Topic 17: Organic II

Question 1

Optical isomers

Answer 1

Type of stereoisomerism (other type is geometric E-Z)
Non-superimposable mirror images formed by chiral molecules
Opposite optical activity
50-50 mixture of two optical isomers: racemic mixture

Question 2

Chiral molecules

Answer 2

Carbon bonded to 4 different GROUPS

Question 3

Optical Activity

Answer 3

Chiral molecules rotate plane polarised light
Different isomers rotate it in different directions
Racemic mixes have ni effect as rotation is countered by other half

Question 4

SN1

Answer 4

tertiary & secondary
trigonal planar carbocation formed
50-50 probability of nucleophile attack from above or below
So racemic mixture formed ALWAYS
if product does not interact with light: it is SN1

Question 5

SN2

Answer 5

primary and secondary
forms intermediate step with [] and …
STERIC HINDERANCE: nucleophile only attacks from one side (doesn’t fit in the other because halogen is too big) so 100% probability of backside attack
Forms one isomer only = optical activity

Question 6

Carbonyl compounds

Answer 6

C=O
Ketones & aldehydes
Permanent dipole-dipole interaction (O- to C+)
can hydrogen bond with water
Soluble in water (up to 5 carbons in chain)
KETONES DONT OXIDATE

Question 7

Nucleophilic addition

Answer 7

Reduction of aldehydes & ketones
Reducing agent LiAL4 (:H- radical and H+ ion) in DRY ETHER (Also works with cyanide )
acidified potassium dichromate
OH dative covalent bond
Aldehyde makes primary alcohol, ketone makes secondary

Question 8

CARBONYL TEST: Tollen’s

Answer 8

[Ag(NH3)]+
warm
result: silver mirror
reduction of silver (+1 to 0), oxidation of aldehyde ONLY to carboxylic acid

Question 9

CARBONYL TEST: Fehling’s solution

Answer 9

Cu2+
Warm
Result: blue to brick red precipitate of Cu2O
Cu reduced from 2+ to 1+
Only for aldehydes

Question 10

CARBONYL TEST: Acidified Potassium Dichromate

Answer 10

K2Cr2O7 / H+ (H2SO4)
Head, acidified
Orange to green (Cr3+ ions formed from Cr6+ = reduction)
Positive result for 1ª & 2ª alcohols and aldehyde oxidation

Question 11

CARBONYL TEST: 2,4-DNPH

Answer 11

2,4-dinitrophenyl hydrazine
Methanol and sulfuric acid
Result: carbonyl shows yellow-red precipitate
Purify by crystallisation then measure melting point

Question 12

CARBONYL TEST: Iodine Solution

Answer 12

Sodium hydroxide solution
Result: pale yellow precipitate (triiodomethsne = iodoform = -CHI3)
Positive results only if CH3-C=O group is present (methyl ketones or ethanal)