Organic Chemistry Second Edition Chapter 1 A Review of General Chemistry: Electrons, Bonds, and Molecular Properties David Klein Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-2

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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-3

Look at the Group A number (Roman Numeral) on the periodic table. Number of electrons of neutral atom. Counting Valence Electrons Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-4 Group # Element Atomic # ???

Simple Lewis Structures For simple Lewis Structures… 1.Draw the individual atoms using dots to represent the valence electrons. 2.Put the atoms together so they share PAIRS of electrons to make complete octets. WHAT is an octet? Take NH 3, for example… Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-5

Simple Lewis Structures For simple Lewis Structures… 1.Draw the individual atoms using dots to represent the valence electrons. 2.Put the atoms together so they share PAIRS of electrons to make complete octets. WHAT is an octet? Let’s draw the structure for C 2 H 2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-6

To Chalkboard (more examples)

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” Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-8

Formal Charge Analyze the formal charge of the oxygen atom. or Oxygen should have 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-9

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-10

Atomic Orbitals Electrons behave as both particles and waves. – Like a wave on a lake, an electron’s wavefunction can be (+), (–), or ZERO. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-11

Atomic Orbitals Atomic orbital are wavefunctions; can also be (–), (+), or ZERO – The sign of the wave function has nothing to do with electrical charge. p-orbital, there is a nodal plane. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-12

Atomic Orbitals Electrons are most stable (lowest in energy) if they are in the 1s orbital 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. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-13

Once the 2s is full, electrons fill into the three degenerate 2p orbitals Where are the nodes in each of the 2p orbitals? Atomic Orbitals Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-14

Atomic Orbitals Common elements and their electron configurations Practice with SkillBuilder 1.6 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-15 1s 1 1s 2 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p 2 1s 2 2s 2 2p 3 1s 2 2s 2 2p 4

Hybridized Atomic Orbitals Given the electron configuration for C and H, imagine how their atomic orbitals might overlap Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-16 Would such orbital overlap yield methane of the structure shown?

Recall Geometry of p-Orbitals Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-17

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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-18

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-19

Hybridized Atomic Orbitals The shape of an sp 3 orbital looks has 1-part s character and 3-parts p-character? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-20

Hybridized Atomic Orbitals To make CH 4, the 1s atomic orbitals of four H atoms will overlap with the four sp 3 hybrid atomic orbitals of C Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-21

Consider ethene (ethylene). Each carbon in ethene must bond to three other atoms, so only three hybridized atomic orbitals are needed Hybridized Atomic Orbitals Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-22

Hybridized Atomic Orbitals An sp 2 hybridized carbon will have three equal-energy sp 2 orbitals and one unhybridized p orbital Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-23

Hybridized Atomic Orbitals The sp 2 atomic orbitals overlap to form sigma (σ) bonds Sigma bonds provide maximum HEAD-ON overlap Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-24

Hybridized Atomic Orbitals The unhybridized p orbitals in ethene form pi (π) bonds, SIDE-BY-SIDE overlap Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-25

Hybridized Atomic Orbitals Consider ethyne (acetylene). Each carbon in ethyne must bond to two other atoms, so only two hybridized atomic orbitals are needed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-26

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-27

Hybridized Atomic Orbitals Which should be stronger, a pi-bond or a sigma-bond? WHY? Which should be longer, an sp 3 – sp 3 sigma bond overlap or an sp – sp sigma bond overlap? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e1-28

1.10 Molecular Geometry Valence shell electron pair repulsion (VSEPR theory) – Valence electrons (bonded and lone pairs) repel each other To determine molecular geometry… 2.Predict the hybridization of the central atom If the Steric number is 4, then it is sp 3 If the Steric number is 3, then it is sp 2 If the Steric number is 2, then it is sp Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-29

1.10 sp 3 Geometry The molecular geometry is different from the electron group geometry. HOW? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-30

1.10 sp 2 Geometry Calculate the Steric number for BF 3 Electron pairs that are located in sp 2 hybridized orbitals will form a trigonal planar electron group geometry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-31

1.10 sp 2 Geometry Analyze the steric number, hybridization, electron group geometry and molecular geometry for this imine? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-32 Let’s practice BeH 2 CO 2

1.10 Geometry Summary Practice with SkillBuilder 1.8 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-33

Covalent Bonds Covalent bonds are electrons pairs that exist in an orbital shared between two atoms. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-34 Just like an atomic orbital, the electrons could be anywhere within that orbital region.

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-35

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. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Difference < 0.5Difference 0.5 – 1.7 Difference > 1.7

Molecular Polarity For molecules with multiple polar bonds, the dipole moment is the vector sum of all of the individual bond dipoles Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-37

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) Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-38

Dipole-Dipole Dipole-dipole forces result when polar molecules line up their opposite charges. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-39 The dipole-dipole attractions BETWEEN acetone molecules affects acetone’s boiling and melting points. HOW?

Dipole-Dipole Why do isobutylene and acetone have such different MP and BPs? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-40

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-41

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-42 Ethanol

Hydrogen Bonding Which of the following solvents are protic (capable of H- bonding), and which are not? Acetic acid Diethyl ether Methylene chloride (CH 2 Cl 2 ) Dimethyl sulfoxide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-43

Hydrogen Bonding Explain why the following isomers have different boiling points Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-44

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-45 fleeting attractions are generally weak, but several are significant

London Dispersion Forces More mass generally have higher boiling points. Why? Higher branched molecules generally have lower BPs. Why? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-46

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-47

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 1-48