

This reaction increases the length of the carbon chain.
CH3CH2CHO + HCN –H2SO4/NaCN–> CH3CH2CHOHCN
The organic product formed contains a hydroxyl group -OH and a nitrile group C≡N, classified as hydroxynitriles.

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13
Q

What is the mechanism for the reaction of carbonyls with HCN by nucleophilic addition?

A

(Draw)
The cyanide ion :CN- attacks the electron deficient carbon atom in the carbonyl.
The lone pair of electrons from :CN- is attracted and donated to the S+ carbon atom in the C=O bond, a dative covalent bond forms.
The π-bond in the C=O bond breaks by heterolytic fission, forming a negative intermediate.
The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the product, a hydroxynitrile.

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14
Q

What is the trick for finding the origin of hydroxynitriles?

A

If it forms a cross shape, it was a ketone.
If it is triangular, it was an aldehyde.

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15
Q

How are carbonyl compounds detected?

A

A solution of 2,4-dinitrophenylhydrazine (2,4-DNPH) or brady’s reagent will form a yellow or orange precipitate in the presence of an aldehyde or ketone.
It is usually dissolved in methanol and sulfuric acid as a pale orange solution called brady’s reagent.
The mixture can then be recrystallised and melting points compared to known values.

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16
Q

How are aldehydes and ketones distinguished from one another?

A

Tollen’s reagent - a solution of silver nitrate in aqueous ammonia, will produce a silver mirror in the presence of an aldehyde.
It contains Ag+ ions, which act as an oxidising agent in the presence of ammonia, to form a carboxylic acid from an aldehyde.

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17
Q

What is the practical method for using Tollen’s reagent?

A

To form Tollen’s: Add AgNO3, then add aq NaOH until a brown precipitate of silver oxide is formed.
Add dilute ammonia solution to form a clear colourless solution.
To test for aldehyde: Add the substance to tollens and heat to see if a silver mirror forms.

18
Q

What is a carboxylic acid?

A

An organic acid with the COOH functional group.
Found in medicines, fruit juices, vinegar, and rhubarb leaves.
Carboxylic acids and its derivatives are used in organic synthesis as starting materials or intermediates of more useful compounds.
Aspirin is synthesised from salicylic acid.

19
Q

How are carboxylic acids named?

A

The suffix -oic acid is added to the stem of the longest carbon chain.
The carboxyl carbon atom is always carbon-1.

20
Q

What is the solubility of carboxylic acids?

A

The C=O and O-H bonds are polar allowing carboxylic acids to form hydrogen bonds with water molecules.
Carboxylic acids with up to 4 carbon atoms are soluble in water.
As the number of carbon atoms increases, the solubility decreases as the non-polar chain has a greater effect on the overall polarity of the molecule.
Dicarboxylic acids are solid at RTP and dissolve readily in water.

21
Q

What is the strength of carboxylic acids?

A

Carboxylic acids are weak acids.
When dissolved in water, they only partially dissociate.
HCOOH <—> H+ + HCOO-

22
Q

What are redox reactions of carboxylic acids with metals?

A

Aqueous solutions of carboxylic acids react with metals in redox to form hydrogen gas and the carboxylate salt.
Observe the metal disappearing and effervescence.
2CH3CH2COOH(aq) + Mg(s) –> (CH3CH2COO-)2Mg2+(aq) + H2(g)

23
Q

What is the reaction of carboxylic acids with metal oxides (base)?

A

Carboxylic acid + metal oxide –> Salt + water
2CH3COOH(aq) + CaO(s) –> (CH3COO)2Ca(aq) + H2O(l)

24
Q

What is the reaction of carboxylic acids with alkalis?

A

Carboxylic acid + alkali –> Salt + water
Might not observe any reaction as two solutions react together to form an aqueous solution of the salt.
CH3COOH(aq) + NaOH(aq) –> CH3COONa(aq) + H2O (l)

25
Q

What is the reaction of carboxylic acids with carbonates (base)?

A

Carbonate + carboxylic acid –> CO2 + salt + H2O
If the carboxylic acid is in excess a solid carbonate would disappear, bubbles form.
2CH3COOH(aq) + Na2CO3(aq) –> 2CH3COONa(aq) + H2O (l) + CO2(g)

26
Q

What is the test for carboxylic acid?

A

The neutralisation reaction of carboxylic acids with carbonates, forms bubbles.
Carboxylic acids are the only organic compounds acidic enough to react with carbonates.
Useful to distinguish from phenols, which are not acidic enough.

27
Q

What is a derivative of a carboxylic acid?

A

A compound that can be hydrolysed to form the parent carboxylic acid.
They have a common sequence of atoms in their structure - an acyl group. RC=O(R)
Esters, acyl chlorides, acid anhydrides and amides.

28
Q

What is the reactivity of carboxylic acid derivatives?

A

Acyl chloride
Acid anhydride
Ester
Amide
Carboxylic acid

29
Q

What is an ester?

A

To name, remove the -oic acid suffix and replace with -oate.
The alkyl chain attached to the oxygen atom of the COO group is added as the first word.
E.g. Methyl ethanoate.
Sweet smelling liquids, used in perfumes, and flavourings.

30
Q

What is esterification?

A

The reaction of an alcohol with a carboxylic acid to form an ester.
Warmed alcohol + carboxylic acid, with a small amount of concentrated H2SO4 as a catalyst.
Forms water as a by-product.
Propanoic acid + ethanol –> ethyl propanoate + water.

31
Q

What is acid hydrolysis of esters?

A

The ester is heated under reflux with dilute aqueous acid.
The ester is broken down by water, with the acid acting as a catalyst.
Forms carboxylic acid and alcohol, and is reversible.

32
Q

What is alkaline hydrolysis of esters?

A

Saponification, irreversible.
Ester is heated under reflux with aqueous hydroxide ions.
Forms a carboxylate ion (or salt if using NaOH) and an alcohol.
CH3COOCH3 + NaOH –> CH3COONa + CH3OH
Methyl ethanoate + sodium hydroxide –> sodium ethanoate + methanol

33
Q

What are acyl chlorides?

A

To name, remove the -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride.
Propanoic acid –> propanoyl chloride

34
Q

What is the preparation of acyl chlorides?

A

Carboxylic acid + thionyl chloride, SOCl2.
SO2 and HCl products are evolved as gases, leaving only the acyl chloride.
It should be done in a fume cupboard as the products are harmful.

35
Q

What are the reactions of acyl chlorides?

A

They are very reactive and very useful in organic synthesis as they are easily converted into carboxylic acid derivatives, with good yields.
Acyl chlorides react with nucleophiles by losing the chloride ion whilst retaining the C=O bond.

36
Q

What is the reaction of acyl chlorides with alcohols?

A

Acyl chlorides + alcohol –> ester + HCl
Ethanoyl chloride + propan-1-ol –> propyl ethanoate + Hydrogen chloride

37
Q

What is the reaction of acyl chlorides with phenols?

A

Carboxylic acids are not reactive enough to form esters with phenols, but acyl chlorides and acid anhydrides are, to produce phenyl esters, without acid catalysts.
Ethanoyl chloride + phenol –> Phenyl ethanoate + HCl

38
Q

What is the reaction of acyl chlorides with water?

A

A violent reaction occurs with the evolution of dense steamy HCl fumes, to form a carboxylic acid.
Ethanoyl chloride + H2O –> Ethanoic acid + HCl

39
Q

What is the reaction of acyl chlorides with ammonia?

A

Ammonia acts as a nucleophile by donating the lone pair of electrons on the nitrogen atom to an electron deficient species, to form amides.
Ammonia and acyl chlorides forms a primary amide.
The Nitrogen atom is attached to one carbon atom.
Ethanoyl chloride + 2NH3 –> Ethanamide + NH4Cl (ammonium chloride)

40
Q

What is the reaction of acyl chlorides with amines?

A

Amines act as a nucleophile by donating the lone pair of electrons on the nitrogen atom to an electron deficient species, to form amides.
A primary amine reacts with acyl chloride to form a secondary amide.
The nitrogen atom is attached to 2 carbon atoms.
Ethanoyl chloride + methylamine (2CH3NH2) –> N-methylethanoamide + CH3NH3Cl (methylammonium chloride)

41
Q

What is an acid anhydride?

A

It is formed by the removal of water from two carboxylic acid molecules.
2ethanoic acid –> ethanoic anhydride + water

42
Q

What are the reactions of acid anhydrides?

A

React similarly to acyl chlorides with alcohols, phenols, water, ammonia and amines.
They are less reactive than acyl chlorides and are useful for lab prep where acyl chlorides would be too reactive.
2(CH3CO)2O + C6H5OH –> CH3COOC6H5 + CH3COOH
ethanoic anhydride + phenol –> phenyl ethanoate + ethanoic acid

Chemistry (24 decks)

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