Review of Organic I Chem 3331 1 CHEMISTRY 3332 ORGANIC II Review Wade Chapters 1 13 See www chem uh edu courses thummel Chem3331 Wade REVIEW Chapter 1 Intro Review Consider a simple picture of an ATOM N N N Nucleus consists of Mass Protons 1 1 N Neutrons 1 0 Charge Electron ca 0 1 12C has 6 protons 6 neutrons 13C has 6 protons 7 neutrons 14C has 6 protons 8 neutrons t1 2 5730 years tell ages up to 50K yrs Isotopes of an element have different numbers of neutrons in their nucleus For example This picture for the nucleus is OK but not so good for the electrons They cannot be represented as point charges This problem was solved by the birth of quantum mechanics Shapes of Atomic Orbitals 3 types s p d 1s orbital is lowest in energy and closest to the nucleus Y Z X S orbital spherically symmetrical p orbitals have a node at the nucleus and two lobes which extend on either side Review of Organic I Chem 3331 2 P orbital has a node at the nucleus Y Y Z X 2px Z X Y Z X 2pz 2py Rules for Filling Atomic Orbitals 1 Aufbau Principle orbitals of the lowest energy are filled first 2 Pauli Exclusion Principle only two electrons can occupy the same orbital and they must have the opposite spins 3 Hund s Rule if two or more empty orbitals of equal energy are available one electron is placed in each until all are half full Energy Levels of Atomic Orbitals Energy Levels of Atomic Orbitals High Energy Low 4d 5s 4p 3d 4s 3p 3s 2p 2s 1s These 24 orbitals can account for the first 48 elements Note energy is always released when a bond is formed i e a more stable lower energy situation results Chemical Bonds are the forces that hold atoms together in a molecule 1915 G N Lewis proposed the octet rule Atoms will tend to transfer or share electrons to acquire a stable complete outer shell configuration similar to He Ne and Ar This leads us to the two types of chemical bonds Review of Organic I Chem 3331 3 1 Ionic Bonds result from the transfer of electrons creating oppositely charged species which then attract one another Ionization Energy tendency of an atom to loose an electron Electropositive elements i e Na sodium have low I E and hence can easily loose an electron to become positively charged Electron Affinity tendency of atom to gain an electron Electronegative elements i e F fluorine have low large negative E A and hence can easily add an electron to become negatively charged Sodium Fluoride NaF How about carbon 2 Covalent Bonds result from the sharing of electrons to give a complete outer shell configuration Designate this process using Lewis Dot Structures methanol H O H H C H These are called Kekul structures when an atom looses an electron in forming a covalent bond it acquires a when an atom gains an electron it acquires a Formal charges a consider the protonation of water H O H O has 6 electrons H O now has only 5 electrons Has lost 1 e H O H Notice how we use curved arrows to designate the movement of 2 electrons These arrows do not show the movement of atoms VERY IMPORTANT Polarization H Review of Organic I Chem 3331 4 i e the electron distribution in the bond is unsymmetrical Bonds between non identical atoms are very often polar This polarity is due to the intrinsic electronegativity of atoms on the right side of the periodic table electrons are unequally shared by both nuclei X X symmetric covalent bond d X Y X Y polar covalent bond ionic bond Resonance Structures Some cmpds are not adequately represented by a single Lewis structure Two or more valence bond structures which differ only in the arrangement of electrons are called resonance forms structures H H C N H H C N H H Resonance structures d C H H d N H H Resonance Hybrid O O H3C C O H H3C C O H H H H3C C H3C C H3C H3C C H3C C H3C O O O O O C O O C O Hybrid Hybrid General Rules for Resonance 1 All resonance structures must be valid Lewis structures 2 Structures differ only in arrangement of electrons Atoms and bond angles cannot change 3 Number of unpaired odd electrons must remain the same 4 Major resonance contributor is one with lowest energy Neg charge more stable on electroneg atom H H O O O O d Review of Organic I Chem 3331 5 5 The more resonance forms the better Acid Base Theory 1 Bronsted Lowry Definition Acid proton donor Base proton acceptor K HA H2O Acid H3O A Conjugate base of HA Base K H3O A HA H2O Ka H3O A HA Large constant Acidity Constant pKa log Ka The stronger the acid the lower the value of pKa Explain Lewis Definition Acid Electron pair acceptor Base Electron pair donor This is more general and covers the Bronsted def but also includes others H Cl H O H Cl Acid Base H H Empty orbital F F B F Acid CH3 O CH3 Base F B F CH3 O CH3 H O F Compounds with N O are good Lewis bases because they have lone pair In general electrons which they can donate Note Another name for Lewis acid is electrophile electron loving Review of Organic I Chem 3331 6 Another name for Lewis base is nucleophile nucleus loving Chapter 2 Structure and Properties of Organic Molecules We have used atomic orbital A O theory to explain how electrons are to be arranged in atoms When 2 AO s combine to form a covalent bond we have a new molecular orbital MO H 1s H 1s H H 104 kcal mole H2 molecular orbital M O cylindrically symmetrical d bond Note energy is released a lot This energy is the bond strength of the H H bond This much energy is required to split H2 2 H Node Sideways Overlap antibonding MO bonding MO Let s consider the structure of methane CH4 Shape of new sp3 hybrid Review of Organic I Chem 3331 7 6C 2p 2s 1s How can carbon form 4 bonds if it has only 2 unpaired electrons to share promote 2s 2p 2p 2s 1s mix together 3 2p and 1 2s orbital to get 4 new orbitals we call sp3 hybridize mix sp3 1s Shape of new sp3 hybrid Arrange four of these so that they all originate from a common point and are oriented as far apart as possible They point to the corners of a regular tetrahedron C Ethylene sp2 Hybridization The carbons in ethylene C2H4 are each connected to 3 other atoms The fourth valence of the ethylene involves a C C double bond H H C C H H How do we explain this structure in terms of orbitals Review of Organic I Chem 3331 8 6C 2p 2s 1s Promote …