Sigma ( ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General...

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Sigma () and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11 Slide 1 of 57

Transcript of Sigma ( ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General...

Page 1: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Sigma () and pi (π) bonding in C2H4

FIGURE 11-14

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11 Slide 1 of 57

Page 2: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Class examples

• 4. The ethanoic acid (“vinegar”) moleculae and the methyl ethanoate molecule (an ester) shown on the next slide contain C=O double bonds. What is the hybridization of the C and O atoms in these double bonds? (Mention acetone, acetaldehyde, formaldehye?)

Page 3: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Esters

CHCH33COCO22CHCH22(CH(CH22))66CHCH33

The distinctive aroma and flavor of oranges are due in part to the ester octyl acetate,

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Page 4: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Carbon-Carbon Triple Bonds

• The C≡C triple bond is explained using an sp hybridization scheme for C. We imagine distributing the 4 valence electrons (again singly) over the 2s and three 2p orbitals. One s orbital and one p orbital are combined to form two hybrid sp orbitals. Two p orbitals on C (each containing a single electron) can be used to form two pi bonds.

Page 5: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

and π bonding in C2H2

FIGURE 11-16

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General Chemistry: Chapter 11 Slide 5 of 57

Page 6: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Other Molecules with Triple Bonds

• Carbon monoxide: C≡O

• Hydrogen cyanide: H-C≡N

• Methyl cyanide: H3C-C≡N

• Cyanoacetylene:H-C≡C-C≡N

• Aside: The organic cyanides (or nitriles) are found wherever people are smoking tobacco. Many cyanoacetylenes are found in dusty interstellar clouds (so is ethanol!).

Page 7: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Hybridization Summary for C Atoms

Hybridization Scheme

Types and Number of Covalent Bonds

Example Molecules

sp3 4 sigma bonds CH4, C2H6,H3C-O-CH3

sp2 3 sigma bonds, 1 pi bond

H2C=O, H2C=CH2

sp 2 sigma bonds, 2 pi bonds

H-C≡C-H, O=C=O

Page 8: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Alkenes and Alkynes

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Page 9: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Hybridization – Class Examples

• We will draw structures for a range of organic molecules and determine which hybridization scheme can be used to describe the bonding for each C atom. These molecules will include saturated hydrocarbons, unsaturated hydrocarbons, alcohols, carboxylic acids, amines, aromatic compounds…….

Page 10: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals and Wave Properties of Electrons

• We’ve mentioned that atomic orbitals can combine constructively to form a bonding molecular orbital. In the simplest case two H atoms are joined using a bonding molecular orbital. The H2 molecule has lower potential energy (or, is more stable) than the two isolated H atoms. “Destructive” combinations of atomic orbitals are also possible.

Page 11: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbital Theory

• Atomic orbitals are isolated on atoms.• Molecular orbitals span two or more

atoms.• LCAO – Linear combination of atomic orbitals.

Ψ1 = φ1 + φ2 Ψ2 = φ1 - φ2

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11 Slide 11 of 57

Page 12: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Electron Density in Bonding and Antibonding Orbitals

• Bonding orbitals – considerable electron density between the bonded atoms.

• Non-bonding orbitals – very little electron density between the bonded atoms (energetically unfavourable result).

• Bonding and antibonding sigma orbitals are represented on the next slide.

Page 13: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Formation of bonding and antibonding orbitals

FIGURE 11-20

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Page 14: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

The interaction of two hydrogen atoms according to molecular theory

FIGURE 11-21

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Page 15: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Basic Ideas Concerning MOs

Copyright © 2011 Pearson Canada Inc. Slide 15 of 57General Chemistry: Chapter 11

1. Number of MOs = Number of AOs.

2. Bonding (lower energy) and antibonding (higher energy) MOs formed from AOs.

3. e- fill the lowest energy MO first (aufbau process)

4. Maximum 2 e- per orbital (Pauli Exclusion Principle)

5. Degenerate orbitals fill singly before they pair up (Hund’s Rule).

Page 16: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals – Learning Objectives

• 1. Construct molecular orbital diagrams for diatomic molecules composed of elements from the first period elements (H and He) and the second period elements (Li, Be, B, C, N, O F and Ne). This includes species with +ve and -ve charges. (Eg. O2+ and CN-).

• 2. Label MOs in the MO diagram and show their relative energies. Indicate whether MOs are bonding or anti-bonding.

Page 17: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals – Learning Objectives

• 3. Use the molecular formula (for neutral molecules and diatomic ions) and charge to determine the total number of electrons that we must accommodate using the MO picture.

• 4. Distribute all of the electrons among the available MOs – starting with the lowest energy MOs (sound familiar?).

Page 18: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals – Learning Objectives

• 5. After counting the number of electrons in both bonding and anti-bonding orbitals determine the bond order.

• 6. Use the MO diagram (and the number of electrons in the various molecular orbitals) to determine whether a molecule is diamagnetic or paramagnetic.

Page 19: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals – Learning Objectives

• 7. Understand a surprising feature of molecular orbital theory. We can accommodate all of the valence electrons in various molecular orbitals for a diatomic species and end up with a bond order of zero!

Page 20: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Bond Order

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Slide 20 of 57General Chemistry: Chapter 11

• Stable species have more electrons in bonding orbitals than antibonding.

Bond Order = No. e- in bonding MOs - No. e- in antibonding MOs

2

Page 21: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals – Nomenclature:

• For the simplest atoms (H, He, Li, Be) only 1s and 2s orbitals are occupied in the ground electronic state. The overlap of two 1s orbitals can only produce a sigma (σ) bond. In the H2 molecule, for example, two 1s atomic orbitals can combine to form a σ1s bonding molecular orbital and a σ1s

* anti-bonding molecular orbital. When 2p orbitals come into play we can form both σ and π molecular orbitals.

Page 22: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Simplest Diatomics – MO Diagrams

• MO diagrams are initially a bit confusing because they represent the formation of chemical bonds using both a “before picture” (showing the relative energies of the various atomic orbitals) and an “after picture” (showing the relative energies of the molecular orbitals). We’ll illustrate this with the molecules H2, He2, H2

+ and He2+.

Page 23: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Diatomic Molecules of the First-Period

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FIGURE 11-22

•Molecular orbital diagrams for the diatomic molecules and ions of the first-period elements

BO = (1-0)/2 = ½ H2+

BO = (2-0)/2 = 1 H2

BO = (2-1)/2 = ½ He2+

BO = (2-2)/2 = 0 He2

BO = (e-bond - e-

antibond )/2

Page 24: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Class Examples

• Draw molecular orbital diagrams for Li2 and Be2. Using the MO diagrams determine the bond order for both molecules and, as well, indicate from the MO diagrams whether the molecules are diamagnetic or paramagnetic.

Page 25: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecular Orbitals of the Second Period Elements

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• First period use only 1s orbitals.• Second period have 2s and 2p orbitals

available.

• p orbital overlap:– End-on overlap is best – sigma bond (σ).– Side-on overlap is good – pi bond (π).

Page 26: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Molecules with 2nd Period Atoms

• The simplest possible molecular orbital diagram that one could imagine for second row elements having 2p electrons is shown on the next slide. This slide would necessarily apply only to homonuclear diatomics. Note the “symmetrical disposition” of bonding and nonbonding orbitals.

Page 27: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

Possible molecular orbital energy-level scheme for diatomic molecules of the second-period elements

FIGURE 11-25 (PART A)

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General Chemistry: Chapter 11 Slide 27 of 57

Page 28: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

MO Diagrams - Surprises

• The MO diagram presented on the previous slide does not adequately explain all properties of diatomic molecules formed from second period elements. Overlap of 2p atomic orbitals produces six MOs whose order energy order can vary with atomic number of the bonded atoms.

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Page 29: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

MO Diagrams – Surprises – C2:

• Two possible MO diagrams are illustrated for the C2 molecule on the next slide. The presentation of MOs here is similar to that used in drawing orbital diagrams for atoms. By experiment we know that the C2 molecule (4 valence electrons contributed by each C atom for a total of 8) is diamagnetic. Which of the MO diagrams accounts for this diamagnetism?

Page 30: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.

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Slide 30 of 57General Chemistry: Chapter 11

•Experiment shows C2 to be diamagnetic, supporting a modified energy-level diagram

Expected MO Diagram for C2

Modified MO Diagram for C2

Page 31: Sigma (  ) and pi (π) bonding in C 2 H 4 FIGURE 11-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 1 of 57.