Carbonyl vs. Carboxyl Compounds

The compounds having a carbonyl (>C=O) group are called carbonyl compounds. In a carbonyl group, there is a double bond between carbon and oxygen atom, and the carbon is further attached to hydrogens or carbons. Whereas the compounds having the (-COOH) function group i.e. hydroxyl (OH) group attached to the carbonyl (C=O) group are called carboxylic compounds.

Carbonyl vs Carboxyl compounds

Carbonyl compounds Carboxylic compounds
The compounds having C=O functional group are called carbonyl compounds The compounds having COOH functional group are called carboxylic compounds
Compounds with only carbonyl (C=O) functionality constitute carbonyl compounds Compounds with carbonyl (C=O) functional group attached to hydroxyl group constitute carboxylic compounds
Carbonyl compounds do not have hydrogen bonding with in their molecules Carboxylic compounds have strong hydrogen bonding
They have relatively low melting and boiling points They have high melting and boiling points
The suffix “al” or “one” is used when naming carbonyl compounds When naming carboxylic compounds, a suffix “oic acid” is used
They cannot form dimers They can form dimers
Carbonyl compounds cannot donate a proton (H+). They are not acidic Carboxylic compounds can donate a proton (H+) and that is why they are called acids
Carbonyl groups can be placed anywhere in the hydrocarbon chain except terminals Carboxylic acid groups are terminal functional groups
The carbon atom of carbonyl group can be attached to either hydrogen or carbon The carboxyl group can be attached with one alkyl group or hydrogen atom because -OH covers one of the sides
For example Acetone, formaldehyde, and acetaldehyde, etc For example Acetic acid, benzoic acid and formic acid, etc

Summary of the differences between carbonyl and carboxyl compounds

Here is the detailed summary of the parameters of comparison in between carbonyl and carboxylic compounds:

1. Hybridization of carbonyl and carboxylic compounds

Both carbonyl and carboxylic compounds are sp2 hybridized. Three sp2 hybridized orbital forms three σ bonds, one with oxygen and two with other groups attached to it. The fourth orbital of carbon is unhybridized and forms a pi (π) bond with a p orbital of the oxygen atom.

Pi-bonding overlap between pz orbitals

2. Reactivity of carbonyl vs carboxyl compounds

The carbonyl group is a relatively reactive functional group of organic compounds. This is due to the difference in electronegativity between carbon and oxygen. As oxygen is more electronegative than carbon, it attracts the shared pair of electrons more towards itself and becomes partially negative, making carbon partially positive. Thus, the carbonyl group becomes polar and a nucleophile or an electrophile can attack it.

Partial positive charge on carbon and partial negative charge on oxygen

The carbonyl group has a double probability of attachment of reacting species because of its planer shape. Reacting species can attack above or below the plane of a carbonyl group.

The polar nature of the carbonyl group also increases its reactivity.

Related resources

3. Nomenclature of carbonyl and carboxylic compounds

Nomenclature of carboxylic compounds

Common name

Many carboxylic compounds are better known by their common name rather than systematic or IUPAC names. Common names are derived from Greek or Latin words. They are generally derived from the source of the compounds. For example,

  • Formic acid is derived from the Latin word ‘formica‘ meaning ant, as it was obtained from an ant bite.
  • Acetic acid is derived from the Latin word ‘acetum‘ meaning vinegar.
  • Butyric acid is derived from the Latin word ‘butyrum‘ meaning butter.

IUPAC Names

Rules for assigning IUPAC names to carboxylic compounds are:

  • Select the longest chain containing the carboxylic group.
  • Change the ‘e’ of the corresponding alkane to ‘oic acid’.
  • The numbering of carbons starts from carboxylic carbon.
  • Since the carboxylic group is always at the end of the chain, its position is not mentioned.
  • Substituents are named and their position is indicated.

IUPAC naming of carboxylic acids

Nomenclature of aldehydes

Common names

Aldehydes are commonly named according to the corresponding carboxylic acids. The end ‘-ic’ is replaced by the word aldehydes. For example,

  • Formaldehyde from formic acid
  • Acetaldehydes from acetic acid

IUPAC names

Rules for assigning IUPAC names to aldehydes are

  • Select the longest chain containing the aldehydes group.
  • Change the ‘e’ of the corresponding alkane to ‘al’.
  • The numbering of carbons starts from the carbonyl carbon.
  • Since the aldehyde group is always at the end of the chain so its position is never mentioned.
  • Substituents are named and their position is indicated.

IUPAC naming of an aldehyde

Nomenclature of aromatic aldehydes

Aromatic aldehydes are not generally named by the IUPAC system. They are given special names. For example, benzaldehyde, etc.

Aromatic aldehydes naming

Nomenclature of ketones

The compounds in which the carbonyl group is directly bonded to two carbon atoms are called ketones.

In ketones, the carbonyl group occurs within the chain. Ketones are mostly present in sugars. For example, fructose, tagatose, dihydroxyacetone, etc.

Common names

Ketones are commonly named after the alkyl group attached to the carbonyl group and then the word ketone is added.

Alkyl groups are named alphabetically.

If two alkyl groups are the same, then the prefix di- is used before the name of an alkyl group. For example, dimethyl ketone and ethyl methyl ketone, etc.

IUPAC names

Rules for assigning IUPAC names to ketones are:

  • Select the longest chain containing the ketone group.
  • Chain the ‘e’ of the corresponding alkane to ‘one’.
  • Number the carbon in such a way that the carbonyl group gets the smallest possible number.
  • The position of the carbonyl group is indicated by the number of its carbon.
  • Substituents are named and their position is indicated with numbers.

Ketones nomenclature by IUPAC rules

Aromatic ketones

Aromatic ketones are not generally named by the IUPAC system. They are given special names. For example, acetophenone, benzophenone, etc.

IUPAC naming of aromatic ketones

4. The difference in physical properties of carbonyl and carboxylic acids

Physical properties of carbonyl compounds

  • Most simple aldehydes and ketones are liquids (except formaldehyde).
  • Formaldehyde is a gas and is usually used as a 37 to 40 percent solution in water called formalin.
  • Most of the carbonyl compounds are volatile liquids.
  • Aldehydes have a pungent odor.
  • Carbonyl compounds are used as organic solvents e.g. acetone, 2-butanone, etc.
  • Carbonyl compounds are polar compounds.
  • They have higher boiling points than corresponding alkanes and alkenes and relatively lower boiling points than alcohols.
  • Acetaldehyde and acetone are miscible with water.
  • Carbonyl compounds are soluble in organic solvents.

Physical properties of carboxylic acids

  • Lower molecular weight carboxylic acids are volatile liquids.
  • Higher molecular weight carboxylic acids are solid at room temperature.
  • They have an unpleasant odor.
  • They are polar because of the presence of a highly polar (C=O) and (C-O) bond.
  • They exist as dimers because of strong hydrogen bonding.
  • Aliphatic carboxylic acids are soluble in water while aromatic acids are insoluble in water.
  • Lower members of acids are colorless liquids with an unpleasant and pungent smell.
  • Their solubility generally decreases with the increase in molecular mass.
  • Boiling points are high due to the presence of hydrogen bonding.
  • Boiling points increase with the increase in the chain length, e.g. boiling points for HCOOH is 100 °C, for CH2COOH is 118 °C, and for C2H5COOH it is 151 °C.

5. Uses of carbonyl and carboxylic compounds

Uses of carbonyl compounds

  • Carbonyl compounds are used as organic solvents.
  • Carbonyl compounds are starting materials for the synthesis of many resins and plastics.
  • Acetone is a starting material for the synthesis of some organic polymers.
  • Formalin is used as a disinfectant, antiseptic, and preservative for biological specimens.
  • Carbonyl compounds are used in the silvering of mirrors.
  • Acetone is used as a nail paint remover and paint thinner, etc.
  • Benzaldehydes is used as bee repellent, etc.

Uses of carboxylic acids

  • Carboxylic acids are used in pharmaceutical drugs.
  • In labs, carboxylic acids are being used as solvents.
  • They are used in the food industry.
  • Many carboxylic acids are used as food preservatives.
  • They are used in the manufacturing of soap (saponification).
  • They are used in the production of polymers and polyesters.

Key takeaway(S)

Carbonyl vs Carboxyl Compounds

Concepts Berg

Are carbonyl compounds and carboxylic acids the same?

Carbonyl compounds and carboxylic acid are not the same. They have different functional groups and have different physical and chemical properties.

What is the difference between carboxyl and carboxylic?

Carboxyl is a functional group whereas, carboxylic is the term used for the compounds containing this group.

Why are carboxylic acids not considered carbonyl compounds?

Although carboxylic acids have a carbonyl group, they are not considered carbonyl compounds because:

  • Carboxylic acids have a hydroxyl functional group along with the carbonyl group.
  • They have different physical and chemical properties.

What is the general formula of aldehyde?

The general formula of aldehyde is R-(C=O)-H provided that R is either a hydrogen atom or an alkyl group. It is also represented by CnH2nO.

What is a carbonyl functional group?

When sp2 hybridized carbon is double bonded with oxygen and single bonded with two R groups, this is a carbonyl functional group. R may be a hydrogen, alkyl, or aryl group.

What is the difference between carbonyl compounds: aldehydes and ketones?

The compounds in which the carbonyl group is directly bonded to at least one hydrogen atom are called aldehydes.

The compounds in which the carbonyl group is directly bonded to two carbon atoms are called ketones.

What is the difference between an acyl group and a carbonyl group?

Carbonyl is a functional group. Although the acyl group is a functional group as well, it is a derivative of carboxylic acids. They have differences in physical and chemical properties.

What is the reactivity order between an alkene and a carbonyl group?

Carbonyl group is more reactive than alkene because of two main reasons,

  • Polar nature of carbonyl carbon due to (C=O) bond.
  • Planer geometry of carbonyl groups. Although it is a common characteristic between alkenes and carbonyl, alkenes do not have an electrophilic carbon.

What is the difference between fatty acids and carboxylic acid?

Molecules having (-COOH) functional groups with carbon directly bonded R group are called carboxylic acids. Whereas the long chain carboxylic acids which are solid at room temperature are called fatty acids.

Why do aldehyde and carboxyl are terminal while ketone is internal in the chain?

Aldehydes and carboxyl are always terminal. It is because aldehyde and carboxylic acids have an identification of specific groups on one side of the carbonyl carbons. Aldehydes have hydrogen and carboxyl groups have a hydroxyl group attached to one side of the sp2 hybridized carbonyl carbon. Ketones do not have such conditions, It can be placed anywhere in the chain, but never on terminals.

What is the name of the chemical compound derived from a carboxylic acid often in which at least one hydroxyl group OH is replaced by an alkoxy group?

Esters are the chemical compounds that are derived from carboxylic acids by the replacement of the hydroxyl (-OH) group with the alkoxy (-OR) group.

What is the order of the boiling point for carboxylic acids, alcohol, and aldehyde?

Carboxylic acids and alcohols have higher boiling points than aldehydes because both have strong hydrogen bonding while aldehyde has no internal hydrogen bonding at all.

The order of boiling points is:

carboxylic acids > alcohols > carbonyl compounds

Why is carboxylic acid RCOOH not spelled RCO2H instead?

Carboxylic acid is commonly spelled as COOH. However, it can be written as RCO2H.

Which has the higher boiling point and why, carboxylic acid or amide?

Amides have a higher boiling point than carboxylic acids because amides have strong hydrogen bonding than carboxylic acid. Amides have two hydrogen atoms for hydrogen bonding, nitrogen, and oxygen whereas carboxylic acids only have oxygen to make hydrogen bonds.

Why are carbonyl compounds aldehydes and ketones able to undergo nucleophilic addition reactions?

As the oxygen atom is more electronegative than the C atom, it attracts the shared pair of electrons more strongly toward itself and makes carbon electrophilic. That’s why carbonyl compounds give nucleophilic addition reactions.

Why do carboxylic acids behave like fairly strong acids?

According to HSAB theory, the strength of an acid depends on the stability of the conjugate base. The conjugate base of a carboxylic acid is stabilized by resonance. That’s why carboxylic acids behave as strong acids.

What is the difference between hydroxy and hydroxyl groups?

When -OH group is present in a molecule as a functional group, this is called a hydroxyl group. Whereas when present as a substituent, it is called a hydroxy group.

Why is amide the least reactive of a carboxylic acid derivative?

When amide reacts the leaving group is an amine (or ammonia). Amine or ammonia are among the strongest bases and are poor leaving groups because they attack back on the electrophilic carbonyl carbons. So, amides become the least reactive derivatives of carboxylic acid.

Can a carboxylic acid react with another carboxylic acid? If it can, what are the products of this reaction?

The carboxylic acid can react with another carboxylic acid and the product of this reaction is acid anhydride with the elimination of water.

Why do aldehydes and ketones have a lower boiling point than carboxylic acid and alcohols?

Alcohols and carboxylic acids have higher boiling points than aldehydes and ketones because of hydrogen bonding. Carbonyl compounds have no hydrogen bonding for which they have lower boiling points than alcohols and carboxylic acids.

Why do we classify carboxylic acids as acids even though they have an OH on a terminal bond? Isn’t OH a characteristic of a base?

Carboxylic acids can donate H+ ions, that’s why they are considered acids.

Can LiAlH4 reduce acid chlorides or not? What is the reason?

LiAlH4 is a strong reducing agent. It can reduce acid chlorides or acyl chlorides to alcohols.

Why do the angles deviate from 120° in a carboxylic acid?

Carbon is sp2 hybridized in carboxylic acid. However, the angle deviates from 1200 because of the electronic repulsions between lone pairs of oxygen and lone pairs-bond pairs of side atoms (steric hindrance).

Which is more acidic, aldehyde or ketone?

Aldehydes are relatively more electron deficient species than ketones and that is because ketones have two electron donating alkyl groups. That’s why aldehydes are more acidic than ketones. It means that aldehydes have more electron pair accepting capacity than ketones. Obviously, we are considering the Lewis concept for acidity here.

“Electron pair acceptors are acids and donors are bases”

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