Chapter 4 - Isomerism 1: Conformers and Constitutional Isomers

cyclohexane

this cyclic alkane is the standard in reference to heat of combustion because it has no ring strain - followed closely by cyclopentane, but it does have a little bit of ring strain

isomers

two molecules are _______ of each other if they have the same molecular formula, but are different in some way - their atoms can be connected in different ways, they can differ as a result of rotations about single bonds, or they can be mirror images of each other

enantiomers

(configurational isomers) stereoisomers that are mirror images of each other

diastereomers

(configurational isomers) stereoisomers that are not mirror images

disubstituted cyclohexanes, cis and trans isomers, and haworth projections

- a chair flip does not switch a substituent from one side of the plane to the other - the cis-trans relationships between any pair of substituents on a cyclohexane ring is independent of the particular chair conformation the species is in - substituents that are cis to each other on a cyclohexane ring remain cis after chair flip; substituents that are trans remain trans - since the cis-trans relationship of any pair of substituents is unaffected by chair flip, it is often more convenient to represent substituted cyclohexanes using haworth projections

conformational isomers

- also called "conformers" - most similar - differ only by rotations about single bonds - same connectivity - type of stereoisomers

longer molecules and the zig-zag conformation

- for longer molecules, such as hexane, there are several single bonds about which rotation can occur. - this gives rise to many possible conformations - each of these bonds can exist in either a gauche or an anti conformation - the most stable conformation of hexane is the all-anti conformation, also called the zig-zag formation

cyclohexane

- has no ring strain - is not a planar molecule - its lowest energy conformation resembles a chair and is therefore called chair conformation

equivalent

within the different conformations of cyclohexane, all of the carbons are __________; despite all rotations, they are all in the same situation realistically, even if the structure LOOKS like they have different connectivity

newman projection

A method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis. rules: - the two atoms directly connected by the bond of interest are shown explicitly -- the nearer atom is depicted as a point -- the more distance atom is depicted as a circle - bonds to the front atom converge at the point, whereas bonds to the back atom connect to the circle

stereoisomers

Compounds with the same structural formula but with a different arrangement of the atoms in space.

configurational isomers

can only be interchanged by breaking and reforming bonds

saturated

a molecule is said to be _________ if it has the maximum number of hydrogen atoms possible, consistent with: - the number and type of each non-hydrogen atom in the molecule - the octet rule and the duet rule

index of hydrogen deficiency (IHD)

aka degree of unsaturation aka HDI, all the same - half the number of hydrogen atoms missing from that molecule compared to an analogous, completely saturated molecule - each double bond contributes 1 to a molecule's IHD - each triple bond contributes 2 to a molecule's IHD - each ring contributes 1 to a molecule's IHD

constitutional isomers

aka structural isomers - share the same molecular formula but differ in their connectivity - formula is pretty much their only similarity - ex: cyclobutane and but-1-ene are constitutional isomers because they have different connectivity

hydrogen

all of the carbons in the cyclohexane are equivalent, but what we must still consider is the ________ atoms that are bonded to those carbons. - six of these occupy equatorial positions and six occupy axial positions - each carbon is bonded to one equatorial and one axial

steric strain

an increase in energy that results from electron repulsion between atoms or groups of atoms that are not bonded together - if one H atom on each CH3 group in ethane is replaced by another substituent, the energies of the three staggered conformations are no longer the same, because one of those substituents now takes up more space

cyclic alkanes

any difference in heat of combustion per CH2 group for two ______ _______ reflects a difference in ring strain per CH2 group - total ring strain is obtained by multiplying the strain per CH2 group by the number of CH2 groups, n

staggered

at room temperature, _________ conformations constantly interconvert through rotation about the C-C single bond

ring strain

chemists attribute much of the instability of other (not 5 or 6) ring sizes to ____ ______, the increase in energy due to geometric constraints on the ring - quantified using heat of combustion - can occur when bond angles much smaller or larger than normal/preferred are forced upon the atoms in the ring

connectivity

conformational isomers have the same ____________, meaning that the same atoms in both species are bonded together by the same types of bonds

chair flipping

converts axial hydrogens into equatorial hydrogens and vice versa through pseudorotations - even though it interconverts axial and equatorial positions on a cyclohexane ring, it does not allow substituents to switch sides of the ring's plane

envelope conformation

cyclopentane is relatively stable in the ________ ____________ - four of its five carbon atoms lie in one plane, with the fifth carbon outside of that plane - bond angles range from 102° to 106° in this conformation (it has some angle strain) - strain is minimized by adopting a nonplanar configuration (if it stays planar, it would have an eclipsed conformation, which is less stable and has lots of torsional strain and steric strain)

rotational conformations

describe molecules having different angles of rotation about single bonds - newman projections offer a convenient way to illustrate these

constitutional isomers

differ in the order of attachment of atoms, do not have the same connectivity

dihedral angle

each angle of rotation defines a particular ________ _____ (Θ) corresponding to the angle between the C-X and C-Y bonds in a Newman projection - you don't really need to remember the sign (+/-) but you should remember the magnitude

monosubstituted

if one of the hydrogen atoms in cyclohexane is replaced by another substituent, such as CH3, the result is considered a _______________ cyclohexane - the two chair conformations of this type of cyclohexane are not equivalent - two nonequivalent chair forms are conformational isomers of one another - equatorial position of substituent is preferable because less strain

gauche conformation

least favorable staggered conformation; large substituents are 60° from one another and thus have to deal with steric strain because they are being forced to occupy space that overlaps

unsaturated

molecules that contain double bonds, triple bonds, or rings are said to be ___________ because they have fewer than the maximum number of hydrogen atoms possible

structure

molecules with the same formula do NOT have to have the same _________

anti conformation

most favorable of staggered conformations; large substituents are 180° from one another and do not undergo steric strain

cyclobutane

most stable conformation is slightly puckered, with interior angles of about 88° - puckering of the ring relieves some torsional strain - it's literally a square in a planar conformation, but it staggers one out of the plane to reduce strain

chair conformation

most stable conformation of cyclohexane, named after what it resembles - 111° bond angles, essentially no angle strain on the carbons - totally staggered newman projection conformation, essentially no torsional strain

60

newman projections usually rotate with dihedral angles in units of ___ degrees

pseudorotation

occurs by partial rotations about the C-C bonds - all five possible envelope conformations of cyclopentane interconvent via this method - in a practical sense though, these happen so rapidly that they interconvert almost constantly, so it's nearly impossible to really distinguish which carbon is out of the plane

multiple

rather than occurring in a single step, a cyclohexane chair flip involves ________ independent steps - throughout such a chair flip, cyclohexane assumes conformations known as the "half-chair," the "twist-boat," and the "boat."

heats of combustion

ring strain can be quantified using this measure - the energy given off in the form of heat during a combustion reaction - shown as ΔH°C

cyclic alkanes and ring strain

ring structures can also have conformations - the most common rings found in nature are five- and six-membered rings (more stable cyclics)

more stable

staggered conformations are ____ ______ than eclipsed conformations, because there is reduced steric strain and torsional strain, and electron densities overlapping one another because the atoms are in the same plane and thus shoved closer together than they prefer

axial

substituents bonded to a carbon in the carbon ring and do not occupy the same plane as the carbons are considered in __________ position

equatorial

substituents bonded to a carbon in the carbon ring and occupy the same plane are considered in __________ position

torsional strain

the difference in energy between a staggered conformation and an eclipsed conformation of H3C-CH3 is a result of _________ ______: - an increase in energy (i.e., decrease in stability) that arises in an eclipsed conformation

conformational analysis

the investigation of molecular conformations and their relative energies (specifically in reference to newman projections, in this case)

Haworth projections

the ring is depicted as being planar and substituents are drawn perpendicular to that plane - every substituent on a cyclohexane ring favors an equatorial position rather than an axial position - the most stable chair conformation can usually be predicted for a number of disubstituted cyclohexanes

cyclopropane

there is no alternative to having all three carbon atoms in the same plane, because three points define a plane - all three angles of the ring are exactly 60° and all three C-C bonds are the same length, so the ring forms an equilateral triangle - has a LOT of strain because the 60° angles are significantly less than the 109.5° ideal angle they would prefer

staggered conformations

these conformations occur when C-H (or C-R) bonds on the front carbon bisect those on the rear carbon atom (do not cover one another)

eclipsed conformations

these conformations occur when the C-H (or C-R) bonds on the front carbon cover those on the rear carbon atom in the Newman projections - most energetic of conformation

1,3-diaxial

these interactions are a form of steric strain - no such steric strain exists when the non-H substituent group is in the equatorial position - a monosubstituted cyclohexane is more stable when the substituent is found in an equatorial position - bulky groups favor the equatorial position on a cyclohexane ring all of this is kinda redundant but basically equatorial = more space away from other atoms = less steric strain