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Reaction of O, S and N with H Atoms

Reaction of O, S and N with H Atoms. The complete electron configurations for S could be written as Sulfur 1s 2 2s 2 2p 6 3p x 2 3p y 1 3p z 1

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Reaction of O, S and N with H Atoms

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  1. Reaction of O, S and N with H Atoms • The complete electron configurations for S could be written as • Sulfur 1s22s22p63px23py13pz1 • Again, we can “pair up” all electrons if S and two H atoms combine to form H2S. The valence bond picture suggests that all bond angles in H2O, NH3and H2S should be 90o. This is close to the value seen in H2S (92o) but significantly underestimates bond angles in H2O (105o) and NH3 (107o).

  2. Bonding in H2S represented by atomic orbital overlap FIGURE 11-3 General Chemistry: Chapter 11

  3. The Methane “Problem” • The ground state configurations for C can be written as 1s22s22px12py1 Using the valence bond picture and the concept of paired electrons in molecular orbitals we might expect C to react with H atoms to form CH2. The CH2 molecule does form but is unstable (a transient species). However, carbon “happily” reacts with H to form the methane, CH4.

  4. Methane and Hybridization • By experiment, as previously discussed, methane has a regular tetrahedral geometry – four equal bond distances and all bond angles of 109.5o. The regular geometry of methane and its ability to form four bonds can be explained using the concept of hybridization. How have we explained carbon’s tendency to form four bonds previously?

  5. Methane and Hybridization – cont’d: • In the hybridization picture we imagine methane being formed from C and H atoms in three steps. In the first step we take a ground state C atom and excite one electron (from the 2s orbital) to form the lowest lying ( or first) excited state. • Carbon Ground State: 1s22s22px12py1 • Carbon Excited State: 1s22s12px12py12pz1

  6. Methane and Hybridization – cont’d: • In the second step we imagine “combining” the single occupied 2s orbital and the three occupied 3p orbitals in the excited to form four equivalent sp3 hybrid orbitals(each containing a single unpaired electron). In step three the “hybridized C atom” reacts with four H atoms to form a CH4 molecule. The process is represented on the next few slides.

  7. Hybridization of Atomic Orbitals General Chemistry: Chapter 11

  8. The sp3 hybridization scheme FIGURE 11-6 General Chemistry: Chapter 11

  9. Bonding and structure of CH4 FIGURE 11-7 General Chemistry: Chapter 11

  10. Organic Compounds and Structures: An Overview FIGURE 26-1 • Representations of the methane molecule General Chemistry: Chapter 26

  11. Organic Chemistry • The next two slides illustrate what starts to happen when two or more sp3 hybridized carbons are linked. The chemistry of carbon is infinitely varied and organic compounds are part of all of living things, important energy sources, key pharmaceuticals and so on.

  12. The ethane molecule C2H6 FIGURE 26-2 General Chemistry: Chapter 26

  13. The propane molecule, C3H8 FIGURE 26-3 General Chemistry: Chapter 26

  14. Alkanes from petroleum General Chemistry: Chapter 26

  15. Hybridization in NH3 and H2O • The sp3 hybridization picture can also be used to discuss the bonding in NH3 and H2O. The neutral N and O atoms have more valence electrons than does C. We thus end up putting either one lone pair of electrons (for N) or two lone pairs of electrons (for O) into sp3 hybrid orbitals. The following slides represent the process for N (NH3).

  16. Bonding in H2O and NH3 Notice that hybrid orbitals can accommodate lone-pair electrons as well as bonding electrons. General Chemistry: Chapter 11

  17. sp3 hybrid orbitals and bonding in NH3 FIGURE 11-8 General Chemistry: Chapter 11

  18. Class Examples • 1. Jumping the gun “just a bit” let’s draw structural formulas for ethane (H3C-CH3), methyl amine (H3C-NH2), methanol (CH3-OH) and propane (H3C-CH2-CH3). What structural features do these molecules have in common? Does the “octet rule” still hold?

  19. “Finding” sp3 Hybridized Atoms • 2. Using the molecular structure of the morphine molecule shown on the next slide, find (a) an sp3 hybridized C atom, (b) an sp3 hybridized N atom and (c) an sp3 hybridized O atom. There may be more than one example of each. The example is a little unfair since many “non-terminal” H atoms are not shown in this structure.

  20. Morphine , a very powerful and addictive painkiller, can be isolated from the opium poppy (Papaversomniferum). General Chemistry: Chapter 26

  21. Hybridization in B and Be Compounds • A hybridization scheme can be invoked for B that involves exciting a B atom from its ground electronic state 1s22s22px1 (say) to its first excited state 1s22s12px12py1. The hybrid orbitals formed here (from the combination of a single s orbital and two p orbitals are called sp2 hybrid orbitals. The sp2 hybridization scheme is invoked for the BF3 molecule.

  22. sp2 Hybrid Orbitals General Chemistry: Chapter 11

  23. The sp2 hybridization scheme FIGURE 11-9 General Chemistry: Chapter 11

  24. sp Hybridization • sp hybridization is important for molecules such as H-C≡N and H-C≡C-H. Acetylene (ethyne) is the first member of an important series of organic compounds, the alkynes.

  25. sp Hybridization General Chemistry: Chapter 11

  26. The sp hybridization scheme FIGURE 11-10 General Chemistry: Chapter 11

  27. Class Example • 3. The sp3 hybridization scheme can be invoked to explain the bonding in the silane (SiH4) molecule. What atomic orbitals on silicon would be used to construct hybrid orbitals?

  28. Covalent Bonds – Orbital Overlap – Sigma and Pi Bonds • The formation of both sigma bonds (σ bonds) and pi bonds (π bonds) is likely familiar. • Sigma bonds are formed (for a pair of atoms) by the overlap of atomic orbitals “pointing” towards, in each case, the other bonded atom. We can use s and p orbitals (etc) to form sigma bonds.

  29. Covalent Bonds – Orbital Overlap – Sigma and Pi Bonds – cont’d: • In sigma bonds the two atomic orbitals used to “construct” the molecular orbital overlap in the most spatially “direct” manner possible. • The overlap of a H 1s orbital and S 3p orbitals is shown on the next slide. • As an aside, one can see that it difficult to “point” an s orbital in any direction. Why?

  30. Bonding in H2S represented by atomic orbital overlap FIGURE 11-3 General Chemistry: Chapter 11

  31. Pi Bonds • Pi (π) bonds are formed when the bonding electron pair is placed in a molecular orbital formed (frequently) by p orbitals on adjacent atoms overlapping. The p orbitals on the bonded atoms are oriented perpendicular to the internuclear axis (which makes orbital overlap slightly less favourable).

  32. Carbon-Carbon Double Bonds • The carbon-carbon double bonds in common organic molecules are comprised of one σ bond and one π bond. The simplest molecule of this type is ethylene, H2C=CH2. Ethylene is a planar symmetric molecule (four identical C-H bonds and all bond angles near 120o). • Aside: Hydrocarbons containing C=C double bonds are called unsaturated. They can react with H2 to from saturated hydrocarbons.

  33. Carbon-Carbon Double Bonds – cont’d: • In saturated hydrocarbons (such as propane H3C-CH2-CH3 the bonding behaviour of all C atoms is well explained using the sp3 hybridization scheme. In many unsaturated hydrocarbons the bonding of C atoms joined by double bonds is rationalized using an sp2hybridization scheme.

  34. Carbon-Carbon Double Bonds – cont’d: • For the sp3 hybridization scheme (for carbon!) we imagined distributing four valence electrons among a 2s and the three 2p atomic orbitals and “scrambling” these orbitals together to form four hybrid orbitals. For the sp2 hybridization scheme we will “scramble” the single 2s orbital and two 2p orbitals to form three hybrid sp2 orbitals. A single 2p orbital remaining is used to from a pi bond.

  35. Multiple Covalent Bonds Bonding in C2H4 • Ethylene has a double bond in its Lewis structure. • VSEPR says trigonal planar at carbon. H H C C H H General Chemistry: Chapter 11

  36. Bonding in Ethylene – an Information Packed Slide! • The next slide contains lots of information. • Upper left corner – a representation (for C) showing the structure and disposition in space of the three hybrid sp2 orbitals and the “left over” carbon p atomic orbital. • Upper right corner – picture showing how the five sigma bonds in ethylene are formed using four H 1s orbitals and six carbon sp2 orbitals.

  37. Bonding in Ethylene – an Information Packed Slide – cont’d! • Lower left corner. The sigma bonds are in place and the p orbitals on the two C atoms have “not yet” overlapped to from a pi bond. In fact the p orbitals have been drawn slightly “smaller than life” for clarity. • Bottom center and right. Two representations of the pi bond in ethylene. Note the electron density well away from the C-C internuclear axis.

  38. Sigma (s) and pi (π) bonding in C2H4 FIGURE 11-14 General Chemistry: Chapter 11

  39. 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?)

  40. Esters The distinctive aroma and flavor of oranges are due in part to the ester octyl acetate, CH3CO2CH2(CH2)6CH3 General Chemistry: Chapter 26

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