Bonding Flashcards
Define:
a hybrid orbital
A hybrid orbital is a molecular orbital formed by the combination of two or more atomic orbitals from different subshells.
Hybrid orbitals are named by the atomic orbitals and subshells (s, p, or d) from which they are formed.
sp = one s orbital and one p orbital; sp3 = one s orbital and three p orbitals
Define:
a sigma bond
A sigma bond is a bond formed between two orbitals directly overlaping: head-to-head. Sigma bonds are the strongest type of covalent bond.
All covalent bonds contain exactly one sigma bond, though they may have one or two pi bonds.
Define:
a pi bond
A pi bond is formed between two orbitals oriented parallel to one another, overlapping side-to-side.
Two lobes of one molecular orbital overlap with the two lobes of another molecular orbital.
In order for a pi bond to form, there must first be a sigma bond present.
How many pi and sigma bonds are there in a:
- single bond
- double bond
- triple bond
- A single bond has no pi bonds, it has only a sigma, by default.
- A double bond has one pi bond and one sigma.
- A triple bond has two pi bonds and one sigma.
In order for a pi bond to form, there must first be a sigma bond present.
What is sp3 hybridization?
sp3 hybridization occurs when an s orbital hybridizes with three p orbitals, forming 4 equivalent sp3 orbitals.
A carbon atom with sp3 hybridization will form single bonds to four different atoms. The classic example of this is methane.
What is the geometry around a carbon atom which is sp3 hybridized?
An sp3-hybridized carbon atom has tetrahedral geometry.
Remember from general chemistry: when an atom is surrounded by four single bonds they will be in the most symmetric arrangement, a tetrahedron with bond angles of 109.5º.
What is sp2 hybridization?
sp2 hybridization occurs when an s orbital hybridizes with two p orbitals, forming 3 equivalent sp2 orbitals.
An atom undergoing sp2 hybridization will form bonds to three total atoms. These will be a mix of two single bonds and one double bond. A classic example of this is methanal.
What is the geometry around a carbon atom which is sp2 hybridized?
An sp2-hybridized carbon atom has trigonal planar geometry.
Remember from general chemistry: when an atom is surrounded by three bonds (and no lone pairs) they will be in the most symmetric arrangement, a planar triangle with bond angles of 120º.
What is sp hybridization?
sp hybridization occurs when an s orbital hybridizes with one p orbitals, forming 2 equivalent sp orbitals.
A carbon atom undergoing sp hybridization will make bonds to two atoms. This can be both double bonds, or a single and triple bond. A classic example of this is carbon dioxide.
What is the geometry around a carbon atom which is sp hybridized?
An sp-hybridized carbon atom has linear geometry.
Remember from general chemistry: when an atom is surrounded by two bonds (and no lone pairs) they will be in the most symmetric arrangement, a straight line with bond angles of 180º.
What is the shape of the trichloromethane (HCCl3) molecule?
Trichloromethane is tetrahedral.
Since the carbon is bonded to 4 atoms, it is sp3 hybridized, which means the 4 atoms are arranged around it in a tetrahedron.
What is the shape of the methanal (H2CO) molecule?
Methanal is trigonal planar.
Since the carbon is bonded to 3 atoms, it is sp2 hybridized, which means the 3 atoms are arranged in a planar triangle.
What is the shape of the atoms arranged directly around either of the carbon atoms in 1,2,dibromo-ethane (BrH2C-CBrH2)?
The atoms around the carbons in 1,2,dibromo-ethane are arranged in a tetrahedron.
Each carbon atom is bound to 4 things total (1 bromine, 2 hydrogens and 1 carbon), they are both sp3 hybridized, hence the atoms around each are arranged in a tetrahedron.
What is the shape of the atoms arranged around either of the carbon atoms in 1,1,dichloro-ethene (Cl2C=CH2)?
The atoms around the carbons in 1,1,dichloro-ethene are arranged in a planar triangle.
Each carbon atom is bound to 3 things total (2 hydrogens and a double bond to the other carbon, or 2 chlorines and a double bond to the other carbon), they are both sp2 hybridized, hence the atoms around them are arranged in a planar triangle.
What is the shape of the atoms arranged around the central carbon atom in acetonitrile, also known as cyanomethane (CH3C≡N)?
The atoms around the cental carbon in acetonitrile are arranged in a straight line.
The carbon atom is bound to 2 things total (1 carbon and a triple bond to the nitrogen), this is sp hybridized, hence the atoms around it are arranged in a straight line. The other terminal carbon is sp3 hybridized.
What is the most common shape of the carbon backbone in an alkane like hexane?
The most common shape depicted for a carbon backbone is the carbon atoms in a zig-zag form, with the hydrogens branching off of each carbon.
This is a visual representation of the most stable state of a set of sp3 hybridized carbons.
What is the molecular formula and name for this molecule?
The molecule is C5H12, pentane.
This picture is a structural formula, a 2-D graphical depiction of the molecule’s shape.
What is the molecular name of the molecule depicted below?
CH3-CH2-CH2-CH2-CH2-CH3
This molecule is hexane.
This is an example of the formula nomenclature of organic chemistry, where the geometric information in the structural formula is assumed. It can also be written without the hyphens between carbons, or as CH3(CH2)4CH3
What is the molecular formula and name of the molecule being depicted below?
This molecule is butane, C4H10.
Often the MCAT will present a skeletal formula of an organic molecule, instead of this full structural form. The hydrogens will be assumed, and vertices of the chain each represent a carbon atom.
What is the difference between the two molecules depicted below?
There is no significant difference; they are resonance forms of the same molecule (in this case: analine).
Both of the structures will look indistinguishable from each other, and can be represented by the 3D structure below.
How does resonance in an organic molecule impact overall stability of the structure?
Resonance increases stability.
Resonance is one of the most common ways that a molecule can be stabilized in organic chemistry. Having several resonance forms is a strong indication that the molecule can actually be made in the lab.
What is the primary requirement for an organic molecule to undergo resonance?
The molecule must be conjugated in order for it to undergo resonance.
Conjugation occurs when a molecular system, typically a carbon chain in the case of organic molecules, has an alternating system of pi and sigma electrons.
Why do conjugated systems undergo resonance?
Conjugated systems undergo resonance because the alternating pi-sigma-pi system allows the weakly-bound pi electrons to shift over one position along the chain..
The structures below show how the electrons in very stable benzene ring (with an amine group), move from one position to the next, shifting the pi-sigma-pi system.
How could a non-carbon atom be part of a conjugated system?
Heteroatoms can be included in the system.
Ex: propenal has a terminal double bond (pi) to oxygen, then single (sigma), then double between cabons (pi). This represents the alternating system for conjugation, and includes a non-carbon atom (oxygen). Another common hereroatom is nitrogen.
How could a conjugated system exist among carbons, besides the alternating pi-sigma-pi bonds along a neutral carbon chain?
Lone pairs and carbocations can be included in the system.
Ex: In cyclopropene, there is a single, double, single, cation arrangement. The cation will move over one carbon, and the double bond will shift over one as well. This will create three resonance structures.
Is this system conjugated?
Yes, this 1,3,butadiene system is conjugated.
There is an alternating pi-sigma-pi bond system, that is necessary for conjugation.
The resonance structures shown below add to stability, though each is significantly less stable than the 1,3,butadiene original structure.
Is this system conjugated?
Yes, the non-bonded electrons on the negatively charged carbon are available to participate in a pi system, as shown below.
In this case, the hetereoatom oxygen resonates to accept the negative charge.
What is a Newman projection of an organic molecule?
A Newman projection is a 2-D projection of a molecule, shown as if the molecule was being viewed directly along a C-C bond.
In this Newman projection of ethane, the central point with three black H’s represents the carbon close to you, and the larger circle with three red H’s represents the carbon behind.
What are the two primary conformations of ethane around the C-C single bond?
The primary conformations of ethane around the C-C single bond are staggered and eclipsed.
These conformations are possible around any C-C single bond (propane, butane, etc), but are shown here for ethane for simplicity.
Which conformation of an organic molecule is more stable, staggered or eclipsed?
The staggered conformation of an organic molecule is more stable.
This is due to the electrostatic repulsion of the overlapping functional groups in the eclipsed conformation. This repulsion is known as torsional strain.
What are the two different forms of the staggered conformation of an organic molecule?
The two forms of the staggered conformation of an organic molecule are anti and gauche.
The forms are determined by the location of the two physically largest groups bound to each carbon. Anti configuration has those two groups 180º apart, while gauche configuration has those two groups 60º apart. The example below shows anti and gauche butane.
Which configuration of an organic molecule is more stable, anti or gauce?
The anti configuration is more stable.
Torsional strain makes the largest possible separation of the large groups the most stable configuration. The anti configuration places these groups the furthest apart, and is the most stable.
What are the names of the two geometric isomers of 2-butene?
CH3-CH=CH-CH3
The two geometric isomers of 2-butene are cis-2-butene and trans-2-butene, whose structures are shown below.
Cis and trans nomenclature only applies if there is one hydrogen on each side of the double bond. The cis form has the large groups on the same side, while the trans form has the large groups across from one another.
Which form of 2-bromo-2-butenal is shown below?
This is Z-2-bromo-2-butenal.
For prioritizing functional groups in E/Z nomenclature, the functional group with the higher atomic number takes priority. In this case, the 2-carbon has two groups bound to it: aldehyde and Br. Br has a higher atomic number than carbon, so it is higher priority. Since it is on the opposite side from the CH2-CH3 on the opposite side of the double bond, this is the Z isomer.
Define:
a chiral carbon
A chiral carbon, also known as a stereocenter, is a carbon which has single bonds to four different substituents.
In the image, notice that the mirror molecule of a chiral carbon cannot be superimposed over the original.
Define:
an enantiomer
An enantiomer is an isomer of a molecule which is the non-superimposable mirror image of the original molecule.
A molecule can only have an enantiomer if it contains at least one chiral carbon. Enantiomers differ in their configuration at each and every chiral carbon.
Is the central carbon depicted chiral?
Yes, the carbon depicted is chiral.
Since it is bound to four unique substituents, it cannot be superimposed on its mirror image, and it is chiral.
Is the carbon depicted chiral?
No, the carbon depicted is not chiral.
A chiral carbon must be bound to four unique substituents. As such, any carbon making a double bond cannot possibly be chiral.
Is the circled carbon chiral?
No, the circled carbon is not chiral.
It is does have 4 single bonds (H not shown), and it is possible for a carbon in a ring structure to be chiral. But, in this particular case the ring is identical in both directions from the highlighted carbon, so the carbon is is not chiral.
What does a Fischer projection indicate about the spatial relationships of the substituents around the central carbon?
A Fischer projection is a 2-D “flattening” of a three-dimensional molecule.
The substituents in the horizontal axis are coming out of the plane of the page, while those in the vertical axis are going into the page.
What is the stereochemistry of the following molecule?
The stereochemistry is R.
Br is the highest atomic number = #1, O is next highest = #2, C is next = #3, H is the lowest so is #4 (unranked). Since the H is towards the back already, no rearrangement is necessary.
What is the stereochemistry of the following molecule?
The stereochemistry is R (specifically: 2-R, since it’s on the second carbon).
O is the highest atomic number = #1, propyl is next highest = #2 since there are more bonds to carbons, methyl is next = #3 since there are only bonds to hydrogen, H is the lowest so is #4 (unranked). But, since OH is towards the back, the H must currently be to the front, i.e. we are viewing the back of this molecule. This means that the lines drawn 1-2-3, that appear to be S (counterclockwise), should actually be R (clockwise), if the whole molecule was flipped over to the front.
Define:
a meso molecule
A meso molecule is an isomer of a molecule which has chiral centers and an internal plane of symmetry. The molecule is overall achiral.
Since the chiral centers on one side of the line of symmetry will rotate light in one direction, and the other side exactly opposite, a meso molecule does not rotate plane polarized light.
What is the relationship between the following two molecules?
The molecules are enantiomers.
The molecules differ at one chiral center. The molecule on the left is S and the molecule on the right is R. They are non-superimposable mirror images of each other.
What is the relationship between the following two molecules?
The molecules are diastereomers.
The molecules share one chiral carbon with the same configuration (S) circled at the top, and one chiral carbon with opposite configuration (one is R, one is R) circled at the bottom. Thus, they are non-superimposable and also not mirror images.
What is the relationship between the following two molecules?
They are the same compound and meso.
The molecule has an internal plane of symmetry, therefore the molecule is not chiral. They are the same, since the mirror image of the compound on the left is superimposable on the compound on the right.
