Chemistry NOMENCLATURE & STRUCTURE OF ALDEHYDES AND KETONES

Introduction :

Aldehydes and ketones both possess a carbonyl group `( rangle C=O)` and therefore are called carbonyl compounds. Formaldehyde is the simplest aldehyde, bonded to two hydrogens. In all other aldehydes, the carbonyl group is bonded to a hydrogen and to an alkyl (or an aryl) group. The carbonyl group of a ketone is bonded to two alkyl (or aryl) groups as shown in fig 1.

Aldehydes and ketones undergo two types of characteristic reactions.

(a) Nucleophilic addition reactions :

The carbonyl compounds can be readily attacked by nucleophiles due to the presence of electrophilic carbon. In the overall reaction, nucleophilic addition takes place and not the nucleophilic substitution as aldehydes and ketones possess very poor leaving groups, `R`-and `H`- respectively.

(b) Reactions due to acidity of `alpha`-hydrogen atoms :

Carbonyl compounds with `alpha`-hydrogen atoms can loose `H^+` to a base to give carbanion. which is resonance stabilized. This carbanion acts as nucleophile and can add to the electrophilic carbon of same or different carbonyl compound to give final product.

Nomenclature :

Aldehydes : IUPAC names the longest continuous chain including the `C` of `-CH=O` and replaces -`e` of the alkane name by the suffix `-al`. The `C` of `CHO` is number `1`. For Compounds with two `-CHO` groups the suffix -dial is added to the alkane name. When other functional groups have naming priority `-CHO` is called formyl.

Common names replace the suffix -ic (-oic or - oxylic) and the word acid of the corresponding carboxylic acids by -aldehyde.

Locations of substituents on chains are designated by Greek letters e.g.

`-overset(tau)(C) -overset(delta)(C)-overset(gamma)(C)-overset(beta)(C)-overset(alpha)(C)-underset (underset H (|))C=O`

The terminal `C` of a long chain is designated `omega`

The IUPAC names of aldehydes follow the usual pattern. The Longest chain containing the `-CHO` group is considered the parent structure and named by replaced `-e` of the corresponding alkane by `al`. The position of the substituent is indicated by a number, the carbonyl carbon always being considered `C-1` Here as with the carbonyl acids, the `C-2` of the IUPAC name corresponding to aplha of the common name. Shown in Fig 2a.

Ketones : Common name use the name of `R` or `AR` as separate words, along with the word ketone. The IUPAC system replaces the `-e` of the name of the longest chain by the suffix - one. In molecules with functional goups such as `-COOH`, that have a higher naming priority, the carbonyl group is indicated by the prefix keto. Thus, `CH_3COCH_2CH_2COOH` is `4`- ketopentanoic acid.

The common name of aldehydes are derived from the names of the corresponding carboxylic acids by replacing -oic acid by -al

The simple aliphatic ketone has the common name acetone. For most other aliphatic ketones we name the two groups that are attached to carbonyl carbon and follow these names by the word ketone. A ketone in which the carbonyl group is attached to a benzene ring is named as phenone, all illustrated below. The position of various groups are indicated by numbers Shown in Fig 2b.

Structure of Carbonyl Group :

The carbonyl carbon atom is `sp^2` hybridized and bonded to three other atoms through three coplanar sigma bonds oriented about `120^o` apart. The unhybridized `p` orbital overlaps with a `p` orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is similar to an alkene `C = C` double bond, except that the carbonyl double bond is shorter and stronger. As shown in Fig 3a.

Another difference between the carbonyl and alkene double bonds is the large dipole moment of the carbonyl group. Oxygen is more electronegative than carbon, and the bonding electrons are not shared equally. In particular, the less tightly held pi electrons are pulled more strongly toward the oxygen atom, giving ketones and aldehydes larger dipole moments than most alkyl halides and ethers. We can use resonance structures to symbolize this unequal sharing of the pi electrons as shown in Fig 3b.

The first resonance structure is clearly more important, since it involves more bonds and less charge separation. The contribution of the second structure is evident by the large dipole moments of the ketones and aldehydes shown in Fig 3c.

This polarization of the carbonyl group contributes to the reactivity of ketones and aldehydes : The positively polarized carbon atom acts as an electrophile, and the negatively polarized oxygen acts as a nucleophile.