O.Chem Carey 10th: Ch.3 Conformations

**Resonance: the most stable contributes the most to the true structure**

**Conjugate base: the most stable has the most acidic PROTONATED form**

Notice that ring inversion does not alter the cis relationship between the methyl groups. *what is down is down*

Nor does it alter their up-versus-down quality; substituents that are up in one conformation remain up in the ring inverted form. *ring inversion only changes axial/equatorial NOT up/down*

Two main explanations have been offered for this difference.

One holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation. The other suggests that better electron delocalization stabilizes the staggered conformation. Both effects contribute to the preference for the staggered conformation.

If we assume that there are certain "ideal" values for bond angles, bond distances, and so on, it follows that deviations from them will destabilize a particular structure.

vThe resulting increase in potential energy is referred to as the strain energy (Estrain) of the structure. Other terms include steric energy and steric strain.

equatorial hydrogens,

located approximately along the equator of the molecule

The staggered conformation is the most stable conformation,

the eclipsed is the least stable conformation.

carbons without any double bonds like to orient

109.5d

every neuman projection has

3 staggered and 3 eclipsed always rotating between them as you rotate, creating oscillating energy levels.

Raising the temperature decreases Ea/RT and increases Ae^-Ea/RT, thereby increasing k. Small increases in Ea result in large decreases in rate.

A is a frequency factor related to the collision rate and geometry. The e-Ea/RT term is the probability that a collision will result in reaction, T is the temperature in kelvins, and R is a constant (8.314 kJ/K · mol or 1.987 × 10−3 kcal/K · mol). Ea is calculated by comparing reaction rates as a function of temperature.

Rate constant k:

An experimentally determined proportionality constant that relates the rate of a reaction to the concentrations of the substances present.

When substituents are present, numbering begins in the smaller ring adjacent to the spiro carbon and proceeds consecutively around the smaller ring away from the spiro carbon, through it, then around the larger ring.

As with alkanes, the direction is chosen so as to give the lower locant at the first point of difference, substituents are listed in alphabetical order, and the locants and substituents appear first in the name.

your final drawing must have the following features.

Bonds that are across the ring from each other are parallel, Notice also that the bonds shown in red are drawn with longer lines to show the side-on perspective, Bonds are slanted as indicated.

in addition to angle strain, cyclopropane is destabilized by torsional strain.

Each C—H bond of cyclopropane is eclipsed with two others.

Estrain =

Ebond stretch + Eangle bend + Etorsion + Enonbonded

Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms.

Eclipsed bonds are characterized by a torsion angle of 0°. When the torsion angle is approximately 60° the spatial relationship is gauche; and when it is 180° it is anti.

If we assume that there are certain "ideal" values for bond angles, bond distances, and so on, it follows that deviations from them will destabilize a particular structure. The resulting increase in potential energy is referred to as the strain energy (Estrain) of the structure. Other terms include steric energy and steric strain.

Estrain = Ebond stretch + Eangle bend + Etorsion + Enonbonded "Estrain represents the ideal position, so any E angle changes, can +

Stereoisomers:

Isomers with the same constitution but that differ in respect to the arrangement of their atoms in space. Stereoisomers may be either enantiomers or diastereomers.

Molecules must become energized in order to undergo a chemical reaction or, as in this case, to undergo rotation around a carbon—carbon bond.

Kinetic (thermal) energy is absorbed by a molecule from collisions with other molecules and is transformed into potential energy. When the potential energy exceeds Ea, the unstable arrangement of atoms that exists at that instant relaxes to a more stable structure, giving off its excess potential energy in collisions with other molecules or with the walls of a container.

Ring inversion:

Process by which a chair conformation of cyclohexane is converted to a mirror-image chair. All of the equatorial substituents become axial, and vice versa. Also called ring flipping, or chair-chair interconversion.

The most stable conformation of trans-1,4-dimethylcyclohexane has both methyl groups in equatorial orientations

Remember, it is a general rule that any substituent is more stable in an equatorial orientation than in an axial one.

1,3-Diaxial repulsion:

Repulsive forces between axial substituents on the same side of a cyclohexane ring.

Cyclopropane's three carbon atoms are, of geometric necessity, coplanar, and rotation about its carbon-carbon bonds is impossible

Strong sp3-sp3 σ bonds are not possible for cyclopropane, because the 60° bond angles of the ring do not permit the orbitals to be properly aligned for effective overlap The less effective overlap that does occur leads to what chemists refer to as "bent" bonds. The electron density in the carbon-carbon bonds of cyclopropane does not lie along the internuclear axis but is distributed along an arc between the two carbon atoms.

bicyclo[number.number.number]alkane

The bracketed numbers identify the number of carbons in the three bridges in descending order. Numbering begins at a bridgehead position and proceeds consecutively in the direction of the largest bridge and continues through the next largest. The atoms in the smallest bridge are numbered last.

bridgehead carbons

The carbons which are located at either end of a "bridge" of carbons. *the bridge count does not include bridgehead carbons*

Arrhenius equation:

The expression k = Ae^(-Ea/RT) relating the rate of a chemical process to temperature.

Unlike ethane, in which the staggered conformations are equivalent, butane has two different staggered conformations

The methyl groups of butane are gauche to each other in one, anti in the other. Both conformations are staggered, so are free of torsional strain, but two of the methyl hydrogens of the gauche conformation lie within 210 pm of each other. This distance is less than the sum of their van der Waals radii (240 pm), and there is a repulsive force between the two hydrogens.

Activation energy (Ea):

The minimum energy that a reacting system must possess above its most stable state in order to undergo a chemical or structural change.

Chair conformation:

The most stable conformation of cyclohexane: All of its bonds are staggered, making it free of torsional strain as well.

spiro[number.number]alkane.

The numbers inside the brackets are, in ascending order, the number of carbons unique to each ring.

Transition state:

The point of maximum energy in an elementary step of a reaction mechanism. The eclipsed conformation is the transition state for the conversion of one staggered conformation of ethane to another.

The hydrogen atoms at the ring junctions are on the same side in cis-decalin and on opposite sides in trans-decalin.

The two stereoisomeric bicyclo[4.4.0]decanes, called cis- and trans-"decalin", are important examples.

IUPAC classifies polycyclic structures according to the minimum number of bond cleavages required to generate a noncyclic structure.

The structure is bicyclic if two bond disconnections yield an open-chain structure, tricyclic if three, tetracyclic if four, and so on.

Kinetics:

The study of reaction rates and the factors that influence them.

The preference for an equatorial methyl group in methylcyclohexane is therefore analogous to the preference for the anti conformation in butane.

The zigzag arrangement described by each path mimics the anti conformation of butane.

The shapes of heterocyclic rings are very much like those of their all-carbon analogs.

Thus, six-membered heterocycles such as piperidine exist in a chair conformation analogous to cyclohexane. (one C replaced with an N). The hydrogen attached to nitrogen can be either axial or equatorial, and both chair conformations are approximately equal in stability. (S is also common)

Intermediate:

Transient species formed during a chemical reaction. Typically, an intermediate is not stable under the conditions of its formation and proceeds further to form the product. Unlike a transition state, which corresponds to a maximum along a potential energy surface, an intermediate lies at a potential energy minimum.

A methyl group is less crowded when it is equatorial than when it is axial.

When the methyl group is equatorial, it experiences no significant crowding. (crowding of hydrogens. when on top they repulse some)

Molecular mechanics

a method for calculating the energy of a molecule by comparing selected structural features with those of "unstrained" standards.

The same fundamental principles apply to medium and large rings as apply to smaller ones

but there are more atoms, more bonds, and more conformational possibilities.

The destabilization of a molecule that results when two of its atoms are too close to each other is called van der Waals strain, or steric hindrance, and contributes to the total steric strain.

In the case of butane, van der Waals strain makes the gauche conformation approximately 3.3 kJ/mol (0.8 kcal/mol) less stable than the anti.

Two nonplanar conformations of cyclopentane, the envelope and the half-chair are of similar energy.

In the envelope conformation four of the carbon atoms are coplanar. The fifth carbon is out of the plane of the other four. There are three coplanar carbons in the half-chair conformation, with one carbon atom displaced above that plane and another below it.

when drawing chairs

solid = up, dash = down

the most stable isomer of a cycloalkane

usually has its substituents in equatorial positions, on opposite surfaces (above, below) the ring, higher priority given to larger molecules

molecular models

wedge-and-dash sawhorse drawing Newman projection

gauche of butane

where top CH3 is 1 and the right CH3 is 4th

half-life

which is the length of time it takes for one half of the molecules to have reacted.

standard free energy (ΔG°)

ΔG° = G°products − G°reactants = − RT |nK

Free energy and enthalpy are related by the expression

ΔG° = ΔH° − TΔS° ΔS° is the difference in entropy between the products and reactants. A positive ΔS° is accompanied by an increase in the disorder of a system. A positive ΔS° leads to a ΔG° that is more negative than ΔH° and a larger K than expected on the basis of enthalpy considerations alone

Eclipsed conformation:

Conformation in which bonds on adjacent atoms are aligned with one another. For example, the CH bonds indicated in the structure shown are eclipsed.

Staggered conformation:

Conformation of the type shown, in which the bonds on adjacent carbons are as far away from one another as possible.

conformations

Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. Conformational analysis will help us to visualize organic molecules as three-dimensional objects and to better understand their structure and properties.

Conformers:

Conformations that correspond to potential energy minima

Heterocyclic compound

Cyclic compound in which one or more of the atoms in the ring are elements other than carbon. Heterocyclic compounds may or may not be aromatic.

Steric hindrance

Decrease in reactivity resulting from the presence of bulky groups at the site of a reaction.

Torsional strain:

Decreased stability of a molecule associated with eclipsed bonds.

Steric strain:

Destabilization of a molecule as a result of van der Waals repulsion, distorted bond distances, bond angles, or torsion angles.

Van der Waals strain:

Destabilization that results when two atoms or groups approach each other too closely. Also known as van der Waals repulsion.

In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection.

Viewed from this perspective, the most stable conformations of pentane and hexane have their carbon "backbones" arranged in a zigzag fashion. All the bonds are staggered, and the chains are characterized by anti arrangements of C—C—C—C units.

the heats of combustion per methylene group of the very large rings are all about the same and similar to that of cyclopentane and cyclohexane.

We conclude, therefore, that the bond angles of large cycloalkanes are not much different from the bond angles of alkanes themselves. The prediction of the Baeyer strain theory that angle strain increases steadily with ring size is contradicted by experimental fact.

Higher alkanes having unbranched carbon chains are, like butane, most stable in their

all-anti conformations.

Wedges fail to show conformation

and it's important to remember that the rings of cis- and trans-1,2-dimethylcyclohexane exist in a chair conformation. This fact must be taken into consideration when evaluating the relative stabilities of the stereoisomers.

The most important result of ring inversion is that

any substituent that is axial in the original chair conformation becomes equatorial in the ring-inverted form and vice versa.

draw the substituted carbons and hydrogens in chair drawings as if they are

arranged staggered in a neumann projection

Quantum mechanical calculations

based on the Schrödinger equation. treats electrons as waves. focuses on nuclei and electrons.

Polycyclic compounds

contain more than one ring

With the exception of cyclopropane,

cycloalkanes are nonplanar

At any instant, almost all of the molecules are in staggered conformations;

hardly any are in eclipsed conformations.

axial hydrogens

have their bonds parallel to a vertical axis that passes through the ring's center.

less stable =

higher heat of combustion (heat of energy released when substance undergoes complete combustion)

equatorial takes precedence over trans

i.e. 1-3-5 cis more stable than 1-3-5 with one axial trans

converting line drawings to neumann projections, the orientations of the lines matter

i.e. N/\/O, N and O should be anti

If a disubstituted cyclohexane has two different substituents, then the most stable conformation is the chair that has the larger substituent in an equatorial orientation.

i.e. conformations related by ring inversion are in rapid equilibrium with one another, but the distribution between them strongly favors those in which the tert-butyl group is equatorial.

When substituents are on the same side, we say they are cis to each other; if they are on opposite sides,

if they are on opposite sides, they are trans to each other.

"puckered" conformation

in which adjacent C—H bonds are twisted away from one another. A fully staggered arrangement in cyclobutane is not possible, but eclipsing interactions are decreased in the puckered form.

Angle strain in the planar conformation of cyclopentane

is relatively small because the 108° angles of a regular pentagon are not much different from the normal 109.5° bond angles of sp3-hybridized carbon. The torsional strain, however, is substantial, because five bonds are eclipsed on the top face of the ring, and another set of five are eclipsed on the bottom face

Stereochemistry

is the term applied to the three-dimensional aspects of molecular structure and reactivity.

spiro compound

one atom is common to two rings.

compare number of interactions between opposite side constituents

really only count CH3 interactions

fused-ring compounds

rings that share a common side. but for classification and naming purposes they are placed in the "bridged" category. The bridge in these cases is the common side and is given a value of zero atoms.

Gauche and Anti only occur in

staggered conformations (not eclipsed)

when naming cyclic alkanes with multiple substituents and there are a couple equally good ways of numbering them so as to give them both the lowest number...

start numbering to observe alphabetical order of substituents (1 ethyl 2 methyl)

Coulombic interaction

the attractive force between oppositely charged atoms or the repulsive force between atoms of like charge.

Cyclic structures also exist in inorganic chemistry.

the most stable form of elemental sulfur is an eight-membered ring of sulfur atoms.

Angle strain

the strain a molecule possesses because its bond angles are distorted from their normal values. Baeyer strain theory = flawed

remember that neumann projections are for detailing the rotation of a single bond

therefore detailing the rest of the drawing by adding line drawings is OK *remember to transfer charges*

bridged compound

two atoms are common to two or more rings.