Aldehydes And Ketones (Introduction & Properties)

Nomenclature And Structure Of Carbonyl Group

Aldehydes and ketones are organic compounds characterized by the presence of a carbonyl group ($C=O$).

Nomenclature

General Structure:

• Aldehyde: $R-CHO$, where R can be hydrogen, alkyl, or aryl group. The carbonyl group is attached to at least one hydrogen atom.
• Ketone: $R-CO-R'$, where R and R' can be alkyl or aryl groups, and R and R' are not hydrogen. The carbonyl group is attached to two carbon atoms.

IUPAC Nomenclature:

• Aldehydes: The parent alkane name loses the final '-e' and adds the suffix '-al'. The carbonyl carbon is assigned the number 1.

Examples:

• $HCHO$: Methanal (Common name: Formaldehyde)
• $CH_3CHO$: Ethanal (Common name: Acetaldehyde)
• $CH_3CH_2CHO$: Propanal
• $C_6H_5CHO$: Benzaldehyde
• Ketones: The parent alkane name loses the final '-e' and adds the suffix '-one'. The parent chain is numbered to give the carbonyl group the lowest possible number.

Examples:

• $CH_3COCH_3$: Propanone (Common name: Acetone)
• $CH_3CH_2COCH_3$: Butan-2-one
• $C_6H_5COCH_3$: Phenylethanone (Common name: Acetophenone)
• Cyclic Ketones: Named by adding '-one' to the cycloalkane name, with the carbonyl carbon as C1.

Example: Cyclohexanone

Structure Of The Carbonyl Group

Hybridization: The carbon atom of the carbonyl group ($C=O$) is $sp^2$ hybridized.

Bonding:

• Sigma ($\sigma$) Bond: Formed by the head-on overlap of an $sp^2$ hybrid orbital of carbon and an $sp^2$ hybrid orbital of oxygen.
• Pi ($\pi$) Bond: Formed by the lateral overlap of the unhybridized $p$ orbital of carbon and an unhybridized $p$ orbital of oxygen.

Geometry: The $sp^2$ hybridization results in a trigonal planar geometry around the carbonyl carbon atom.

Bond Angle: The $C-C=O$ bond angles are approximately 120°.

Polarity: Oxygen is much more electronegative than carbon. This causes the $\pi$ bond electrons and the $\sigma$ bond electrons to be unequally shared, resulting in a significant partial negative charge on the oxygen atom ($\delta^-$) and a partial positive charge on the carbon atom ($\delta^+$).

$C^{\delta+}=O^{\delta-}$

Reactivity: The presence of the polar $C=O$ bond makes the carbonyl carbon electrophilic (electron-deficient) and susceptible to nucleophilic attack. The $\pi$ bond is also a site of reactivity.

Preparation Of Aldehydes And Ketones

Aldehydes and ketones can be prepared by various methods, often involving the oxidation of alcohols or specific reactions starting from hydrocarbons or their derivatives.

Preparation Of Aldehydes

1. From Alcohols: Oxidation of primary and secondary alcohols.

• Primary Alcohols: Oxidation yields aldehydes. Using mild oxidizing agents like pyridinium chlorochromate (PCC) or controlled oxidation, the reaction stops at the aldehyde stage. Stronger oxidizing agents (like acidified $K_2Cr_2O_7$ or $KMnO_4$) can further oxidize the aldehyde to a carboxylic acid.

$RCH_2OH \xrightarrow{[O], PCC} RCHO$

• Secondary Alcohols: Oxidation yields ketones.

$R_2CHOH \xrightarrow{[O]} R_2CO$

2. From Hydrocarbons:

• Oxidation of Alkenes (Ozonolysis): Ozonolysis of alkenes followed by reductive workup yields aldehydes (if the double bond involves a terminal carbon) or ketones.

$RCH=CH_2 + O_3 \xrightarrow{1. Zn/CH_3COOH} RCHO + HCHO$

• Oxidation of Alkanes: Controlled oxidation of alkanes is difficult but possible under specific conditions.

3. Rosenmund Reduction: Catalytic hydrogenation of acid chlorides using a poisoned catalyst (like Pd/BaSO$_4$) selectively reduces the acid chloride to an aldehyde.

$RCOCl + H_2 \xrightarrow{Pd/BaSO_4, \ poison} RCHO + HCl$

4. Stephen Reduction: Reduction of nitriles ($RCN$) using stannous chloride ($SnCl_2$) in the presence of $HCl$, followed by hydrolysis, yields aldehydes.

$RCN + SnCl_2 + HCl \rightarrow [RCH=NH_2]^+Cl^- \xrightarrow{H_2O} RCHO$

5. From Nitriles and Esters: Reduction using $DIBAL-H$ (Diisobutylaluminium hydride) at low temperatures.

$RCN \xrightarrow{DIBAL-H, \ low \ T, \ H_2O} RCHO$

$RCOOR' \xrightarrow{DIBAL-H, \ low \ T, \ H_2O} RCHO$

Preparation Of Ketones

1. From Alcohols: Oxidation of secondary alcohols.

$R_2CHOH \xrightarrow{[O]} R_2CO$

2. From Alkenes (Ozonolysis): Ozonolysis of alkenes where the double bond is internal.

$RCH=CHR' + O_3 \rightarrow \dots \rightarrow RCHO + R'CHO$ (if R, R' are H) or $R_2CO$ or $R'CHO$.

For internal alkenes: $RCH=CHR' + O_3 \xrightarrow{1. Zn/CH_3COOH} RCHO + R'CHO$

3. From Carboxylic Acids: Heating calcium salts of carboxylic acids (when the acid is not formic acid).

$CH_3COOCa(s) + (CH_3COO)_2Ca(s) \xrightarrow{heat} 2CH_3COCH_3 + CaO + CO_2$

4. From Nitriles: Reaction of nitriles with Grignard reagents followed by hydrolysis.

$RCN + R'MgX \xrightarrow{ether} R-C(R')=NMgX \xrightarrow{H_2O/H^+} R-CO-R'$

5. From Esters: Reaction of esters with Grignard reagents (two moles of Grignard reagent react with one mole of ester).

$RCOOR' + 2R''MgX \xrightarrow{ether} R-C(OMgX)(R'')_2 \xrightarrow{H_2O/H^+} R-CO-R''$

6. Friedel-Crafts Acylation: Reaction of benzene or substituted benzenes with acid chlorides or acid anhydrides in the presence of a Lewis acid catalyst ($AlCl_3$).

$C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl$

Physical Properties

The physical properties of aldehydes and ketones are influenced by the polarity of the carbonyl group and the presence of the $C=O$ double bond.

1. Boiling Points:

• Comparison with Alcohols: Aldehydes and ketones have lower boiling points than alcohols of comparable molecular weight. This is because they cannot form intermolecular hydrogen bonds with themselves (they lack the $O-H$ bond), whereas alcohols can.
• Comparison with Alkanes: They have higher boiling points than alkanes of comparable molecular weight due to the polarity of the carbonyl group, leading to stronger dipole-dipole interactions.
• Trend with Molecular Weight: Boiling points generally increase with increasing molecular weight within the aldehyde or ketone series.
• Branching: Increasing branching tends to lower the boiling point.

2. Solubility in Water:

• Lower aldehydes and ketones (up to about 4-5 carbon atoms) are soluble in water.
• This solubility is due to their ability to form hydrogen bonds with water molecules, with the carbonyl oxygen acting as a hydrogen bond acceptor.
• Solubility decreases significantly as the size of the hydrophobic alkyl or aryl group increases.

3. Odor:

• Lower aldehydes and ketones (especially formaldehyde, acetaldehyde, acetone) have pungent odors.
• Higher molecular weight aldehydes and ketones often have pleasant floral or fruity odors and are used in perfumes and flavorings.

4. Polarity: The carbonyl group ($C=O$) is highly polar due to the electronegativity difference between carbon and oxygen. This polarity influences their boiling points and solubility.