   Chapter 20, Problem 41PS

Chapter
Section
Textbook Problem

Acetonitrile, CH3CN, is an important solvent. The chemical is normally available as a by-product of the manufacture of acrylonitrile, CH2=CHCN, the building block of polyacrylonitrile, a widely used polymer. Recently, however, the demand for acrylonitrile dropped, so the supply of acetonitrile diminished to a point where users of the solvent were concerned. For that reason new methods were sought for the synthesis of CH3CN, and a new “green” method was reported.Step 1: reaction of ethanol (C2H5OH) with ammonia to give ethylamine (C2H5NH3) and water.Step 2: reaction of ethylamine and oxygen (over a ruthenium oxide/aluminum oxide catalyst) to give acetonitrile and water. (a) Draw Lewis electron dot structures for ethanol, ethylamine, acetonitrile, and acrylonitrile. Specify the molecular geometry around each C atom and, as appropriate, around the O or N atoms. (b) Write balanced equations for each step in the synthesis of acetonitrile from ethanol. (c) Calculate the atom economy for the synthesis of acetonitrile from ethanol. (Atom economy is discussed in Applying Chemical Principles, 4.1, Green Chemistry and Atom Economy, page 207.)

(a)

Interpretation Introduction

Interpretation: The electron dot structures for ethanol, ethylamine, acetonitrile, and acrylonitrile have to be drawn, molecular geometry around the central atom in these molecules should be determined.

Concept introduction:

Lewis dot structure: For a given molecule, the bonding concerning the atoms and also the lone pairs can be shown by a diagram.

In this structure a single bond is represented by one dot pair and double bond is represented by two dot pairs and so on.

VSEPR Theory [Valence Shell Electron Pair Repulsion Theory]:

Geometry of the molecules can be predicted using VSEPR Theory. It is based on the number of electron pairs around the central atom of the molecule. Depending upon the bond pairs and lone pairs, geometries can be classified as linear, bent, pyramidal, and trigonal planar, tetrahedral, and octahedral and so on.

Explanation

In a molecule, Lewis structure shows the position of an atom and bonding is represented by lines or dot pairs.

The Lewis structures for ethanol, ethylamine, acetonitrile, and acrylonitrile are shown below.

Geometry around atoms in the given molecule can be predicted by VSEPR Theory.

In ethanol, the carbon atoms are surrounded by four bond pairs. Therefore, the geometry will be tetrahedral. In ethanol, oxygen atom is surrounded by 2 bond pairs and 2 lone pairs. Therefore the geometry is bent around oxygen atom.

In ethyl amine, the carbon atoms are surrounded by four bond pairs. Therefore, the geometry will be tetrahedral. Nitrogen atom is surrounded by three bond pair and two lone pairs and the geometry is pyramidal...

(b)

Interpretation Introduction

Interpretation: The balanced equations for each step in the synthesis of acetonitrile from ethanol should be written.

Concept introduction:

• For chemical reaction balanced chemical reaction equation written in accordance with the Law of conservation of mass.
• Law of conservation of mass states that for a reaction total mass of the reactant and product must be equal.
• Stoichiometric factor is a relationship between reactant and product which is obtained from the balanced chemical equation for a particular reaction.

(c)

Interpretation Introduction

Interpretation: Atom economy for the synthesis of acetonitrile from ethanol should be determined.

Concept introduction:

Atom economy: The amount of mass of reactants which is converted into useful products.

Atom economy =massofatomsinproductsmassofatomsinreactants×100%

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