Topic 4 -Conformation Analysis and selective additions to carbonyls

Question 1

What are conformational isomers and how do they differ?

Answer 1

Differ by their rotation around single bonds.
same compound but sat differently in 3d shape

Question 2

what are conformational isomers also known as?

Answer 2

Conformers

Question 3

What is the behaviour of two conformers at room temperature?

Answer 3

They rapidly interconvert via rotation around single bonds.

Question 4

What are the two exampls of conformers that we associate with newman projections?

Answer 4

Staggered and eclipsed

Question 5

What are the two conformers that we associate with 6 membered rings?

Answer 5

Chair and Boat

Question 6

What is antiperiplanar?

Answer 6

Where the dihedral angle between two molecules is between 150-180 degrees.
In a newman projection this will be up and down (directly across the circle)

Question 7

In a newman projection how can we tell which bits are antiperiplanar?

Answer 7

In a newman projection this will be up and down (directly across the circle)

Question 8

What is the lowest energy conformation of a newman projection?

Answer 8

staggered

Question 9

In chair conformations, generally are equatorial or axial positions prefered?

Answer 9

equatorial

Question 10

If there is a tBu substituent on a ring, will it go in the axial or equatorial position in the chair conformation?

Answer 10

Equatorial.
tBu must be equatorial. This will overrule other interactions

Question 11

When answering a question involving a chair conforation we must remember what?

Answer 11

that there are 2 chair conformations and they undergo ring flip. We need to draw both to establish which is the lower energy one.

Question 12

Is the lowest energy conformation always the reactive one?

Answer 12

No, it is possible for some of the lowest energy conformations to not have the right requirements for certain reactions. Therefore it may have to undergo a change such as a ring flip to a higher energy conformation to meet these requirements and thus react.

Question 13

what is a regioselective reaction?

Answer 13

the preference of making or breaking bonds in one direction all other possible directions.

Question 14

what is a chemoselective reaction?

Answer 14

a reaction that operates exclusively on one functional group in the presence of other functional groups.

Question 15

What is a stereospecific reaction?

Answer 15

the stereochemistry of the product is dependent on the stereochemistry of the starting material (stereochemistry of the reactant completely determines the stereochemistry of the product).

Question 16

What is a stereoselective reaction?

Answer 16

a reaction in which multiple stereoisomers can be formed however one product is favoured due to its reaction pathway being more favourable than the others. Can also be a reaction where only one (or some) stereoisomer(s) among a mixture reacts.

Question 17

Shape of orbitals defines what?

Answer 17

Angles of attack of nucleophiles and stereoelectronic effects

Question 18

For E2 elimination to occur, the H and the leaving group must be what??

Answer 18

Antiperiplanar (dihedral angle of 150-180)

Question 19

For E2 elimination to occur, the H and the leaving group must be antiperiplanar, why?

Answer 19

To allow sufficient orbital overlap of the sigma (C-H) and the sigma star (C-LG) for the elimination to take place.

Question 20

In an SN2 reaction, the nucleophile attcaks from what direction and why?

Answer 20

From 180 degrees to the leaving group to overlap with the sigma star C-LG orbital.

Question 21

Nucleophillic attack of carbonyls.
Nucelophiles attack crabonyls from what angle?

Answer 21

The Burgi-Dunitz angle (107degrees)

Question 22

Nucleophillic attack of carbonyls.
Nucelophiles attack carbonyls from what angle?
Why?

Answer 22

The Burgi-Dunitz angle (107degrees)
to overlap with the pi star orbital of the C=O bond.