8. Aldehydes, Ketones, Carboxylic Acids

Butyraldehyde

The spelling butyraldehyde includes the letter "r" because of how common names for carboxylic acids and their corresponding aldehydes are derived.

Here is why:

• Etymological Origin: The 4-carbon carboxylic acid—butyric acid—gets its common name from the Latin word butyrum, which means butter, since it was first isolated from rancid butter.

• Naming Convention: According to common nomenclature rules, the common names of aldehydes are derived from their corresponding carboxylic acids by replacing the suffix -ic acid with aldehyde.

• Therefore, replacing "-ic acid" in butyric acid with aldehyde yields butyraldehyde (maintaining the root sound "butyryl- / butyr-"). Leaving out the "r" ("butyaldehyde") would disrupt the standard historical naming root linked to its Latin source.

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IUPAC NOMENCLATURE

According to the IUPAC system of nomenclature, open-chain aliphatic aldehydes and ketones follow specific rules based on the parent alkane. Here are the rules along with examples from the notebook sources:

1. General Suffix Replacement

• Rule: The IUPAC name is derived from the corresponding alkane by replacing the ending -e with -al for aldehydes and -one for ketones.
• Examples:
  • Methanal ($HCHO$)
  • Ethanal ($CH_3CHO$)
  • Pentan-2-one ($CH_3COCH_2CH_2CH_3$)

2. Numbering the Carbon Chain

• For Aldehydes: The longest carbon chain is numbered starting from the carbon atom of the aldehyde group ($\text{-CHO}$), which is always assigned carbon number 1.

  • Example: Pentanal ($CH_3CH_2CH_2CH_2CHO$)

• For Ketones: The numbering of the carbon chain begins from the end that is nearer to the carbonyl group ($\text{C=O}$).

  • Example: Pentan-2-one ($CH_3COCH_2CH_2CH_3$)

3. Handling Substituents and Multiple Bonds

• Rule: Substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain. If double or triple bonds are present, they are incorporated using suffixes like -en or -yn along with their locants.
• Examples:
  • Prop-2-enal ($CH_2=CHCHO$)
  • 4-Methylpent-3-en-2-one ($(CH_3)_2C=CHCOCH_3$)
  • 2-Methylpropanal ($(CH_3)_2CHCHO$)

4. Cyclic Ketones

• Rule: For cyclic ketones, the carbonyl carbon is designated as carbon number 1, and the prefix cyclo- is used before the alkane name.
  • Example: 2-Methylcyclohexanone

5. Aldehydes Attached to a Ring

• Rule: When the aldehyde group ($\text{-CHO}$) is attached directly to a ring, the suffix -carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbons starts from the carbon atom attached to the aldehyde group.
  • Example: Cyclohexanecarbaldehyde or 3-Methylcyclohexanecarbaldehyde

6. Aromatic Aldehydes

• Rule: The simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is named benzenecarbaldehyde, though the common name benzaldehyde is also fully accepted by IUPAC. Other aromatic aldehydes are named as substituted benzaldehydes.
• Examples:
  • Benzenecarbaldehyde (Benzaldehyde)
  • 3-Bromobenzaldehyde (or 3-Bromobenzenecarbaldehyde)

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3 - Oxopentanal

• In IUPAC nomenclature, when a molecule contains multiple functional groups, they are named based on a priority order established by IUPAC rules.
• Here is why 3-oxopentanal is named as an aldehyde rather than a ketone:

1. Functional Group Priority: Aldehydes ($\text{-CHO}$) have a higher priority in the IUPAC functional group hierarchy than ketones ($\text{C=O}$).

2. Principal Functional Group: Because the aldehyde group outranks the ketone group, it dictates the suffix of the main parent chain. The principal suffix becomes -al (for pentanal, indicating a 5-carbon aldehyde).

3. Treatment of Lower-Priority Groups: The lower-priority group (the ketone carbonyl group) is demoted to a substituent rather than giving the compound the main suffix -one. As a substituent, it is designated by the prefix oxo- indicating its position at carbon-3.

• Therefore, even though both functional groups are present, the presence of the higher-priority aldehyde group makes it an oxopentanal instead of a ketone.

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Structure of aldehydes and ketones

1. Hybridization and Geometry

• $sp^2$-Hybridization: The carbonyl carbon atom is $sp^2$-hybridized.

• Trigonal Planar Arrangement: It forms three $\sigma$ (sigma) bonds with the surrounding atoms, lying in the same plane with bond angles of approximately $120^\circ$.

2. Bonding Structure ($\sigma$ and $\pi$ Bonds)

• $\pi$-Bond Formation: The fourth valence electron of the carbon atom resides in an unhybridized $p$-orbital, which overlaps laterally with a $p$-orbital of the oxygen atom to form a $\pi$-bond.

• Electron Distribution: The resulting $\pi$-electron cloud lies above and below the planar skeleton of the carbonyl group.

• Lone Pairs: The oxygen atom possesses two non-bonding electron pairs.

Key differences between sigma ($\sigma$) and pi ($\pi$) bonds:

Comparison Table

3. Polarity and Resonance

• Polarization: Due to the higher electronegativity of oxygen relative to carbon, the carbon-oxygen double bond is heavily polarized.

• Electrophilic and Nucleophilic Centers: The carbonyl carbon acts as an electrophilic (Lewis acid) center, while the carbonyl oxygen acts as a nucleophilic (Lewis base) center.

• Resonance Structures: The high polarity is explained by resonance involving a neutral structure (A) and a dipolar structure (B), giving carbonyl compounds substantial dipole moments that make them more polar than ethers.

Bond Polarisation

Bond polarisation refers to the unequal sharing of bonding electrons between two bonded atoms of differing electronegativities, which causes one atom to pull the electron cloud closer toward itself, acquiring a partial negative charge while leaving the other atom with a partial positive charge.

Example: The Carbonyl Group ($\text{>C=O}$) in Aldehydes and Ketones

• Cause: The carbon-oxygen double bond in carbonyl compounds (such as aldehydes and ketones) is polarized because oxygen has a higher electronegativity relative to carbon.

• Resulting Centers:

  • The carbonyl carbon becomes an electron-deficient, electrophilic (Lewis acid) center.

  • The carbonyl oxygen becomes a nucleophilic (Lewis base) center.

• Resonance: This high degree of bond polarisation and polarity is further explained on the basis of resonance involving both a neutral structure and a dipolar structure.

Examples of Carbonyl Compounds

• Aldehydes: Compounds where the carbonyl group is bonded to a carbon and a hydrogen atom.

  • Examples: Methanal (Formaldehyde) ($\text{HCHO}$) and Ethanal (Acetaldehyde) ($\text{CH}_3\text{CHO}$).

• Ketones: Compounds where the carbonyl group is bonded to two carbon atoms.

  • Examples: Acetone (Propanone) ($\text{CH}_3\text{COCH}_3$) and Pentan-2-one ($\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$).

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Preparation of Aldehyde

1. From Alcohols

• By Oxidation of Primary Alcohols: $\underset{\text{(Primary alcohol)}}{\text{RCH}_2\text{OH}} \xrightarrow{\text{PCC or } \text{CrO}_3} \underset{\text{(Aldehyde)}}{\text{RCHO}}$

• By Dehydrogenation of Alcohols: $\underset{\text{(Primary alcohol)}}{\text{RCH}_2\text{OH}} \xrightarrow{\text{Cu or Ag}, \, 573\text{ K}} \underset{\text{(Aldehyde)}}{\text{RCHO}} + \underset{\text{(Dihydrogen)}}{\text{H}_2}$

2. From Hydrocarbons

• By Ozonolysis of Alkenes: $\underset{\text{(Alkene)}}{\text{Alkene}} + \text{O}_3 \xrightarrow{\text{CCl}_4} \text{Ozonide} \xrightarrow{\text{Zn/H}_2\text{O}} \underset{\text{(Aldehydes/Ketones)}}{\text{Aldehydes/Ketones}}$

• By Hydration of Alkynes: $\underset{\text{(Ethyne)}}{\text{H-C}\equiv\text{C-H}} + \text{H}_2\text{O} \xrightarrow{1\% \text{ HgSO}_4, \, 20\% \text{ H}_2\text{SO}_4} [\text{Enol form}] \xrightarrow{\text{Tautomerisation}} \underset{\text{(Ethanal / Acetaldehyde)}}{\text{CH}_3\text{CHO}}$

3. From Derivatives of Carboxylic Acids and Nitriles

• From Acyl Chlorides (Rosenmund Reduction): $\underset{\text{(Ethanoyl chloride)}}{\text{CH}_3\text{COCl}} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4} \underset{\text{(Ethanal)}}{\text{CH}_3\text{CHO}} + \underset{\text{(Hydrogen chloride)}}{\text{HCl}}$ $\underset{\text{(Benzoyl chloride)}}{\text{C}_6\text{H}_5\text{COCl}} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4, \, -\text{HCl}} \underset{\text{(Benzaldehyde)}}{\text{C}_6\text{H}_5\text{CHO}}$

• From Nitriles (Stephen Reaction): $\underset{\text{(Nitrile)}}{\text{RCN}} + \text{SnCl}_2 + \text{HCl} \rightarrow \underset{\text{(Imine hydrochloride)}}{\text{RCH=NH}\cdot\text{HCl}} \xrightarrow{\text{H}_2\text{O}} \underset{\text{(Aldehyde)}}{\text{RCHO}}$

• From Nitriles and Esters using DIBAL-H: (di iso butyl aluminium hydride) $\underset{\text{(Nitrile)}}{\text{RCN}} \xrightarrow{1. \text{DIBAL-H}} [\text{Intermediate}] \xrightarrow{2. \text{H}_2\text{O}} \underset{\text{(Aldehyde)}}{\text{RCHO}}$ $\underset{\text{(Ester)}}{\text{RCOOR}'} \xrightarrow{1. \text{DIBAL-H}} [\text{Intermediate}] \xrightarrow{2. \text{H}_2\text{O}} \underset{\text{(Aldehyde)}}{\text{RCHO}}$

4. Preparation of Aromatic Aldehydes from Aromatic Hydrocarbons

• Etard Reaction (Oxidation of Toluene with Chromyl Chloride): $\underset{\text{(Toluene)}}{\text{C}_6\text{H}_5\text{CH}_3} + 2\text{CrO}_2\text{Cl}_2 \xrightarrow{\text{CS}_2} \underset{\text{(Chromium complex)}}{\text{Chromium Complex}} \xrightarrow{\text{H}_3\text{O}^+} \underset{\text{(Benzaldehyde)}}{\text{C}_6\text{H}_5\text{CHO}}$

• Oxidation of Toluene using Chromic Oxide ($\text{CrO}_3$): $\underset{\text{(Toluene)}}{\text{C}_6\text{H}_5\text{CH}_3} \xrightarrow{\text{CrO}_3 / (\text{CH}_3\text{CO})_2\text{O}} \underset{\text{(Benzylidene diacetate)}}{\text{Benzylidene diacetate}} \xrightarrow{\text{H}_3\text{O}^+} \underset{\text{(Benzaldehyde)}}{\text{C}_6\text{H}_5\text{CHO}}$

• Side Chain Chlorination Followed by Hydrolysis: $\underset{\text{(Toluene)}}{\text{C}_6\text{H}_5\text{CH}_3} \xrightarrow{\text{Cl}_2, \, \Delta} \underset{\text{(Benzal chloride)}}{\text{C}_6\text{H}_5\text{CHCl}_2} \xrightarrow{\text{H}_2\text{O}} \underset{\text{(Benzaldehyde)}}{\text{C}_6\text{H}_5\text{CHO}} + 2\text{HCl}$

• Gatterman-Koch Reaction:
  $\underset{\text{(Benzene)}}{\text{C}_6\text{H}_6} + \text{CO} + \text{HCl} \xrightarrow{\text{anhydrous AlCl}_3/\text{CuCl}} \underset{\text{(Benzaldehyde)}}{\text{C}_6\text{H}_5\text{CHO}} + \text{HCl}$

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Chemical reactions of Aldehydes

1. Nucleophilic addition reactions

2. Reduction reactions

3. Oxidation and identification reactions

4. Reactions involving alpha hydrogen

5. Reactions without alpha hydrogen

6. Electrophilic substitution in aromatic aldehydes

Last-minute memory points

• Aldol: $\alpha$-hydrogen present.

• Cannizzaro: ($\alpha$)-hydrogen absent.

• Clemmensen: ($\mathrm{Zn(Hg)/HCl}$), acidic.

• Wolff–Kishner: $(\mathrm{NH_2NH_2/KOH})$, basic.

• Tollens’: silver mirror.

• Fehling’s: brick-red precipitate; aromatic aldehydes such as benzaldehyde generally do not respond positively.

• Oxidation: aldehyde → carboxylic acid.

• Reduction: aldehyde → primary alcohol or, under Clemmensen/Wolff–Kishner conditions, hydrocarbon.

• Grignard: methanal → primary alcohol; other aldehydes → secondary alcohol.

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Chemical reactions of Ketones

1. Nucleophilic addition reactions

2. Reduction reactions

3. Oxidation reactions

4. Condensation reactions

5. Electrophilic substitution in aromatic ketones

Last-minute exam pointers

• Ketone + (NaBH_4/LiAlH_4) → secondary alcohol.

• Clemmensen = (Zn(Hg)/HCl), acidic medium.

• Wolff–Kishner = (NH_2NH_2/KOH), heat, basic medium.

• Methyl ketone + $(I_2/NaOH)$ → yellow $(CHI_3)$ precipitate.

• Aldol condensation requires at least one α-hydrogen.

• 2,4-DNP gives a yellow/orange precipitate with carbonyl compounds.

• In aromatic ketones, $(-COR)$ is deactivating and meta-directing.

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Reactions with Ammonia and its derivaties

Table of $\text{N}$-Substituted Derivatives ($\text{>C=N-Z}$)

1. When $\text{Z} = -\text{H}$ (Reagent: Ammonia, $\text{NH}_3$)

• Derivative Formed: Imine

• Example Reaction:

  $\text{CH}_3\text{CHO} + \text{H}_2\text{N-H} \rightleftharpoons \text{CH}_3\text{CH=NH} + \text{H}_2\text{O}$

  (Ethanal + Ammonia $\rightarrow$ Ethanal imine + Water)

2. When $\text{Z} = -\text{R}$ (Reagent: Primary Amine, $\text{RNH}_2$)

• Derivative Formed: Substituted imine (Schiff’s base)

• Example Reaction:

  $\text{CH}_3\text{CHO} + \text{H}_2\text{N-CH}_3 \rightleftharpoons \text{CH}_3\text{CH=NCH}_3 + \text{H}_2\text{O}$

  (Ethanal + Methylamine $\rightarrow$ $N$-Ethylidene-methanamine / Schiff's base)

3. When $\text{Z} = -\text{OH}$ (Reagent: Hydroxylamine, $\text{H}_2\text{NOH}$)

• Derivative Formed: Oxime

• Example Reaction:

  $\text{(CH}_3)_2\text{CO} + \text{H}_2\text{N-OH} \rightleftharpoons \text{(CH}_3)_2\text{C=N-OH} + \text{H}_2\text{O}$

  (Propanone + Hydroxylamine $\rightarrow$ Acetone oxime)

4. When $\text{Z} = -\text{NH}_2$ (Reagent: Hydrazine, $\text{H}_2\text{NNH}_2$)

• Derivative Formed: Hydrazone

• Example Reaction:

  $\text{CH}_3\text{CHO} + \text{H}_2\text{N-NH}_2 \rightleftharpoons \text{CH}_3\text{CH=NNH}_2 + \text{H}_2\text{O}$

  (Ethanal + Hydrazine $\rightarrow$ Acetaldehyde hydrazone)

5. When $\text{Z} = -\text{NHPh}$ (Reagent: Phenylhydrazine)

• Derivative Formed: Phenylhydrazone

• Example Reaction:

  $\text{CH}_3\text{COCH}_3 + \text{H}_2\text{N-NHC}_6\text{H}_5 \rightleftharpoons \text{CH}_3\text{C(CH}_3\text{)=NNHC}_6\text{H}_5 + \text{H}_2\text{O}$

  (Propanone + Phenylhydrazine $\rightarrow$ Acetone phenylhydrazone)

6. When $\text{Z} = \text{2,4-Dinitrophenyl}$ (Reagent: 2,4-DNP)

• Derivative Formed: 2,4-DNP derivative (yellow, orange, or red solid)

• Example Reaction:

  $\text{RCHO} + \text{H}_2\text{N-NHC}_6\text{H}_3(\text{NO}_2)_2 \rightleftharpoons \text{RCH=NNHC}_6\text{H}_3(\text{NO}_2)_2 + \text{H}_2\text{O}$

  (Aldehyde + 2,4-Dinitrophenylhydrazine $\rightarrow$ 2,4-Dinitrophenylhydrazone solid)

7. When $\text{Z} = -\text{NHCONH}_2$ (Reagent: Semicarbazide, $\text{H}_2\text{NNHCONH}_2$)

• Derivative Formed: Semicarbazone

• Example Reaction:

  _(Ethanal + Semicarbazide $\rightarrow$ Acetaldehyde semicarbazone)_

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