Optical isomerism, aldehydes and ketones

Question 1

What is the definition of functional group isomerism?

Answer 1

same molecular formula but different functional group

Question 2

What does enantiomer mean?

Answer 2

non-superimposable mirror image

Question 3

What is a racemic/ racemate mixture?

Answer 3

50/50 mixture of enantiomers (equal number of moles of each one)

Question 4

What is optical isomerism?

Answer 4

enantiomers, which exist as non-super-imposable mirror images and differ in their effect on plane polarised light

Question 5

What is the definition of superimposed?

Answer 5

molecule looks the same as its mirror image

if you put the molecules on top of each other, the molecules will completely match up

Question 6

What does optically active mean?

Answer 6

capable of rotating the plane of vibration of polarized light to the right or left

Question 7

what does chiral mean?

Answer 7

a carbon atom that has four different groups

Question 8

What are the features of optical isomers?

Answer 8

• same chemical and physical properties
• their effect on plane polarised light and how they react with other chiral compounds is different

Question 9

What is plane polarised light and how is it produced?

Answer 9

Some substances have the ability to remove from normal light all the light waves except those vibrating in a single plane = plane-polarised light.

Question 10

What can optical isomers do with plane polarised light?

Answer 10

it can rotate it (optically active)

Question 11

Why are racemates optically inactive?

Answer 11

because it is a 50:50 ratio of enantiomers. One enantiomer rotates it in one direction and the other enantiomer rotates it by the same amount in the opposite direction. This cancels out the effect.

Question 12

Why do planar bonds produce racemates?

Answer 12

Because the reactive intermediate can attack from above or below the plane with an equal probability = equal amount of each enantiomer will form

Question 13

What are the conditions for a nucleophilic addition? Write the equation for the addition of hydrogen cyanide to ethanal?

Answer 13

reagent: HCN or KCN (generated from aqueous NaCN and dil.H2SO4)

Equation: CH3CHO + HCN → CH3CHOHCN

Question 14

Draw the mechanism for HCN + ethanal in nucleophilic addition and what is the name of the product?

Answer 14

Question 15

How do you make lactic acid (synthesis of optically active compounds)?

Answer 15

1) nucleophilic addition of hydrogen cyanide to ethanal
2) convert the product (2-hydroxypropanenitrile) to a carboxylic acid by adding HCL +2H2O

Question 16

What are the issues faced in the drug industry due to optical isomers

Answer 16

one of the optical isomers is complementary to receptors in the body. Sometimes the other optical isomer is actually dangerous.

• separating the two isomers can be difficult an expensive due to similar properties
• selling a drug with a mixture of optical isomers is wasteful as only half of it will be active (other half inactive)

Question 17

Why is HCN not normally used as the reagent in nucleophillic addition with aldehydes and ketones. What is used instead?

Answer 17

• very toxic
• Dilute sulfuric acid and KCN

Question 18

Write the hydrolysis equation for 2-hydroxypropanenitrle.

Answer 18

CH₃CH(OH)CN + 2H₂O + HCl → CH₃CH(OH)COOH + NH₄Cl

Question 19

Write the reduction equation for 2-hydroxypropanenitrle.

Answer 19

CH₃CH(OH)CN + 2H₂ → CH3CH(OH)CH₂NH₂

Question 20

Draw the mechanism for NaBH4 + ethanal in nucleophilic addition and what type of reaction is this?

Answer 20

reduction

Hydride from the NaBH4 attacks the aldehyde and electrons shift to the oxygen. The oxygen now has lone pairs and attacks H+ ion from water molecule

the OH- is a byproduct produced

Question 21

What reducing agents for the reduction of aldehydes and ketones?

Answer 21

NaBH4 or anhydrous LiAlH4 (both need to be in aqueous ethanol)