19.1 Aldehydes and Ketones | Carbonyl Compounds | Chemistry 11

Organic compounds containing the carbonyl functional group ( $> C = O$ ) are called aldehydes and ketones. This section outlines their key characteristics, differences, and structures.

Comparison of Aldehydes and Ketones

The primary distinction between aldehydes and ketones lies in the atoms bonded to the carbonyl carbon.

Feature	Aldehydes	Ketones
Functional Group	The carbonyl carbon is directly attached to at least one hydrogen atom. The functional group is the formyl group ( $- C H O$ ).	The carbonyl carbon is bonded to two other carbon atoms. The functional group is the keto group ( $> C = O$ ).
General Formula	Aliphatic aldehydes: $C_{n} H_{2 n} O$	Aliphatic ketones: $C_{n} H_{2 n} O$
Structural Formula	General structure $R - C H O$ , where $R$ = H or alkyl/aryl group. $O ∥ R - C - H$	General structure $R - C O - R^{'}$ , where $R$ and $R^{'}$ are alkyl or aryl groups. $O ∥ R - C - R^{'}$
Examples	(i) Methanal (Formaldehyde): $H C H O$ <br /><br /> (ii) Ethanal (Acetaldehyde): $C H_{3} C H O$	(i) Propanone (Acetone): $C H_{3} C O C H_{3}$ <br /><br /> (ii) Butanone: $C H_{3} C O C_{2} H_{5}$

Key Points

Carbonyl Compounds: Both aldehydes and ketones contain the carbonyl functional group ( $> C = O$ ), where the carbon is $s p^{2}$ hybridized.
Structural Difference:
- Aldehydes: Terminal carbonyl group ( $R - C H O$ ); carbonyl carbon bonded to at least one H.
- Ketones: Internal carbonyl group ( $R - C O - R^{'}$ ); carbonyl carbon bonded to two carbon atoms.
Functional Isomers: Aliphatic aldehydes and ketones share the general formula $C_{n} H_{2 n} O$ . For example, propanal ( $C H_{3} C H_{2} C H O$ ) and propanone ( $C H_{3} C O C H_{3}$ ) both have the formula $C_{3} H_{6} O$ .
Nomenclature: Aldehydes end with the suffix -al (e.g., methanal, ethanal); ketones end with -one (e.g., propanone, butanone).
Polarity: The $C = O$ bond is polar due to the higher electronegativity of oxygen, giving the carbonyl carbon a partial positive charge ( $δ^{+}$ ) and oxygen a partial negative charge ( $δ^{-}$ ). This makes the carbonyl carbon electrophilic and susceptible to nucleophilic attack.
Significance: Formaldehyde ( $H C H O$ ) is used as a preservative; acetone ( $C H_{3} C O C H_{3}$ ) is a common solvent; many sugars (e.g., glucose) contain an aldehyde functional group.

Comparison of Aldehydes and Ketones

The primary distinction between aldehydes and ketones lies in the atoms bonded to the carbonyl carbon.

Feature	Aldehydes	Ketones
Functional Group	The carbonyl carbon is directly attached to at least one hydrogen atom. The functional group is the formyl group ( $- C H O$ ).	The carbonyl carbon is bonded to two other carbon atoms. The functional group is the keto group ( $> C = O$ ).
General Formula	Aliphatic aldehydes: $C_{n} H_{2 n} O$	Aliphatic ketones: $C_{n} H_{2 n} O$
Structural Formula	General structure $R - C H O$ , where $R$ = H or alkyl/aryl group. $O ∥ R - C - H$	General structure $R - C O - R^{'}$ , where $R$ and $R^{'}$ are alkyl or aryl groups. $O ∥ R - C - R^{'}$
Examples	(i) Methanal (Formaldehyde): $H C H O$ <br /><br /> (ii) Ethanal (Acetaldehyde): $C H_{3} C H O$	(i) Propanone (Acetone): $C H_{3} C O C H_{3}$ <br /><br /> (ii) Butanone: $C H_{3} C O C_{2} H_{5}$

Key Points

Carbonyl Compounds: Both aldehydes and ketones contain the carbonyl functional group (

> C = O

), where the carbon is

s p^{2}

hybridized.

Structural Difference:

Aldehydes: Terminal carbonyl group ( $R - C H O$ ); carbonyl carbon bonded to at least one H.
Ketones: Internal carbonyl group ( $R - C O - R^{'}$ ); carbonyl carbon bonded to two carbon atoms.

Functional Isomers: Aliphatic aldehydes and ketones share the general formula

C_{n} H_{2 n} O

. For example, propanal (

C H_{3} C H_{2} C H O

) and propanone (

C H_{3} C O C H_{3}

) both have the formula

C_{3} H_{6} O

Nomenclature: Aldehydes end with the suffix -al (e.g., methanal, ethanal); ketones end with -one (e.g., propanone, butanone).

Polarity: The

C = O

bond is polar due to the higher electronegativity of oxygen, giving the carbonyl carbon a partial positive charge (

δ^{+}

) and oxygen a partial negative charge (

δ^{-}

). This makes the carbonyl carbon electrophilic and susceptible to nucleophilic attack.

Significance: Formaldehyde (

H C H O

) is used as a preservative; acetone (

C H_{3} C O C H_{3}

) is a common solvent; many sugars (e.g., glucose) contain an aldehyde functional group.