Question

Write structural formulas for all aldehydes with the molecular formula \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}\) and give each its IUPAC name. Which of these aldehydes are chiral?

Short answer

Question: List the possible structural formulas for aldehydes with the molecular formula C6H12O and identify which of these aldehydes are chiral. Answer: The possible structural formulas for aldehydes with the molecular formula C6H12O are Hexanal, 2-Methylpentanal, 3-Methylpentanal, and 2,3-Dimethylbutanal. Among these, only 3-Methylpentanal is chiral.

Step-by-step solution

Know about aldehydes and nomenclature rules

Aldehydes are organic compounds containing a carbonyl functional group (C=O) bonded to a hydrogen atom and an alkyl or an aryl group. In IUPAC nomenclature, aldehydes are named by changing the suffix "-e" of the corresponding alkane to "-al." The positions of other substituents are indicated by numbers, starting from the carbonyl carbon as C1.

Find structural formulas for aldehydes with the molecular formula C6H12O

To draw the structural formulas, we will start with the longest carbon chain, including the aldehyde group, and then examine possible substitutions with shorter carbon chains.  1. Hexanal. \(\mathrm{CH}_{3}\mathrm{-CH}_{2}\mathrm{-CH}_{2}\mathrm{-CH}_{2}\mathrm{-CH}_{2}\mathrm{-CHO}\) We start with 6 carbons in a straight chain with an aldehyde at the end. No other structural isomer is possible with six continuous carbons. 2. Pentanal isomers 2a. 2-Methylpentanal. \(\mathrm{CH}_{3}\mathrm{-CH}\left(\mathrm{CH}_{3}\right)\mathrm{-CH}_{2}\mathrm{-CH}_{2}\mathrm{-CHO}\) One carbon can be replaced by a methyl group, which can be placed on C2 or C3 (if placed at C4, it would be identical to the structure of 2-Methylpentanal). 2b. 3-Methylpentanal. \(\mathrm{CH}_{3}\mathrm{-CH}_{2}\mathrm{-CH}\left(\mathrm{CH}_{3}\right)\mathrm{-CH}_{2}\mathrm{-CHO}\) 3. Butanal isomers 3a. 2,3-Dimethylbutanal. \(\mathrm{CH}_{3}\mathrm{-C}\left(\mathrm{CH}_{3}\right)\mathrm{CH}\left(\mathrm{CH}_{3}\right)\mathrm{-CHO}\) With an aldehyde at the end of a butane chain, the remaining carbon can form two methyl groups attached to C2 and C3. No other substitution pattern is possible.

Identify chiral molecules

Now, let's examine each identified aldehyde and check for chirality. Chiral molecules must have a carbon center with four different groups attached. 1. Hexanal. No chiral carbons, as there is no carbon with four different groups. 2. Pentanal isomers 2a. 2-Methylpentanal. No chiral carbons, as there is no carbon with four different groups. 2b. 3-Methylpentanal. Chiral. The carbon at position 3 has four different groups: \(\mathrm{-CH}_{2}\mathrm{-CH}_{3}\) to the left, \(\mathrm{-CH}_{2}\mathrm{-CHO}\) to the right, -H, and \(\mathrm{-CH}_{3}\) at the bottom. 3. Butanal isomers 3a. 2,3-Dimethylbutanal. No chiral carbons, as there is no carbon with four different groups. In conclusion, the aldehydes with the molecular formula C6H12O are Hexanal, 2-Methylpentanal, 3-Methylpentanal, and 2,3-Dimethylbutanal. Among these, only 3-Methylpentanal is chiral.

Key concepts

Structural Formulas of Aldehydes

Understanding the structural formulas of aldehydes is crucial for identifying different aldehyde compounds. An aldehyde typically consists of a carbonyl group (C=O) with a hydrogen atom and an alkyl or an aryl group attached to it. The simplest structural formula for an aldehyde can be represented as R-CHO, where R is any alkyl or aryl group.

For instance, with a molecular formula of cd=C}_{6}cd=H}_{12}cd=O}, we can derive several structural isomers. The simplest is Hexanal, which is a straight chain aldehyde with no branches: HO}. For aldehydes with branching, such as 2-Methylpentanal and 3-Methylpentanal, the structures introduce side methyl groups HO} and HO} respectively, illustrating how branching occurs at different positions of the carbon chain.

Aldehyde Chirality

Chirality is a property of a molecule that has a non-superimposable mirror image, often compared to how our left and right hands are mirror images but not identical. In aldehydes, chirality occurs when there is at least one carbon atom connected to four different groups, making it a chiral center.

For aldehydes with the molecular formula HO}, most structures do not have a chiral center and are not chiral. However, the aldehyde 3-Methylpentanal possesses a chiral carbon at the third position because it is bonded to four different groups: a hydrogen atom, a methyl group CHO}, and two different alkyl chains CHO} and CHO}. This unique feature makes it the only chiral aldehyde among the isomers derived from HO}, showcasing the importance of closely examining molecular connectivity for identifying chiral centers in organic compounds.

Organic Chemistry Nomenclature

The IUPAC nomenclature in organic chemistry is a systematic way of naming chemical compounds so that each name is unique and descriptive of the molecule's structure. In the case of aldehydes, the name usually ends with the suffix '-al,' providing an immediate indication of the presence of an aldehyde functional group in the molecule.

Following the IUPAC system, the longest carbon chain containing the aldehyde group is identified, and the chain is numbered starting from the aldehyde carbon. For instance, Hexanal is named based on its six-carbon chain with no branching. When side chains or substituents are present, their position and name are included, as seen with 2-Methylpentanal and 3-Methylpentanal . The proper naming of these molecules requires recognizing the parent chain and the location of substituents, ensuring that the name reflects the molecule's precise structure. This systematic approach aids in providing a clear and universally understood language among chemists.