Organic Chemistry (QA)

Question 1

Pale Yellow Solution

Answer 1

Cl2(aq)

Question 2

Brown solution

Answer 2

I2 and excess I- to form I3

Question 3

What colour is Br2 in denser and less dense hexane (organic layer)

Answer 3

orange and red-orange respectively

Question 4

What forms esters

Answer 4

salt of carboxylic acid (-CO2H) and alcohol

CONDENSATION RXN

Question 5

What is sweet smelling / nail polish smell

Answer 5

esters

Question 6

Neutral FeCl3 + ______ gives violet colouration

Answer 6

phenoxide ion

Question 7

add sodium hydrogen carbonate / sodium carbonate to ______ gives effervescence of colourless, odourless gas that forms a white ppt in limewater

Answer 7

carboxylic acid / acid chloride
[neutralisation]
Note : add 1/2cm3 of limewater only

Question 8

add aq silver nitrate and aq sodium hydroxide and ammonia until precipitate JUST dissolves (say : brown ppt dissolves in NH3 to form a colourless solution) (Tollen’s reagent - [Ag(NH3)2]+ OH- ) to the ________ forms a silver mirror

Answer 8

ALL aldehydes (differentiate aldehydes from ketones)
[oxidation]

Question 9

_____ + I2 (aq) + NaOH (aq), warm, gives yellow ppt/solid (NOT WHITE)

Answer 9

only alcohols with R-CH(OH)CH3 where R can be H/CH3 and aldehydes (R can only be H) / ketones R-COCH3 (R can be H/CH3)
[oxidation] IODOFORM Reaction

Question 10

unknown compound gives orange ppt with 2,4 - DNPH (dinitrophenylhydrazine)

Answer 10

ketones / aldehydes (including benzaldehyde) differentiate these 2 fn groups (carbonyl =C) from alcohols or other fn groups [condensation]

Question 11

unknown compound added to K2Cr2O7 or KMnO4 and dilute H2SO4, heat under reflux,
orange solution (Cr2O7-) turns green (Cr3+) / purple solution (MnO4-) turns colourless (Mn2+)

Answer 11

aldehydes / primary and secondary alcohol (differentiate from ketones and phenols and carboxylic acid)
[oxidation]
only for KMnO4, positive reaction also shows alkenes/methyl benzenes

Question 12

unknown compounds gives a positive Fehling’s solution test (turns blue (Cu2+) to brick red / red brown)

Answer 12

aliphatic aldehydes (not benzaldehyde) 
[oxidation] to Cu2O

Question 13

unknown compound decolourises red brown Br2

Answer 13

alkenes [electrophilic addition] (NO METHYLBENZENE)

phenols (forms white ppt) , phenylamines (white ppt) [electrophilic substitution]

Question 14

add 2-3 pieces of sodium metal into the filtrate to unknown compound results in effervescence / bubbles SAY : H2 gas liberated extinguishes a lighted (not glowing) splint with a “pop” sound

Answer 14

alcohol group present (carboxylic acid, phenols, 1 and 2nd degree alchohol)

Question 15

unknown compound added to PCl5 (at room temp) results in violent reaction and produces clouds of hydrogen chloride gas

Answer 15

all alcohols (good way to test for tertiary alcohol) except phenol fn group

Question 16

unknown compound added to AgNO3

Answer 16

acid chlorides present

Question 17

unknown compound added to H2O

Answer 17

acid chloride / acyl chloride present

Question 18

unknown compound to NaOH (aq), heat then test gas with DAMP red litmus paper (turns blue)

Answer 18

primary amides present (NH4+ ions present)

Question 19

add unknown compound to NaOH (aq), heat then acidify with HNO3(aq) followed by AgNO3 (aq) forming a precipitate

Answer 19

halogenalkanes present
Cl- white precipitate (Ag-X)
Br- very pale cream precipitate
I- very pale yellow precipitate

Question 20

add unknown compound to NaOH(aq), heat, collect distillate, test with K2Cr2O7, dilute H2SO4

Answer 20

esters fn grp present

Question 21

how to test for each of the 6 gases

Answer 21

SO2 - turns aq acidified potassium manganate (KMnO4) from purple to colourless

Question 22

Lucas Test

Answer 22

Add conc HCl and ZnCl2
T : immediate cloudiness appears in solution
S : cloudiness appears within 5 minutes
P : no cloudiness appears unless warmed

Question 23

How might you distinguish between a primary and secondary alcohol?

Answer 23

Warm and distill with acidified pottasium dichromate. Then test with tollens reagent. The acidified pottasium dichromate will have oxidised the primary alcohol to an aldehyde, which will form a silver mirror with Tollen’s reagent. The secondary alcohol will have been oxidised to a ketone, which does not react with Tollen’s reagent.