Optical isomerism, aldehydes and ketones

Question 1

optical isomerism

Answer 1

occurs in a compound with an asymmetrical carbon atom (chiral centre), which has four different groups of atoms attached to it in a tetrahedral fashion

Question 2

enantiomers

Answer 2

two non-superimposable isomers with the same molecular and structural formula but only differ in their spatial arrangement

Question 3

racemic mixture

Answer 3

mixing equal amounts of the 2 enantiomers gives an optically inactive mixture which has no effect on plane-polarised light as the two rotating effects cancel out

Question 4

how can a racemic mixture be formed

Answer 4

• Occurs when reactant/ intermediate has a trigonal planar group (e.g. carbonyl C=O)
• Attack by nucleophile CN- at the electron deficient carbon (δ+) of the trigonal planar
• Equally likely to be attacked from either side of the plane so equal chance of each enantiomer being formed

Question 5

properties of enantiomers

Answer 5

Same physical and chemical properties except they rotate the plane-polarised light in opposite directions by equal amounts

Question 6

types of forces between aldehydes and ketones

Answer 6

Carbonyl group (C=O) is strongly polar so there is are permanent dipole-dipole forces between the molecules

Question 7

boiling point comparison of aldehydes + ketones, alkanes, alcohols and alkenes

Answer 7

alkanes + alkenes < aldehydes + ketones < alcohols (H bonds)

Question 8

what does the reduction aldehydes produce

Answer 8

alcohols

Question 9

types of reduction reactions of aldehydes and ketones

Answer 9

catalytic hydrogenation

nucleophilic addition :H- nucleophile

Question 10

catalytic hydrogenation of aldehydes and ketones

reagents, conditions, and a fun fact

Answer 10

• Reagents: H2, Ni catalyst
• Conditions: 140°C, high pressure
• Not selective so reduces both C=C and C=O present

Question 11

nucleophilic addition of aldehydes and ketones to form alcohols
reagents, conditions, what they each produce

Answer 11

• Reagents: NaBH4 (Source of hydride :H- which is the nucleophile)
• Conditions: room temperature and pressure, aqueous in ethanol
• Aldehydes reduced to primary alcohols
• Ketones reduced to secondary alcohols

Question 12

what can not be nucleophilically additioned

Answer 12

• Alkenes cannot be reduced due to electron-rich double bond repels hydride ions

Question 13

nucleophilic addition with HCN

reagents, conditions, products

Answer 13

• Reagents: dilute H2SO4 and KCN (HCN is toxic so not used)
• Conditions: room temperature and pressure
• Can sometimes form enantiomers of hydroxyl-nitrile