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Chemistry > (7) Aldehydes and Ketones > Flashcards

(7) Aldehydes and Ketones Flashcards

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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

example of aldehydes

A

methanal- CH₂O
ethanal- CH₃CHO

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4
Q

examples of ketones

A

propanone- CH₃COCH₃
butanone- CH₃COCH₂CH₃

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5
Q

bpt

compare aldehydes and ketones to alanes regarding bpt and relative mass

A

both series are considerably less volatile* than alkanes even with same Mᵣ

*less likely to vaporise

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6
Q

bpt

what intermoleculars do aldehydes and ketones have

A

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

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7
Q

bpt

compare bpt of ethanal to ethanol

A

H-bonds in ethanol&raquo_space;> pd-pd in ethanal

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8
Q

describe aldehydes and ketones solubity in water

A

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

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9
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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10
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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11
Q

preparation

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

A

acidified potassium dichromate; Kr₂Cr₂O₇/H+

orange –> green

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12
Q

preparation

summarise reactions of alcohols with oxidising agent

A
  • primary (1°) alcohol -[O]-> aldehyde -[O]-> carboxylic acid
  • secondary (2°) alochol -[O]-> ketone
  • tertiary -X-> doesnt oxidise
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13
Q

preparation

equations for primary alcohol

oxidising

A
  1. ethanol + [O] -> ethanal + H₂O
  2. ethanal + [O] -> ethanoic acid + H₂O
  3. (overall) ethanol + 2[O] -> Ethanoic acid + H₂O
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14
Q

preparation

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

oxidation of 1° alcohol

A

aldehyde = distilled and heated, distilling -al produced
carboxylic acid = prolonged reflux*

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

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15
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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16
Q

reaction with hydrogen cyanide

reaction of aldehydes and ketones with hydrogen cyanide

A

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

17
Q

what is hydrogen cyanide

A

HCN, toxic gas

18
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

19
Q

what is a nucleophile

A

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

attracted to positive charge

20
Q

what is addition

A

double covalent bond breaks and something is added

pi bond breaks

21
Q

reaction with hydrogen cyanide

describe CN- attack on aldehydes

A

attacks central carbon equally from either side of double bond giving an equilmolar mixture of two optical isomers (racemic mixture)

apart from methanal as no chiral carbon (optically inactive)

product = 2-hydroxypropanenitrile

22
Q

reaction with hydrogen cyanide

describe CN- attack on ketones

A

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

23
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

24
Q

2,4-dinitrophenylhydrazine

how to distinguish between aldehyde and ketone

A
  • add aldehyde/ketone to bradys reagent
  • orange or yellow precipatate shows C=O bond
25
Q

structure of 2,4-dinitrophenylhydrazine

A

reindeer with O₂N, NO₂ antlers

26
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

27
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

28
Q

properties of 2,4-dinitrophenylhydrazone

A

orange, crystalline solid with a well defined mpt

29
Q

preparation of 2,4-dinitrophenylhydrazone

describe the suction filtration method

A
  • place filter paper in buchner funnel
  • place funnel in buchner flask
  • attach flask to suction pump and suck air through the flask
30
Q

preparation of 2,4-dinitrophenylhydrazone

describe diagram for suction filtration

A
  • buchner funnel and filter paper
  • buchner flask
  • no gaps
  • suction indicated with arrow
31
Q

preparation of 2,4-dinitrophenylhydrazone

describe the mpt method

A
  • place solid in capillary tube at one end
  • heat slowly (using mpt apparatus)
  • record temp when solid starts and finishes melting
  • repeat and average
  • compare temp with data book
32
Q

distinguishing aldehydes and ketones

describe experiment with acidified dichromate (VI) solution

A

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

-CHO oxidised to -COOH

33
Q

distinguishing aldehydes and ketones

describe fehlings soltution experiment

A

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)

34
Q

distinguishing aldehydes and ketones

describe tollens’ agent experiment

A

tollens’ agent= colourless solution of silver(I)nitrate
* aldehyde = silver mirror on test tube
* ketone = no change

aldehyde reduces Ag(I) ion to Ag

Ag+ + e- -> Ag

35
Q

reduction

what are aldehydes and ketones reduced to

A

aldehydes = primary alcohols
ketones = secondary alcohols

36
Q

reduction

what is lithal (lithium tetrehydridoaluminate)

A

powerful reducing agent that produces H- ion
* reduces water vigorously so must be in anhydrous conditions
* usually dissolved in dry ether (no water present)

37
Q

reduction

equations for reduction of ethanal and propanone

A
  • (-al) CH₃CHO + 2[H] -> CH₃CH₂OH (1
  • CH₃COCH₃ + 2[H] -> CH₃CH(OH)CH₃

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