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Hybridization The Ins and Outs

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Are you good to go? Can you answer the following questions? If you can, then you ARE good to go… Can you explain why atoms hybridize to form sp 3, sp 2 or sp orbitals instead of just using s and p orbitals? Can you identify the hybridization of an atom in a molecule? Do you recall the geometries and bond angles for the hybrid orbitals? Can you compare bonds lengths and strengths?

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So - ARE you good to go? If you think you are good to go, skip to the end and see if you can answer the questions. If you think its been too long, just start clicking through… Hopefully this will get you up to speed…

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Why bother with hybridization? There are several reasons why hybridization makes sense… First, atoms in their elemental state aren’t prepared to make the necessary bonds. In order to make covalent bonds (the sharing of a pair of electrons), the atoms must have unpaired electrons to share. Consider carbon – its electronic configuration is 1s 2 2s 2 2p 2. How many unpaired electrons does it have? Only 2…

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Why bother with hybridization? Looking at the valence electrons in the second shell, it has four total electrons but only two unpaired electrons: If you need to make four covalent bonds, you don’t have enough unpaired electrons!

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Why bother with hybridization? Clearly having enough unpaired electrons to make bonds with is important… Now for the second reason for hybridization. The orbitals that contain those electrons in the atoms’ elemental states are not in what we would call the “best spatial orientation”. What does that mean? It means that the orbitals with the electrons are putting those negatively charged electrons too close to each other. And we know that like charges repel like charges. The farther away, the better!

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Why bother with hybridization? See if you can picture what the elemental Carbon atom would look like if we used an s and three p orbitals:

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Why bother with hybridization? When we un-pair the 2s electrons, and promote one into the higher energy 2p z orbital, the spatial orientation is not any better:

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Why bother with hybridization? Notice how you now have THREE electrons that are 90º apart from each other. And one electron in an s orbital desperately trying to get as far away as possible. That electron will be on the far side of the atom, trying to avoid the other electrons.

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Why bother with hybridization? Spatial orientation is a major driving factor for hybridization to occur. Those electrons want to get as far away from each other as possible. The third and final reason to hybridize – you will make stronger bonds using hybrid orbitals. How you ask? (Okay – so maybe you didn’t ask, but let me tell you…) Hybrid orbitals overlap much better head-to-head to form sigma bonds.

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Why bother with hybridization? Consider a normal p orbital simply overlapping with an s orbital: There’s a small bit of overlap to make a bond and so a sigma bond can certainly form.

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Why bother with hybridization? Now consider a hybrid orbital overlapping with an s orbital and remember that hybrid orbitals are asymmetrical and have one larger lobe for overlapping: There’s a a lot better overlap to make a bond with the larger lobe and a stronger sigma bond will form. Stronger bonds means more stable and lower energy system!

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Why bother with hybridization? So, now you know. You get the correct number of unpaired electrons for the making of bonds and they will be better oriented in space relative to each other. With the asymmetric hybrid orbitals (that one big lobe for bonding), you will form stronger bonds and more stable compounds. All excellent reasons for atoms to hybridize away from their elemental state. Now… Let’s consider each possible type of hybridization… When (and how) do they occur…?

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Sp3 Hybrid Orbitals First things first…. In any hybridization process, the elemental carbon atom much first unpair the pair of electrons in the s orbital so you have four unpaired electrons to make four covalent bonds. This does take some energy…

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Sp 3 Hybrid Orbitals Now we can combine the four orbitals to make four new hybrid orbitals. Orbitals are only math equations. Add them together and divide by four:

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Sp 3 Hybrid Orbitals These orbitals are all equal and the same. Geometrically, the large end of each hybrid orbital points to a corner of a tetrahedron.

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Sp 3 Hybrid Orbitals The bond angle for an sp 3 hybridized atom is 109.5º. The orbitals are 75% p and 25% s. The larger bonding orbital is wider than a p orbital and a bit shorter. Better to bond with!

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Sp 3 Hybrid Orbitals The orbitals are 75% p and 25% s. The larger bonding orbital is wider than a p orbital and a bit shorter. Better to bond with! Note that the s orbital is the lowest in energy (electrons are closest to the positively charged nucleus) and p is the highest in energy (electrons are furthest from the nucleus). The sp 3 hybrid orbital is about 25% more stable (lower energy!) than a p orbital.

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Sp 3 Hybrid Orbitals Question: When is sp 3 hybridization going to occur? Answer: Any time an atom has four groups of electrons around it. What are “groups”? Groups are pairs of electrons found in a sigma bond or a lone pair of electrons. Where do you find sigma bonds? Every single, double or triple bond has one sigma bond.

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Sp 3 Hybrid Orbitals The following compounds all contain atoms that are sp 3 hybridized: Make sure you can clearly see the FOUR GROUPS - the sigma bonds (in single bonds for these) and lone pairs of electrons.

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Sp2 Hybrid Orbitals How do sp 2 hybrid orbitals form? Sp 2 hybrid orbitals form from the combination of one s orbital and two p orbitals resulting in three sp 2 orbitals. You will find sp 2 orbitals with atoms that are part of a double bond. A double bond is comprised of one sigma bond and one pi bond. When we combine orbitals, we need to set aside one p orbital to form a pi bond with and combine the remaining s and two p orbitals to form the new hybrid.

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Sp 2 Hybrid Orbitals As before, in any hybridization process, the elemental carbon atom much first un-pair the pair of electrons in the s orbital so you have four unpaired electrons to make four covalent bonds. This does take some energy…

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Sp 2 Hybrid Orbitals Now we can combine the s orbital and two of the p orbitals orbitals to make three new hybrid orbitals, leaving a p orbital available on the atom to form a pi bond.

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Sp 2 Hybrid Orbitals The sp 2 orbitals are all equal and the same. Geometrically, they are all contained on the same plane (PLANAR or trigonal planar) The remaining p orbital sits perpendicular to the plane of the sp 2 hybrid orbitals.

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Sp 2 Hybrid Orbitals The bond angle for an sp 2 hybridized atom is 120º. The orbitals are 67% p and 33% s. The larger bonding orbital is wider than a p or sp 3 orbital and even shorter due to the increased s character of the orbital. Even better to bond with!

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Sp 2 Hybrid Orbitals Keeping everything relative, remember again that the s orbital is the lowest in energy (electrons are closest to the positively charged nucleus) and p is the highest in energy (electrons are furthest from the nucleus). The sp 3 hybrid orbital is about 25% more stable than a p orbital. The sp 2 hybrid orbital is about 33% more stable than a p orbital. More stable, lower in energy. Rounder orbital forming stronger bonds…

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Sp 2 Hybrid Orbitals Question: When is sp 2 hybridization going to occur? Answer: Any time an atom has three groups of electrons around it. Remember you are only counting the sigma bonds and lone pairs of electrons. The molecule shown below has two atoms that are sp 2 hybridized:

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Sp Hybrid Orbitals How do sp hybrid orbitals form? Sp hybrid orbitals form from the combination of one s orbital and one p orbital resulting in two sp orbitals. You will typically find sp orbitals with atoms that are part of a triple bond. A triple bond is comprised of one sigma bond and two pi bonds. When we combine orbitals, we need to set aside two p orbitals to form two pi bonds with and combine the remaining two s and p orbitals to form the new hybrid sp orbitals.

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Sp Hybrid Orbitals As before, in any hybridization process, the elemental carbon atom much first un-pair the pair of electrons in the s orbital so you have four unpaired electrons to make four covalent bonds. This does take some energy…

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Sp Hybrid Orbitals Now we can combine the s orbital and one of the p orbitals orbitals to make two new hybrid orbitals, leaving two p orbitals available on the atom to form two pi bonds.

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Sp Hybrid Orbitals The two sp orbitals are equal and the same. Geometrically, they face away from each other in opposite directions (LINEAR). Only the large lobes of the hybrids are shown here (in blue). The remaining p orbitals sit perpendicular to each other ready to form two pi bonds.

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Sp Hybrid Orbitals The bond angle for an sp hybridized atom is 180º. The orbitals are 50% p and 50% s. The larger bonding orbital is wider than a p or sp 3 or sp 2 orbital and even shorter due to the increased s character of the orbital. Even better to bond with!

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Sp Hybrid Orbitals Keeping everything relative, remember again that the s orbital is the lowest in energy (electrons are closest to the positively charged nucleus) and p is the highest in energy (electrons are furthest from the nucleus). The sp 3 hybrid orbital is about 25% more stable than a p orbital. The sp 2 hybrid orbital is about 33% more stable than a p orbital. The sp hybrid orbital is 50% more stable than a p orbital. More stable, lower in energy. Rounder orbital forming an even stronger bond.

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Sp Hybrid Orbitals Question: When is sp hybridization going to occur? Answer: Any time an atom has two groups of electrons around it. Remember you are only counting the sigma bonds and lone pairs of electrons. The molecule shown below has two atoms that are sp hybridized:

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So… Now What? We’ve reviewed all the possible hybridizations that we commonly see for the atoms found in an organic molecule. Hopefully you have a sense for WHY they are utilized and where they come from (yes, I know – that whole orbital recombination thing is a higher mathematical nightmare – just remember that orbitals are math equations of where you’ll most likely find an electron around an atom and math equations can be added, subtracted, multiplied and divided.) You need to be able to recognize hybridizations of atoms and answer some basic questions.

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So… Now What? “What kinds of questions will be on the test?”, you ask. Let me give you some examples: The very basic “What is that atom’s hybridization?” is a good one to start with. You may also be asked to determine the geometry of an atom, what orbitals make up a bond, what is the bond angle for a given set of atoms or what bond is stronger/weaker/longer/shorter. All of these questions pertain to the hybridization of the atoms involved.

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Now… Try a Few… Determine the hybridization of the highlighted atoms shown below:

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Now… Try a Few… How did you do? C 1 is sp 3, N is sp 3, O is sp 3 and C 5 is sp hybridized.

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Now… Try a Few… Can you recognize your orbitals? Determine what orbitals comprise the following sigma bonds: C 1 -C 2, C 2 -N, C 3 –C 4 and C 5 -H

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Now… Try a Few… How did you do? C 1 -C 2 is comprised of sp 3 and sp 2 orbitals, C 2 -N is comprised of sp 2 and sp 3 orbitals, C 3 –C 4 is comprised of sp 3 and sp orbitals and C 5 -H is formed from sp and s orbitals. Hydrogen is the only atom that will not hybridize. Remember that pi bonds are always made of p orbitals!

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Now… Try a Few… This time, try some bond angles… Determine the bond angles for C 1 -C 2 -N, C-O-C 3 and C 3 -C 4 - C 5.

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Now… Try a Few… Remember that when determining bond angles, its the central atom in the sequence that decides the bond angle. The bond angle for C 1 -C 2 -N is 120º, C-O-C 3 is 109.5ºand C 3 - C 4 -C 5 is 180º.

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Now… Try a Few… Compare bond lengths and strengths. Determine which bond is longer: C 1 -C 2 or C 3 -C 4 ? Which bond is stronger? O 1 -C 2 or O-C 3 ?

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Now… Try a Few… Remember that longer orbitals form longer bonds but shorter rounder orbitals make stronger bonds because of better overlap. Determine which bond is longer: C 1 -C 2 or C 3 -C 4 ? C 1 -C 2 is formed from sp 3 and sp 2 orbitals and C 3 -C 4 is formed from sp 3 and sp orbitals. Since the sp 2 orbital is longer than the sp orbital, C 1 -C 2 is the longer bond. Which bond is stronger? O 1 -C 2 or O-C 3 ?

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Now… Try a Few… Remember that longer orbitals form longer bonds but shorter rounder orbitals make stronger bonds because of better overlap. O 1 -C 2 is formed from sp 2- sp 2 overlap while O-C 3 is formed from sp 3 -sp 3 overlap. The sp 2 orbitals are shorter and rounder and will overlap better to form stronger bonds.

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And that’s about it… That’s all you need to know… if you can answer those basic questions, then you are good to go for what you need to be able to do in Orgo I. The shape and structure of a molecule often determines its reactivity so you’ll be identifying hybrid orbitals throughout the course.

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