Ch.14 - Chemical Kinetics

Problem 123a

Chapter 14, Problem 123a

Many primary amines, RNH₂, where R is a carboncontaining fragment such as CH₃, CH₃CH₂, 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').

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Identify the hybridization of the nitrogen atom in a primary amine. Since nitrogen in amines typically forms three sigma bonds and has one lone pair, it is sp3 hybridized.

Visualize the sp3 hybridization: The nitrogen atom uses its sp3 hybrid orbitals to form bonds. These orbitals are arranged in a tetrahedral geometry.

Draw the nitrogen atom at the center with its sp3 hybrid orbitals extending outwards. Each orbital forms a bond with either a hydrogen atom or the carbon atom (R group).

Represent the lone pair on nitrogen: One of the sp3 hybrid orbitals contains a lone pair of electrons, which is not involved in bonding.

Illustrate the bonding: Draw three sigma bonds from the nitrogen atom, two to hydrogen atoms and one to the carbon atom (R group), showing the tetrahedral arrangement.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate the bonding requirements of atoms in molecules. In the case of primary amines, the nitrogen atom undergoes sp3 hybridization, resulting in four equivalent hybrid orbitals that allow for the formation of sigma bonds with hydrogen and carbon atoms.

Tetrahedral Geometry

Tetrahedral geometry refers to the spatial arrangement of four electron groups around a central atom, resulting in bond angles of approximately 109.5 degrees. In primary amines, the tetrahedral arrangement around the nitrogen atom is crucial for understanding the molecular shape and the nature of the transition state during reactions.

Transition State

The transition state is a high-energy, unstable arrangement of atoms that occurs during a chemical reaction, representing the point at which reactants are transformed into products. In the context of primary amines, the tetrahedral transition state indicates that the nitrogen's hybrid orbitals play a significant role in stabilizing this state as the reaction proceeds.

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Textbook Question

The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride:

Reaction 1: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

This reaction is very slow in the absence of light. However, Cl₂(g) can absorb light to form Cl atoms:

Reaction 2: Cl₂(g) + hv → 2 Cl(g)

Once the Cl atoms are generated, they can catalyze the reaction of CH₄ and Cl₂, according to the following proposed mechanism:

Reaction 3: CH₄(g) + Cl(g) → CH₃(g) + HCl(g)

Reaction 4: CH₃(g) + Cl₂(g) → CH₃Cl(g) + Cl(g)

The enthalpy changes and activation energies for these two reactions are tabulated as follows:

Reaction ΔH° (kJ/mol) E_a (kJ/mol)

3 +4 17

4 -109 4

(b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4.