Bonding Flashcards
Having trouble remembering the difference between E and Z isomers? Finding it tough to identify meso compounds? Use these cards to master these topics, as well as resonance and hybridization, as they are tested on the MCAT.
Define:
hybrid orbital
A molecular orbital formed by combining two or more atomic orbitals from different subshells.
Hybrid orbitals are named for the atomic orbitals and subshells (s, p, or d) from which they are formed. For example, sp hybridization includes one s orbital and one p orbital, while sp3 hybridization includes one s orbital and three p orbitals.
What is a sigma bond?
A bond formed between two orbitals that directly overlap 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 as well.
What is a pi bond?
A bond formed between two orbitals oriented parallel to one another, overlapping side-to-side. In other words, two lobes of one molecular orbital overlap with two lobes of another molecular orbital.
In order for a pi bond to form, a sigma bond must also be present.
How many pi and sigma bonds does a single bond have?
A single bond has no pi bonds; it contains only a single sigma bond.
How many pi and sigma bonds does a double bond have?
A double bond has one pi bond and one sigma bond.
In order for a pi bond to form, a sigma bond must first be present.
How many pi and sigma bonds does a triple bond have?
A triple bond has two pi bonds and one sigma bond.
In order for a pi bond to form, a sigma bond must first be present.
When does sp3 hybridization occur?
It is displayed when one s orbital hybridizes with three p orbitals, forming four equivalent sp3 orbitals.
A carbon atom with sp3 hybridization forms single bonds to four different atoms. A classic example of this bond arrangement is methane. For other central atoms, note that lone pairs also contribute to overall hybridization.
What is the molecular geometry around a carbon atom that is sp3 hybridized?
It has tetrahedral geometry.
When an atom is surrounded by four single bonds, they will exist in the most symmetric arrangement possible, a tetrahedron with bond angles of 109.5º.
When does sp2 hybridization occur?
It is displayed when an s orbital hybridizes with two p orbitals, forming three equivalent sp2 orbitals.
A carbon atom undergoing sp2 hybridization will form bonds to three total atoms, with a mix of two single bonds and one double bond. A classic example of this bond arrangement is methanal. For other central atoms, note that lone pairs also contribute to overall hybridization.
What is the molecular geometry around a carbon atom which is sp2 hybridized?
It has trigonal planar geometry.
When an atom is surrounded by three bonds (and no lone pairs), they will exist in the most symmetric arrangement possible, a planar triangle with bond angles of 120º.
When does sp hybridization occur?
It is displayed when an s orbital hybridizes with one p orbital, forming two equivalent sp orbitals.
A carbon atom undergoing sp hybridization will form bonds to two atoms. These can include either two double bonds or a single and triple bond. A classic example of this bond arrangement is carbon dioxide.
What is the molecular geometry around a carbon atom which is sp hybridized?
it has linear geometry.
When an atom is surrounded by two bonds (and no lone pairs), they will exist in the most symmetric arrangement possible, a straight line with bond angles of 180º.
What is the shape of the trichloromethane (HCCl3) molecule?
tetrahedral
Since the central carbon is bonded to four atoms, it is sp3 hybridized. This means that the four atoms are arranged around it in a tetrahedron.
What is the shape of the methanal (H2CO) molecule?
trigonal planar
Since the central carbon is bonded to three atoms, it is sp2 hybridized. This means that the three 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-dibromoethane (BrH2C-CBrH2)?
tetrahedron
Since each carbon atom is bound to four other atoms (one bromine, two hydrogens, and one carbon), they are both sp3 hybridized. Hence, the atoms around each are arranged tetrahedrally.
What is the shape of the atoms arranged around either of the carbon atoms in 1,1-dichloroethene (Cl2C=CH2)?
planar triangle
Since each carbon atom contains three bonds (two single bonds to the hydrogens and a double bond to the other carbon, or two single bonds to the chlorines and a double bond to the other carbon), they are both sp2 hybridized. Hence, the atoms around each one is 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)?
straight line
Since the carbon atom in question contains two bonds (one single bond to the carbon and one triple bond to the nitrogen), it is sp hybridized. Hence, the atoms around it are arranged in a straight line. The other terminal carbon is sp3 hybridized.
Describe the shape of the carbon backbone in a straight-chain alkane, like hexane.
It is commonly depicted as a zig-zag line of carbon atoms with hydrogens branching from each.
This is a visual representation of the most stable state of a set of sp3 hybridized carbons.
Give the molecular formula and name for this molecule.
C5H12 or pentane
This picture is a structural formula, which is a two-dimensional graphical depiction of the molecule’s shape.
Give the molecular formula and name for the following molecule.
CH3-CH2-CH2-CH2-CH2-CH3
hexane
This is an example of formula nomenclature, in which the geometric information in the structural formula is assumed. It is commonly given without the hyphens between carbons, which would be CH3(CH2)4CH3 in this case.
Give the molecular formula and name for this molecule.
C4H10 or butane
Often, the MCAT will present a skeletal formula of an organic molecule, instead of this full structural form. In such a depiction, the hydrogens can be assumed, and each vertex of the chain represents a carbon atom.
What is the difference between the two molecules depicted below?
Structurally, no significant difference exists. Both molecules are resonance forms of the same molecule (in this case, analine).
The two structures will look indistinguishable from each other. They can be represented by the 3D structure above.
In an organic molecule, how does resonance impact the overall stability of the structure?
It increases stability.
Resonance is one of the most common ways that an organic molecule can be stabilized. Having several resonance forms is a strong indication that the molecule can be synthesized easily.
Define:
conjugation
It occurs when a molecular system, typically a carbon chain, contains an alternating system of pi and sigma electrons.
Conjugation drastically increases stability. For example, benzene is fully conjugated and is extremely stable.
Why do conjugated systems generally have many resonance forms?
It undergoes resonance because the alternating pi-sigma-pi system allows the weakly bound pi electrons to shift one position along the chain.
The structures above show how the electrons in aniline, a benzene ring with an attached amine group, move from one position to the next. This shifts the pi-sigma-pi system.
How can a non-carbon atom be part of a conjugated system?
Heteroatoms, or non-carbon atoms in a carbon chain, can be included in the system.
For example, propenal contains a terminal double (pi) bond to oxygen, then a single (sigma) bond between carbons, then another double bond. This alternating system represents conjugation even though it includes oxygen, a non-carbon atom. Another common heteroatom is nitrogen.
How can a conjugated system exist among carbons that lack the characteristic alternating pi-sigma-pi bonds along their chain?
Lone pairs and carbocations can be included in the system.
For example, cyclopropene has a single, double, single, cation arrangement. The cation can move to the adjacent carbon, and the double bond can then shift over one position as well. This creates three resonance structures.
Is this system conjugated?
Yes, this 1,3,butadiene system is conjugated.
It contains the alternating pi-sigma-pi bond system that is characteristic of conjugation.
The resonance structures shown above add to overall stability, though each is significantly less stable than 1,3-butadiene’s 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 heteroatom oxygen resonates to accept the negative charge.
Describe a Newman projection and how it depicts an organic molecule.
A two-dimensional 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 the viewer, while the larger circle with three red H’s represents the carbon behind it.
In ethane, what two primary conformations can exist around the C-C single bond?
- staggered
- eclipsed
These conformations are not specific to ethane; they can exist around any C-C single bond (in propane, butane, etc).
Which conformation of an organic molecule is more stable?
Choose between staggered or eclipsed.
staggered
Staggered arrangements are more stable because they “spread out” the functional groups. In the eclipsed conformation, on the other hand, a larger amount of electrostatic repulsion exists between the closely overlapping groups. This repulsion is known as torsional strain.
What two different forms can exist in an organic molecule with a staggered conformation?
- anti conformation
- gauche conformation
These forms are determined by the location of the two largest groups bound to each carbon. In the anti configuration, these groups are 180º apart; in the gauche configuration, they are only 60º apart. The example above shows anti and gauche butane.
Which configuration of an organic molecule is more stable?
Choose between anti conformation or gauche conformation.
anti conformation
Since it minimizes torsional strain, the maximum possible separation of the large groups will be the most stable conformation. The anti configuration places these groups the farthest apart, and is thus more stable.
Name the two geometric isomers that can exist for 2-butene, shown below.
CH3-CH=CH-CH3
- cis-2-butene
- trans-2-butene
Their structures are shown below:
Cis and trans nomenclature only applies if one hydrogen is present on each side of the double bond. The cis form contains the large groups on the same side of the bond, while the trans form has the large groups positioned across from one another.
Which form of 2-bromo-2-butenal is shown below?
Z-2-bromo-2-butenal
In E/Z nomenclature, the functional group with the higher atomic number takes priority. In this case, the 2-carbon is bound to two groups: an aldehyde and bromine; Br has a higher atomic number than carbon, giving it a higher priority. Since it is positioned on the same side of the double bond as the methyl group, this is the Z isomer.
What is the absolute configuration of the stereocenter shown below?
This chiral center has an R configuration.
Br, with the highest atomic number, is prioritized #1. O, the next highest, is #2, while C is #3. H has the lowest atomic number, giving it a priority of #4. The first-, second-, and third-priority substituents are arranged in a clockwise direction (R). Since this hydrogen is already pointing backwards, no rearrangement is necessary.
What is the absolute configuration of the stereocenter shown below?
This chiral center has an R configuration.
O, with the highest atomic number, is given #1 priority. The propyl group is the next highest, giving it #2 (since it contains bonds to carbons instead of the hydrogens present on methyl, which is given #3). Finally, H is #4. However, since OH is pointing towards the back, the hydrogen must currently be facing forwards; in other words, we are viewing the back of this molecule. This means that when a line is drawn from 1 to 2 to 3 (in the counterclockwise, or S, direction), the actual configuration would be R (clockwise) if that large substituent were flipped to the front.
Define:
chiral carbon
Also known as a stereocenter, is a carbon that contains single bonds to four different substituents.
In this image, notice that the mirror molecule of a chiral carbon cannot be superimposed over the original.
Define:
enantiomer
A type of stereoisomer that is the non-superimposable mirror image of an original molecule.
Molecules can only have enantiomers if they contain at least one chiral carbon. Note that enantiomers must differ in their configuration at each and every stereocenter.
Is the central carbon depicted below chiral?
Yes
Since this carbon is bound to four unique substituents, it cannot be superimposed on its mirror image, making it chiral.
Is the carbonyl carbon depicted below chiral?
No
A chiral carbon must be bound to four unique substituents. As such, any carbon that makes a double bond cannot possibly be chiral.
Is this carbon (circled below) chiral?
No
This carbon does have 4 single bonds (including one to a hydrogen that is not shown), and a carbon in a ring structure certainly can be chiral. In this particular case, however, the ring is identical on both sides of the carbon in question, making it achiral.
What does a Fischer projection indicate about the spatial relationships of the substituents around a central carbon?
A two-dimensional “flattening” of a three-dimensional molecule.
The substituents in the horizontal axis point out of the plane of the page, while those along the vertical axis point into the page.
Define:
meso compound
A molecule that contains both chiral centers and an internal plane of symmetry. Though its carbons are chiral, the overall molecule is achiral.
The chiral centers on one side of the line of symmetry will rotate light in one direction, while those on the other side will rotate it the opposite way. So, a meso molecule does not rotate plane polarized light.
What is the relationship between the following two molecules?
enantiomers
These molecules differ at their only chiral center; the molecule on the left is S while the molecule on the right is R. As enantiomers, they are non-superimposable mirror images of each other.
What is the relationship between the following two molecules?
diastereomers
The molecules share one chiral carbon with the same configuration (S, circled at the top), and one chiral carbon with opposite configurations (one is R, one is S, circled at the bottom). Thus, they are non-superimposable and are not mirror images.
What is the relationship between the following two molecules?
They are the same compound and meso.
In both depictions, this molecule has an internal plane of symmetry, making it achiral. The two compounds are the same, since the mirror image of the compound on the left is superimposable over the compound on the right.
