Structural Biochemistry/Organic Chemistry/Organic Functional Group/Carbonyl/Aldehyde
The aldehyde is a functional group underneath the general group known as carbonyls. An aldehyde is a compound which contains a terminal carbon that is bound to any generic “R” group, doubly bonded to an oxygen, and single-bonded to a hydrogen. The “R” group is normally a simple aliphatic chain. When the “R” group is hydrogen, the resulting aldehyde is the simplest one, and is known as formaldehyde.
The carbon doubly bound to the oxygen is labeled as the alpha-carbon. Thus, the hydrogen bound to that carbon is the alpha-hydrogen. Because of the resonance stabilization from the carbonyl oxygen, the hydrogen is more acidic and has a pKa around 17. Of course, this pKa varies on the “R” group that the carbon is bound to.
When naming aldehydes use the IUPAC rules. For some aldehydes, it is much easier to use their common name.
1. The carbonyl carbon is designated as carbon #1.
2. The longest alkyl chain including the carbonyl carbon is designated and named according to normal alkyl rules.
3. The suffix for an aldehyde is –al. The “e” in the alkane chain is replaced with “-al” to designate that the molecule is an aldehyde.
4. Exception: If the carbonyl carbon of an aldehyde is part of a ring, the compound is now a cycloalkane carbaldehyde. The carbon of the ring to which the aldehyde is attached is now carbon #1.
5. An aldehyde takes higher priority than an alcohol group.
6. If an aldehyde is a substituent because there are multiple carbonyl groups present, the carbonyl with the longer alkyl chain predominates and then presence of the other carbonyl group is designated by –oxo.
Several Common Names:
In some cases, the aldehyde can be named by removing the suffix and replacing it with aldehyde.
HCHO is named formaldehyde.
CH3CHO is named acetaldehyde.
C6H5CHO is named benzaldehyde.
Like all the carbonyl compounds, the carbonyl carbon in the aldehyde is positively charged due to the electronegative atom. Thus, the carbonyl carbon is an electrophile.
Nucleophilic Attack: Because of the polarity of the carbonyl carbon, nucleophilic attack is a typical reaction aldehydes undergo. Thus, the carbon is subject to nucleophilic attack from other excellent nucleophiles such as cyanide and hydroxide ions. In nucleophilic attack, the lone pair of electrons on the nucleophile attacks the positively charged carbon, which moves one of the two bonds shared between the carbon and oxygen onto the oxygen. Now the oxygen bears a full negative charge. This oxygen can be worked up with acid resulting in the alcohol.
Reducing and Oxidizing Aldehydes
1. Aldehydes can be reduced to primary alcohols by using reducing agents such as sodium borohydrate and lithium aluminum hydride. For example, ethanol is reduced to ethanol by using one of those two reducing agents.
2. Aldehydes can also be oxidized to carboxylic acids using reducing acids such as permanganate or nitric oxide.