4.2 carbonyl compounds

Question 1

carbonyl compounds contain

planar 120 degree

Answer 1

> C=O

C is bonded by O by a sigma and a pi bond

bond is polar due to to diff in electronegativity

Question 2

two types of carbonyl compounds

Answer 2

aldehyde - at least one H attached to the carbonyl group

ketone - two carbons attached to the carbonyl

Question 3

physical properties of carbonyl compounds

boiling point

carbonyls have higher bp and mp than hydrocarbons of similar relative molecular masses

why?

Answer 3

carbonyls have london forces and permanent dipole - dipole force between their molecules

hydrocarbons have london forces

carbonyl s have stronger intermoleculaar force than hydrocarbons. more energy is required to break it

Question 4

physical properties of carbonyl compounds

boiling point

carbonyls have lower bp and mp than alcohols why?

Answer 4

alcohols have hydrogen bonding

hydrogen bonding is stronger than london forces and permanent dipole - dipole force

Question 5

physical properties of carbonyl compounds

solubility

the lower members of both series are soluble in water why?

Answer 5

the weak intermolecular forces btwn lower members of carbonyl can be overcome by the hydrogen bonding btwn carbonyl and water

Question 6

physical properties of carbonyl compounds

smell

lower members of aldehyde
ketone

Answer 6

lower members of aldehyde
- pungent odours

ketone
- sweeter than aldehyde

Question 7

show how hydrogen bonds act btwn ethanl and water

explain why ethanal is more soluble than pentanal in water

Answer 7

refer notes pg 2

Question 8

nucleophilic addition of hydrogen cyanide

reaction with ethanal

eq
reagent
condition
product
reaction type

Answer 8

eq notes pg3

reagent: hydrogen cyanide HCN in KCN ( catalyst )
condition: reflux in alkaline solution pH8
product: 2 - hydroxypropanitrile

reaction type: nucleophilic addition

Question 9

nucleophilic addition of hydrogen cyanide

reaction with ethanal

important step to?

Answer 9

increase the carbon skeleton of the original molecule

Question 10

nucleophilic addition of hydrogen cyanide

reaction with ethanal

mechanism

Answer 10

refer notes pg 3

Question 11

nucleophilic addition of hydrogen cyanide

reaction with aldehyde or asymmetric ketone

product is a racemic mixture

draw mechanism

Answer 11

refer to pg 4 exercise 2

Question 12

nucleophilic addition of hydrogen cyanide

reaction with ethanal

product is a racemic mixture of both optical isomers of 2-hydroxypropanitrile

draw mechanism

Answer 12

refer pg 4

Question 13

nucleophilic addition of hydrogen cyanide

reaction with ethanal

product is a racemic mixture of both optical isomers of 2-hydroxypropanitrile

why?

Answer 13

the reactant is planar

do not confuse with reasons from SN1 where carbocation is planar

Question 14

C=O is nucleophilic addition

C=C is electrophilic addition

why

Answer 14

C=O is polar. nucleophiles attack slightly+ C. will repel electrophiles

C=C is non polar. electrophiles attack the electron cloud above and below the sigma bond

Question 15

nucleophilic addition of hydrogen cyanide

reaction with ethanal

HCN is a weak acid why the reaction occurs more slowly at both high and low concentrations of hydrogen ions

high H+ low pH
low H+ high pH

Answer 15

low pH

• not enough CN- from the base to attack slightly + C in slow reaction
• less nucleophiles

high pH

• not enough HCN to regenerate CN- back in fast reaction
• position of equilibrium shifts to the right

Question 16

why ketones are less reactive than aldehyde

Answer 16

ketones have 2 electron donating group ( CH3 )

steric hindrance

slower reaction

Question 17

detecting presence of carbonyl

using?
eq
reagent
condition
product
reaction type
observation

Answer 17

eq refer notes pg 5

reagent: 2,4 - dinitrophenylhydrazine solution / brady’s reagent
condition: room temp
product: 2,4 - dinitrophenylhydrazone solid

reaction type: nucleophilic addition / elimination

observation: orange to yellow percipitate

Question 18

detecting presence of carbonyl

reagent: 2,4 - dinitrophenylhydrazine solution
condition: room temp

positive only for?

Answer 18

ketone

aldehyde

Question 19

oxidation of aldehydes

acidic condition :
alkaline condition:

what forms

Answer 19

acidic: carboxylic acid

alkaline condition: salt of carboxylic acid / carboxylate anion

Question 20

oxidation of aldehydes

common oxidising agent

strong?
weak?

Answer 20

strong: KMnO4 and K2Cr2O7 (acidic) specify the acid used

weak:
- blue fehling’s solution
- colourless tollen’s reagent
alkaline

Question 21

oxidation of aldehydes

observations?

K2Cr2O7 acidified with dilute H2SO4
Fehling’s reagent in alkaline and warm gently
Tollen’s reagent in alkaline and warm gently

Answer 21

K2Cr2O7 acidified with dilute H2SO4
- orange solution turns green / Cr 3+

Fehling’s reagent in alkaline and warm gently
- blue solution turns colourless and into brick red percipitate / Cu2O cooper (I) oxide

Tollen’s reagent in alkaline and warm gently
- colorless solution turns into a silver mirror in test tube / Ag

Question 22

eq for

oxidation of ethanal in acidified potassium dichromate (VI) solution

oxidation of ethanal in alkaline tollen’s

Answer 22

refer pg 7

Question 23

half eq

for silver ions acting as oxidising agent

for copper (II) ions acting as oxidising agent

Answer 23

refer pg 7

Question 24

reduction

aldehyde to?

ketone to?

Answer 24

primary alcohol

secondary alcohol

Question 25

reduction of aldehyde and ketone using?

Answer 25

LiAlH4 | lithium aluminium hydride in dry ether

Question 26

LiAlH4 lithium aluminium hydride in dry ether what s so good about it?

Answer 26

powerful reducing agent | provides strong nucleophiles hydride ion H- to attack C of C=O

Question 27

LiAlH4 lithium aluminium hydride in dry ether why dry ether

Answer 27

LiAlH4 is a powerful reducing agent | provides strong nucleophiles hydride ion H- to attack H slightly + of water instead of C slightly + of C=O

Question 28

reduction of ethanal eq

Answer 28

refer pg8

Question 29

reduction of propanone eq

Answer 29

refer pg 8

Question 30

tri-iodomethane reaction involves what group?

Answer 30

CH3-C=O Refer pg 8

Question 31

tri-iodomethane reaction ``` eq reagent condition product reaction type observation ```

Answer 31

eq refer pg 8 reagent: iodine in aq NaOH condition: warm gently product: sodium salt of carboxylic acid and triiodomethane observation: yellow ppt and antiseptic smell

Question 32

tri-iodomethane reaction invovles two steps/reactions

Answer 32

substitution - 3H in CH3 are replaced by iodine atoms hydrolysis - the intermediate is hydrolysed to form a yellow ppt of CHI3

Question 33

yellow ppt from unkown compounds from tri - iodomethane what is detected?

Answer 33

CH3CO-

Question 34

tri-iodomethane reaction who are involve?

Answer 34

methyl ketone ethanal ethanol 2nd methyl alcohol

Question 35

why ethanol and 2nd methyl alcohol give a positive result for tri-iodomethane reaction

Answer 35

initially oxidised by alkaline iodine solution to ethanal and methyl ketone respectively react further to form triiodomethane

Question 36

test and identification of a carbonyl compound how to distinguish two carbonyls with similaar boiling points

Answer 36

determining the melting points of their 2,4 - dinitrophenylhydrazone derivatives

Question 37

determining the melting points of their 2,4 - dinitrophenylhydrazone derivatives test and identification of a carbonyl compound how to distinguish two carbonyls with similaar boiling points method

Answer 37

- react carbonyls with 2,4 - dinitrophenylhydrazine solution - products are purified by recrystallization and filtered then washed - dry the purified product and measure its melting point - refer data booklet and compare this melting temp with those known 2,4 - dinitrophenylhydrazine derivatives - the melting derivatives can be used to identify the original ketone / aldehyde

Question 38

test to differentiate aldehyde and ketone

Answer 38

oxidation using warmed K2Cr2O7 with dilute H2SO4 aldehyde turns orange to green ketone remains orange

Question 39

test to differentiate ketone and ketone pentan - 2 - one and pentan - 3 - one

Answer 39

warm gently with iodine in aq NaOH pentan - 2 - one forms yellow ppt and pentan - 3 - one remains brown solution

Question 40

test to differentiate aldehyde and aldehyde ethanal and hexanal

Answer 40

warm gently with iodine in aq NaOH ethanal forms yellow ppt hexanal remains brown solution

Question 41

test to differentiate alcohol and alcohol methanol and ethanol

Answer 41

warm gently with iodine in aq NaOH ethanol forms yellow ppt methanol remains brown solution

Question 42

reactions of aldehyde

Answer 42

oxidation reduction addition

Question 43

reactions of ketone

Answer 43

reduction addition