Question

Many primary amines, \(\mathrm{RNH}_{2}\), where \(\mathrm{R}\) is a carboncontaining fragment such as \(\mathrm{CH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2},\) and so on, undergo reactions where the transition state is tetrahedral. (a) Draw a hybrid orbital picture to visualize the bonding at the nitrogen in a primary amine (just use a C atom for "R"). (b) What kind of reactant with a primary amine can produce a tetrahedral intermediate?

Short answer

The bonding in a primary amine involves sp3 hybridization of the nitrogen atom, with three sigma bonds and one lone pair, creating a tetrahedral geometry. To form a tetrahedral intermediate, reactants need to accept the lone pair from the nitrogen atom of the primary amine. One such reactant is an acid chloride (\(\mathrm{RCOCl}\)), which can form a new bond with the nitrogen atom, creating a tetrahedral transition state that ultimately leads to the formation of an amide.

Step-by-step solution

(a) Drawing Hybrid Orbital Picture of Primary Amine

To visualize the bonding in a primary amine molecule with a nitrogen atom, start by drawing a simple Lewis structure. The primary amine, \(\mathrm{RNH_{2}}\), consists of a nitrogen atom with two hydrogen atoms and one R carbon-containing fragment attached to it. Now, focusing specifically on the nitrogen atom, it has three bonding orbitals that form sigma bonds with the other atoms in the molecule, and one non-bonding orbital that carries the lone pair electrons. As we're considering a hybrid orbital picture, the nitrogen atom undergoes sp3 hybridization, resulting in four sp3 orbitals. Here is a representation of the hybrid orbital picture of a primary amine (using a C atom for R): ``` H | R-C-N | H ``` Three of the sp3 orbitals form sigma bonds with the three surrounding atoms (R and two hydrogen atoms), while the fourth sp3 orbital houses the lone pair electrons.

(b) Identifying Reactants To Produce Tetrahedral Intermediate

To produce a tetrahedral intermediate with a primary amine, we need to find reactants that are compatible with the lone pair on the nitrogen atom. One such type of reactant are electrophilic compounds, which have a deficiency of electrons and can accept the lone pair during a reaction to create a new bond while retaining the tetrahedral geometry. A common reactant that can produce a tetrahedral intermediate with primary amines is an acid chloride (\(\mathrm{RCOCl}\)). In this system, the lone pair on the nitrogen atom of the amine can attack the electrophilic carbon atom in the acid chloride, leading to the release of the chloride ion and the formation of a new bond between nitrogen and carbon. As a result, a tetrahedral transition state is formed, which upon further rearrangements leads to the formation of an amide as the final product. In conclusion, the bonding in a primary amine involves sp3 hybridization of the nitrogen atom, with three sigma bonds and one lone pair. To form a tetrahedral intermediate, reactants need to accept the lone pair from the nitrogen atom of the primary amine, with acid chlorides being one such example.

Key concepts

Primary Amine

A primary amine, represented as \( \mathrm{RNH}_2 \), is a type of organic compound where a nitrogen atom is bonded to two hydrogen atoms and a carbon-containing fragment or alkyl group, denoted here by \( \mathrm{R} \). The carbon fragment can be something like \( \mathrm{CH_3} \) or \( \mathrm{CH_3CH_2} \). These compounds are foundational in organic chemistry due to their variety of reactions and roles in forming larger molecules.

Primary amines are characterized by the presence of one nitrogen atom, which is key in their chemical behavior. The lone pair of electrons on the nitrogen atom makes primary amines nucleophilic, meaning they can donate these electrons to an electron-deficient atom or a group in a chemical reaction.

• Due to their nucleophilicity, primary amines participate in a range of reactions, including the formation of amides and the synthesis of more complex organic compounds.
• This nucleophilic characteristic is also why primary amines engage in reactions resulting in a tetrahedral intermediate, particularly with electrophilic reagents.

The nitrogen in primary amines is crucial not only to their reactivity but also to the structural integrity of these molecules.

Hybridization

In chemistry, hybridization is the concept of mixing atomic orbitals into new hybrid orbitals suitable for the pairing of electrons to form chemical bonds. In primary amines, the nitrogen atom undergoes \( \mathrm{sp^3} \) hybridization.

This type of hybridization involves one s orbital and three p orbitals from the nitrogen's electron shell combining to form four \( \mathrm{sp^3} \) hybrid orbitals.

• Three of these \( \mathrm{sp^3} \) orbitals are involved in forming sigma bonds with other atoms in the molecule. Such bonds are represented in a primary amine by the bonds to the carbon atom of the \( \mathrm{R} \) group and the two hydrogen atoms.
• The remaining \( \mathrm{sp^3} \) orbital holds the lone pair of electrons, which gives the nitrogen atom its nucleophilic characteristics.

Understanding hybridization is key to visualizing molecular geometry, which for a primary amine is roughly a modified tetrahedron resulting from the electron pair arrangement.

Tetrahedral Intermediate

A tetrahedral intermediate is a transient structural state that occurs during certain chemical reactions. It is called 'tetrahedral' because it resembles a tetrahedron—a shape with four triangular faces.

In reactions involving primary amines, the transition to a tetrahedral intermediate often involves the nitrogen atom using its electron pair to form a new bond with an electron-poor atom or group in a reactant. A classic example is when an amine reacts with an acid chloride:

• The nucleophilic nitrogen atom attacks the carbonyl carbon of the acid chloride, which has a partial positive charge due to its eyes on bonding with other species.
• This attack leads to the formation of a new bond and a transition state that has four groups around the central atom—the defining feature of a tetrahedral intermediate.

This intermediate state eventually leads to the stable product, such as an amide when working with acid chlorides.

Electrophilic Reaction

An electrophilic reaction is a type of chemical reaction where an electrophile—an atom or molecule that is electron-poor—seeks out regions of high electron density to form bonds.

In the context of primary amines, an electrophilic reaction is often seen when these compounds interact with electron-deficient species. These species can accept electrons and usually possess a positive charge or a highly polarized bond that creates a partial positive charge.

• Acid chlorides, often used in these reactions, are one such electrophilic compound, ready to accept electrons from nucleophiles like primary amines.
• During such a reaction, the lone pair on the nitrogen atom in a primary amine attacks the electron-deficient site, forming a new bond and sometimes resulting in a tetrahedral intermediate.

This step is crucial in various synthesis processes, demonstrating the versatility and importance of electrophilic reactions in creating complex organic structures.

Sigma Bond

A sigma bond (\( \sigma \)-bond) is a type of covalent bond that is the strongest possible type of joint between two atomic nuclei, forming by the head-on overlapping of atomic orbitals.

In primary amines, sigma bonds are formed between the nitrogen atom and other atoms such as hydrogen or carbon in its \( \mathrm{R} \) group.

• The overlapping of \( \mathrm{sp^3} \) hybrid orbitals from nitrogen with the orbitals of hydrogen atoms results in strong sigma bonds.
• This overlap facilitates a stable bonding interaction, providing the structural integrity necessary for the chemical functionality of the amine.

Sigma bonds are essential to the stability and structure of molecules, as they allow for free rotation around the bond axis, contributing to molecular flexibility. This bond type positions itself diametrically opposed to pi bonds, which involve side-by-side overlapping of p orbitals and feature above and below the plane of the atoms.