Conformation of saturated rings

what is ring strain energy

extra energy a molecule has by virtue of being constrained to a ring, determined from heats of combustion

what are small rings

3-4 members, ~25 kcal/mol, e.g cyclopropane, aziridine

what are normal rings

5-7 membered, ~5 kcal/mol, include piperidine and cyclopentane

what are medium rings

8-14 membered, 5-15 kcal/mol include cyclooctane and cyclononane

what are large rings

>14 membered, very low energy due to being almost linear, include cyclohexadecane

what are contributions to conformational energy unique to rings

baeyer (angle) strain and transannular strain (medium rings only)

what is baeyer (angle) strain

atoms are distorted from their ideal bond angle in order to form a ring

what is transannular strain

only effects medium rings (8-14 membered), substituents point at each other across the ring leading to additional steric strain

describe the conformation of cyclopropane

all Cs in one plane, all C-C bonds are eclipsed so has high pitzer strain, much smaller bond angles than ideal so high baeyer strain, results in curvy weak C-C bonds with high P charecter

describe the conformation of cyclobutane

they pucker (decreasing bond angles) increasing baeyer strain, and make sure no bonds are fully eclipsed, reducing (but still high) pitzer strain, forming butterfly conformation

what is this conformation

butterfly conformation, found in cyclobutane

describe the conformation in cyclopentane

forms an envelope conformation, has some pitzer strain and some baeyer strain

what is this conformation?

envelope conformation found in cyclopentane

describe conformation of cyclohexane

forms chair conformation, all bonds are staggered and essentially stain free, staggered conformation

what are the cyclohexane conformations with the highest energy to lowest

half chair, then boat, then twist boat, then chair

what cyclohexane conformation is this?

boat

what cyclohexane conformation is this?

half chair

what cyclohexane conformation is this?

twist boat

what cyclohexane conformation is this?

chair

what position is preferred for substituents on mono-substituted cyclohexane, why?

equatorial positions, this avoids 1,3-diaxial interactions

what is this showing

1,3 diaxial interactions on mono-substituted cyclohexanes

what is A-value showing

its a parameter to define size of substitutients, the E differance in kcal/mol bet

how do you find A-value

change in E (between highest and lowest conformation energy)

what is a locking group

a substituent on cyclohexane that locks it in one conformation, is a trimethyl group

when are di-substituted rings more stable?

when conformer has the largest group (measured by A value) in equatorial position

what are the 2 locking groups?

tB (tributyl methyl), and trans decalin

what is transdecalin

only conformer that exists is with central hydrogen in axial positions

what is the J value for Hax,Hax?

10

what is the J value for Hax,Heq?

2-5

what is the J value for Heq,Hax?

2-5

what is the J value for Heq,Heq?

2-5

what is tetrahydropyran?

6 membered heterocyclic ring containing one oxygen, derived from pyran by saturation of double bond

what is the anomeric effect?

electronegative group at the anomric position, despite destabilising 1,3 diaxial interactions due to strong hyper-conjugation

what molecules is anomeric effect observed on?

molecules with OR, Cl, Br or F at anomeric position