Chp. 1 Bonding and Molecular Geometry (1).pdf

 Molecules Spread Out Generally speaking, molecules spread out all their atoms as far away from each other as possible. For example, in the molecule CCl4...... The furthest apart that the four chlorine atoms can get on a flat, two-dimensional surface is 90 degrees.  Molecules...

 Molecules Spread Out Generally speaking, molecules spread out all their atoms as far away from each other as possible. For example, in the molecule CCl4...... The furthest apart that the four chlorine atoms can get on a flat, two-dimensional surface is 90 degrees.  Molecules Spread Out In real life, however, molecules do not exist in a two-dimensional world. They exist in a three-dimensional world, where each of these chlorine atoms can actually spread out further than 90°, like this: You can see that the chlorines (green balls) can spread out from their central carbon (the gray ball) to have a 109.5° angle between each chlorine atom. This shape is called a tetrahedron.  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Number of “things” Geometry Bond angle around central atom 2 Linear 180⁰ 3 Trigonal planar 4 Tetrahedral  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Number of “things” Geometry Bond angle around central atom 2 Linear 180⁰ 3 Trigonal planar 4 Tetrahedral  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Number of “things” Geometry Bond angle around central atom 2 Linear 180⁰ 3 Trigonal planar 120⁰ 4 Tetrahedral  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Number of “things” Geometry Bond angle around central atom 2 Linear 180⁰ 3 Trigonal planar 120⁰ 4 Tetrahedral  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Number of “things” Geometry Bond angle around central atom 2 Linear 180⁰ 3 Trigonal planar 120⁰ 4 Tetrahedral 109.5⁰  Three Simple Geometries At the simplest level of chemistry, there are three different shapes –or geometries– that you should know: Electron Geometry Bond angle Domains 2 Linear 180⁰ 3 Trigonal planar 120⁰ 4 Tetrahedral 109.5⁰  Hybridization Hybridization is a change atoms undergo to form bonds. Electron Geometry Bond angle Hybridization Domains 2 Linear 180⁰ sp 3 Trigonal planar 120⁰ sp2 4 Tetrahedral 109.5⁰ sp3  Hybridization What is the hybridization and bond angle of each atom indicated? hybridization? sp2 ~120⁰ O sp 2 hybridization? (trigonal planar) sp 3 H 120⁰ hybridization? 109.5⁰ (trigonal planar) C C H H H (tetrahedral) H H sp sp hybridization? sp 180⁰ 180⁰ 180⁰ (linear) (linear) (linear) H C C H  Condensed Formulas Remember, a condensed formula is one that can written on one line of type. For example, the Lewis structure of ammonia looks like this: But I obviously cannot write it down this in a single line of print. To do that, I have to write its condensed formula, which is NH3.  Condensed Formulas Line-Bond (Line- Condensed Formula Structural Formula Angle) Formula CH3CH2CH2CH2CH2CH3 CH3(CH2)4CH3 C6H6 CH3COCH3  Condensed Formulas Write the condensed formula for each of the following structures. (CH3)3CO CH3CO2CH2C H H3 CH3CO2C (CH3)2CHCH2CH2CH2 H3 CH (CH33)2CH(CH2)3CH3 How many Sigma (σ ) and Pi (π ) bonds are there? Single covalent bonds = one sigma (σ) Double bonds = one sigma (σ) + one pi (π) Triple bonds = one sigma (σ) and two pi (π) How many sigma (σ) and pi (π) bonds are there in the following molecule? 14 sigma (σ) How many Sigma (σ ) and Pi (π ) bonds are there? Single covalent bonds = one sigma (σ) Double bonds = one sigma (σ) + one pi (π) Triple bonds = one sigma (σ) and two pi (π) How many sigma (σ) and pi (π) bonds are there in the following molecule? one sigma (σ) + one pi (π) 10 sigma (σ) 1 pi (π) How many Sigma (σ ) and Pi (π ) bonds are there? Single covalent bonds = one sigma (σ) Double bonds = one sigma (σ) + one pi (π) Triple bonds = one sigma (σ) and two pi (π) How many sigma (σ) and pi (π) bonds are there in the following molecule? one sigma (σ) + two pi (π) 9 sigma (σ) 2 pi (π)  Bond Lengths & Strengths 1σ 1σ 2π 1π 1σ Shorter and stronger This is because a triple bond has 1 sigma (σ) and 2 pi (π), while a double bond has 1 sigma (σ) and 1 pi (π), and a single bond has only 1 sigma (σ). Even though any individual sigma is shorter and stronger than any individual pi between the same two atoms, the addition or combination of pi bonds on top of sigma bonds adds together to make the final, overall bond (triple vs. double vs. single) shorter and stronger.  Bond Lengths & Strengths Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds: are shorter and which are shorter and stronger than... stronger than... What orbitals make up σ and π bonds? e- e- e-. 2e- 109.5° e- e- 90° e- 90° Carbon’s electron configuration: 1s22s22p2  109.5° + 4 = One 2s orbital Four sp3 orbitals Three 2p orbitals Carbon’s electron configuration: 1s22s22p2  e- e- 109.5° 109.5° = e- e- e- e- 109.5° e- e- Four sp3 orbitals What orbitals make up σ and π bonds? 120° sp2 120° 3 + = Two 2p orbitals One 2s orbital Three sp2 orbitals What orbitals make up σ and π bonds? e- e- 2e- e- e- e- 2e- e- carbon oxygen What orbitals make up σ and π bonds? π σ e- e- e- 2e- e- e- 2e- e- e- carbon oxygen What orbitals make up σ and π bonds? sp 180° 2 + = One 2p orbital One 2s orbital Two sp orbitals What orbitals make up σ and π bonds? sp 180° e- e- e- e- e- e- e- e- carbon carbon What orbitals make up σ and π bonds? 180° π e- σ e- e- e- e- e- e- e- π e- carbon carbon  Resonance Structures There are some molecules that have pi electrons that can move around from one atom to another. For example, the following molecules (A and B) are both different forms of acetate: Structures A and B are called resonance structures (or resonance contributors). In reality, acetate actually exists somewhere in-between A and B, with the – charge being shared equally by the two oxygens. We sometimes draw acetate, then, like this:  Resonance Structures Drawing different resonance structures is kind of like moving doors on a hinge. To show you this, I’d like to draw resonance structures for a few different molecules on some upcoming slides.  Resonance Structure Rules When drawing different resonance structures, remember: 1. Only electrons move. Specifically, only pi electrons, lone-pair electrons, or negative charges can move. In other words, do NOT atoms. 2. You CAN move electrons toward or into an atom that does NOT have a full octet, such as carbocations. 3. If an atom already HAS a full octet, then you can move electrons into it ONLY IF you push electrons out the opposite side (electrons in, electrons out). 4. Do not move or break sigma bonds, only pi bonds.  Examples Resonance structures (resonance contributors) Resonance hybrid Major/Greatest Resonance Contributor? When different resonance contributors are possible, which one will be the “major” or “greatest” contributor? The short answer is: the one that is most stable. Here’s how that breaks down, in order of priority: 1. The most stable resonance structure will have a full octet on every atom. 2. The most stable resonance structure will have the smallest possible number of charges. 3. The most stable resonance structure will have negative charges on the most electronegative atoms and positive charges on the least electronegative atoms.  Examples resonance hybrid (benzene) resonance structures (resonance contributors)  Examples Allyl carbocation Allyl carbanion  Resonance for Radicals In chemistry, radicals are molecules or atoms that have unpaired electrons, drawn as single dots. Radicals also resonance-delocalize much like positive- or negatively-charged centers. To following radical movement, you just have to remember that any bond, which we draw as a line between two atoms, really represents two shared electrons. For example: = Resonance for Radicals = Newman Projections Convert to “3D” Sawhorse Representation Newman Projections rotate * * Most stable (staggered) Least stable (eclipsed)  Cycloalkanes and Ring Strain Alkanes are hydrocarbon molecules that have all single bonds, as opposed to alkenes, which have C=C double bonds, and alkynes, which have C≡C bonds. Cycloalkanes are alkanes that are cyclic –in other words, ringed alkanes, or alkanes with rings in them. Obviously, cyclopropane is the smallest cycloalkane. Alkane carbons have four single bonds around them. As you 109.5⁰ should remember, the ideal geometry (shape) of such carbons is tetrahedral, with a bond angle of 109.5⁰.  Cycloalkanes and Ring Strain You may remember from geometry class that the sum of all interior angles in a triangle is 180⁰. The sum of all the angles in each subsequent polygon is determined by just adding 180 more degrees for each additional side: 360⁰ 540⁰ 720⁰ 180⁰  Cycloalkanes and Ring Strain You may remember from geometry class that sum of all interior angles in a triangle is 180⁰. The sum of all the angles in each subsequent polygon is determined by just adding 180 more degrees for each additional side: 60⁰ 90⁰ 120⁰ 108⁰ strained 109.5⁰  Cycloalkanes and Ring Strain You may remember from geometry class that sum of all interior angles in a triangle is 180⁰. The sum of all the angles in each subsequent polygon is determined by just adding 180 more degrees for each additional side: 60⁰ 90⁰ 120⁰ 108⁰ angle 109.5⁰ strain  Cyclohexane is the Most Stable Cyclohexane rings are very special because they’re found in many real-life organic compounds. Cyclohexanes are shaped like this (instead of being in a flat plane) because it allows the bond angles around each carbon to be almost exactly 109.5º.  Cyclohexane is the Most Stable Cyclohexane rings are very special because they’re found in many real-life organic compounds. Although cyclohexane’s atoms can move around to deviate from the shape shown here, this shape –called a chair conformation– is its most stable form. Because this allows a near-perfect 109.5⁰ bond angle, cyclohexane is the most stable cycloalkane.  Cyclohexane is the Most Stable Chair conformations are drawn on paper like this: I’ll now teach you how to draw chair conformations properly. Drawing Chair Structures “mirror image” bowtie Axial vs. Equatorial Drawing the Axials Drawing the Equatorials Axial vs. Equatorial Equatorial positions are more favorable than axial for larger groups because of 1,3-diaxial interactions. Axial vs. Equatorial Please remember: if you have a cyclohexane ring with appendages other than hydrogen attached to it, the most stable form of that ring will be the one that places the largest number of its appendages in the equatorial (not axial) positions. If it isn’t possible to place all the substituents equatorial, then placing the largest substituents in the equatorial positions will usually achieve the greatest stability.  Trans vs. Cis Cyclohexanes If you have a ring with two substituents: if the two substituents are going in the same direction –either both up or both down– then they are cis to each other. If they go in opposite directions (one up and one down), then they are trans to each other.