CHM 221 Quiz 4

Terpenes

- odiferous steroid precursors steroid precurson made from isoprene - one unit is a monoterpene Terpenes are constructed in plants from isoprene units

Rings larger that cyclohexane have more strain

.Bond angle distortion ·Partial eclipsing of hydrogens ·Transannular steric repulsions ·Medium sized rings adopt several conformations that are very close in energies, such as cyclodecane:

Drawing chair conformations

1. Draw the carbon chair 2. Add the axial hydrogens 3. Draw the C1 and C4 equatorial hydrogens 4. Draw the remaining equatorial hydrogens.

Tetrahedral

109.5 degrees, sp3

trigonal planar

120 degrees, sp2

Polycyclic Alkanes

A bridged bicycloalkane is a bicycloalkane whose molecule has two carbon atoms shared by all three rings identifiable in the molecule. The two carbon atoms shared by the three rings are called bridgehead carbon atoms. A bond or a chain of bonds connecting the bridgehead carbon atoms is called a bridge

Cyclohexane has axial and equatorial hydrogen atoms

In the chair form, cyclohexane has two types of hydrogens: Axial: 6 H parallel to the principle molecular axis Equitorial: 6 H perpendicular to the principle molecular axis

torsional strain

Increased energy that results when molecules assume eclipsed or gauche staggered conformations.

Cyclohexane also has several less stable conformations

A second, less stable conformation of cyclohexane is the boat form, which is less stable than the chair form by 6.9 kcal mol-1 The higher energy due to eclipsing of 8 hydrogens and the transannular (steric crowding across a ring).

The properties of the cycloalkanes differ from those of their straight chain analogs.

A. Cycloalkanes have higher BPs, MPs, and densities than their straight-chain analogs. ·Increased London forces: due to more rigid & symmetric cyclic systems. B. Smaller cycloalkanes with an odd # of carbons have lower MP than their even numbered neighbors. ·Attributed to crystal packing

Angular Strain

Angle strain is the increase in potential energy of a molecule due to bond angles deviating from the ideal values. eg: Cyclopropane Due to the rigidity of the cyclopropane ring it could assume only one conformation, namely the planar conformation.

Cis/trans isomers are non-superimposable molecules having the same molecular formula and connectivity.

Conformers,on the other hand, are also stereoisomers but can be interconverted by rotations about C-C bonds. (gauche, anti, ...)

Ring Strain & Structure of Cycloalkanes B

Cyclohexane has essentially no ring strain.

Cyclohexane: A Strain-Free Cycloalkane

Cyclohexane has several conformations, one of which is called the chair conformation.

Ring Strain & Structure of Cycloalkanes: The heats of combustion of the cycloalkanes reveal the presence of ring strain.

Cyclopropane (60o) and cyclobutane (90o) possess C-C-C bond angles significantly different from 109.5o and are strained. This is termed ring strain.

Strain affects the structures and conformational function of the smaller cycloalkanes.

Cyclopropane: All methylene hydrogens are eclipsed (eclipsing strain) No bond rotation to achieve a staggered conformation. The C-C-C bond angles are 60o, far from the unstrained value of 109.5o. The C-C bond energy is about 65 kcal mol-1 The ring of cyclopropane is easily opened, for instance by hydrogenation:

Disubstituted cycloalkanes can be stereoisomers

Disubstituted cycloalkanes having substituents on different carbons possess cis (same side) / trans (opposite side) isomers.

The ring strain in cycloalkanes can be estimated by comparing the theoretical heats of combustion to the measured heats of combustion:

Four groupings: Small (3,4) Common (5,6,7) Medium (8-12) Large (>12)

Carbocyclic products in nature

Natural products are organic compounds produced by living organisms. Four classification schemes are used for natural products: ·Chemical structure ·Physiological activity ·Organism or plant specificity (taxonomy) ·Biochemical origin

Polycyclic Alkanes

Polycyclic alkanes may contain fused or bridged rings. The fused system, decalin, can be compared to the disubstituted molecule, 1,2-diethylcyclohexane.

A regular pentagon has interior angles of 108o (close to tetrahedral). However, cyclopentane is puckered, not planar.

Puckering relieves some stress (H eclipsing).

ring strain in cycloalkanes

Ring Strain occurs because the carbons in cycloalkanes are sp3 hybridized, which means that they do not have the expected ideal bond angle of 109.5o ; this causes an increase in the potential energy because of the desire for the carbons to be at an ideal 109.5o. An example of ring strain can be seen in the diagram of cyclopropane below in which the bond angle is 60o between the carbons. The reason for ring strain can be seen through the tetrahedral carbon model. The C-C-C bond angles in cyclopropane (diagram above) (60o) and cyclobutane (90o) are much different than the ideal bond angle of 109.5o. This bond angle causes cyclopropane and cyclobutane to have a high ring strain. However, molecules, such as cyclohexane and cyclopentane, would have a much lower ring strain because the bond angle between the carbons is much closer to 109.5o.

Naming Example

Root name presents the longest carbon chain the molecule. Example For 2-methylbutane, root name is "but". Prefix represents the substituents present in the longest carbon chain. Example: For 2-methylbutane, the prefix is "2-methyl". Suffix represents the functional groups in the molecule. Example: For 2-methylbutane, the suffix is "ane". Infix is present between prefix and suffix and only present in cyclo compounds. Example: Cyclo, bicyclo, tricyclo... etc....

Cycloalkanes: parent compound

The alkyl or cyclo- group with the greatest number of carbons is the parent compound. The smaller unit is generally treated as a substituent to the larger unit. ·Propylcyclopentane (not cyclopentylpropane) ·Cyclohexyloctane (not octylcyclohexane) The prefixes di-, tri, tetra- are not considered in alphabetizing the substituents. 2-Ethyl-1,1-dimethylcyclobutane

Cyclobutane is puckered with a bending angle of about 26o

The bent cyclobutane molecules flips rapidly from one puckered conformation to another. The puckered conformation partially relieves the strain caused by the otherwise eight eclipsing hydrogens. The C-C bond strength is about 63 kcal mol-1. Cyclobutane also undergoes ring opening but is less reactive than cyclopropane.

boat conformation

The boat-shaped conformation of cyclohexane that has no angle strain but does have some steric strain and some torsional strain.

bridgehead carbons

The carbon atoms shared by two or more rings. Three chains of carbon atoms (bridges) connect the bridgeheads.

chair conformation

The chair-shaped conformation of cyclohexane that has no angle strain and has no torsional strain because it is perfectly staggered about all the C-C bonds. It is strain free. Most Stable Chair conformation: H's are staggered; 109.5o bonding

Different Types of Strain

There are many different types of strain that occur with cycloalkanes. In addition to ring strain, there is also transannular strain, eclipsing, or torsional strain and bond angle strain. Transannular strain exists when there is steric repulsion between atoms. Eclipsing (torsional) strain exists when a cycloalkane is unable to adopt a staggered conformation around a C-C bond, and bond angle strain is the energy needed to distort the tetrahedral carbons enough to close the ring. The presence of angle strain in a molecule indicates that there are bond angles in that particular molecule that deviate from the ideal bond angles required (i.e., that molecule has conformers).

Eclipsing strain

Torsional strain or eclipsing strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom. eg: Consider two conformations of ethane: The smallest dihedral angle is 60º in 1; it is 0º in 2.

Numbering Cycloalkanes

Use the lowest possible numbering sequence for polysubstituted cycloalkanes. If two identical numbering sequences are possible, use alphabetical precedence (put all substituents in alphabetical order).

substituted cyclohexanes

When the cyclohexane ring bears a substituent, the two chair conformers are not the same. ... In examining possible structures for substituted cyclohexanes, it is useful to follow two principles. Chair conformations are generally more stable than other possibilities. Understand the axial and equatorial positions of substituents.

twist boat (or skew-boat) conformation

a nonplanar conformation of a cyclohexane ring that is twisted from and slightly more stable than a boat conformation

London forces Cycloalkanes

alkanes are subject to intermolecular forces called London dispersion forces. These weak intermolecular interactions result in relatively low melting and boiling points. Like alkanes, cycloalkanes are not particularly reactive, with the exception of the smallest, most strained cycloalkanes

Cycloalkanes

alkanes in which the carbon atoms are arranged in a ring, or cyclic, structure

cis isomer

an isomer of an alkene in which the hydrogen atoms in the double bond are on the same side

types of strain

angular strain, torsional strain, steric strain, transannular strain

Norborane is an example of a ______________

bridged bicyclic ring system Two non-adjacent carbon atoms belong to both rings and are called bridgehead carbon atoms.

conformers (conformational isomers)

different spatial arrangements of the atoms that result from rotation about a single bond

Names of cycloalkanes

follow IUPAC rules To name a cycloalkane- prefix its alkane name with cyclo- The cardinal sin The pentavalent carbon Doomed to fail O-chem -UCLA student Numbering a cycloalkane ring is necessary only when there is more than one substituent.

Decalin is an example of a ______________

fused bicyclic ring system.

The shared carbon atoms are called ___________

ring-fusion carbons

Groups attached to the ring-fusion carbons are called __________________

ring-fusion substituents.

transannular strain

strain resulting from steric crowding of two groups across a ring The chair conformation drawing is more favored than the boat because of the energy, the steric hindrance, and a new strain called the transannular strain. ... This is known as the transannular strain, which means that the strain results from steric crowding of two groups across a ring.

trans isomer

substituents other than hydrogen on opposite side of double bond

steric strain

the interference between two bulky groups that are so close together that their electron clouds experience a repulsion

1,3-diaxial interactions

the strong steric strain between two axial groups on cyclohexane carbons with one carbon between them

The boat form of cyclohexane is flexible & actually represents a transition state between two slightly more stable

twist-boat (or skew-boat) conformations