10/14-21, 203J

Front 1

Types of Ring Strain

Back 1

Bond length strain E(r)

Bond angle strain E(θ)

Torsional strain E(φ)- (repulsion from electrons and substituents on adjecent atoms (eclipsed, staggered etc))

Non-bonded interactions E(d) - if atoms or groups not directly bonded together are pushed close together (less than sum of Van der Waals radii)

Front 2

How can molecular strain be measured?

Back 2

examining trends in heats of combustion (ΔHcomb) for a homologous series

Front 3

Which is more costly in energy bond length or bond angle?

Back 3

Bond angle deformation is less costly than changes in bond length

Front 4

Conformational isomer?

Back 4

different shapes of molecules resulting from the deformation of bonds (almost always rotation about single bonds) (no bonds are broken when converting between conformers)

Front 5

Configurational isomer?

Back 5

Different connectivity of atoms within stereoisomers

Front 6

Draw cyclohexane chair conformation now

Back 6

Front 7

Total Strain in Cycloalkanes (graph)

Back 7

Front 8

why is cyclopropane highly strained?

Back 8

High bond angle stain + eclipsed C-H bonds(torsional strain)

Front 9

Cyclobutane strain (comments)

Back 9

bond angle starin is high, but distortion away from planarity means C-H bonds don't have to be eclipsed

Front 10

Cyclopentane strain(comments)

Back 10

although closest bond angle to 109.5, planar configuration results in eclipsed CH2 bonds therefore forms envelope or twist conformations (envelope is lower in energy)

Front 11

Cycloheptane strain (comments)

Back 11

Rings with > 6 atoms tend to be inherently more flexible
4 low-energy conformations

Front 12

Medium sized ring strain C8-C11 (comments)

Back 12

all significantly less stable than cyclohexane because of bond angle strain, torsional strain and transannular clashing e.g. cyclodecane -all internal H-atoms are 1.8A apart, less than sum of their Van der Waals radii (which is about 2.2A)

Front 13

Large sized ring strain >11

Back 13

Called macrocycles, Transannular strain is diminished can adopt any number of conformers to become staggered and achieve optimum bond angle.

Front 14

What two distinct locations for substituents are there in cyclohexane chair conformation?

Back 14

Axial and Equatorial positions

Front 15

Covalent bond homolysis (rough energies)

Back 15

200-460KJmol⁻¹

Front 16

Hydrogen bond strength (rough energies)

Back 16

4-125 KJmol⁻¹

Front 17

Cyclohexane chair flip barrier (rough energies)

Back 17

43 KJmol⁻¹

Front 18

Butane rotational barrier (rough energies)

Back 18

6 KJmol⁻¹

Front 19

NMR of cyclohexane signals C₆D₁₁H₁

Back 19

Front 20

What conformers of monosubstituted cyclohexane where X≠H have the lowest energy?

Back 20

Conformers with the largest substituent in an equatorial position are the generally the lowest energy

Front 21

How do we quantify conformational equilibrium?

Back 21

Front 22

What is an 'A value'?

Back 22

Energy difference between two conformers

Front 23

What do the A values (and equlibrium constants) depend on?

Back 23

Front 24

What is a conformational lock?

Back 24

where it is energetically costly to occupy a particular conformer therefore doesn't really happen

Front 25

What position does a tBu group prefere to take on a cyclohexane ring?

Back 25

equitorial, as axial positions are very energetically costly

Front 26

What is an E1 reaction?

Back 26

Front 27

What is an E2 reaction?

Back 27

Front 28

What is an E1cB reaction?

Back 28

Front 29

What nucleophiles favour elimination over substitution?

Back 29

Front 30

What geometry intermediate does E1 form?

Back 30

Trigonal planar carbocation

Front 31

What happens if the geometry of a molecule prevents planarization of C-center in E1 reactions?

Back 31

E1 is not possible

Front 32

What do you need for a E1cB mechanisms to occur?

Back 32

a poor leaving group -OH or -OR and an acidic hydrogen (hydrogen on carbon atom adjacent to a carbonyl carbon atom or a H on a OH)

Front 33

What conformations gives good orbital overlap for E2 elimination?

Back 33

Front 34

E2 elimination from cyclohexanes information (possibly break up into multiple questions in future)

Back 34

Front 35

What products are most favoureud in elimination reactions under most circumstances?

Back 35

Most subsituted double bond products

Front 36

1-chloro-1-methylcycloHexane, where will the double bond form on E2 elimination witha sterically hindered base?

Back 36

Front 37

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Front 38

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Front 39

structure of cyclohexanone?

Back 39

Front 40

What isomer of cyclohexene is stable?

Back 40

cis as trans would be too strained

Front 41

what positions do hydrogens in cyclohexene adopt?

Back 41

Front 42

how many atoms lie in a plane in cyclohexene?

Back 42

4

Front 43

How do we know if a substituent on a di substituted cyclohexane is axial or equatorial?

Back 43

We need to draw out the chair diagram, to give us a 3d representation from which we can then start to determine what the ideal position is

Front 44

How can we predict lowest energy conformation?

Back 44

In general, the conformation with fewer axial substituents is lower in energy.

If there are equal numbers of axial and equatorial substituents in two conformations, refer to the A values

Front 45

Anomalies to usual conformations in cyclohexane?

Back 45

Front 46

What are spiro compounds?

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Front 47

What are decalins?

Back 47

Front 48

Difference between cis and trans decalins?

Back 48

Trans-decalins are conformationally locked, while cis-decalins are able to undergo ring flipping.

Front 49

what is the base structure of all steroids?

Back 49