topic 17b: carbonyls

Question 1

aldehyde

Answer 1

r-c=o-h

Question 2

ketone

Answer 2

r-c=o-r’

Question 3

naming aldehyde

Answer 3

• no need to include number
• prefix-anal

Question 4

naming ketone

Answer 4

-prefix-an-number-one

Question 5

carbonyls IMFs

Answer 5

• LFs
• permanent dipole due to delta positive C, delta negative O

Question 6

compare a carbonyl to an alcohol with similar MR?

Answer 6

• same LF
• alcohol has HYDROGEN BONDING

Question 7

CARBONYLS H bonding with water

Answer 7

• delta negative O, lone pairs H bond with delta positive H on water

Question 8

small carbonyls solubility

Answer 8

soluble in water due to H bonds; delta h in water attracted to lone pair on oxygen atom of carbonyl

Question 9

LARGER carbonyls solubility

Answer 9

• less soluble
• longer hydrocarbon chains which are hydrophobic and NON POLAR(can’t h bond w water)
• proportion of molecule that can H bond w water smaller
• strength of H bond with water weaker than IMFs between each molecule
• so energetically unfavourable

Question 10

aldehydes + acidified potassium dichromate (VI)

Answer 10

• OXIDATION
• to form carboxylic acid
• orange to green (cr2o7 2- to cr3+)

Question 11

ketone + acidified potassium dichromate (vi)

Answer 11

NO CHANGE, stays orange
- ketons are resistant to oxidation
- no readily available h atom

Question 12

tollens reagent is …

Answer 12

ammoniacal silver nitrate

Question 13

tollens + aldehyde

Answer 13

• heat
• oxidation of aldehyde
• silver mirror formed

Question 14

what is produced with tollens

Answer 14

• reduction of Ag+ to Ag
• aledhyde oxidised to carboxylic acid

Question 15

what is fehlings

Answer 15

• cu2+ in sodium hydroxide

Question 16

what is benedicts

Answer 16

cu2+ in sodium carbonate

Question 17

aldehyde + fehling/benedicts

Answer 17

HEAT
- reduction. of cu2+ to cu+
-blue to brick red ppt
- aldehyde oxidised to carboxylic acidf

Question 18

aldehydes reduce to…

Answer 18

primary alcohol

Question 19

ketones reduce to

Answer 19

secondary alcohol

Question 20

reduction of carbonyl

Answer 20

• LiAlH4 in dry ether
• EQN: + 2[H]
• nucleophilic addition

Question 21

carbonyl + HCN in the presence of KCN

Answer 21

• nucleophilic addition
• c(OH)(CN)(R2)

Question 22

carbonyl + 2,4 DNP

Answer 22

• orange ppt
  USEFUL AS COXYLIC ACIDS AND ESTER DONT GIVE POSITIVE

Question 23

Iodoform test

Answer 23

• iodine + carbonyl + add sodium hydroxide until iodine decolourises
• pale yellow PPT formed
• ‘medical’ Smell

Question 24

iodoform test product

Answer 24

CHI3

Question 25

hazard of KCN

Answer 25

toxic

Question 26

why presence of KCN

Answer 26

HCN alone too weak; KCN dissociates more to produce CN- nucleophile

Question 27

if the aldehyde is not the principle group it is?

Answer 27

oxo

Question 28

what is formed in the HCN addition

Answer 28

hydroxy alkane-nitrile

Question 29

effect of pH on the HCN nucleophilic addition reaction

Answer 29

NEED HIGH PH
- shift position of equilibrium to the right to get a high conc of CN- nucleophile to attack c +
NEED LOW PH
- need acid so that the anion intermediate doesnt revert to carbonyl
USE PH 6-8

Question 30

nucleophile

Answer 30

a species containing a lone pair of e- with a tendency to donate them

Question 31

HCN mechanism (draw)

Answer 31

• lone pair on CN attacks delta + carbon of c=o
• c=o breaks
• INTERMEDIATE: C-O:-, C—N
• lone pair on o goes to h. h-cn breaks, goes to cn
• final is sa hydroxy nitrile

Question 32

chirality eith hcn addition

Answer 32

• trigonal plana carbonyl group
• attack by CN- nucleophile can happen above or below plane w equal probability
• racemic mixture produced

Question 33

why LiAlH4 in dry ether

Answer 33

water would destroy reagent

Question 34

what must you do to generate final compound for LiAlH4

Answer 34

-add acid or water

Question 35

what feature of carbonyl make snucleophilic attack possible

Answer 35

c=o
- o more elctroneagtive so delta - and you have delta + c
- so c+ vulnerable to attack from nucleophile

Question 36

why can c=c not undergo nucleophilic addition

Answer 36

• non polar bond
• rich electron density so nucleophile not attracted

Question 37

why dont ketones readily oxidise

Answer 37

no H

Question 38

redox equation eg methanal and dichromate ion (ACIDIC)

Answer 38

ORANGE TO GREEN
3CH3CHO + Cr2O72- + 8H+ -> 3CH3COOH +2Cr3+ + 4H2O

Question 39

other reducing agent

Answer 39

NaBH4

Question 40

Why do NaBH4 and LiAlH4 not reduce C=C but do reduce C=O

Answer 40

• C=C non polar so nucleophile not attracted
• o more eneg than c so h- nucleophile attracted to delta + c

Question 41

iodoform only works for

Answer 41

methyl ketones
- ETHANAL
- secpndary alcohol

Question 42

iodoform reaction equation

Answer 42

carbonyl + I2 + alkali = CHI3 + carboxylate salt + ionic salt + water

Question 43

iodoform only tests for…

Answer 43

ketones

Question 44

give the steps for how 2,4 DNP can distinguish between aldehyde and ketone (4)

Answer 44

• add DNP to solutions
• forms a yellow orange preceipitate
• filter and recrystallise
  -determine melting temperature
• compare to database

Question 45

why is there a reacemic mixture formed in the HCN mechanisms? (2)

Answer 45

• planar around carbonyl carbon
• equal likelihood of nucleophile attack above and below plane