Isomers and Stereochemistry

Question 1

Isomers are different compounds with the same molecular formula

Answer 1

They have the same number of the same atoms and have different connectivity of those atoms.

Question 2

Constitutional/structural isomers

Answer 2

Molecules which have the same molecular formula but different atom-atom connectivity

Question 3

Stereoisomers

Answer 3

Molecules with the same atom-atom bonds but differ in their orientation. Different orientations are known as configurations

Question 4

Conformational isomers

Answer 4

Stereoisomers in which diff isomers are produced by rotation of a single bond (all non-cyclic bonds are free to rotate)

Question 5

Types of stereoisomers

Answer 5

• Enantiomers
  
  • contain a centre of chirality
• Diastereomers
  
  • geometric isomers
  • rotamers

Question 6

Chirality

Answer 6

An inability to superimpose a molecule on its mirror image by an combination of rotations and translations.

Question 7

An enantiomer will only have one other enantiomer- never more, never less.

Answer 7

A molecule with 1 chiral centre exists as 2 stereoisomers. Enantiomers have identical physical and chemical properties in an achiral environment.
Enantiomers are very difficult to separate from each other as they have the same physical and chemical properties.

Question 8

Enantiomer nomenclature

Answer 8

• When naming a chiral compound, the config of the chiral centre is indicated at the beginning of the name, italicised and surrounded by brackets (R)-2-butanol
• When multiple chirality centres are present each config must be preceded by a locant to indicate the config at that centre (2R,3S)-3-Methyl-2-pentanol

Question 9

A solution or sample containing only one enantiomer of a chiral is said to be optically or enantiomerically pure.

Answer 9

A solution or sample containing equal amounts of both enantiomers is known as a racemic or a racemate.

Question 10

Achiral

Answer 10

A molecule with no chiral centres.

Question 11

A molecule with 2 chiral centres can exist as 4 stereoisomers. 2 pairs of enantiomers

Answer 11

Each stereoisomer has 1 enantiomer and 2 diastereomers. Diastereomer molecules are non mirror images and are non superimposable. They exhibit different physical properties.

Question 12

Multiple chiral centres

Answer 12

As the number of stereoisomers increase, the maximum number of stereoisomers also increases by 2^n. N= number of chiral carbons

Question 13

Meso compounds are often observed with cyclic structures. What are the requirements of a meso compound?

Answer 13

To be a meso compound a compound must:

• contain at least 2 chiral centres
• possess centres with the same substituents
• have an internal mirror plane which bisects the molecule in half (1 half should reflect the other half perfectly) -chiral centres will have opp config

Question 14

Geometric isomers

Answer 14

Another set of stereoisomers which are diastereomers can be found in substituted alkenes (compounds that contain a C=C)

Question 15

Cis/trans isomerism in geometric isomers

Answer 15

• Cis is used to indicate an isomer where 2 identical groups are pointing in the same direction.
• Trans is used to indicate an isomer where 2 identical groups are pointing in opposite directions.

Question 16

In order for cis/trans isomerism to exist, we need 2 conditions:

Answer 16

• 2 adjacent carbons of a ring or double bond bearing non-identical groups on either carbon
• Each carbon has at least one substituent identical to each other

Question 17

Stability in geometric isomers

Answer 17

-In trans isomers the large substituents are facing away  
 from each other
  -more stable
  -less reactive 
-In cis isomers the large substituents are on the same 
 side of the double bond 
  -less stable
  -places bond strain on molecule 
  -more reactive

Question 18

E/Z

Answer 18

If priority groups are on the opposite sides of the double bond- E isomer
If priority groups are on the same side of the double bond- Z isomer