Chapter 17- chirality

Question 1

Define the term ‘chiral’

Answer 1

A molecule that’s asymmetric in such a way that the structure and its mirror image are not superimposable.

Question 2

Define the term ‘achiral’

Answer 2

A molecule that’s symmetric in such a way that it can be superimposed on its mirror image

Question 3

Describe the difference between constitutional isomers and stereoisomers

Answer 3

Constitutional isomers and stereoisomers make up two broad categories of isomers. Constitutional isomers have different connectivities and stereoisomers have the same connectivities but differ in spatial arrangements.

Question 4

What are the two types of stereoisomers?

Answer 4

Enantiomers

Diastereomers

Question 5

Which type of stereoisomer is chiral?

Answer 5

Enantiomers

Question 6

Which type of stereoisomer aren’t mirror images?

Answer 6

Diastereomers. An example is cis-trans isomerism in cycloalkenes

Question 7

In a molecule, how do you find a chiral carbon?

Answer 7

A chiral carbon has four different groups bonded to it.

Question 8

What’s the difference between a stereocentre and a chiral centre?

Answer 8

The difference is that stereocentre is the more general term, whereas a chiral centre normally applies to carbon.

Question 9

How do you name enantiomers using the R,S system?

Answer 9

1: Assign priorities to the four groups, with #1 being the highest priority and #4 the lowest. Priorities are based on the atomic number.
2: Orient the molecule so that the lowest priority group is directed away from you.
3: Trace a circle from #1 to #2 to #3.
4: A clockwise circle corresponds to the R configuration, while an anticlockwise circle corresponds to the S configuration.

Question 10

What’s the maximum number of stereoisomers possible for a molecule with n stereocentres?

Answer 10

2^n

Question 11

Which type of stereoisomer has almost identical chemical and physical properties?

Answer 11

Enantiomers

Question 12

Define the term ‘meso compound’

Answer 12

An achiral compound with two or more stereocentres. It’s also a diastereomer of the other stereoisomers.

Question 13

When don’t compounds have as many stereoisomers as the 2^n rule predicts?

Answer 13

When one of the stereoisomers is a meso compound

Question 14

Describe the difference between polarised light and ordinary light

Answer 14

Polarised light can only oscillate in one angle. If it can oscillate in all angles, then it’s ordinary light. Light can be polarised using a filter which only allows light in that’s a certain angle

Question 15

Define the term ‘Dextrorotatory’

Answer 15

A compound that rotates the plane of a polarised light ray to the right, i.e. clockwise facing the oncoming radiation.

Question 16

Define the term ‘laevorotatory’

Answer 16

A compound that rotates the plane of a polarised light ray to the left, i.e. anticlockwise facing the oncoming radiation.

Question 17

What’s optical activity?

Answer 17

The ability to rotate a plane of polarised light

Question 18

Why is optical activity useful for chemists?

Answer 18

It allows chemists to detect enantiomers in the laboratory. Each member of a pair of enantiomers rotates a plane of polarised light in the opposite direction.

Question 19

Define the term ‘observed rotation’

Answer 19

The number of degrees, α, through which the analysing filter must be rotated to restore darkness to the field of view

Question 20

What does the observed rotation for a particular compound depend on?

Answer 20

Its concentration, the length of the sample tube, the temperature, the solvent and the wavelength of the light used.

Question 21

Specific rotation=

Answer 21

observed rotation (degrees) / (length (dm) x concentration (g/ml))

Question 22

The values of the specific rotation for a pair of enantiomers are …?

Answer 22

are exactly the same, but the signs are opposite.

Question 23

What’s a racemic mixture?

Answer 23

An equimolar mixture of two enantiomers