Knights content L1-5

Question 1

What is the name given to isomers which DIFFERENT connectivity

Answer 1

Constitutional isomers

Question 2

What is the name given to isomers with the same connectivity

Answer 2

Stereoisomers

Question 3

What is the name of isomers which interconvert by rotation

Answer 3

Conformational isomers

Question 4

What is the name of isomers which do not interconvert by rotation

Answer 4

Configurational isomers

Question 5

What two groups do stereoisomers divide into

Answer 5

Configurational and conformational Isomers

Question 6

What is the name give to isomers that do not interconvert by rotation but are not mirror images

Answer 6

Diasteroisomers

Question 7

What is the name given to isomers that do not interconvert by rotation but are mirror images

Answer 7

Enantiomers

Question 8

What are the two groups within Configurational isomers

Answer 8

Diasteroisomers and enantiomers

Question 9

When is a molecule chiral

Answer 9

A molecule is chiral if it has a non superimposable mirror image

Question 10

What are enantiomers

Answer 10

The two mirror image forms of a molecule

Question 11

What do enantiomers possess

Answer 11

Enantiomers have identical melting and boiling points, nmr spectra and IR spectra

Question 12

When would enantiomers not have identical physical and chemical properties

Answer 12

When placed in a chiral environment ie when interacting with other chiral, non racemic molecules

Question 13

Describe enantiomers in the presents of plane polarised light

Answer 13

When a beam of plane-polarized light is passed through a chiral compound the plane of the polarization is rotated through an angle. One enantiomer will rotate the plane clockwise and the other enantiomer will rotate the light anticlockwise by the same amount

Question 14

What does the size of angle rotated by the enatiomer in plane polarised light depend on

Answer 14

Path length, solvent concentration, temperature and wavelength

Question 15

What is the observed angle of rotation denoted by and what do we call this

Answer 15

Denoted by [a]
a is alpha
This is also known as as specific rotation

Question 16

What is the equation to calculate specific rotation and what do the letters stand for

Answer 16

[a] = a/c.l x100
a is observed rotation
C is concentration in g per 100ml
l is path length in decimetres

Question 17

What is the D/L nomenclature used for

Answer 17

The D/L nomenclature is nowadays only used for certain classes of
compound, such as carbohydrates and amino acids.

Question 18

How do we assign D or L to molecules

Answer 18

In order to assign a molecule as D or L, we simply need to compare the configuration to that of glyceraldehyde

Question 19

What configuration are all of the naturally occurring amino acids and which is the exception

Answer 19

All of the naturally occurring amino acids are L except glycine which is achiral

Question 20

Define enatiomerically pure

Answer 20

If a sample of a chiral compound consists of a single enantiomer it can be referred to as enatiomerically pure

Question 21

Define racemic

Answer 21

An equal mix of enantiomers is a racemic mixture

Question 22

Define scalemic

Answer 22

If the mixture of enatiomers is not 50:50 then it is called scalemic

Question 23

How to we report the enantiomeric purity of a compound

Answer 23

The enantiomeric purity of a compound reported by quoting the enantiomeric excess (ee)

Question 24

How do we calculate the enantiomeric excess

Answer 24

ee = (moles of one enantiomers) - (moles of another enantiomer) / total number of moles X 100

Question 25

Can enantiomeric excess be measure by optical rotation

Answer 25

The optical purity should equal the enantiomeric excess We calculate optical purity by the following equation [a]observed/[a]enantiopure x100

Question 26

List the methods for measuring the enantiomeric excess of a sample and what do these methods rely on

Answer 26

Optical rotation Chiral high performance liquid chromatography Chiral gas chromatography NMR All of these methods rely on placing the compound into a chiral environment

Question 27

What do diastereoisomer have

Answer 27

Diastereoisomers have different physical and chemical properties

Question 28

What is the maximum number of stereoisomers for a compound with n chiral centres

Answer 28

In general a compound with n chiral centres chan exist in a maximum of 2^n stereoisomers

Question 29

What are configurational isomers

Answer 29

configurations are stereoisomers which can only be interconverted by breaking one or more bonds

Question 30

Describe chiral centres in nitrogen molecules

Answer 30

simple amines racemise very rapidly by an umbrella inversion mechanism. This inversion is geometrically impossible in some bicycling structures and does not occur in ammonium salt so these nitrogen atoms can be chiral centres. Troger’s base and ammonium salt is chiral

Question 31

Describe chiral centres in phosphorus molecules

Answer 31

phosphorus atoms carrying 4 different substituents tetrahedrally are chiral centres E.g phosphine oxide, phosphonium salt, phosphine

Question 32

Describe how phosphines can be chiral

Answer 32

Phosphines can be chiral because pyramidal inversion which would convert enantiomers is very slow for phosphorus. This is because of a high orbital reorganisation energy needed to reach the planar transition state for pyramidal inversion.

Question 33

Give examples of chiral centres for sulfur molecules

Answer 33

sulfonium salt Sulfoxide Sulfoximine

Question 34

Describe how a sulfone can be chiral

Answer 34

a sulfone can be chiral if the oxygen atoms are different isotopes

Question 35

When does a meso form occur

Answer 35

Any compound containing two chiral centres with the same 4 ligands attached to each will have a meso form

Question 36

What is the E nomenclature for alkenes

Answer 36

higher priority groups on opposite sides of the alkene

Question 37

What is the Z nomenclature for alkenes

Answer 37

higher priority groups on the same sides of the alkene

Question 38

If an alkene only has two substituents what terms do we use to describe them

Answer 38

cis - groups on the same sides of of the alkene Trans - groups on opposite sides of the alkene

Question 39

Describe how geometrical isomers can occur in cyclic systems

Answer 39

it is a general rule that a ring and a double bond are stereochemically equivalent so geometrical isomers can occur in cyclic systems

Question 40

Describe how you would assign cis and trans to cyclic compounds

Answer 40

Cis - on the same face of the ring Trans - on opposite faces of the ring

Question 41

For butane what does the 3 bond NMR coupling constant 3J between two hydrogens depend on

Answer 41

the torsion/ dihedral angle between them