Question

In the following compounds C1CCNCC1 C1COCCN1 c1cc[nH]c1 c1cc[nH]c1 (1) (2) (3) (4) The order of basicity is (a) \(4>1>3>2\) (b) \(3>1>4>2\) (c) \(2>1>3>4\) (d) \(1>3>2>4\)

Short answer

The order of basicity is (b) \(3>1>4>2\).

Step-by-step solution

Identify the Functional Groups

The given compounds in SMILES notation need to be translated to understand their functional groups. 1. Piperidine (hexahydro-1H-azepine): cyclohexane with a nitrogen atom. 2. Morpholine: a 4-membered ring with one oxygen, one nitrogen, and four carbons. 3. Pyrrole: a 5-membered aromatic ring with nitrogen. 4. Indole: similar to pyrrole, but with an additional benzene fused to it.

Understand Basicity Criteria

Basicity of amines is determined by the availability of the lone pair on the nitrogen atom for donation. This is influenced by the nitrogen's hybridization, resonance, and electronegativity of adjacent atoms.

Evaluate Piperidine

Piperidine (Compound 1) has a nitrogen with a lone pair in an sp3 hybridized orbital, making it highly available for protonation. This makes it quite basic.

Evaluate Morpholine

Morpholine (Compound 2) has a nitrogen lone pair which is less available due to the electronegative oxygen in the ring, reducing its basicity compared to piperidine.

Evaluate Pyrrole

In pyrrole (Compound 3), the nitrogen's lone pair is part of the aromatic sextet, decreasing its availability to act as a base, so it is significantly less basic.

Evaluate Indole

Indole (Compound 4) also has its nitrogen involved in the aromatic system, similar to pyrrole, making it less basic. However, the additional benzene ring slightly influences the electron density compared to pyrrole.

Compare Basicity

Comparing all four, the general order of basicity is determined by the availability of the nitrogen's lone pair for proton donation: Piperidine > Indole > Pyrrole > Morpholine.

Key concepts

Functional Groups

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Identifying functional groups helps us understand the behavior and reactivity of compounds.
For instance, in this exercise:

• Piperidine (Compound 1): This compound is a cyclic secondary amine characterized by a six-membered carbon ring with a nitrogen atom.
• Morpholine (Compound 2): This structure features a heterocyclic ring comprising nitrogen and oxygen atoms alongside carbon.
• Pyrrole (Compound 3): Known for its aromaticity, this is a five-membered ring with one nitrogen atom, highly involved in electron delocalization.
• Indole (Compound 4): Similar to pyrrole but incorporates an additional benzene ring which alters its reactivity.

Recognizing these groups is essential for understanding how chemicals react and interact in various conditions.

Basicity of Amines

Basicity in amines revolves around the ease with which the nitrogen atom can donate its lone pair of electrons. This is affected by several factors:

• Lone Pair Availability: The more accessible the lone pair on the nitrogen, the stronger the base it forms.
• Hybridization: Nitrogen atoms in sp³ hybridized orbitals have lone pairs that are more available than those in sp² or sp hybridized states.
• Resonance: Participation in resonance can delocalize the lone pair, reducing basicity. For instance, pyrrole's lone pair is part of its aromatic sextet, diminishing its basic nature.
• Electronegativity of Neighboring Atoms: An atom like oxygen in morpholine can draw electron density, making nitrogen less willing to donate its lone pair.

In these compounds, the basicity order bears considering how the nitrogen's environment affects its ability to donate electrons.

SMILES Notation

SMILES (Simplified Molecular Input Line Entry System) notation is a concise way to represent a chemical structure using letters for atoms and symbols for chemical connections. This notation is powerful for computational analysis and database storage.
Here's how SMILES applies in our examples:

• Piperidine is shown as C1CCNCC1, describing a cyclic (indicated by numbers "1") chain of carbon atoms plus nitrogen.
• Morpholine appears as C1COCCN1, reflecting both oxygen and nitrogen within the ring structure.
• Pyrrole is represented by c1cc[nH]c1, with lowercase 'c' denoting an aromatic carbon and '[nH]' specifying nitrogen bonded to hydrogen.
• Indole follows similar representation due to its aromatic nature.

Understanding SMILES can simplify digital processing of chemical structures, crucial for chemoinformatics.

Nitrogen Hybridization

Nitrogen atoms can exhibit various hybridization states—sp³, sp², and sp—each impacting the atom's electronic and reactive characteristics.

• sp³ Hybridization: Found in piperidine, where nitrogen's lone pair in an sp³ orbital is more extended and available for donation, contributing to stronger basicity.
• sp² Hybridization: In pyrrole and indole, nitrogen's lone pair participates in the aromatic π-system, reducing its ability to accept protons.
• Resonance and Delocalization: The overlap of p-orbitals in aromatic systems like pyrrole delocalizes the lone pair, significantly affecting basicity.

This understanding of hybridization informs both the electronic structure and the potential reactions nitrogen-containing organic compounds might undergo.