Chapter 2.SE, Problem 42AP

Draw electron-dot structures for the following molecules, indicating any unshared electron pairs. Which of the compounds are likely to act as Lewis acids and which as Lewis bases?

(a) AlBr₃

(b) CH₃CH₂NH₂

(c) BH₃

(d) HF

(e) CH₃SCH₃

(f) TiCl₄

Interpretation Introduction

a) AlBr₃

Interpretation:

Electron dot structure for AlBr₃ indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. Thus aluminum has three dots to represent its 3s²3p¹electrons, bromine has seven dots to represent its 4s²4p⁵ electrons. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

To give:

The electron dot structure for AlBr₃ indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for AlBr₃ indicating any unshared electron pairs can be given as

It will act as a Lewis acid.

Explanation of Solution

Aluminum atom has three electrons (3s²3p¹) in its valence shell. It has used all the three electrons in forming three Al- Br bonds. So it has no lone pair of electrons. The bromine atom has seven electrons in its valence shell (4s²4p⁵). Out of these seven electrons, each bromine has used one electron for forming Al-Br bond and thus has the remaining six electrons as three lone pairs. AlBr_{3 will act as Lewis acid since the central aluminum atom requires two more electrons to complete its octet.}

Conclusion

The electron–dot structure for AlBr_{3 indicating any unshared electron pairs can be given as}

It will act as a Lewis acid.

Interpretation Introduction

b) CH₃CH₂NH₂

Interpretation:

Electron dot structure for CH₃CH₂NH₂ indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. Thus hydrogen has one dot to represent its 1s¹ electron, carbon has four dots to represent its 2s²2p² electrons, and nitrogen has five dots to represent its 2s²2p³ electrons. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

To give:

The electron dot structure for CH₃CH₂NH₂ indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for CH₃CH₂NH₂ indicating any unshared electron pairs can be given as,

It will act as a Lewis base.

Explanation of Solution

Carbon atom has four electrons (2s²2p²) in its valence shell. Both carbons have used all the four electrons in forming four bonds with other atoms. So they do not possess any lone pair of electrons. The nitrogen atom has five electrons in its valence shell (2s²2p³). Out of these five electrons, nitrogen has used three electrons for forming one C-N and two N-H bonds and thus has the remaining two electrons as a lone pair. CH₃CH₂NH₂ will act as Lewis base since the nitrogen atom can donate a pair of electrons to other species.

Conclusion

The electron–dot structure for CH₃CH₂NH₂ indicating any unshared electron pairs can be given as,

It will act as a Lewis base.

Interpretation Introduction

c) BH₃

Interpretation:

Electron dot structure for BH₃ indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. Thus boron has three dots to represent its 2s²2p¹electrons, hydrogen has one dot to represent its 1s¹ electron. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

To give:

The electron dot structure for BH₃ indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for BH₃ indicating any unshared electron pairs can be given as,

It will act as a Lewis acid.

Explanation of Solution

Boron atom has three electrons (2s²2p¹) in its valence shell. It has used all the three electrons in forming three B- H bonds. So it has no lone pair of electrons. Each hydrogen atom has one electron in its valence shell (1s¹) which they utilize in forming the B-H bonds. BH₃ will act as Lewis acid since the central boron atom requires two more electrons to complete its octet.

Conclusion

The electron–dot structure for BH₃ indicating any unshared electron pairs can be given as,

It will act as a Lewis acid.

Interpretation Introduction

d) HF

Interpretation:

Electron dot structure for HF indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. Thus fluorine has seven dots to represent its 2s²2p⁵ electrons, hydrogen has one dot to represent its 1s¹ electron. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors. To give: The electron dot structure for HF indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for HF indicating any unshared electron pairs can be given as

It will act as a Lewis acid.

Explanation of Solution

Fluorine atom is the most electronegative element and hence the H-F bond is highly polarized. The electrons in the bond are more concentrated on F rather than on H. The hydrogen is thus electron deficient and it can accept a pair of electrons from a Lewis base. Hence HF is likely to behave as Lewis acid.

Conclusion

The electron–dot structure for HF indicating any unshared electron pairs can be given as

It will act as a Lewis acid.

Interpretation Introduction

e) CH₃SCH₃

Interpretation:

Electron dot structure for CH₃SCH₃ indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. Thus hydrogen has one dot to represent its 1s¹ electron, carbon has four dots to represent its 2s²2p² electrons and sulfur has six dots to represent its 3s²3p⁴ electrons. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

To give:

The electron dot structure for CH₃SCH₃ indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for CH₃SCH₃ indicating any unshared electron pairs can be given as,

It will act as a Lewis base.

Explanation of Solution

Carbon atom has four electrons (2s²2p²) in its valence shell. Both carbons have used all the four electrons in forming four bonds with other atoms. So they do not possess any lone pair of electrons. The sulfur atom has six electrons in its valence shell (3s²3p⁴). Out of these six electrons, sulfur has used two electrons for forming two C-S bonds and thus has the remaining four electrons as two lone pairs. CH₃SCH₃ will act as Lewis base since the sulfur atom can donate a pair of electrons to other species.

Conclusion

The electron–dot structure for CH₃SCH₃ indicating any unshared electron pairs can be given as,

It will act as a Lewis base.

Interpretation Introduction

f) TiCl₄

Interpretation:

Electron dot structure for TiCl₄ indicating any unshared electron pairs is to be drawn. Whether it will act as Lewis acid or Lewis base is also to be stated.

Concept introduction:

A covalent bond formed by sharing of two electrons between two atoms in a molecule is usually indicated by electron-dot structures in which valence electrons are represented as dots. A stable molecule results whenever all the atoms except hydrogen in a molecule achieve in their valence shell eight dots and hydrogen achieve two dots. The electrons present in the inner shells of the atoms are not shown.

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

To give:

The electron dot structure for TiCl₄ indicating any unshared electron pairs and to state whether it will act as Lewis acid or Lewis base.

Answer to Problem 42AP

The electron–dot structure for TiCl₄ indicating any unshared electron pairs can be given as,

It will act as a Lewis acid.

Explanation of Solution

The chlorine atom has seven electrons in its valence shell (3s²3p⁵). Out of these seven electrons, each chlorine has used one electron for forming Ti-Cl bond and thus have the remaining six electrons as three lone pairs. Titanium is a transition element. Transition elements utilize the d orbitals in the penultimate shell also along with orbitals of the valence shell during bonding. It has the configuration 3d²4s². It has used all the four electrons in forming four Ti-Cl bonds.

The d orbitals can have a maximum of ten electrons. In TiCl₄, the titanium atom has empty d orbitals to accommodate electrons donated by a Lewis base. Hence TiCl₄ will act as Lewis acid.

Conclusion

The electron–dot structure for TiCl₄ indicating any unshared electron pairs can be given as,

It will act as a Lewis acid.