CHEM 257-chapter 4: organic compounds/cycloalkenes and their steriochemistry

cycloalkanes

saturated cyclic hydrocarbons with carbon atoms joined in a ring, general formula is CnH2n

cycloalkane nomenclature rules

1) find the parent (longest chain, not always the ring)

2) number the substituents (attachment point is carbon 1, use direction that gives lowest numbers) tiebreaker: alphabetize

3) write out full name per usual

locked conformation

rings are less flexible than open-chain alkanes, more rigid which means less bond rotation can take place

cycloalkanes have 2 faces and sides

"top" or "bottom" faces, wedged bonds represent a top face, dashed bonds represent a bottom face

"cis" and "trans" sides, cis is on the same side, trans is across

isomers

compounds with the same molecular formula but different structures and properties

constitutional isomers

compounds with the same molecular formula but different connections among their atoms

sterioisomers

compounds with the same connectivity between atoms but different atom arrangement/3-D orientation

cyclic molecules

rings of carbon that can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering

angle strain

the strain that arises when a bond angle is either compressed or expanded compared to its optimal value, non-ideal bond angles results when bond angles are forced to deviate from the ideal 109 tetrahedral value

torsional strain

results when cyclic molecules must assume conformations that have eclipsed interactions, eclipsing of bonds on neighboring atoms

steric strain

repulsion between neighboring nonbonded atoms, due to large size of non-hydrogen atoms that approach each other too closely

cyclopropane

most strained of all rings because of the 60 degree bond angles from the C-C-C-and the torsional strain because all C-H bonds are eclipsed, sigma bonds are bent (weakened) which causes less bond energy

cyclobutane

less angle strain than cyclopropane but more torsional strain because there are more hydrogens, almost same total level of strain as cyclopropane, slightly bent to relieve torsional strain, but that increases angle strain

cyclopentane

no angle strain but very high torsional strain because of the 10 eclipsing hydrogen bonds, nonplanar to reduce torsional strain which results in 4 carbons in a plane, fifth carbon is bent

cyclohexane

most common of cycloalkanes, ring is STRAIN FREE because it adopts a different conformation, tetrahedral angles between all carbons

chair conformation

most stable conformation of cyclohexane, every atom is 60 degrees apart, all C-C-C bond angles are near the 109-tetrahedral value, and all neighboring C-H bonds are staggered

non-ring bonds

either axial or equatorial, each carbon has 1 axial and 1 equatorial hydrogen, each face has 3 axial and 3 equatorial hydrogens which alternate

axial position

perpendicular to ring, every carbon is either pointing up or down

equatorial position

in plane of the ring, every carbon is either pointing left or right

ring flips

the ring turns, the positions of the substituents swap, "top face" groups stay top face and "bottom face" stay bottom face

equatorial conformation

most stable conformation, has less strain and lower in energy, maximize equatorial substitutes and put largest group in equatorial

axial conformation

have steric strain from 1,3-diaxial interactions