7.1: Question 7.1.7 PASS - label hybridized atomic orbitals, identify sigma and pi bonds

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Exercise 7.1.7

For the molecule acetonitrile:

Acetonitrile structure question.png

a) How many sigma and pi bonds does it have?

b) What orbitals overlap to form the C-H sigma bonds?

c) What orbitals overlap to form the C-C sigma bonds?

d) What orbitals overlap to form the C-N sigma bonds?

e) What orbitals overlap to form the C-N pi bonds?

f) What orbital contains the lone pair electrons on nitrogen?

g) Sketch the molecule showing the hybridized atomic orbital ‘cloud’ overlap.

Answer

a) 5 sigma and 2 pi.

b) A sp³ hybrid orbital from carbon and an s orbital from hydrogen

c) A sp³ hybrid orbital from one carbon and a sp hybrid orbital from another carbon.

d) A sp hybrid orbital from carbon and a sp orbital from nitrogen.

e) A p_y and a p_z orbital from carbon and a p_y and p_z orbital from nitrogen.

f) A sp hybrid orbital.

g) 7.1.7.hybridized atomic orbial overlap sketch_AJ.png

See LibreText 7.1.7 sp hybrid orbitals and the structure of acetylene

Strategy Map

Step	Hint
1. Count the number of single bonds (sigma) in the molecule	There is a single bond between every single atom. Recall Lewis Structures (see LibreText section 4.4.2) and Valence Bond Theory (see LibreText section 5.2)
2. Count the number of double and triple bonds (pi) in the molecule.	Double/triple bonds are represented as second and third lines along a single bond.
3. Identify hybridization of center atoms (the two carbon atoms).	The hybridization of the center atoms will depend on their electron domain geometry. Recall VSEPR Theory (see LibreText section 5.5.1)
4. Identify the hybridization of the outer atoms, if any.
5. Identify which atoms are overlapping. The hybrid orbitals that overlap will depend on steps 3 and 4.
6. Sketch your diagram showing all overlaps and ensure you label everything.

Solution

a) 5 sigma (single bonds) and 2 pi (one double and one triple bond).

b) A sp³ hybrid orbital from carbon and an s orbital from hydrogen. (The hydrogen atoms are all unhybridized.)

c) A sp³ hybrid orbital from one carbon and a sp hybrid orbital from another carbon. (Both carbons are hybridized, but their electron domain geometries are not the same, meaning their hybridizations are different.)

d) A sp hybrid orbital from carbon and a sp orbital from nitrogen. (Both carbon and nitrogen have a sp hybridization.)

e) A p_y and a p_z orbital from carbon and a p_y and p_z orbital from nitrogen. (the ‘Y’ and ‘Z’ just mean they have slightly different orientations)

f) A sp hybrid orbital. (Nitrogen has a linear electron domain geometry meaning it has a sp hybridization in both directions.)

g) Hand drawn sketch:

7.1.7.hybridized atomic orbial overlap sketch_AJ.png

Guided Solution

Download Guided Solution as a pdf

Guided Solution	Hint
This is a theory type problem that tests your knowledge on atomic orbital hybridization. You must be able to correctly identify the hybridization of multiple atoms in a molecule and sketch the orbital overlaps.	See LibreText Section 7.1.7 (sp Hybrid Orbitals and the Structure of Acetylene)
For the molecule acetonitrile: a) How many sigma and pi bonds does it have? b) What orbitals overlap to form the C-H sigma bonds? c) What orbitals overlap to form the C-C sigma bonds? d) What orbitals overlap to form the C-N sigma bonds? e) What orbitals overlap to form the C-N pi bonds? f) What orbital contains the lone pair electrons on nitrogen? g) Sketch the molecule showing the hybridized atomic orbital ‘cloud’ overlap.
What are sigma and pi bonds? Sigma bonds are your single bonds. Pi bonds are your double and triple bonds.	There is at least one sigma bond between each of the connected atoms. Look at the bonds between your carbon and your nitrogen. How many lines do you see??
How do you identify atomic orbital hybridization? Identify the electron domain geometry of your central atom. (in this case, the two center atoms are carbons) Your electron domain geometry will tell you the number of bonding directions attached to your atom. Each bonding direction represents an orbital needed. Identify if your outer atoms require hybridization (if they do, they will have multiple bonding directions as well, this includes lone pairs)	For instance, a tetrahedral electron domain geometry must have four electron domains so it will have an s+p+p+p (sp³) hybridization.
How do you know which orbitals are overlapping?	Each bond that is between the atoms will be an overlap. The hybridization of the orbitals will depend on the two atoms that the bond is between.
How do you sketch the orbital overlaps? Begin by drawing your center atom with its orbitals in the shape of its electron domain geometry. (it's okay if they just look like “sausages” if you label them with their hybridization) Overlap these orbitals with orbitals representing the outer atoms. Label these as sigma bonds. The orbitals that overlap to form pi bonds are longer and skinnier than the sigma bonds. They overlap sideways and therefore connect above and below the sigma bond.	Using different colours in your drawing for orbital type can be helpful to keep track.
Complete Solution: a) There will always be a single bond between each atom in the molecule. A second or third line next to the single bond represents a double or triple bond. You can count these in your Lewis structure form to identify the sigma and pi bonds. answer 5 sigma (single bonds) and 2 pi (one double bond and one triple bond). b) This carbon atom makes bonds in four directions which gives it a sp³ hybridization. Three of these directions are hydrogen atoms. All of these hydrogen atoms are unhybridized and have s orbitals. When the orbitals from the hydrogen and carbon overlap, they form a sigma bond. answer A sp³ hybrid orbital from carbon and an s orbital from hydrogen. c) Although both atoms are hybridized, they have a different number of bonding directions therefore one is sp³ and the other is sp. answer A sp³ hybrid orbital from one carbon and a sp hybrid orbital from another carbon. d) Both atoms make bonds in two different directions giving them sp orbital hybridization. answer A sp hybrid orbital from carbon and a sp orbital from nitrogen. e) These are the pi bonds. They are labeled with ‘Y’ and ‘Z’ as they have slightly different orientations on the axis. Pi orbitals make bonds above and below the sigma bond as they overlap sideways. answer A p_y and a p_z orbital from carbon and a p_y and p_z orbital from nitrogen. f) Nitrogen has two electron domain directions. One is towards the carbon in a triple bond and the other is towards its lone pair therefore, It has two hybridized sp orbitals. The orbital that contains nitrogens lone pair is an sp orbital. answer A sp hybrid orbital. g) Hand drawn sketch below: answer
Check your work! Go back over your answers with the sketch and the structure to make sure you have tracked all the bonds and orbitals correctly. There are sigma bonds between each of the connected atoms, and the two pi bonds are the second and third bond between the C and N. Make sure that your identified orbitals match the electron domain geometry. Why does this answer make chemical sense? Hybridized orbitals are created so that all single bonds are made by the same orbitals and are equal. This is why hybridization will depend on the number of bonding directions made by the atom. The hybridization will likely vary between the atoms in the bond but will remain consistent within an atom itself.

Guided Solution

Hint

This is a theory type problem that tests your knowledge on atomic orbital hybridization. You must be able to correctly identify the hybridization of multiple atoms in a molecule and sketch the orbital overlaps.

See LibreText Section 7.1.7 (sp Hybrid Orbitals and the Structure of Acetylene)

For the molecule acetonitrile:

Acetonitrile structure question.png

a) How many sigma and pi bonds does it have?

b) What orbitals overlap to form the C-H sigma bonds?

c) What orbitals overlap to form the C-C sigma bonds?

d) What orbitals overlap to form the C-N sigma bonds?

e) What orbitals overlap to form the C-N pi bonds?

f) What orbital contains the lone pair electrons on nitrogen?

g) Sketch the molecule showing the hybridized atomic orbital ‘cloud’ overlap.

What are sigma and pi bonds?

Sigma bonds are your single bonds.

Pi bonds are your double and triple bonds.

There is at least one sigma bond between each of the connected atoms.

Look at the bonds between your carbon and your nitrogen. How many lines do you see??

How do you identify atomic orbital hybridization?

Identify the electron domain geometry of your central atom. (in this case, the two center atoms are carbons)
Your electron domain geometry will tell you the number of bonding directions attached to your atom.
Each bonding direction represents an orbital needed.
Identify if your outer atoms require hybridization (if they do, they will have multiple bonding directions as well, this includes lone pairs)

For instance, a tetrahedral electron domain geometry must have four electron domains so it will have an s+p+p+p (sp³) hybridization.

How do you know which orbitals are overlapping?

Each bond that is between the atoms will be an overlap. The hybridization of the orbitals will depend on the two atoms that the bond is between.

How do you sketch the orbital overlaps?

Begin by drawing your center atom with its orbitals in the shape of its electron domain geometry. (it's okay if they just look like “sausages” if you label them with their hybridization)
Overlap these orbitals with orbitals representing the outer atoms.
Label these as sigma bonds.
The orbitals that overlap to form pi bonds are longer and skinnier than the sigma bonds. They overlap sideways and therefore connect above and below the sigma bond.

Using different colours in your drawing for orbital type can be helpful to keep track.

Complete Solution:

a) There will always be a single bond between each atom in the molecule. A second or third line next to the single bond represents a double or triple bond. You can count these in your Lewis structure form to identify the sigma and pi bonds.

answer 5 sigma (single bonds) and 2 pi (one double bond and one triple bond).

b) This carbon atom makes bonds in four directions which gives it a sp³ hybridization. Three of these directions are hydrogen atoms. All of these hydrogen atoms are unhybridized and have s orbitals. When the orbitals from the hydrogen and carbon overlap, they form a sigma bond.

answer A sp³ hybrid orbital from carbon and an s orbital from hydrogen.

c) Although both atoms are hybridized, they have a different number of bonding directions therefore one is sp³ and the other is sp.

answer A sp³ hybrid orbital from one carbon and a sp hybrid orbital from another carbon.

d) Both atoms make bonds in two different directions giving them sp orbital hybridization.

answer A sp hybrid orbital from carbon and a sp orbital from nitrogen.

e) These are the pi bonds. They are labeled with ‘Y’ and ‘Z’ as they have slightly different orientations on the axis. Pi orbitals make bonds above and below the sigma bond as they overlap sideways.

answer A p_y and a p_z orbital from carbon and a p_y and p_z orbital from nitrogen.

f) Nitrogen has two electron domain directions. One is towards the carbon in a triple bond and the other is towards its lone pair therefore, It has two hybridized sp orbitals. The orbital that contains nitrogens lone pair is an sp orbital.

answer A sp hybrid orbital.

g) Hand drawn sketch below:

answer

Acetonitrile structure question.png

Check your work!

Go back over your answers with the sketch and the structure to make sure you have tracked all the bonds and orbitals correctly. There are sigma bonds between each of the connected atoms, and the two pi bonds are the second and third bond between the C and N. Make sure that your identified orbitals match the electron domain geometry.

Why does this answer make chemical sense?

Hybridized orbitals are created so that all single bonds are made by the same orbitals and are equal. This is why hybridization will depend on the number of bonding directions made by the atom. The hybridization will likely vary between the atoms in the bond but will remain consistent within an atom itself.

(question source from page titled 1.9: sp Hybrid Orbitals and the Structure of Acetylene, shared under a CC BY-SA 4.0 license, authored, remixed, and/or curated by Steven Farmer, Dietmar Kennepohl, Krista Cunningham, Tim Soderberg, William Reusch, & William Reusch. Access for free at https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/01%3A_Structure_and_Bonding/1.09%3A_sp_Hybrid_Orbitals_and_the_Structure_of_Acetylene)

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