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Organic Chemistry Second Edition

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  1. Organic Chemistry Second Edition David Klein Chapter 1 A Review of General Chemistry: Electrons, Bonds, and Molecular Properties Klein, Organic Chemistry 2e

  2. Organic Chemistry • The study of carbon-containing molecules and their reactions • What happens to a molecule during a reaction? • A collision • Bonds break/form • The BIG question: WHY do reactions occur? • We will need at least 2 semesters of your time to answer this question • FOCUS ON THE ELECTRONS Klein, Organic Chemistry 2e

  3. Atomic Structure • General Chemistry Review • Nucleus • Protons (+1) and neutrons (neutral) • Outside the nucleus. • Electrons (-1) • Some electrons are close to the nucleus and others are far away, WHY? • Why are valence electrons important? Klein, Organic Chemistry 2e

  4. Counting Valence Electrons • Look at the Group Anumber (Roman Numeral) on the periodic table. Number of electrons of neutral atom. Element Group # Atomic # ??? Klein, Organic Chemistry 2e

  5. Simple Lewis Structures • For simple Lewis Structures… • Draw the individual atoms using dots to represent the valence electrons. • Put the atoms together so they share PAIRS of electrons to make complete octets. WHAT is an octet? • Take NH3, for example… Klein, Organic Chemistry 2e

  6. Simple Lewis Structures • For simple Lewis Structures… • Draw the individual atoms using dots to represent the valence electrons. • Put the atoms together so they share PAIRS of electrons to make complete octets. WHAT is an octet? • Let’s draw the structure for C2H2 Klein, Organic Chemistry 2e

  7. To Chalkboard(more examples)

  8. Formal Charge • Atoms in molecules (sharing electrons) can also have unbalanced charge, which must be analyzed, because it affects stability • To calculate FORMAL charge for an atom, compare the number of valence electrons that the atom should“own” to the number of valence electrons it actually“owns” Klein, Organic Chemistry 2e

  9. Formal Charge • Analyze the formal charge of the oxygen atom. or • Oxygen shouldhave 6 valence electrons, because it is in group VIA on the periodic table. • It actually has 8 valence electrons. It needs 8 for its octet, but only 7 count towards its charge. WHY? • If it actually has 7, but it should only have 6, what is its formal charge? Klein, Organic Chemistry 2e

  10. Atomic Orbitals General Chemistry review • The type or orbital be identified by its shape • An orbital is a region where there is a calculated 90% probability of finding an electron. The remaining 10% probability tapers off as you move away from the nucleus Klein, Organic Chemistry 2e

  11. Atomic Orbitals • Electrons behave as both particles and waves. • Like a wave on a lake, an electron’s wavefunction can be (+), (–), or ZERO. Klein, Organic Chemistry 2e

  12. Atomic Orbitals • Atomic orbital are wavefunctions; canalso be (–), (+), or ZERO • The sign of the wave function has nothing to do with electrical charge. • p-orbital, there is a nodal plane. Klein, Organic Chemistry 2e

  13. Atomic Orbitals • Electrons are most stable (lowest in energy) if they are in the 1sorbital • The 1s orbital is full once there are two electrons in it. Pauli Exclusion Principle • The 2s orbital is filled next. The 2s orbital has a node. Klein, Organic Chemistry 2e

  14. Atomic Orbitals • Once the 2s is full, electrons fill into the three degenerate2porbitals • Where are the nodes in each of the 2porbitals? Klein, Organic Chemistry 2e

  15. Atomic Orbitals • Common elements and their electron configurations • Practice with SkillBuilder 1.6 1s2 1s22s1 1s1 1s22s2 1s22s22p1 1s22s22p2 1s22s22p3 1s22s22p4 Klein, Organic Chemistry 2e

  16. Hybridized Atomic Orbitals • Given the electron configuration for C and H, imagine how their atomic orbitals might overlap Would such orbital overlap yield methane of the structure shown? Klein, Organic Chemistry 2e

  17. Recall Geometry of p-Orbitals Klein, Organic Chemistry 2e

  18. 1.9 Hybridized Atomic Orbitals • To make methane, the C atom must have 4 equal atomic orbitals available for overlapping • If an electron is excited from the 2s to the 2p, will that make it suitable for making methane? Klein, Organic Chemistry 2e

  19. Hybridized Atomic Orbitals • The carbon must undergo hybridization to form 4 equal atomic orbitals • The atomic orbitals must be equal in energy to form four equal-energy symmetrical C-H bonds Klein, Organic Chemistry 2e

  20. Hybridized Atomic Orbitals • The shape of an sp3 orbital looks has 1-part scharacter and 3-parts p-character? Klein, Organic Chemistry 2e

  21. Hybridized Atomic Orbitals • To make CH4, the 1s atomic orbitalsof four H atoms will overlap with the four sp3 hybrid atomic orbitals of C Klein, Organic Chemistry 2e

  22. Hybridized Atomic Orbitals • Consider ethene (ethylene). • Each carbon in ethene must bond to three other atoms, so only three hybridized atomic orbitals are needed Klein, Organic Chemistry 2e

  23. Hybridized Atomic Orbitals • An sp2 hybridized carbon will have three equal-energy sp2 orbitals and one unhybridized porbital Klein, Organic Chemistry 2e

  24. Hybridized Atomic Orbitals • The sp2 atomic orbitals overlap to form sigma (σ) bonds • Sigma bonds provide maximum HEAD-ON overlap Klein, Organic Chemistry 2e

  25. Hybridized Atomic Orbitals • The unhybridizedporbitals in ethene form pi (π) bonds, SIDE-BY-SIDE overlap Klein, Organic Chemistry 2e

  26. Hybridized Atomic Orbitals • Consider ethyne (acetylene). • Each carbon in ethyne must bond to two other atoms, so only two hybridized atomic orbitals are needed Klein, Organic Chemistry 2e

  27. Hybridized Atomic Orbitals • The sp atomic orbitals overlap HEAD-ON to form sigma (σ) bonds while the unhybridized p orbitals overlap SIDE-BY-SIDE to form pi bonds • Practice with SkillBuilder 1.7 Klein, Organic Chemistry 2e

  28. Hybridized Atomic Orbitals • Which should be stronger, a pi-bond or a sigma-bond? WHY? • Which should be longer, an sp3 – sp3 sigma bond overlap or an sp – sp sigma bond overlap? Klein, Organic Chemistry 2e

  29. 1.10 Molecular Geometry • Valence shell electron pair repulsion (VSEPR theory) • Valence electrons (bonded and lone pairs) repel each other • To determine molecular geometry… • Predict the hybridization of the central atom • If the Steric number is 4, then it is sp3 • If the Steric number is 3, then it is sp2 • If the Steric number is 2, then it is sp Klein, Organic Chemistry 2e

  30. 1.10 sp3 Geometry • The molecular geometry is different from the electron group geometry. HOW? Klein, Organic Chemistry 2e

  31. 1.10 sp2 Geometry • Calculate the Steric number for BF3 • Electron pairs that are located in sp2 hybridized orbitals will form a trigonal planar electron group geometry Klein, Organic Chemistry 2e

  32. 1.10 sp2 Geometry Analyze the steric number, hybridization, electron group geometry and molecular geometry for this imine? Let’s practice BeH2 CO2 Klein, Organic Chemistry 2e

  33. 1.10 Geometry Summary • Practice with SkillBuilder 1.8 Klein, Organic Chemistry 2e

  34. Covalent Bonds • Covalent bonds are electrons pairs that exist in an orbital shared between two atoms. Just like an atomic orbital, the electrons could be anywhere within that orbital region. Klein, Organic Chemistry 2e

  35. 1.5 Polar Covalent Bonds • Covalent bonds are either • Nonpolar Covalent –bonded atoms share electrons evenly • Polar Covalent – One of the atoms attracts electrons more than the other • Electronegativity - how strongly an atom attracts shared electrons and causes covalent bond polarization Klein, Organic Chemistry 2e

  36. Polar Covalent Bonds • Electrons tend to shift away from lower electronegative atoms to higher electronegative atoms. • The greater the difference in electronegativity, the more polar the bond. Difference < 0.5 Difference 0.5 – 1.7 Difference > 1.7

  37. Molecular Polarity • For molecules with multiple polar bonds, the dipole moment is the vector sum of all of the individual bond dipoles Klein, Organic Chemistry 2e

  38. Intermolecular Forces • Many properties such as solubility, boiling point, density, state of matter, melting point, etc. are affected by the attractions BETWEEN molecules • Neutral molecules (polar and nonpolar) are attracted to one another through… • Dipole-dipole interactions • Hydrogen bonding • Dispersion forces (a.k.a. London forces or fleeting dipole-dipole forces) Klein, Organic Chemistry 2e

  39. Dipole-Dipole • Dipole-dipole forces result when polar molecules line up their opposite charges. The dipole-dipole attractions BETWEEN acetone molecules affects acetone’s boiling and melting points. HOW? Klein, Organic Chemistry 2e

  40. Dipole-Dipole • Why do isobutylene and acetone have such different MP and BPs? Klein, Organic Chemistry 2e

  41. Hydrogen Bonding (Dipole-Dipole) • Strong dipole-dipole attraction • because the partial + and – charges are relatively large • Only when a hydrogen attached to highly electronegative atom (O, N, F) is it δ+ • δ+ on the H atom attracts large δ– charges on other molecules Klein, Organic Chemistry 2e

  42. Hydrogen Bonding • Note: Even with the large partial charges, H-bonds are still ~20 times weaker than covalent bonds • Compounds with H atoms that are capable of forming H-bonds are called protic Ethanol Klein, Organic Chemistry 2e

  43. Hydrogen Bonding • Which of the following solvents are protic(capable of H-bonding), and which are not? • Acetic acid • Diethyl ether • Methylene chloride (CH2Cl2) • Dimethylsulfoxide Klein, Organic Chemistry 2e

  44. Hydrogen Bonding • Explain why the following isomers have different boiling points Klein, Organic Chemistry 2e

  45. London Dispersion Forces • Two nonpolar molecules will attract one another • Electrons are in constant random motion within their Molecular orbitals • molecule will sometimes produce uneven electron distribution that produces a temporary dipole fleeting attractions are generally weak, but several are significant Klein, Organic Chemistry 2e

  46. London Dispersion Forces • More mass generally have higher boiling points. Why? • Higher branched molecules generally have lower BPs. Why? Klein, Organic Chemistry 2e

  47. Solubility • We use the principle, like-dissolves-like • Polar compounds generally mix well with other polar compounds • If the compounds mixing are all capable of H-bonding and/or strong dipole-dipole, then there is no reason why they shouldn’t mix • Nonpolar compounds generally mix well with other nonpolar compounds • If none of the compounds are capable of forming strong attractions, then no strong attractions would have to be broken to allow them to mix Klein, Organic Chemistry 2e

  48. Solubility • We know it is difficult to get a polar compound (like water) to mix with a nonpolar compound (like oil) • We can’t use just water to wash oil off our dirty cloths • To remove nonpolar oils, grease, and dirt, we need soap Klein, Organic Chemistry 2e