Essential Organic Chemistry. Chapter 1

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1 Essential Organic Chemistry Paula Yurkanis Bruice Chapter 1 Electronic Structure and Covalent Bonding

2 Periodic Table of the Elements

3 1.1 The Structure of an Atom Atoms have an internal structure consisting of one or more subatomic particles: protons, neutrons, and electrons. proton positive charge mass = x kg neutron no charge mass = x kg electron negative charge mass = x kg

4 1.1 The Structure of an Atom Most of the mass of an atom is concentrated in the nucleus. The nucleus contains one or more positively charged protons, and one or more neutrons with no electrical charge.

5 1.1 The Structure of an Atom One or more negatively charged electrons are in constant motion somewhere outside the nucleus. The number of electrons is equal to the number of protons; the atom has no overall electrical charge.

6 1.1 The Structure of an Atom An atom is mostly free space because the volume of the nucleus and the electrons outside the nucleus are extremely small compared to the overall volume of the atom.

7 1.1 The Structure of an Atom Most of the mass of an atom is in its nucleus. Most of the volume of an atom is occupied by its electrons, and that is where our focus will be because it is the electrons that form chemical bonds.

8 1.1 The Structure of an Atom Atomic number equals the number of protons in its nucleus, and is also the number of electrons that surround the nucleus. Mass number the sum of its protons and neutrons. Isotopes have the same atomic number, but different mass numbers because they have different numbers of neutrons (i.e., 12 C, 13 C, and 14 C). Atomic weight (Atomic mass) of a naturally occurring element is the average mass of its atoms. Molecular weight of a compound is the sum of the atomic weights of all the atoms in the molecule.

10 1.2 How the Electrons in an Atom are Distributed The electrons in an atom can be thought of as occupying a set of shells that surround the nucleus. The first shell is the smallest and the one closest to the nucleus. Atomic orbitals The probability distribution about one atomic nucleus (each shell contains subshells known as atomic orbitals). Table 1.1 Distribution of electrons in the first three shells First shell Second shell Third shell Atomic orbitals s s, p s, p, d Number of atomic orbitals Maximum number of electrons 1 1, 3 1, 3,

11 Atomic Orbitals s orbital p orbitals p x p y p z

12 Other Important Points Pauli Exclusion Principle No two electrons can have identical quantum numbers. Each atomic orbital can have, at most, two electrons. Aufbau Principle Atomic orbitals are filled with electrons from lowest potential energy to highest. Degenerate Orbitals Orbitals that have the same energy (e.g., 2p x, 2p y, 2p z ). Hund s Rule The electronic configuration with the highest multiplicity (most unpaired spins) is more stable.

13 Atomic Number = 6 Carbon 2p 2s 1s

14 Atomic Number = 8 Oxygen 2p 2s 1s

16 Electron configuration Core electrons Electrons in inner shells (those below the outermost shell) Valence electrons Electrons in the outermost shell. Elements in the same column of the periodic table have the same number of valence electrons, thus having similar chemical properties. The chemical behavior of an element depends on its electronic configuration.

18 1.3 Ionic and Covalent Bonds Octet rule (proposed by G. N. Lewis) An atom is most stable if its outer shell is either filled or contains eight electrons and it has no electrons of higher energy.

19 Chemical Bonding Chemical Bonds The forces holding atoms together in compounds. Only valence electrons are used in bonding. Lewis Dot Representation of Atoms Dots around the chemical symbol of an atom represent the valence electrons.

20 Examples Atom Electronic Structure Electronic Configuration Lewis Dot Structure 3p 3s Boron 1s 2 2s 2 2p 1 2p 2s B 1s 3p 3s Phosphorus [Ne] 3s 2 3p 3 2p 2s P 1s

21 The Ionic Bond Transfer of Electrons from One Atom to Another Example: Sodium chloride Na Na + 1 e Cl + 1 e Cl

22 The Ionic Bond The electrostatic attraction between oppositely charged ions.

23 The Covalent Bond Some atoms do not transfer electrons from one atom to another to form ions. Instead they form a chemical bond by sharing pairs of electrons between them. A covalent bond consists of a pair of electrons shared between two atoms.

24 Hydrogen, H 2 * E * 1s 1s E

25 Fluorine, F 2 F + F F F F 2 * F 2p F 2p F 2

26 Nitrogen, N 2 N + N N N Carbon Dioxide, CO 2 O + C + O O C O

28 Electronegativity (EN) Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Electronegativity differences in covalently bonded atoms result in polar covalent bonds.

29 Polar Covalent Bonds The polarity of a bond is determined by the difference in electronegativity values. If the electronegativities are the same the bond is nonpolar and the electrons are shared equally. If the atoms have greatly differing electronegativities the bond is very polar.

30 Polar Covalent Bonds F 2 F F A nonpolar covalent bond HF H F A polar covalent bond

31 Polar Covalent Molecules A polar molecule is a molecule that is electrically asymmetrical, causing it to be oppositely charged at two points. The molecule possesses a molecular dipole. H Cl O H H O C H 3 C CH 3

32 Polar Covalent Molecules Care must be taken to distinguish between polar bonds and polar molecules!! Carbon dioxide: O C O

33 Polar Covalent Molecules

36 Electrostatic potential maps

38 1.4 How The Structure of A Compound Is Represented Nonbonding or lone-pair electrons: Electrons not used in bonding. Formal charge: The difference between the number of valence electrons an atom has when it is not bonded to any other atoms and the number of electrons it owns when it is bonded. Formal charge = number of valence electrons (number of lone-pair electrons + ½ number of bonding electrons)

40 1.4 Representation of Structure

42 Writing Lewis Dot Structures Find the total number of valence electrons supplied by all of the atoms in the structure. Write down the skeletal arrangement of the atoms and connect them with a single covalent bond (two electrons). Normal bonding: H: one covalent bond halogens: one covalent bond O: two covalent bonds N: three covalent bonds C: four covalent bonds

44 Writing Lewis Dot Structures Distribute pairs of electrons (pairs of dots) around each atom (except H) to give each atom eight electrons around it (the octet rule). If there are not enough electrons to give these atoms eight electrons, change single bonds between atoms to double or triple bonds by shifting non-bonded pairs of electrons as needed. Compute formal charges and evaluate the structure.

50 s orbital 1.5 Atomic Orbitals p orbitals

51 1.6 How Atoms form Covalent Bonds Sigma (σ) bond The covalent bond that is formed when the two orbitals overlap

52 1.6 How Atoms form Covalent Bonds

53 1.7 How Single Bonds Are Formed in Organic Compounds: Methane and Ethane

54 Methane However, mixing the carbon 2s orbital and the 3 carbon 2p orbitals gives 4 sp 3 hybrid orbitals: s 2p 2p 2p sp 3 sp 3 sp 3 sp 3 2p sp 3 2s

55 Methane

56 Ethane

58 1.8 How A Double Bond Is Formed: The Bond in Ethene Trigonal planar geometry around each carbon atom. The molecular orbitals of ethene: H H 1s orbitals of the four hydrogen atoms C 2s, 2p x, 2p y, and 2p z orbitals of the two carbon atoms C H H

59 Ethene The 2s, 2p x, and 2p y orbitals on carbon form three sp 2 hybrid orbitals on each carbon atom. The three sp 2 hybrid orbitals point toward the corners of a triangle. A trigonal planar atom has sp 2 hybridization.

60 Ethene The sp 2 hybrids and the 1s orbitals on hydrogen make up the -framework bonds that are relatively low in energy. A trigonal planar atom has sp 2 hybridization.

61 Ethene The sp 2 hybrids and the 1s orbitals on hydrogen make up the -framework bonds that are relatively low in energy. A trigonal planar atom has sp 2 hybridization.

62 1.9 How A Triple Bond Is Formed: The Bond in Ethyne

63 1.9 How A Triple Bond Is Formed: The Bond in Ethyne

64 1.9 How A Triple Bond Is Formed: The Bond in Ethyne

67 1.10 Bonding in the Methyl Cation, the Methyl Radical, and the Methyl Anion The Methyl Cation

68 The Methyl Radical

69 The Methyl Anion

70 1.11 The Bonds in Water

72 1.12 The Bonds in Ammonia and in the Ammonium Ion

74 1.13 The Bond in a Hydrogen Halide

Organic Chemistry. Review Information for Unit 1. Atomic Structure MO Theory Chemical Bonds

Organic Chemistry. Review Information for Unit 1. Atomic Structure MO Theory Chemical Bonds Organic Chemistry Review Information for Unit 1 Atomic Structure MO Theory Chemical Bonds Atomic Structure Atoms are the smallest representative particle of an element. Three subatomic particles: protons