Week 4 Textbook Reading

Question 1

The single bond between 2 carbon atoms in a molecule is the result of …

Answer 1

overlap between an sp^3 hybrid orbital on each atom

Question 2

Because the sp^3 orbitals are symmetric around the internuclear axis, the rotation of one methyl group around the C-C sigma bond …

Answer 2

does not change the overlap of these orbitals
So the sigma bond remains intact

Question 3

conformations

Answer 3

The different spatial arrangements produced by rotation around single bonds are called conformations

Question 4

conformers/rotamers

Answer 4

The term conformers or rotamers refer to the different shapes a molecule can adopt by rotations around its single bonds

Question 5

newman projection

Answer 5

The conformation around a single bond can be clearly represented by a Newman projection
These projections show the arrangement of the atoms as they would appear when viewed from one end of the axis through the single bond

Question 6

torsion angle/dihedral angle

Answer 6

Rotations around a particular bond is described by the torsion angle or dihedral angle, which’s the angle between one plane defined by the single bond and a substituent on one of the atoms in the bond and a second plane defined by the single bond and a substituent on the other atom of the bond

Question 7

eclipsed conformation

Answer 7

Eclipsed conformation is when the substituents on the front atom aligned with those on the back atom
The 3 bonds of the front atom are close enough to the 3 bonds at the back that the electron pairs of these aligned bonds repel each other
This repulsion makes the eclipsed conformation of ethane the least stable → that’s the highest energy conformation

Question 8

torsional strain

Answer 8

The repulsion generates torsional strain which’s a force that drives rotations away from eclipsed conformations
Rotation around the C-C bond of ethane in an eclipsed conformation moves the front and rear atoms further apart, reducing the torsional strain

Question 9

in which conformation is torsional strain at a minimum

Answer 9

This torsional strain is at a minimum in a staggered conformation, where the 3 bonds of the atoms at the front are positioned midway among the 3 bonds at the back, making a torsion angle of 60 deg

Question 10

which conformation causes the torsional strain to increase

Answer 10

Rotation around the C-C bond to a torsion angle of 120deg raises the energy of the molecule because torsional strain increases as the molecules moves into another eclipsed conformation

Question 11

conformational analysis

Answer 11

The exploration of the geometries and resulting energies of molecular conformations is called conformational analysis

Question 12

how do molecules rotate from one energy minimum to another

Answer 12

Torsional strain creates energy barriers, which molecules must overcome in order to rotate from one energy minimum to another

Question 13

steric strain

Answer 13

This occurs when atoms that are not directly bonded to one another are close enough that their electron clouds interpenetrate
The resulting repulsion forces the atoms away from each other if they are free to move
Often, the bonds to the atoms limit their movement, so the repulsive force strains these bonds
This steric strain raises the energy level of the conformation, making it less stable

Question 14

antiperiplanar conformation or anti-conformation

Answer 14

The staggered conformation that has a Cl–C–C–Cl torsion angle of 180° is called the antiperiplanar conformation or anti-conformation
Since the substituent bonds are staggered, the anti-conformation has no torsional strain
The steric strain in this conformation is also negligible because the separation between each chlorine atom and the other substituents is large enough to effectively eliminate repulsion between their electron clouds
Consequently, the anti-conformation is the lowest energy conformation of 1,2-dichloroethane.

Question 15

gauche conformation

Answer 15

The gauche conformation of a molecule CH2X-CH2Y have a torsion angle of either 60 deg or 300 deg between substituents X and Y

Question 16

what causes the strain to increase

Answer 16

Larger atoms and groups increase the amount of strain

Question 17

angle strain

Answer 17

Angle strain arises when the atoms of a ring have bond angles that deviate from the normal bond an

Question 18

strains in cyclic molecules

Answer 18

Because the carbon atoms of propane are sp3 hybridized, they achieve maximum atomic orbital overlap with their neighbors at tetrahedral bond angles of about 109.5°
Since the triangular geometry of cyclopropane requires C–C–C bond angles of 60°, maximum overlap of sp3 orbitals cannot occur
Instead, each carbon atom has “bent” bonds with reduced orbital overlap to the two adjacent carbon atoms

-3 membered rings also possess torsional strain

Question 19

four membered rings

Answer 19

Rings with 4 or more single-bonded atoms have non-planar conformations
A limited amount of rotation is possible in a four-membered ring, so the angle and torsional strain pushes cyclobutane into a non-planar butterfly conformation
The butterfly conformation is an energy minimum because the decrease in torsion strain is greater than the slight increase in angle strain

Question 20

ring inversion

Answer 20

Ring inversion is the conversion of a cyclic molecule from one conformation to another by rotation around the single bonds of the ring

Question 21

conformations of five membered rings

Answer 21

For five-membered rings, the planar conformation is a regular pentagon
Little angle strain exists because the internal angle of a regular pentagon is 108 deg, very close to the 109.5 deg angle required for maximum orbital overlap
The planar structure has significant torsional strain because all the C-H bonds are eclipsed
The atoms in five-membered rings can rotate farther around the single bonds than the atoms in four-membered rings, so five-membered rings have conformations that reduce the torsional strain by a greater amount

Question 22

most stable conformation for cyclopentane

Answer 22

For cyclopentane, the most stable conformation is the envelope conformation, which has one carbon atom of the ring out of the plane of the other four ring atoms, giving a structure that resembles an envelope with its flap open
Though some torsional strain still exists in the planar part of the envelope conformation, the almost staggered arrangement of C–H bonds around the carbon atom of the flap reduces torsional strain

Question 23

conformations of six-membered rings

Answer 23

Deviate further from planarity to form a conformation that’s virtually free of torsional and angle strain
In chair conformation, the ring atoms are displaced alternately above and below the equatorial plane of the molecule
Note the equatorial plane of a ring compound is the same as the plane of the planar conformation

-no torsional strain in chair conformation

Question 24

axial hydrogens

Answer 24

These axial hydrogens are displaced alternately above and below the equatorial plane

Question 25

equatorial hydrogens

Answer 25

The remaining six hydrogens (colored red) are termed equatorial because their positions lie close to the equatorial plane of the ring

Question 26

drawing substituents

Answer 26

Axial substituents should be vertical Each equatorial substituent is parallel to a ring bond Each carbon has one axial and one equatorial position Two bonds should have gaps to show that they lie behind other bonds

Question 27

Six-membered rings flip their chairs

Answer 27

The six carbon-carbon bonds of cyclohexane are constantly rotating, causing the structure to wiggle The molecules spend most of their time in a chair conformation or in conformations close to it Occasionally, the rotations are large enough to flip the chair into other conformations, including a different chair A ring inversion from one chair to another also flips each axial substituent to an equatorial position and each equatorial substituent to an axial position

Question 28

half-chair conformation

Answer 28

The half-chair conformation sits at an energy maximum

Question 29

twist-boat conformation

Answer 29

The twist-boat conformation is an energy minimum attained as the bonds in the half-chair continue to rotate

Question 30

The rate at which ring inversion occurs depends on...

Answer 30

how frequently stable chair conformations can overcome the barrier to inversion by attaining the high-energy conformation of the half-chairs

Question 31

Monosubstituted six-membered rings

Answer 31

A six-membered ring bearing one substituent, such as methylcyclohexane, exists as a mixture of two rapidly interconverting chair conformations The two chair conformations of cyclohexane have the same geometry and, therefore, the same energy, but those of methylcyclohexane do not In one chair conformation of methylcyclohexane, the methyl group is positioned equatorially; in the other chair conformation, the methyl group has an axial position

Question 32

The conformation with an equatorial methyl group has almost _____ steric strain as the methyl group is well separated from the hydrogen atoms of the ring

Answer 32

no

Question 33

1,3-dixaxial interactions

Answer 33

1,3-diaxial interactions are steric strains that arise from interpenetration of the electron clouds of ring substituents separated by 2 carbon atoms Note that 1,3-diaxial interactions between two hydrogens are negligible because the electron clouds of the hydrogens are so small The steric strain from 1,3-diaxial interactions depends on the size and nature of the ring substituents Greater steric strain increases the ratio of the equatorial conformation to the axial conformation

Question 34

the conformation with an axial methyl group has...

Answer 34

However, the conformation with an axial methyl group has significant steric strain, as the methyl group is close to the two axial hydrogens on the same face of the ring

Question 35

Disubstituted six-membered rings

Answer 35

1,4-Dimethycyclohexane has two isomers that differ in the arrangement of the methyl groups on the ring The cis isomer has both methyl groups bonded to the same face of the ring, while the trans isomer has the two methyl groups bonded to opposite faces of the ring In the top view of the cis isomer in the following diagram, both methyl groups project toward the viewer One of these methyl groups is oriented equatorially, and the other is oriented axially

Question 36

when can conformations have equal energies

Answer 36

Ring inversion of a chair conformation of the cis isomer moves the equatorial methyl group to an axial position and the axial methyl group to an equatorial position So, both chair conformations of cis-1,4-dimethylcyclohexane have one equatorial methyl group and one axial methyl group The conformations have identical 1,3-diaxial interactions and, therefore, have equal energies