JEE Chemistry Aldehydes Ketones and Carboxylic Acids Notes Set 04

Carbonyl Group: Structure and Classification

Compounds that contain the carbonyl group \(\ce{C=O}\) are called carbonyl compounds, with general formula \(\ce{C_nH_{2n}O}\) for saturated aldehydes and ketones. The carbonyl carbon is sp2 hybridised and forms three sigma bonds in a plane with bond angles close to 120 degrees. The unhybridised p orbital overlaps sideways with a p orbital of oxygen to make the pi bond.

Compounds such as acids, esters, amides and acid halides also contain a \(\ce{C=O}\) group but are not called carbonyl compounds in this sense, because a lone pair on the atom attached to the carbonyl carbon is delocalised into the group by resonance.

Polarity and Reactivity

Oxygen is more electronegative than carbon, so the pi electrons are pulled towards oxygen: \(\ce{C^{\delta+}=O^{\delta-}}\). The carbon is electrophilic and is attacked by nucleophiles, while the oxygen is attacked by electrophiles such as \(\ce{H+}\). This gives aldehydes and ketones a dipole moment larger than that of ethers and makes nucleophilic addition their typical reaction. The C=O double bond is stronger than a C=C bond.

1. Aldehydes are more reactive than ketones because they have less steric hindrance and only one electron-donating alkyl group.
2. Electron-withdrawing groups on the carbon (as in chloral, or a nitro group on the ring of benzaldehyde) increase reactivity; electron-donating groups reduce it.
3. Aromatic carbonyl compounds are less reactive than aliphatic ones because the ring donates electrons by resonance.

Preparation of Benzaldehyde

Reactions of Benzaldehyde

Carboxylic Acids

A carboxylic acid contains the \(\ce{-COOH}\) group, a carbonyl group joined to a hydroxyl group. The group behaves in its own way, not simply as the sum of the two. Monocarboxylic acids have formula \(\ce{C_nH_{2n}O2}\), and the higher members are called fatty acids. Acids are also classified as mono-, di- or tricarboxylic acids (for example ethanoic acid, ethanedioic acid and citric acid).

The last three (caproic, caprylic, capric) are found in goat fat, which is the origin of the memory aid Caper-Goat.

Why Carboxylic Acids Are Acidic

The carbonyl group of \(\ce{-COOH}\) withdraws electrons (the -I effect) and weakens the O-H bond. After the proton leaves, the carboxylate ion is stabilised by resonance: the negative charge is shared equally by the two oxygens, and both C-O bonds have the same length (about 127 pm). An alkoxide ion has no such delocalisation, so alcohols are almost neutral.

Benzoic Acid

Acid Derivatives and Nucleophilic Acyl Substitution

Replacing the \(\ce{-OH}\) of an acid by a group Z (such as \(\ce{Cl}\), \(\ce{OCOR}\), \(\ce{OR}\) or \(\ce{NH2}\)) gives an acid derivative \(\ce{RCOZ}\), whose group \(\ce{RCO-}\) is called the acyl group. The typical reaction is nucleophilic acyl substitution: the nucleophile adds to the carbonyl carbon to give a tetrahedral intermediate (the carbon changes from sp2 to sp3), and then Z leaves.

1. The better the leaving group, the more reactive the derivative. The weaker the base Z is, the better it leaves.
2. Basicity of the leaving groups increases in the order chloride, carboxylate, alkoxide, amide ion.
3. Hence reactivity decreases in the order acyl chloride, anhydride, ester, amide.

Useful Name Reactions of this Chapter