Hybridized Atomic Orbitals CH4 (methane): C needs 4 atomic orbitals for C-H bonds Does excitation below give geometry consistent with that observed for CH4? Expected Geometry: Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals CH4 (methane): C needs 4 atomic orbitals for C-H bonds Does excitation below give geometry consistent with that observed for CH4? Expected Geometry: Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Methane looks like this: 4 equal C-H bond lengths Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals C atomic orbitals hybridize to form 4 symmetrical equal-energy atomic orbitals Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Note: the shape of an sp3 orbital. It’s 3 parts p orbital, 1 part s orbital Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals To make CH4 the 1s atomic orbital of each H atoms will overlap with the four sp3 hybrid atomic orbitals of C (4 sigma (σ) bonds are made) σ σ σ σ Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Draw a picture that shows the necessary atomic orbitals and their overlap to form ethane (C2H6). (Don’t worry about geometry for now) Draw a picture that shows the necessary atomic orbitals and their overlap to form H2O. (Don’t worry about geometry for now) Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Consider ethene (ethylene). Each carbon in ethene must bond to 3 other atoms, so only 3 hybridized atomic orbitals are needed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals An sp2 hybridized carbon will have three equal-energy sp2 orbitals and one unhybridized p orbital Which is lower in energy, the sp2 or the p? Why? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals The sp2 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 2e

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

Hybridized Atomic Orbitals Why is sp2 hybridization not appropriate for methane (CH4)? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Consider ethyne (acetylene). Each carbon in ethyne is bonded to 2 other atoms, only 2 hybridized atomic orbitals are needed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 2e

Hybridized Atomic Orbitals Which should be stronger, a pi bond or a sigma bond? WHY? Pi bonds involve sideways overlap sigma bonds involve head on (i.e. axial overlap) Axial overlap is stronger than sideways overlap Which should be longer, an sp3 – sp3 sigma bond overlap or an sp – sp sigma bond overlap? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hybridized Atomic Orbitals Explain the different strengths and lengths below. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Molecular Geometry Valence shell electron pair repulsion (VSEPR theory) Valence electrons (bonded and lone pairs) repel each other To determine molecular geometry… Determine the Steric number Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

sp3 Geometry For any sp3 hybridized atom, the 4 valence electron pairs will form a tetrahedral electron group geometry Methane has 4 equal bonds, so the bond angles are equal How does the lone pair of ammonia affect its geometry? The bond angels in oxygen are even smaller, why? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

sp3 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

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 What will be the molecular geometry? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

sp2 Geometry How many electrons are in Boron’s unhybridized p orbital? Does this geometry follow VSEPR theory? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

sp2 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

1.10 sp Geometry Analyze the Steric number, the hybridization, the electron group geometry, and the molecular geometry for the following molecules Can you draw the Lewis structures? BeH2 CO2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

1.10 Geometry Summary Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Molecular Polarity Electronegativity Differences cause induction Induction (shifting of electrons WITHIN a molecular orbitals) creates a dipole moment. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Molecular Polarity Dipole moment = [amount of partial charge] x (distance charges are separated] Dipole moment (μ) is reported in units of Debye (D) 1 debye = 10-18 esu ∙ cm esu (electrostatic unit) 1 e- charge = 4.80 x 10-10 esu Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Molecular Polarity Consider the dipole for CH3Cl Dipole moment (μ) = charge (e) x distance (d) While we will not calculate dipole moment values, we should be able to draw a vector showing the direction of a molecular dipole Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Molecular Polarity What atom will be attacked by OH– Draw 2 curved arrows to show movement of electrons to demonstrate the “attack” Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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

Molecular Polarity 1) Determine correct molecular geometry BEFORE analyzing polarity. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Molecular Polarity Electrostatic potential maps are often used to give a visual depiction of polarity Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Intermolecular Forces Many physical properties such as: Solubility Boiling Point Density State of matter Melting Point are affected by the attractions BETWEEN molecules Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Intermolecular Forces 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

Dipole-Dipole Dipole-dipole forces result when polar molecules line up their opposite charges. Acetone (from e.g., Lowes) has permanent dipole. Recall: The dipole-dipole attraction affets b.p. and and m.p. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

Hydrogen Bonding Hydrogen bonds are strong dipole-dipole attraction Hydrogen bonds have relatively large δ+ and δ– Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hydrogen Bonding A (H)ydrogen attached to a highly electronegative atom (O, N, F) will have a large δ+ charge The H (δ+) can attract large δ– (such as lone pair e–) on other molecules Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hydrogen Bonding H-bonds are about 20 X 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

Hydrogen Bonding Which of the following solvents are protic (have an H capable of H-bonding) Acetic acid Diethyl ether Methylene chloride Dimethyl sulfoxide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

Hydrogen Bonding H-bonds help cause DNA to form a double helix Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces Nonpolar molecules (dipole = 0 D) will they attract one another HOW? Since nonpolar molecules normally have their e– spread out evenly around the nuclei (+) completely balancing the charge Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces However, electrons are in constant random motion within their Molecular Orbitals This causes a temporary electron distribution that is NOT evenly balanced with the positive charge of the nuclei This induces a temporary dipole in a neighboring molecule https://www.youtube.com/watch?v=J3ulfzEdafA Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces London Dispersion Forces are short-lived and are generally weak Enough of them can add up to a lot Recall last part of video https://www.youtube.com/watch?v=J3ulfzEdafA Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces London dispersion forces are strong enough to support the weight of the Gecko Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces Explain why molecules with more mass generally have higher boiling points Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

London Dispersion Forces Explain why more highly branched molecules generally have lower boiling points Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

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

Solubility Soap molecules organize into micelles in water, which form a nonpolar interior to carry away dirt. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e