1. 1 Organic Chemistry CHM 211 Summer 2006 Dr. John Tyrell Office: Dobo 227 Telephone: 962-7299(campus) 793-3361(home) Email: tyrellj@uncw.edutyrellj@uncw.edu http://people.uncw.edu/tyrellj/chem211/index. htm

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3. 3 Texts Organic Chemistry, 6 th edition, McMurry Optional Study Guide and Solutions Manual for McMurry's organic Chemistry, 6 th edition Molecular model kit

4. 4 Grading Policy Four 45-minute exams, each at 100 points. One comprehensive final exam at 100 points. The lowest 45-minute exam will be dropped. An absence at a 45-minute exam counts as the dropped exam. There will be no make up exams. Each of the exams will likely include at least one problem from the homework assignments. Exams 2- 4 will likely contain one review question from the previous test

5. 5 Attendance & Homework Attendance is expected, but not officially monitored for grading purposes. Missing 1 day in the summer is like missing 1 week during a regular semester! Homework problems are assigned, but not collected. Actively working on the homework problems allows you to test whether you understand the material and serves as a review guide for the exams.

6. 6 Keys to Success Memorization alone is not sufficient Reasoning alone is not sufficient Study three times: Before the lecture After the lecture Before the test Actively do problems. (keep a notebook) Cooperate – study groups

7. 1. Structure and Bonding Based on McMurry’s Organic Chemistry, 6 th edition, Chapter 1

8. 8 Organic Chemistry is the study of carbon containing compounds Pronunciation is important Pronounced “CAHBON”

9. 9 C (Carbon) 6 protons Atomic number = 6 12 C has 6 protons, 6 neutrons and 6 electrons amu = 12 13 C has 6 protons, 7 neutrons and 6 electrons amu =13 Carbon atomic weight = 12.011

10. 10 Atom Protons and neutrons are in the center or nucleus of the atom Electrons move around the nucleus. Orbitals describe where the electrons are s orbitals (spherical), p orbitals(dumbbell), d orbitals Electrons are grouped in different layers or shells

11. 11 1.1 Atomic Structure Structure of an atom Positively charged nucleus (very dense, protons and neutrons) and small (10 -15 m) Negatively charged electrons are in a cloud (10 -10 m) around nucleus Diameter is about 2  10 -10 m (200 picometers (pm)) [the unit angstrom (Å) is 10 -10 m = 100 pm]

12. 12 1.2 Atomic Structure: Orbitals Quantum mechanics: describes electron energies and locations by a wave equation Wave function solution of wave equation Each Wave function is an orbital,  A plot of  2 describes where electron most likely to be Electron cloud has no specific boundary so we show most probable area

13. 13 Shapes of Atomic Orbitals for Electrons Four different kinds of orbitals for electrons based on those derived for a hydrogen atom Denoted s, p, d, and f s and p orbitals most important in organic chemistry s orbitals: spherical, nucleus at center p orbitals: dumbbell-shaped, nucleus at middle

14. 14 p-Orbitals There are three perpendicular p orbitals, px, py, and pz, of equal energy Lobes of a p orbital are separated by region of zero electron density, a node

15. 15 1.5 The Nature of the Chemical Bond Atoms form bonds because the compound that results is more stable than the separate atoms Ionic bonds in salts form as a result of electron transfers Organic compounds have covalent bonds from sharing electrons (G. N. Lewis, 1916) Lewis structures shown valence electrons of an atom as dots Hydrogen has one dot, representing its 1s electron Carbon has four dots (2s 2 2p 2 ) Stable molecule results at completed shell, octet (eight dots) for main-group atoms (two for hydrogen)

16. 16 Number of Covalent Bonds to an Atom Atoms with one, two, or three valence electrons form one, two, or three bonds Atoms with four or more valence electrons form as many bonds as they need electrons to fill the s and p levels of their valence shells to reach a stable octet

17. 17 Valences of Carbon Carbon has four valence electrons (2s 2 2p 2 ), forming four bonds (CH 4 )

18. 18 Valences of Oxygen Oxygen has six valence electrons (2s 2 2p 4 ) but forms two bonds (H 2 O)

19. 19 Valences of Nitrogen Nitrogen has five valence electrons (2s 2 2p 3 ) but forms only three bonds (NH 3 )

20. 20 Non-bonding electrons Valence electrons not used in bonding are called nonbonding electrons, or lone-pair electrons Nitrogen atom in ammonia (NH 3 ) Shares six valence electrons in three covalent bonds and remaining two valence electrons are nonbonding lone pair

21. 21 1.6 Valence Bond Theory Covalent bond forms when two atoms approach each other closely so that a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom Electrons are paired in the overlapping orbitals and are attracted to nuclei of both atoms H–H bond results from the overlap of two singly occupied hydrogen 1s orbitals H-H bond is cylindrically symmetrical, sigma (  ) bond

22. 22 Bond Energy Reaction 2 H·  H 2 releases 436 kJ/mol Product has 436 kJ/mol less energy than two atoms: H–H has bond strength of 436 kJ/mol. (1 kJ = 0.2390 kcal; 1 kcal = 4.184 kJ)

23. 23 Bond Length Distance between nuclei that leads to maximum stability If too close, they repel because both are positively charged If too far apart, bonding is weak

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25. 25 1.7 Hybridization: sp 3 Orbitals and the Structure of Methane Carbon has 4 valence electrons (2s 2 2p 2 ) In CH 4, all C–H bonds are identical (tetrahedral) Why??

26. 26 1.7 Hybridization: sp 3 Orbitals and the Structure of Methane sp 3 hybrid orbitals: s orbital and three p orbitals combine to form four equivalent, unsymmetrical, tetrahedral orbitals (sppp = sp 3 ), Pauling (1931)

27. 27 Tetrahedral Structure of Methane sp3 orbitals on C overlap with 1s orbitals on 4 H atom to form four identical C-H bonds Each C–H bond has a strength of 438 kJ/mol and length of 110 pm Bond angle: each H–C–H is 109.5°, the tetrahedral angle.

28. 28 1.8 Hybridization: sp 3 Orbitals and the Structure of Ethane Two C’s bond to each other by  overlap of an sp 3 orbital from each Three sp 3 orbitals on each C overlap with H 1s orbitals to form six C–H bonds C–H bond strength in ethane 420 kJ/mol C–C bond is 154 pm long and strength is 376 kJ/mol All bond angles of ethane are tetrahedral

29. 29 1.9 Hybridization: sp 2 Orbitals and the Structure of Ethylene sp 2 hybrid orbitals: 2s orbital combines with two 2p orbitals, giving 3 orbitals (spp = sp 2 ) sp 2 orbitals are in a plane with120° angles Remaining p orbital is perpendicular to the plane 90  120 

30. 30 Bonds From sp 2 Hybrid Orbitals Two sp 2 -hybridized orbitals overlap to form a  bond p orbitals overlap side-to-side to formation a pi (  ) bond sp 2 –sp 2  bond and 2p–2p  bond result in sharing four electrons and formation of C-C double bond Electrons in the  bond are centered between nuclei Electrons in the  bond occupy regions are on either side of a line between nuclei

31. 31 Structure of Ethylene H atoms form  bonds with four sp 2 orbitals H–C–H and H–C–C bond angles of about 120° C–C double bond in ethylene shorter and stronger than single bond in ethane Ethylene C=C bond length 133 pm (C–C 154 pm)

32. 32 1.10 Hybridization: sp Orbitals and the Structure of Acetylene C-C a triple bond sharing six electrons Carbon 2s orbital hybridizes with a single p orbital giving two sp hybrids two p orbitals remain unchanged sp orbitals are linear, 180° apart on x-axis Two p orbitals are perpendicular on the y-axis and the z-axis

33. 33 Orbitals of Acetylene Two sp hybrid orbitals from each C form sp–sp  bond pz orbitals from each C form a p z –p z  bond by sideways overlap and p y orbitals overlap similarly

34. 34 Bonding in Acetylene Sharing of six electrons forms C  C Two sp orbitals form  bonds with hydrogens

35. 35 1.11 Hybridization of Nitrogen and Oxygen Elements other than C can have hybridized orbitals H–N–H bond angle in ammonia (NH 3 ) 107.3° N’s orbitals (sppp) hybridize to form four sp 3 orbitals One sp 3 orbital is occupied by two nonbonding electrons, and three sp 3 orbitals have one electron each, forming bonds to H

36. 36 Hybridization of Oxygen in Water The oxygen atom is sp 3 -hybridized Oxygen has six valence-shell electrons but forms only two covalent bonds, leaving two lone pairs The H–O–H bond angle is 104.5°

37. 37 1.12 Molecular Orbital Theory A molecular orbital (MO): where electrons are most likely to be found (specific energy and general shape) in a molecule The two atomic orbitals make two molecular orbitals. Additive combination (bonding) MO is lower in energy Subtractive combination (antibonding) forms MO is higher

38. 38 Molecular Orbitals in Ethylene The  bonding MO is from combining p orbital lobes with the same algebraic sign The  antibonding MO is from combining lobes with opposite signs Only bonding MO is occupied

39. 39 Summary Organic chemistry – chemistry of carbon compounds Atom: positively charged nucleus surrounded by negatively charged electrons Electrons occupy orbitals around the nucleus. Different orbitals have different energy levels and different shapes s orbitals are spherical, p orbitals are dumbbell- shaped Covalent bonds - electron pair is shared between atoms Valence bond theory - electron sharing occurs by overlap of two atomic orbitals

40. 40 Summary Molecular orbital (MO) theory, - bonds result from combination of atomic orbitals to give molecular orbitals, which belong to the entire molecule Sigma (  ) bonds - Circular cross-section and are formed by head-on interaction Pi (  ) bonds – “dumbbell” shape from sideways interaction of p orbitals

41. 41 Summary Carbon uses hybrid orbitals to form bonds in organic molecules. In single bonds with tetrahedral geometry, carbon has four sp 3 hybrid orbitals In double bonds with planar geometry, carbon uses three equivalent sp 2 hybrid orbitals and one unhybridized p orbital Carbon uses two equivalent sp hybrid orbitals to form a triple bond with linear geometry, with two unhybridized p orbitals Atoms such as nitrogen and oxygen hybridize to form strong, oriented bonds The nitrogen atom in ammonia and the oxygen atom in water are sp 3 -hybridized

42. 42 Quick Review Carbon s and three p orbitals hybridize to form four sp 3 orbitals Methane, ethane – C is tetrahedral, 109.5° Ethylene- s and two p orbitals hybridize to form three sp 2 orbitals. Remaining p orbitals overlap to form a  bond. The bonds between the nuclei are the three  bonds from the overlapped sp 2 orbitals.