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A Level Chemistry > module 6 (chapter 26) - carbonyls > Flashcards

module 6 (chapter 26) - carbonyls Flashcards

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1
Q

carbonyl

A

have a C=O functional group

-the C=O isn’t the functional group on it own though, they form part of the functional group

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2
Q

shape

A

trigonal planar

-planar dipole as the O is delta negative and the C is delta positive

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3
Q

uses of aldehydes and ketones

A
  • methanal is used to preserve biological specimen
  • octanal is responsible for the smell in oranges
  • aldehydes are used in perfume industry
  • propanone is used as nail varnish remover
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4
Q

reactions

A

-undergo nucleophilic addition reactions due to the polarity of the bond. nucleophiles attract the delta positive carbon
nucleophiles are atoms or groups of atoms that are attracted to an electron deficit centre. they often contain an electronegative atom with a lone pair

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5
Q

oxidation

A
  • aldehydes can be oxidised to form carboxylic acids in the presence of acidified potassium dichromate and in a reflux set up
  • reflux needs to be an open system as this allows any build up of gases to escape.
  • ketones can not due to a lack of reactivity
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6
Q

reduction of aldehydes

A
  • use sodiumhydridoborate NaBH4
  • aldehydes are reduced to primary alcohols, ketones secondary
  • [H] is used to represent reducing agent
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7
Q

what is the mechanism stages for reduction using NaBH4

A
  • there is a dipole present on the carbonyl leaving it susceptible to nucleophilic attack
  • lone pair on the hydride ion is attracted and donated to delta positive carbon, forming a dative covalent bond
  • this causes the pi elections in the C=O bond to break by heterolytic fission and the production of an intermediate
  • oxygen of the intermediate donates a lone pair of electrons to a hydrogen atom in the water molecule
  • this propionates the intermediate to form an alcohol
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8
Q

carbonyl with hydrogen cyanide

A
  • used as a reducing agent
  • it is a colourless and poisonous liquid so can’t be used in an open lab
  • sodium cyanide and sulphuric acid are used to provide the HCN (still hazardous)
  • reducing agent forms a weak acid in solution 9partially ionises to form CN- and H3O+)
  • the CN- ions need to be acidified to react directly. this make it more reactor as the C=O becomes polar
  • PH should be no lower than 4 as this can cause side reactions
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9
Q

mechanism for hydrogen cyanide

A
  • lone pair from cyanide ion is attracted to delta positive carbon atom, forming a dative covalent bind
  • pi bond in C=O bond breaks by heterolytic fission forming a negatively charge intermediate
  • delta negative O donates a lone pair of electrons to the hydrogen ion, propagating the intermediate
  • forma a hydroxynitrile
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10
Q

how could a low PH affect the production of hydroxynitriles

A
  • HCN is very weak so hardly ionises at all
  • it would mean that more of the ionised CN ions will be converted into HCN
  • having fewer CN ions will slow the reaction down
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11
Q

carbonyl test - 2,4-DNPH

A
  • add a few drops of the unknown to the DNPH
  • if not crystals form add a few drops of sulfuric acid
  • if a yellow ppt forms then an aldehyde or ketone is present
  • can be hazardous as friction can cause it to explode
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12
Q

2,4-DNPH - determining the melting point of crystals

A
  • filter using a Buchner funnel
  • redissolve in hot ethanol
  • place in ice to reform
  • remove and filter
  • apply heat to evaporate solvent
  • place in capillary tube and into a melting point apparatus to find the melting point
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13
Q

acidified potassium dirchromate to distinguish

A
  • add a few drops of unknown solution
  • if nothing happens warm mixture gently in a test tube in a water bath for a couple of minutes
  • ketone remains orange
  • aldehyde turns green as orange dichromate ions are reduced to green chromium ions by the aldehyde
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14
Q

tollens reagant

A
  • contains diamminesilver ion
  • to make tollens add sodium hydroxide to silver nitrate(made freshly as can quickly become explosive)
  • a brown precipitate of silver oxide will form then add ammonia to redissolve the precipitate
  • add a few drops of the unknown to it and gently warm
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15
Q

results of tollens

A
  • ketone has no change remains colourless
  • aldehyde forms a grey precipitate (silver mirror)
  • the aldehyde reduced the diamine silver ion of metallic silver as the aldehyde is oxidised to form a carboxylic acid
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16
Q

why doesn’t DNPH react with other carbonyl containing functional groups?

A
  • with a carboxylic acid or amide, the compounds act as a base, leaving the carboxylate ion negatively charged and unable to attack the nucleophile
  • in aldehydes and ketones the carbocation is positively charged so can be attacked by a nucleophile.
17
Q

solubility of carboxylic acids

A

the C=O and O-H bonds in carboxylic acids are polar allowing carboxylic acids to form hydrogen bonds with water molecules

  • carboxylic acids up to four chains are soluble
  • as the number of carbon atoms increases, solubility deceases as the non-polar chain has a greater effect on the overall polarity
18
Q

strength of carboxylic acids and shape

A
  • they are classified as weak acids as they partially dissolute
  • have a planar arrangement and are isometric with esters
19
Q

redox reactions fo carboxylic acids

A
  • they react with metals to form hydrogen gas and a caroboxylate salt
  • metal disappears and effervescing
  • forms (CH3Ch2COO-)2 Mg2+
20
Q

reaction with alkalis and metal oxides

A
  • react with alkalis to form a salt and water
  • CH3COO-Na+
  • (CH3COO-)2 Ca2+
21
Q

reaction with carbonates

A
  • carbon dioxide gas is formed
  • if in excess would disappear
  • 2CH3COO-Na+
22
Q

test for a carboxyl group

A
  • carboxylic acids are the only common organic compound sufficiently acidic to react with carbonates
  • this helps to distinguish them from phenol which can’t
23
Q

what are derivatives of carboxylic acids

A

a compound which can be hydrolysed to form the parent carboxylic acid
-esters, acyl chlorides, acid anhydrides and amides

24
Q

esters uses and properties

A 
fairly unreactive
-used as flavourings 
solvents 
plasticides 
-they are insoluble in water as has an organic group attached to the O instead of an H. it therefore doesn't form hydrogen bonds with water
25
Q

acid anhydrides

A

-formed from the removal of water from two carboxylic acid molecules

26
Q

esterification

A
  • the reaction between an alcohol and a carboxylic acid to form an ester
  • heat under reflect with concentrated sulphuric acid
  • the acid is a dehydrating agent. it removes water causing the equilibrium to move to the right and this increases the yield of the ester
27
Q

hydrolysis of esters

A
  • ester is heated under reflect with dilute aqueous acid
  • the ester is broken down by water, with the ester acting as a catalyst
  • products are a carboxylic acid and alcohol
  • it is a reversible reaction but not dynamic equilibrium
28
Q

alkaline hydrolysis

A
  • it is irreversible as would produce an alcohol and carboxylic acid which were not the starting reactants
  • it is first hydrolysed to form a carboxylic acid and water. it is then neutralised to form a carboxylate salt
  • ester is heated under reflux with aqueous hydroxide ions
29
Q

acyl chlorides

A
  • can be prepared from their parent carboxylic acid from reaction with thionyl chloride -also known as sulphur dichloride oxide, it is a liquid at room temperature
  • also produces sulphur dioxide and hydrogen chloride
  • carry out in a fume cupboard as HCl is toxic
  • product isn’t pure as may be excess thionyl chloride or acid
  • they react with nucleophiles by losing their Cl- ion and retaining the C=O bond
30
Q

evaluation of acyl chloride reaction

A
  • very reactive and so a catalyst is not required

- produce good yields

31
Q

acyl chloride reactions

A
  • react with alcohols to form esters and HCl
  • react with phenol to form esters and HCl (carboxylic acid isn’t reactive enough to do this)
  • react with water to give esters and HCl
32
Q

reaction of acyl chlorides with ammonia and amines to form amides

A
  • ammonia and amides can act as nucleophiles by donating the lone pair of electrons in the nitrogen atom to an electron deficient species
  • these form amides (e.g. ethanamide) and HCl
  • it can form either primary or secondary amides
33
Q

amides

A

contain O=C-NH2

34
Q

acid anhydrides

A
  • react in a similar way to acyl chlorides with alcohols, phenols, water, ammonia and amides
  • they are less reactive so safer and more useful
  • 2(CH3CO)2O + C6H5OH – CH3COOC6H5 + CH3COOH
  • formation of phenyl ethanoate and ethnic acid from ethnic anhydride and phenol
35
Q

mechanism for reaction of acyl chlorides and nucleophiles

A
  • lone pair of electrons form nucleophile is attracted to the delta positive carbon atom in the C=O
  • a dative covalent bond is formed between the nucleophile and carbonyl carbon atom
  • Pi bond of C=O breaks forming a negatively charged intermediate
  • a long pair of electrons on oxygen reforms the C=O bond, removing the chloride ion
  • proton is also lost

A Level Chemistry (21 decks)

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