Chapter 26: Carbonyls

what is a carbonyl functional group?

C=O

Aldehyde written in structural formula:

CHO

Ketone written in structural formula:

CO

ketone suffix:

-one

aldehyde:

-al

oxidation of aldehydes:

reflux with acidified dichromate ions

do ketones undergo oxidation reactions?

no

the C=O double bond is made up of…

a ⫪-bond

a σ-bond

is a C=C bond polar? how about a C=O double bond?

• C=C = non-polar

  • C=O = polar → Oxygen is more electronegative than carbon

    • electron density in the double bond lies closer to the oxygen than to the carbon

    • this makes the carbon end of the bond slightly positive and the oxygen end slightly negative

In the nucleophilic attack of a carbonyl, what is the nucleophile attracted to?

the slightly positive carbon atom in the C=O double bond

What is NaBH₄ used as?

a reducing agent?

NaBH₄ + aldehyde →

reduced to a primary alcohol

reduction of an aldehyde equation:

what are ketones reduced by NaBH₄?

secondary alcohols

how are reducing agents represented?

[H]

Hydrogen Cyanide:

• colourless

• extremely poisonous

  • boils slightly above room temperature

carbonyl + HCN →

hydroxynitrile (requires H₂SO₄ / HCN)

reaction conditions for the reduction of carbonyls:

NaBH₄ / H₂O

reaction conditions for the reaction of HCN + carbonyl

H₂SO₄ / NaCN

NaBH₄ contains what ion, acting as a nucleophile?

hydride ion → :H^-

mechanism for the reaction of NaBH₄ with a carbonyl:

1. lone pair from hydride ion, :H^-, is attracted and donated to the δ+ carbon atom in the aldehyde or ketone C=O double bond.

2. a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond.

3. The ⫪-bond of the C=O double bond breaks by heterolytic fission, forming a negatively charged intermediate.

4. The oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in water molecule.

5. The intermediate has been then protonated to form an alcohol.

drawn out mechanism for the reduction of a carbonyl with NaBH_4:

addition of a carbonyl compound usually consists of 2 steps:

1. nucleophilic attack on the carbonyl group to form a negatively charged intermediate

2. protonation of the intermediate to form an alcohol

reaction of NaCN/H⁺ steps:

1. long pair of electrons from the cyanide ion, :CN^-, is attracted and donated to the δ+ carbon atom in the aldehyde or ketone C=O double bond. A dative covalent bond forms.

2. The ⫪-bond in the C=O double bond breaks by heterolytic fission, forming a negatively charged intermediate.

3. The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion, to form the product.

4. The product is a hydroxynitrile.

which atom is the negative charge of a cyanide ion on?

the carbon → -CN → needs to be shown in the mechanism

drawn out mechanism of HCN + carbonyl:

what is used to detect a carbonyl group? what colour does it go?

2,4-dinitrophenlyhydrazine → 2,4-DNP

forms a yellow or orange precipitate

In practical work, 2,4-DNP is usually dissolved in…

in methanol and sulphuric acid as a pale orange solution called Brady’s reagent.

Once 2,4-DNP has been used to establish that a compound is a carbonyl, how can we distinguish between aldehydes and ketones?

Tollen’s reagent → solution of silver nitrate in aqueous ammonia.

aldehyde = silver mirror is produced

2 reactions take place during the use of a Tollens’ reagent:

1. oxidation of aldehyde → carboxylic acid

2. reduction of Ag⁺_(aq) ions in the Tollens’ reagent to form Ag_(s) (silver mirror)

solubility of carboxylic acids:

• the C=O and O-H bonds in carboxylic acids 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.

what are even more soluble carboxylic acids?

dicarboxylic acids

carboxylic acids are classified as … acids

weak

when dissolved in water, methanoic acid partially dissolves in water. Draw the reversible reaction established:

HCOOH_(aq) ⇌ H⁺_(aq) + HCOO^-_(aq)

carboxylic acids take place in redox and neutralisation reactions. What are these, and with what?

• redox with metals

• neutralisation with bases (alkalis, metal oxides, carbonates)

After being involved in a neutralisation / redox reaction, what does a carboxylic acid form?

carboxylate salts → COO^-

neutralisation reaction of carboxylic acids with bases:

carboxylic acids react with all bases → metals, alkalis, carbonates.

redox reactions of carboxylic acids with metals:

aqueous solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and the carboxylate salt.

metal = disappears

hydrogen gas = produced

2CH₃CH₃COOH_(aq) + Mg_(s) → (CH₃CH₂COO^-)₂Mg²⁺_(aq) + H_2(g)

carboxylic acids + metal oxides →

salt + water

2CH₃COO_(aq) + CaO_(s) → (CH₃COO^-)₂Ca²⁺_(aq) + H₂O_(l)

carboxylic acid + alkalis →

salt + water

overall: CH₃COOH_(aq) + NaOH_(aq) → CH₃COO^-Na⁺_(aq) + H₂O_(l)

ionic: H⁺_(aq) + OH^-_(aq) → H₂O_(l)

carboxylic acid + carbonate →

salt + water + carbon dioxide

2CH₃COOH_(aq) + Na₂CO_3(aq) → 2CH₃COO^-Na⁺_(aq) + H₂O_(l) + CO_2(g)

text for the carboxyl group?

compound + carbonate → carbon dioxide

what is a derivative of a carboxylic acid?

a compound that can be hydrolysed to from the parent carboxylic acid.

carboxylic acids have a common sequence of atoms in their structure:

acyl group

in an ester, the name comes from

prefix = alcohol

suffix = carboxylic acid

naming of an acyl chloride:

remove the -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride

acid anhydride:

removal of water molecules from 2 carboxylic acid molecules

• an -OH taken from one

• an H⁺ from the other

esterification:

• alcohol + carboxylic acid → ester + water (with conc. sulphuric acid catalyst)

reaction conditions for esterification

small amount of concentrated sulphuric acid → catalyst

what hydrolyses esters?

aqueous acid or alkali

what is hydrolysis?

the chemical breakdown of a compound in the presence of water or in an aqueous solution

acid hydrolysis:

reverse of esterification

• heat ester under reflux with dilute aqueous acid

• ester broken down by water, with acid acting as catalyst

• products = carboxylic acid + alcohol

• reversible

alkaline hydrolysis:

• aka saponification and is irreversible

• ester is heated under reflux with aqueous hydroxide ions

• forms carboxylate ion and an alcohol

alkaline hydrolysis: NaOH + methyl ethanoate →

CH₃COO^-Na⁺ + CH₃OH

preparation of acyl chloride (should be done in a fume cupboard)

parent carboxylic acid + thionyl chloride (SOCl₂) → acyl chloride + SO_2(g) + HCl_(g)

acyl chloride + alcohol →

ester + HCl

acyl chloride + phenols →

phenyl ester + HCl

acyl chloride + water →

carboxylic acid + HCl gas

acyl chlorides + ammonia →

primary amide + ammonium chloride

(the ammonia is a nucleophile)

amide group:

-NH2

primary amide:

the nitrogen atom is attached to 1 carbon atom

what does the name N-methylamide show us?

the methyl group is attached to the nitrogen rather than a carbon atom of the main carbon chain

2 x primary amine + acyl chloride →

secondary amide + ammonium chloride salt

ethanoyl chloride + methylamine →

N-methylethanamide + methylammonium chloride

acyl chlorides are…

very reactive

acid anhydrides are…

less reactive than acyl chlorides:

acid anhydride + alcohol →

ester + carboxylic acid

mechanism of acyl chlorides + nucleophiles →

addition then elimination

if NaOH is used as the alkali in alkaline hydrolysis, what forms?

alcohol + salt: e.g. sodium ethanoate

acid anhydrides react with …

-OH groups

acid anhydrides give…