Question

Propose structures for substances that fit the following descriptions: (a) A chiral quaternary ammonium salt (b) A six-membered heterocyclic diamine (c) A secondary amine, \(\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{~N}\)

Short answer

(a) N,N,N-trimethyl-1-phenylpropan-1-aminium bromide; (b) Piperazine; (c) Cyclohexylamine.

Step-by-step solution

Understanding Chirality and Quaternary Ammonium Salts

A chiral molecule has a non-superimposable mirror image, often involving an asymmetric carbon. A quaternary ammonium salt has four alkyl groups attached to a positively charged nitrogen. To achieve chirality, one of these groups must make the nitrogen center chiral. A common example could be N,N,N-trimethyl-1-phenylpropan-1-aminium bromide, where asymmetry arises from different groups around the nitrogen.

Identifying a Six-Membered Heterocyclic Diamine

A heterocyclic compound includes at least one atom in the ring structure that is not carbon. In a six-membered ring with two nitrogen atoms, a common structure is piperazine. Piperazine has the formula C₄H₁₀N₂ with nitrogen atoms at opposite positions in a hexagonal ring, making it a heterocyclic diamine.

Determining the Secondary Amine Structure

A secondary amine has the general structure R₂NH where R is a hydrocarbon group. For a compound with the formula C₆H₁₁N, the remaining formula suggests a cyclohexyl group. Cyclohexylamine, with a cyclohexyl group bonded to the secondary nitrogen (NH), fits this description. It has a cyclic hydrocarbon ring (C₆) with an amine group (NH) attached, making it a secondary amine.

Key concepts

Chiral Quaternary Ammonium Salt

At the heart of a quaternary ammonium salt is a positively charged nitrogen atom bonded to four carbon-based groups. This structure grants it unique properties, especially in the realm of chirality. Chirality refers to the property of an object being non-superimposable on its mirror image. Think of your left and right hands—similar, yet undeniably different in structure. In chemistry, this often involves a central atom attached to four different groups. For a quaternary ammonium salt to be chiral, the nitrogen atom must have four distinct substituents. A classic example of a chiral quaternary ammonium salt is N,N,N-trimethyl-1-phenylpropan-1-aminium bromide. To visualize its chirality, imagine a nitrogen atom at the center of a pyramid, with each vertex occupied by a different group. The key to its chirality is the arrangement of these groups, making the molecule's mirror image non-superimposable. Such compounds are vital in various applications, including as chiral catalysts in chemical reactions.

Six-Membered Heterocyclic Diamine

When we speak about heterocyclic compounds, we refer to rings that contain more than just carbon atoms. Specifically, a six-membered heterocyclic diamine includes a ring of six members—some of which are nitrogen atoms rather than carbon. One common example fitting this description is piperazine. Piperazine is structurally characterized by a six-membered ring with two nitrogen atoms situated directly opposite each other. The molecular formula is C₄H₁₀N₂. What makes it particularly fascinating is that it's an integral part of many pharmaceutical compounds, functioning as a building block for medications that treat various conditions. This dual nitrogen structure contributes to its unique chemical properties, facilitating its use in different chemical reactions and as a synthesis intermediate.

Secondary Amine Structure

Secondary amines are characterized by the presence of two carbon groups ( R ) connected to a nitrogen ( N ). The simplest way to imagine this is as a nitrogen atom bonded to two carbon-containing groups and a hydrogen atom. In the case of the secondary amine with the formula C₆H₁₁N, this corresponds to cyclohexylamine. Cyclohexylamine embodies the secondary amine structure with its main feature being a six-membered carbon ring—cyclohexane—bound to an amine group (NH). The structure of cyclohexylamine includes a nitrogen atom bonded to a cyclohexyl group, making it a valuable component in various chemical syntheses. This structure allows flexibility and stability, serving as an intermediate or a basic unit in dye manufacturing, pharmaceuticals, and the production of herbicides, due to its capability to interact and bond with various organic compounds.