Org Chem Unit 1 - Chapter 2

Can monosubstituted cycloalkanes have cis/trans isomers? Will a cycloalkane be cis/trans if both substituents are on the same carbon? Do the substituents need to be the same?

-No, because only one substituent will be present. -No, since both groups are bonded to same carbon only one arrangement is possible for them. There need to be substituents on 2+ carbons of the ring. -No, they just need to be on stereocenters on the ring

What is the difference between a cyclic hydrocarbon and a cycloalkane? What is a bicycloalkane? What makes it different than standard cycloalkanes?

Cyclic hydrocarbon = hydrocarbon that forms a ring Cycloalkane = a saturated cyclic hydrocarbon Bicycloalkane = an alkane that contains two rings that share two carbon atoms (connected by a "bridge") -shared carbon atoms are called bridgehead carbons

Give an example of a type of stereocenter that isn't a chiral center?

sp2 cis/trans carbon bond

What structural feature of cycloalkanes makes cis, trans possible?

the atoms are connected in such a way that rotation isn't possible, and therefore the groups on the ring have a fixed relationship with respect to each other (either cis or trans)

What is torsional energy?

the energy required to overcome repulsions brought about by torsional strain

What is the cis,trans isomerism like for decalin? In what positions are the H atoms at the ring junction?

Left = trans decalin (both are axial) Right = cis decalin (top is equatorial to ring A; bottom is axial to ring B)

What is a monosubstituted cyclohexane? What is it's implication? What type of interaction does this cause?

MSC = a hydrogen atom has been replaced by an alkyl group that occupies an equatorial position in one conformation and an axial in the other, so 2 chairs are no longer equivalent -causes a diaxial interaction, which is steric strain from the interaction between the axial alkyl and axial H atoms

What is a racemic mixture? What is optical purity? What are the 2 formulas for the optical purity of a mixture?

Racemic Mixture = a mixture of equal amounts of two enantiomers that is optically inactive (has a specific rotation of 0)

How to calculate the degrees of unsaturation? What exactly does this measure?

The number of bonds made by p-orbitals (double/triple) and/or the number of rings

What generalization can you make about the densities of alkanes relative to the density of water?

all alkanes that are liquid at room temp are less dense than water, because the oxygen atom of H2O has a larger atomic mass than the C atom of alkanes

Is cis-trans isomerism possible in alkanes?

because the single bonds can rotate 360°, it isn't possible

What does ring flipping refer to? What is the difference between an axial and an equatorial position? Which one is more favorable, why?

-cyclohexane has 2 chair conformations that are constantly flipping, which changes all of its axial and equatorial positions to the opposite Axial = straight up and down Equatorial = pointing out towards sides; parallel to the 2 bonds from which it is once removed -equatorial position is more favorable due to less steric interaction (make sure larger group is equatorial)

What are the 3 requirements of meso compounds?

1) 2+ chiral centers 2) Symmetrical/achiral (R,S or S,R) 3) Identical groups on the chiral centers

What is a hydrogenation reaction? What elements are NEEDED for it to occur. Which bonds are typically broken? Which bonds are typically made? What general type of reaction is hydrogenation? What does this mean for the stability of the bonds before and after?

1) Adding H2 gas and a catalyst (heat) to an alkene to produce an alkane. 2) (2 bonds) --- On the H2 gas, 1 H-H bond sigma bond is broken. On the alkene, 1 C-C pi bond is broken. 3) (2 bonds) --- On the alkane, 2 sigma C-H bonds are made. 4) Because bonds are broken, they are typically exothermic reactions. The bonds on the alkene are more stable (because energy is given off, and less energy means increased stability).

What are the 2 types of staggered conformations?

1) Anti (180°) = groups are max distance apart 2) Gauche (60°, 300°) = closer together, can be related by reflection

What are the steps of cyclohexane interconversion for a ring flip? Define each and rank them in terms of stability.

1) Chair = left side is up and right side is down [most stable conformation] -no angle strain; minimal torsional strain (all C-C bonds staggered) 2) Half Chair = right half of molecule is dragged up and flattened [least stable conformation] -results in both angle strain (half of bonds are 120°) and torsional strain (2 C-C bonds eclipsed) 3) Twist Boat = slight rotation of bonds to relieve some steric strain from flagpole atoms [more stable conformation] 4) Boat = right half of molecule pulled up fully [less stable conformation] -minimal angle strain -some torsional strain (4 C-H bonds eclipsed) -2 flagpole H atoms on top cause steric strain

What is the determination of the following: 1) No chiral centers 2) One chiral center 3) 2+ chiral centers 4) An internal mirror plane 5) An internal mirror plane and chiral centers

1) Compounds with no chiral centers are usually achiral 2) Compounds with exactly one chiral center are always chiral (with a pair of enantiomers), unless there is a mirror plane 3) May or may not be chiral 4) achiral 5) achiral (meso compounds)

What are the different types of isomers?

1) Constitutional Isomer = same MF, different connectivity 2) Stereoisomer = same MF and connectivity but different spatial arrangement of atoms/bonds 2A) Enantiomer = stereoisomers that are non-superimposable mirror images; same physical properties but differ in optical activity 2B) Diastereomer = stereoisomers that aren't mirror images of each other and have different chemical/physical properties 2b*) Configurational (Cis/Trans - E/Z - RS) = same connectivity but a different spatial arrangement given a stereocenter, double bond or ring 2C) Conformational isomers = different 3D arrangement of atoms that can be interconverted by rotating a single bond -can be either diastereomer or enantiomer 2D) Meso Compound = a molecule with a mirror plane (and therefore not chiral), but can have chiral centers;

Describe the conformations for the following cyclic structures with small ring strain: 1) Cyclopropane 2) Cyclobutane 3) Cyclopentane

1) Cyclopropane = planar conformation -60° bond angles (highest angle strain) -6 pairs of fully eclipsed C-H bonds (lowest torsional strain) 2) Cyclobutane = puckered butterfly conformation -88° bond angles -adopts non planar conformation to minimize torsional strain (which is greater than the tradeoff of increased angle strain) 3) Cyclopentane = puckered envelope -108° bond angles (lowest angle strain) -folds a carbon upwards out of the plane to decrease torsional strain

When comparing molecules with multiple chiral centers, how do you determine their relationship?

1) Determine R,S stereochemistry 2) Use table 3) If molecule is symmetric, check for meso compounds considering only R,S or S,R molecules

How to tell enantiomers and diastereomers apart?

1) Enantiomers = opposite configurations at ALL chirality centers 2) Diastereomers = opposite configurations at only some chirality centers

What are the 3 methods for determining the configuration of a stereocenter? Differences?

1) For single bonds: -all 4 connected groups must be different -assign priorities based on atomic # -R = clockwise; S = counterclockwise 2) For double bonds where groups separated by double bond are the same: -cis = same side; trans = opposite side 3) For double bonds where groups separated by double bond are different -rank groups on both sides from 1-2 -E = important groups on opposite sides; Z = important groups on same side

What does it mean for enantiomers to be optically active? What is plane-polarized light? How can optical activity be observed? What is the specific rotation? What is the formula?

1) It means that a pair of enantiomers rotates the plane of polarized light in opposite directions -Dextrorotatory = rotates light to the right -Levorotatory = rotates light to the left 2) the plane of polarized light that gets rotated in opposite directions is essentially light oscillating in only a single plane 3) Using a polarimeter 4) Observed rotation at a specific cell length and sample concentration either to the right (dextro = +) or to the left (levo = -) -specific rotation of a pair of enantiomers is same but opposite

What are the 4 criteria for aromaticity (very stable molecules)?

1) Molecule must be cyclic 2) Molecule must be fully conjugated (all carbons must be sp2) 3) Molecule must be planar (otherwise 2p orbitals wouldn't overlap well) 4) Must obey Huckel's rule (the # of pi electrons in the molecule must be equal to [4n + 2], where "n" is any integer)

What are the 5 key physical properties of alkanes/cycloalkanes?

1) Non-polar compounds with only the weakest intermolecular force (dispersion forces) present resulting from the interaction between temporary induced dipoles 2) Very hydrophobic (and insoluble in water) due to non polarity 3) Very low boiling points/melting points due to weak dispersion forces; both tend to increase as molecular weight increases 4) As branching increases, alkane becomes more compact and surface area decreases, decreasing strength of dispersion forces and decreasing boiling/melting point 5) As branching increases, heat of combustion decreases (energy given off from oxidation rxn), which makes it more stable

What are the 2 general types of conformations and their subtypes?

1) Staggered Conformation: 1a) Anti = largest groups 180 degrees from each other, most stable conformation 1b) Gauche = largest groups 60 degrees from each other 2) Eclipsed Conformation = groups are extremely close to each other; most unstable when both large groups are extremely close to each other

Define the following: 1) Stereocenter 2) Chiral Center

1) Stereocenter = any atom in a molecule for which exchanging two groups creates a different stereoisomer 2) Chiral Center = any tetrahedral C atom with 4 different substituents

What does it mean to be chiral vs achiral? Plane of symmetry? Center of symmetry?

Chiral = object and mirror image are not superimposable; lack both a mirror plane and an inversion center; optically active Achiral = object and mirror image are superimposable; symmetric; has an internal mirror plane; optically inactive Plane of symmetry = an imaginary plane passing through an object so that one half is the mirror image of the other half Center of symmetry = a point in which identical components are located on opposite sides equidistant from that point

What is the difference between conformation and configuration?

Configuration = arrangement of atoms around a stereocenter Conformation = 3D spatial arrangement that results from rotation of a single bond

What is a conformation? What are the 2 general types of conformations? What are the differences in terms of energy?

Conformation = any 3D arrangement of atoms in a molecule that results from rotation around a single bond 1) Staggered Conformation = the groups/atoms on a carbon are as far apart as possible from the atoms/groups on an adjacent carbon -lowest energy/strain (most stable) 2) Eclipsed Conformation = the opposite of staggered (close as possible) -highest energy/strain (least stable)

A long polymer chain (like polyethylene) can potentially exist in solution as a chiral object. Give two examples of chiral structures that such a chain could adopt

Despite it not having any chiral centers, it could exist in chiral conformations such as: -chiral helix -chiral knot

What is the relationship between strain, energy, heat of combustion per bond and stability?

High strain ---> high energy ---> high heat of combustion per bond ---> low stability

Are there any diaxial interactions between equatorial and H atoms?

No

What is strain? What are the 3 types of strain found in conformations?

STRAIN = instability (and therefore energy) stored in a compound due to structural distortion 1) Torsional Strain = strain that comes from having eclipsed bonds -60 and 180 degrees have lowest torsional strain as they are both staggered conformations, which means that they also have the lowest torsional energy 2) Angle Strain = when cyclic tetrahedral bonds are forced out of their ideal bond angle of 109.5 degrees -bigger angle change = greater strain = less stability = higher energy 3) Steric Strain = when nonbonded atoms separated by 4+ bonds are forced closer to each other than their atomic radii would allow