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. - презентация

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

2 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?)

3 Esters CH 3 CO 2 CH 2 (CH 2 ) 6 CH 3 The distinctive aroma and flavor of oranges are due in part to the ester octyl acetate, Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 26Slide 3 of 75

4 Carbon-Carbon Triple Bonds The CC 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.

5 and π bonding in C 2 H 2 FIGURE Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 5 of 57

6 Other Molecules with Triple Bonds Carbon monoxide:CO Hydrogen cyanide:H-CN Methyl cyanide:H 3 C-CN Cyanoacetylene:H-CC-CN Aside: The organic cyanides (or nitriles) are found wherever people are smoking tobacco. Many cyanoacetylenes are found in dusty interstellar clouds (so is ethanol!).

7 Hybridization Summary for C Atoms Hybridization Scheme Types and Number of Covalent Bonds Example Molecules sp 3 4 sigma bondsCH 4, C 2 H 6, H 3 C-O-CH 3 sp 2 3 sigma bonds, 1 pi bond H 2 C=O, H 2 C=CH 2 sp2 sigma bonds, 2 pi bonds H-CC-H, O=C=O

8 Alkenes and Alkynes Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 26Slide 8 of 75

9 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…….

10 Molecular Orbitals and Wave Properties of Electrons Weve 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 H 2 molecule has lower potential energy (or, is more stable) than the two isolated H atoms. Destructive combinations of atomic orbitals are also possible.

11 Molecular Orbital Theory Atomic orbitals are isolated on atoms. Molecular orbitals span two or more atoms. LCAO – Linear combination of atomic orbitals. Ψ 1 = φ 1 + φ 2 Ψ 2 = φ 1 - φ 2 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 11 of 57

12 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.

13 Formation of bonding and antibonding orbitals FIGURE Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 13 of 57

14 The interaction of two hydrogen atoms according to molecular theory FIGURE Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 14 of 57

15 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 (Hunds Rule).

16 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. O 2 + and CN -). 2. Label MOs in the MO diagram and show their relative energies. Indicate whether MOs are bonding or anti-bonding.

17 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?).

18 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.

19 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!

20 Bond Order Copyright © 2011 Pearson Canada Inc. 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

21 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 H 2 molecule, for example, two 1s atomic orbitals can combine to form a σ 1s bonding molecular orbital and a σ 1s * anti-bonding molecular orbital. When 2p orbitals come into play we can form both σ and π molecular orbitals.

22 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). Well illustrate this with the molecules H 2, He 2, H 2 + and He 2 +.

23 Diatomic Molecules of the First-Period Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 23 of 57 FIGURE Molecular orbital diagrams for the diatomic molecules and ions of the first- period elements BO = (1-0)/2 = ½ H2+H2+ BO = (2-0)/2 = 1 H2H2 BO = (2-1)/2 = ½ He 2 + BO = (2-2)/2 = 0 He 2 BO = (e - bond - e - antibond )/2

24 Class Examples Draw molecular orbital diagrams for Li 2 and Be 2. 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.

25 Molecular Orbitals of the Second Period Elements Copyright © 2011 Pearson Canada Inc. Slide 25 of 57General Chemistry: Chapter 11 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).

26 Molecules with 2 nd 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.

27 Possible molecular orbital energy-level scheme for diatomic molecules of the second-period elements FIGURE (PART A) Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 27 of 57

28 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. Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 11Slide 28 of 57

29 MO Diagrams – Surprises – C 2 : Two possible MO diagrams are illustrated for the C 2 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 C 2 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?

30 Copyright © 2011 Pearson Canada Inc. Slide 30 of 57General Chemistry: Chapter 11 Experiment shows C 2 to be diamagnetic, supporting a modified energy-level diagram Expected MO Diagram for C 2 Modified MO Diagram for C 2

31