do you remember..? - towson university · 1/30/14 1 hybridization the ins and outs do you...

1/30/14

1

Hybridization The Ins and Outs

Do you remember..?

!   Can you answer the following questions? If you can, then you are good to go…

!   Can you explain why atoms hybridize s and p orbitals to form sp3, sp2 or sp 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?

So - ARE you good to go?

!   If you think you are comfortable with the topic of hybridization, skip to the end (Slide 36) 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…

Why atoms hybridize…

!   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 to form covalent bonds.

!   Consider carbon – its electronic configuration is 1s22s22p2. With four valence electrons, how many unpaired electrons does it have?

!   Only 2…


!   Looking at the valence electrons of carbon 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!

2s

2p


!   Clearly having enough unpaired electrons to make bonds with is important…

!   Now for the second reason for hybridization. The orbitals that contain those valence electrons in the atoms’ elemental states are not in what we would call the “best spatial orientation”.

!   What does that mean? “Best spatial orientation”? It means that in the elemental state, the s and p orbitals are putting those negatively charged electrons too close to each other. And we know that like charges repel like charges… The farther away the electrons can get, the more stable the system.

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!   See if you can picture what the elemental Carbon atom would look like if we used the one s and three p orbitals. The s orbital in the center and three p orbitals, each placed 90º apart, like the axes on a graph:

!   Put an electron in each…

Px

Py

Pz

90º

90º

s


!   Ground State: Valence = 2s22p2

!   The pz orbital is empty. The pair of electrons in the s orbital must then un-pair to make four covalent bonds, placing one electron in the pz orbital .

Px

Py

Pz

90º

90º


!   With a single electron in each orbital:2s12px12py

12pz1:

!   Notice how the electrons are pretty much all clustered together 90º apart with the s electron close to the center. A lot of electron repulsions would occur in this arrangement. High energy!

Px

Py

Pz

90º

90º


!   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 stronger sigma bonds.


!   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. This picture doesn’t really do it justice: the p orbital is VERY narrow, relative to the hybrids we will see.


!   Now consider a hybrid orbital overlapping with an s orbital and remember that hybrid orbitals are asymmetrical and have one larger lobe for overlapping to form bonds:

!   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 atoms hybridize… !   So, now you know… You get: !   (1) the correct number of unpaired electrons for the

making of bonds and they will have !   (2) a better spatial orientation of electrons relative to

each other for less electron repulsions and you will form

!   (3) stronger bonds using asymmetric larger orbital lobes and thus 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…?

sp3 Hybrid Orbitals

!   First things first…. 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 – the p orbital is higher in energy than the s:

2s

2p

2s

2p

sp3 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. The four new orbitals are all the same energy.

2s

2p2sp3

sp3 Hybrid Orbitals

!   These sp3 hybrid orbitals are all equal and the same. Geometrically, the large end of each hybrid orbital points to a corner of a tetrahedron.

sp3 Hybrid Orbitals

!   The bond angle for an sp3 hybridized atom is 109.5º.

109.5º

sp3 Hybrid Orbitals !   The sp3 orbitals are 75% p and 25% s. The larger bonding lobe is

wider than a p orbital and a bit shorter. Better to bond with!

!   Note that the s orbital is the lowest energy orbital (electrons are closest to the positively charged nucleus) and p is the highest energy orbital (electrons are furthest from the nucleus).

!   The sp3 hybrid orbital is more stable (lower energy!) than a p orbital. Its shorter, holding electrons closer to the nucleus.

ps

subtractive additive

1 s + 3 p =

25% s, 75% p

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sp3 Hybrid Orbitals !   Q: When is sp3 hybridization going to occur?

!   A: Any time an atom has four groups of electrons around it.

!   Q: What are “groups”?

!   A: Groups are pairs of electrons found in a sigma bond or a lone pair of electrons.

!   Q: Where do you find sigma bonds?

!   A: There is one sigma bond inside every single, double or triple bond.

sp3 Hybrid Orbitals !   Each of the following compounds all contain atoms

(highlighted in red) that are sp3 hybridized:

!   Make sure you can clearly see the FOUR GROUPS - the sigma bonds (in single bonds for these) and lone pairs of electrons.

C

H

H

HH O HH N

H

HH

sp2 Hybrid Orbitals

!   How do sp2 hybrid orbitals form? Sp2 hybrid orbitals form from the combination of one s orbital and two p orbitals resulting in three sp2 orbitals. You will typically find sp2 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 hybrids.

sp2 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…

2s

2p

2s

2p

sp2 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.

2s

2p

2sp22p

sp2 Hybrid Orbitals

!   The sp2 orbitals are all equal and the same. Geometrically, they are all contained on the same plane (called trigonal planar or even just “planar”).

!   The remaining p orbital sits perpendicular to the plane of the sp2 hybrid orbitals. Only the larger bonding lobes of the sp2 orbitals (outlined in red) are shown above, while both lobes of the p orbital (gray and white) are represented.

side view:

sp2

sp2

sp2

p orbital

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sp2 Hybrid Orbitals

!   The bond angle for an sp2 hybridized atom is 120º as you can see from the top view (only larger bonding lobes shown):

!   The orbitals are 67% p and 33% s. The larger bonding orbital is wider than a p or sp3 orbital and even shorter due to the increased s character of the orbital. Even better to bond with!

120º

top view

sp2 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 sp3 hybrid orbital is about 25% more stable than a p orbital.

!   The sp2 hybrid orbital is about 33% more stable than a p orbital. More stable, lower in energy. Shorter, rounder orbital forming stronger bonds…

sp2 Hybrid Orbitals !   Q: When is sp2 hybridization going to occur?

!   A: 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 (shown in red) that are sp2 hybridized:

HC

HN

H

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 s and p orbitals to form the new hybrid sp orbitals.

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…

2s

2p

2s

2p

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.

2s

2p

2sp

2p

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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 (one in blue and one in red).

!   The remaining p orbitals sit perpendicular to each other, ready to form two pi bonds. Both lobes (gray and white) for both p orbitals are shown above.

sp hybrid orbitals(shown in blue andred)

py

pz

sp Hybrid Orbitals

!   The bond angle for an sp hybridized atom is 180º. Again, only the large bonding lobes are shown below.

!   The orbitals are 50% p and 50% s. The larger bonding orbital is wider than a p or sp3 or sp2 orbital and even shorter due to the increased s character of the orbital. Even better to bond with!

180º

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 hybrid orbital is 50% more stable than a p orbital. More stable, lower in energy. Shortest, Roundest orbital forming the strongest sigma bond.

sp Hybrid Orbitals !   Q: When is sp hybridization going to occur?

!   A: 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 (in red) that are sp hybridized:

H C N

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 defining areas in space 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.

What can you expect? !   “What kinds of questions might 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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Try a Few Questions… !   Determine the hybridization of the highlighted atoms shown

below:

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Hybridizations? !   How did you do?

!   C1 is sp3, O1 is sp2, N is sp3, O2 is sp3 and C5 is sp hybridized.

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Orbitals? !   Can you recognize the hybrid orbitals that make up specific

sigma bonds?

!   Determine what orbitals comprise the following sigma bonds: C1-C2, N-H, C3–C4 and C5-H

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Orbitals… !   How did you do?

!   C1-C2 is comprised of sp3 and sp2 orbitals, respectively

!   N-H is comprised of sp3 and s orbitals,

!   C3–C4 is comprised of sp3 and sp orbitals and

!   C5-H is formed from sp and s orbitals.

!   Remember that pi bonds are always made of p orbitals, should that question arise!

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Bond Angles… !   This time, try some bond angles…

!   Determine the bond angles for C1-C2-N, H-C-O2 and C4-C5-H.

!   In order to do so, you must identify the hybridization of the central atom

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Bond Angles… !   Remember that when determining bond angles, it is the

central atom in the sequence that decides the bond angle.

!   The bond angle for C1-C2-N is 120º because C2 is sp2.

!   The bond angle for H-C-O2 is 109.5º because C is sp3.

!   The bond angle for C4-C5-H is 180º because C5 is sp.

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

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Lone Pairs… !   The lone pairs of most atoms will be contained in the hybrid

orbitals of the atom itself.

!   What hybrid orbital contains the lone pair on O1 and O2?

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Lone Pairs… !   How did you do?

!   O1 is sp2 hybridized therefore the lone pair on O1 is in an sp2 hybrid orbital and

!   O2 is sp3 hybridized therefore the lone pair on O2 is in an sp3 hybrid orbital.

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Bond Lengths… and Strengths…

!   Compare bond lengths and strengths.

!   Determine which sigma bond is longer: C1-C2 or C3-C4?

!   Determine which sigma bond is stronger? O1-C2 or O2-C3?

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Lengths and Strengths… !   Remember that longer orbitals form longer, weaker bonds but

shorter, rounder orbitals make shorter, stronger bonds because of better overlap.

!   #1: Which bond is longer: C1-C2 or C3-C4? C1-C2 is formed from sp3 and sp2 orbitals and C3-C4 is formed from sp3 and sp orbitals. Since the difference is the comparison of sp2 for one and sp for the other, the sp2 orbital is longer than the sp orbital, C1-C2 is the longer bond.

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

Lengths and Strengths… !   And finally:

!   #2: The sigma bond of O1-C2 is formed from sp2-sp2 overlap while sigma bond of O2-C3 is formed from sp3-sp3 overlap. The sp2 orbitals are shorter and rounder and will overlap better to form stronger bonds.

H C C1 C2 N

H

H

H

H

O1

C

H

O2

H

H

C3

H

C4

H

C5 H

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.

do you remember..? - towson university · 1/30/14 1 hybridization the ins and outs do you...

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