Why aldehydes and ketones have higher boiling points than alcohols?

This video introduces a new group of organic compounds – carbonyl compounds, including the structure, properties and reactions of

• aldehydes
• ketones and
• carboxylic acids (organic acid).

 Structure and Nomenclature

• Aldehyde, ketone and carboxylic acids all contain a carbonyl carbon that is sp2 This means both functional groups contain a C=O bond, of which one is a reactive π-bond, the other is an unreactive s-bond.

Functional group

Suffix

Prefix

Generic structure

Example

Aldehyde

-al

Formyl-

Ketone

-one

Oxo-

Carboxylic acid

-oic acid

Carboxyl-

Nomenclature priority

- In order of decreasing priority: carboxylic acid, aldehyde, ketone, alcohol, alkene, alkyne and alkanes.

- In the presence of carboxylic acid, aldehyde or ketone functional group, an alcohol will be referred to by its prefix ‘hydroxyl’ 

Properties of Aldehydes and Ketones

Boiling and Melting Points

• Smaller aldehydes and ketones are polar molecules and there can form dipole-dipole forces on top of dispersion forces.
• Aldehydes and ketones generally have stronger intermolecular forces than hydrocarbons of similar molecular mass. Thus, they have higher boiling and melting points
• Compared to alcohols, aldehydes and ketones generally have weaker intermolecular forces because they cannot form hydrogen bonds, unlike alcohol molecules. Alcohol molecules contain hydroxyl (–OH) groups that can participate in hydrogen bonding as either a donor or acceptor.

• In their own homologous series, boiling and melting points of aldehydes and ketones increase with molecular mass due to stronger dispersion forces.

Solubility in water

• Aldehydes and ketones are polar compounds, primarily due to the presence of an electronegative oxygen atom in their functional groups. Smaller aldehyde and keton molecules are soluble in water.
• Aldehydes and ketones cannot form hydrogen bonds with themselves (among molecules of only aldehyde and ketone), they can however form hydrogen bonds with water molecules.
• This enables small aldehyde and ketone molecules to dissolve in water (molecules with low number of carbon atoms).
• As the length of non-polar carbon chain increases, the polarity of aldehydes and ketones decreases. This reduces the solubility of aldehydes and ketones in water.

Table: melting and boiling points of aldehydes and ketones increase with molecular weight (size) while their solubilities decrease with molecular weight.

Properties of Carboxylic Acid

Acidity of Carboxylic Acids

• Carboxylic acids are organic weak acids.
• The deprotonation of hydrogen from a carboxylic acid forms a carboxylate ion.

• The proton or hydrogen atom attached to oxygen is acidic because:
  • O–H bond is polarised and weak due to oxygen’s high electronegativity.
  • Resonance stabilisation of the conjugate base (carboxylate ion)

• When carboxylic acids are halogenated, the O–H bond becomes more polarised. This means pKa decreases and acidity increases.

• As the carbon chain of carboxylic acids increases in length, acidity decreases and pKa This is because alkyl groups have the opposite effect to that of halogens.

Boiling and Melting Points

• Carboxylic acids are polar compounds. They are generally considered more polar than aldehydes and ketones due to the presence of an additional electronegative oxygen atom.

• Carboxylic acids can form hydrogen bonds – hydrogen atom attached to oxygen acts as a bond donor while an electron lone pair acts as a bond acceptor.
• The electron lone pair can either be from –OH or C=O.

• Carboxylic acids have much higher boiling and melting points than hydrocarbons, alcohols, aldehydes and ketones of similar molecular weight.

• The formation of two hydrogen bonds between two molecules of carboxylic acid forms a dimer configuration which further increases the strength of dispersion forces between the two molecules.

• As a result of this dimer configuration, carboxylic acids have higher boiling points than alcohols of similar molar mass, despite both functional groups being able to form hydrogen bonds.

Table: compounds that can form hydrogen bonds have, in general, stronger intermolecular force and higher boiling and melting points than those that do not.

Compound

Functional group

Molar mass

(g mol–1)

Type of intermolecular force

Boiling point (ºC)

Alkane

58

Dispersion

–1

Aldehyde

72

Strong dipole

49

Ketone

72

Stronger dipole

56

Alcohol

74

Hydrogen bonding

97

Carboxylic acid

88

Hydrogen bonding

118

Solubility in Water

• Similar to alcohols, aldehydes and ketones, small carboxylic acids are soluble in water.

- Carboxylic acids are more soluble in water than alcohol, aldehydes and ketones of similar molecular weight because they can form more hydrogen bonds.

• Carboxylic acids’ solubilities in water decrease with molecular weight (number of carbons in its chain). The extension of the carbon chain decreases the overall polarity of the molecule.

Reactions of Aldehydes, Ketones and Carboxylic acids

Oxidation

• Oxidation of alcohols produces aldehyde, ketone and carboxylic according to the following table

Reactant

Reagent/catalyst/condition

Product

Mild oxidising agent

• pyridinium chlorochromate (PCC)

Strong oxidising agent

• Acidified potassium permanganate (H+/KMnO4)
• Acidified sodium dichromate (H+/NaCr2O7)
• Jones Reagent (CrO3/H+)
• Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)

Carboxylic acid

Any oxidising agent

• Acidified potassium permanganate (H+/KMnO4)
• Acidified sodium dichromate (H+/NaCr2O7)
• Jones Reagent (CrO3/H+)
• Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)

• Oxidation of an aldehyde produces a carboxylic acid

Carboxylic Acid and Base Reactions

• Carboxylic acids are weak acids that react with Arrhenius and Brønsted-Lowry bases
• Each carboxylic acid functional group is monoprotic i.e. donates one proton
• Carboxylic acid + metal hydroxide  salt + water

Example: reaction between acetic acid (C2H4O2) and sodium hydrogen carbonate to produce sodium acetate, carbon dioxide and water

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