Topic 17A - Stereochemistry: Chirality

Question 1

What is a chiral molecule?

Answer 1

One that cannot be superimposed on its mirror image and that contains a chiral carbon that is assymetric, bonded to four different groups.

Question 2

What’s the definition of non-superimposable?

Answer 2

Mirror images that cannot be the same even when rotated

Question 3

What is the definition if an asymmetric molecule?

Answer 3

One that contains a carbon that is bonded to four different groups

Question 4

What are enantiomers/optical isomers?

Answer 4

Molecules that occur in pairs made up of a chiral molecule and its non-superimposable mirror image

Question 5

What is a racemic mixture?

Answer 5

Admixture containing equal amounts of each enantiomer/optical isomer and is, therefore, optically inactive

Question 6

Draw the optical isomers of Lactic acid, CH3CH(OH)COOH

Answer 6

• Label the chiral centre (the 2nd carbon)
• The chiral carbon should be drawn in the middle with the: CH3 on the plane diagonally down and to the left, H on the plane and up, COOH in to the plane and to the right, OH out of the plane and diagonally to the right
• Draw a dotted line down the centre to act as the mirror and draw the reflection of the molecule

Question 7

What is the definition of plane polarised light?

Answer 7

Light that has been passed through a polaroid sheet causing it to only vibrate in one plane

Question 8

What do enantiomers do to plane polarised light?

Answer 8

They rotate it by equal amounts but in opposite directions

Question 9

What would a racemic mixture do to plane polarised light?

Answer 9

Nothing, as the mixture is optically inactive

Question 10

Describe how an observer can determine the degree of rotation of plane polarised light due to an optically active sample

Answer 10

First light that is vibrating in all directions is passed through a polaroid to make it plane polarised. The plane polarised light is then passed through he same of enantiomer and rotated by some degree. A second polaroid is then rotated until it matches the degree of rotation and this is recorded bt the observer.

Question 11

What type of halogenoalkanes react via the SN2 mechanism?

Answer 11

primary and secondary

Question 12

What are the meanings of the individual symbols: S, N and 2 in the name SN2 mechanism?

Answer 12

S - substitution
N - Nucleophilic
2 - The number of species in the first step

Question 13

Why can we not mention about primary halogenoalkanes when talking about chirality?

Answer 13

As they have no chiral carbon

Question 14

Describe the SN2 mechanism for the secondary halogenoalkane 2-bromobutane and, therefore, draw the process

Answer 14

1. The C-Br bond in 2-bromobutane is polar and so the C atom has a partially + charge and the Br has a partially - charge.
2. The OH nucleophile attacks the chiral carbon from the opposite direction to the halogen leaving group.
3. This is then the transition state where the C-Br bond becomes a dotted line to show that it is just breaking and there is a dotted line between the C-OH to show the bond is just forming. The molecule is surrounded by brackets.
4. The new product butan-2-ol has formed. However, comparing the structure of this molecule to the original one, there has been an inversion around the chiral carbon, the molecules coming off of the chiral carbon have been reflected.

Question 15

What is the definition of stereospecific?

Answer 15

Stereospecific means that if you start with one enantiomer of a halogenoalkane you will only ever produce one enantiomer of the alcohol. Therefore, if you begin with an optically active compound your product will also be optically active.

Question 16

Can you predict whether the isomer produced from an SN2 mechanism is the + or - enantiomer?

Answer 16

No, as there is no simple relationship between the three dimensional shape of a chiral compound and the direction it rotates plane polarise light.

Question 17

What type of halogenalkanes react via the SN1 mechanism?

Answer 17

tertiary

Question 18

What are the meanings of the individual symbols: S, N and 1 in the name SN1 mechanism?

Answer 18

S - substitution
N - nucleophilic
1 - The number of species in the first step

Question 19

Describe the SN1 mechanism for the secondary halogenoalkane 2-bromo-2methylpentane and, therefore, draw the process

Answer 19

1. First the electrons move to the bromine due to the partially negative charge on it.
2. The molecule is not a carbocation intermediate with a positive charge on the chiral carbon. This carbocation intermediate has 3bp’s and 0lp’s so it has a trigonal planar shape. A these carbocation intermediate is flat, the OH nucleophile has an equal probability of attacking from above or below.
3. Both enantiomers are formed from this mechanism even when you begin with only one therefore this mechanism is not stereospecific.

Question 20

Why does the SN1 mechanism produce an optically inactive mixture?

Answer 20

The carbocation intermediate has a trigonal planar shape, therefore the nucleophile has an equal probability of attacking from above or below. Therefore, if the reactant is one isomer of a chiral halogenoalkane, the product will be an equal mixture of the two optical isomers the alcohol. Thus the product will have no optical activity since it is a racemic mixture.

Question 21

What are the positives to using racemic mixtures as drugs?

Answer 21

There is no need to separate out the enantiomeric products. This reduces the difficulty and cost of producing the drug.

Question 22

What are the negatives to using racemic mixtures as drugs?

Answer 22

Only one enantiomer will fit into he active site desired. This means the other enantiomer may fit into another active site and could cause harmful side effects or have no effect at all.