(7) Aldehydes and Ketones Flashcards

1
Q

general formula for aldehyde

A

CₙH₂ₙO

. O
. ||
R–C–H

carbonyl group on end (terminal carbon)

-al

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2
Q

general formula for ketones

A

CₙH₂ₙO

. O
. ||
R–C–R’

carbonyl group in middle of carbon chain

-one

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3
Q

bpt

what intermolecular forces do aldehydes and ketones have

A

permanent dipole of C=O, pd-pd attractions between molecules (stronger than vdW)

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4
Q

bpt

compare bpt of ethanal to ethanol

A

H-bonds in ethanol > pd-pd in ethanal

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5
Q

describe aldehydes and ketones solubity in water

A

both soluble in water and will dissolve polar and non-polar solutes

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6
Q

solubility

why are aldehydes and ketones soluble in water

A

permanent dipole of C=O allows molecules to break H-bonds in water molecules and form H-bonds with water molecules

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7
Q

solubility

what happens when carbon chain increases

regarding C=O

A

-C=O has less influence on physical properties

  • short chain = polar due to C=O
  • large chain = non-polar as far from C=O
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8
Q

preparation

oxidising agent used for oxidation of alcohols (+colour change)

A

acidified potassium dichromate; Kr₂Cr₂O₇/H+

orange –> green

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9
Q

preparation

summarise reactions of alcohols with acidified potassium dichromate (VI)

A
  • primary (1°) alcohol -[O]-> aldehyde -[O]-> carboxylic acid
  • secondary (2°) alochol -[O]-> ketone
  • tertiary -X-> doesnt oxidise
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10
Q

preparation

what is the practical difference between producing an aldehyde and a carboxylic acid

oxidation of 1° alcohol

A

aldehyde = distilled and heated
carboxylic acid = prolonged reflux*

*continously heating and cooling (vapour turns back into liquid)

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11
Q

preparation

how are aldehydes, ketones and carboxylic acids produced

practically

A

aldehyde- distillation(1° alcohol)
carboxylic acid- reflux (1°/ -al)
ketone- reflux (2° alcohol)

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12
Q

reaction with hydrogen cyanide

reaction of aldehydes and ketones with hydrogen cyanide

A

both produce 2-hydroxynitriles (-CN, -OH)
* aldehyde/ketone + HCN -> cyanohydrin

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13
Q

what is hydrogen cyanide

A

HCN, toxic gas

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14
Q

reaction with hydrogen cyanide

nucleophillic addition with HCN

A
  1. :CN- breaks C=O and attaches to C*
  2. :O- goes to H
  3. CN and OH both are attached to C

“goes to” = arrow in flow scheme

*CN has higher priority than OH

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15
Q

what is a nucleophile

A

ion/molecule with lone pair that attacks low electron density region

attracted to positive charge

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16
Q

what is addition

A

double covalent bond breaks and something is added

pi bond breaks

17
Q

reaction with hydrogen cyanide

describe CN- attack on aldehydes

A

attacks central carbon equally from either side of double bond giving a racemic mixture

apart from methanal as no chiral carbon (optically inactive)

product = 2-hydroxy(no. of carbons)nitrile

18
Q

reaction with hydrogen cyanide

describe the products in CN- attack on ketones

A

if ketone symmetrical = product optically inactive
if ketone asymmetrical = product optically active

19
Q

reaction with 2,4-dinitrophenylhydrazine

how to produce bradys reagent

A

solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulfuric acid

20
Q

2,4-dinitrophenylhydrazine

how to tell if something is an aldehyde/ketone

A
  • add aldehyde/ketone to bradys reagent
  • orange or yellow precipatate shows C=O bond
21
Q

structure of 2,4-dinitrophenylhydrazine

A

reindeer with NO₂ antlers

22
Q

what type of reaction is aldehyde/ketone with 2,4-dinitrophenylhydrazine

A

condensation reaction- two molecules join together losing a small molecule in the process

water formed

23
Q

describe reaction between aldehyde/ketone with 2,4-dinitrophenylhydrazine

A

-al/-one + 2,4-dinitro –
> -al/-one attached to 2,4 dinitro by C≡N + water

water: O comes from C=O and H₂O from tail and back leg of 2,4-nitro

product is called 2,4-dinitrophenylhydrazone for -al and -one

24
Q

properties of 2,4-dinitrophenylhydrazone

A

orange, crystalline solid with a well defined mpt

25
# preparation of 2,4-dinitrophenylhydrazone describe the suction filtration method
* place filter paper in buchner funnel * place funnel in buchner flask * attach flask to suction pump and suck air through the flask
26
# preparation of 2,4-dinitrophenylhydrazone describe diagram for suction filtration
* buchner funnel and filter paper * buchner flask * no gaps * suction indicated with arrow
27
# distinguishing aldehydes and ketones describe experiment with acidified dichromate (VI) solution
small amount of potassium dichromate (VI) acidified with dilute sulfuric acid, a few -al/-one drops are added. Mixture warmed gently in warm water bath * **aldehyde** = orange -> green* * **ketone** = no change | *orange dichromate ions reduced to green chromium ions ## Footnote -CHO oxidised to -COOH
28
# distinguishing aldehydes and ketones describe fehlings soltution experiment
mixture of fehlings solutions A and B give blue solution with Cu(II) ions, warm in hot bath * **aldehyde** = reddish precipitate of copper (I) ions * **ketone** = no change | aldehyde reduces Cu(II) to Cu(I)
29
# distinguishing aldehydes and ketones describe tollens' agent experiment
tollens' agent= colourless solution of silver(I)nitrate * **aldehyde** = silver mirror on test tube * **ketone** = no change | aldehyde reduces Ag(I) ion to Ag ## Footnote Ag+ + e- -> Ag
30
# reduction what are aldehydes and ketones reduced to
aldehydes = primary alcohols ketones = secondary alcohols
31
# reduction what is lithal (lithium tetrehydridoaluminate)
powerful reducing agent that produces H- ion * reduces water vigorously so must be in anhydrous conditions * usually dissolved in dry ether (no water present)
32
# reduction equations for reduction of ethanal and propanone
* (-al) CH₃CHO + 2[H] -> CH₃CH₂OH (1 * CH₃COCH₃ + 2[H] -> CH₃CH(OH)CH₃