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chemistry > carbonyls and carboxylic acids > Flashcards

carbonyls and carboxylic acids Flashcards

1
Q

What are acidified dichromate ions

A

H+/Cr2O72-

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2
Q

What is a common name and use for methanal

A

Formaldehyde, commonly used in solution to preserve biological specimens

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3
Q

How is the aldehyde functional group written

A

CHO

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4
Q

What is the structural formulae for methanal

A

HCHO

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5
Q

How is the ketone functional group written in displayed formulae

A

CO

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6
Q

What is another name for propanone and a use

A

Acetone , commonly used in nail varnish remover

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7
Q

How do you write the oxidising agent in an oxidation reaction

A

Square brackets o ( depending on whether it’s an aldehyde or carboxylilic acid you may need to put 2 os

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8
Q

Why will tertiary alcohols not oxidise

A

Have no hydrogens freely attached so cannot oxidise

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9
Q

How do you name ketones

A

Eg alkane then one , only write number if it cannot go on just 1 carbon

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10
Q

What is the rule for naming aldehydes

A

The aldehyde is carbon 1 , whereas a ketone needs to be numbered

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11
Q

What is the C=O made of

A

1 pi bond and 1 sigma bond , the pi bond is formed by the overlap of p orbitals above and below the plane .

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12
Q

What is special about the C=O bond that makes it different to alkanes

A

The C=O is polar , whereas C=C is non polar . This is because the oxygen atom is more electronegative than the carbon atom which creates a dipole in the C=O bond therefore electron density lies closer to the oxygen atom and a nucelophille is attracted to the delta positive carbon atom

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13
Q

What is a carbonyl group

A

C=O

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14
Q

What mechanism do carbonyls undergo

A

Nucleophillic addition

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15
Q

What is the reduction of aldehydes and ketones

A

-reduction of aldehydes and ketones is done in the prescence of NaBH4 which is used as a reducing agent which reduces aldehydes and ketones into alcohols ( opposite of alcohols into ketones and aldehydes )also known as sodium tetrahydrioborate(III) , which is done with a warm carbonyl with NaBH4 in aqueous solution .

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16
Q

What are aldehydes reduced into

A

Primary alcohols

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17
Q

What are ketones reduced to

A

Secondary alcohols

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18
Q

Why is NaBH4 used in the reaction

A

Provides the hydride ion :H- which is the nucleophile in the nucleophillic addition reaction

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19
Q

What happens in the reduction of ethanal

A

Reduced into an primary alcohol

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20
Q

What is used to represent the reducing agent in the equation

A

Square brackets H

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21
Q

What are the steps to the mechanism of reduction of carbonyls with NaBH4 and draw it

A
  • drawing
    1) lone pair of electrons from the hydride ion is attracted to and donated to the carbon atom in the carbonyl C=O double bond
    2) a dative covalent bond is formed between the hydride ion and the delta positive carbon atom in the C=O bond
    3) the pi bond in the C=O bond breaks by heterolyitc fission forming a negatively charged intermediate
    4) the oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule ( from the aqueous solution ) the intermediate has then been protonated to form an alcohol
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22
Q

Will NaBH4 reduce carboxylic acids

A

No , as they are less prone to attack

23
Q

What happens with the reaction of carbonyls with HCN ( hydrogen cyanide)

A

This is very useful as it provides a way to extend the length of the carbon chain
- hydrogen cyanide adds across the C=O bond of aldehydes and ketones
Eg you can use an aldehyde and HCN to produce a hydroxynitrile - contain both alcohol functional group and nitrile functional group CN

24
Q

What is hydrogen cyanide

A

Hydrogen cyanide is a colourless poisonous liquid . It boils just above room temperature. So it is incredibly unsafe , so HCN cannot be used in an open laboratory instead cyanide and sulfuric acid are used to provide HCN for this reaction

25
Q

Draw the dot and cross diagram for HCN

A

Double bond between CN and single between CH with a lone pair on the N

26
Q

Describe and draw the mechanism for the reaction with HCN

A

Draw

Steps
1) lone pair of electrons from the cyanide ion :CN- is attracted and donated to the carbon atom in the carbonyl C=O double bond
2) a dative covalent bond is formed between the cyanide ion and the delta positive carbon atom in the c=o bond
3) the pi bond in the C=O bond breaks by heterolytic fission , forming a negatively charged intermediate
4) the oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom
5) the intermediate has been protonated to form a hydroxynitrile product

27
Q

How do you detect the prescence of the carbonyl group

A
  • a solution of 2,4 dinitrophenylhydrazine(2,4-DNP) in a mixture of methanol and sulfuric acid is known as bradys reagent.( it is dissolved in methanol and sulfuric acid because solid 2,4-DNP can be very hazardous because friction or a sudden blow can cause it to explode)
    -bradys reagent is a pale orange solution
    -when bradys reagent is added to an aldehyde or a ketone , a yellow or orange precipitate is formed . this precipitate is a 2,4-dinitrophenyldrazone derivative and confirms the prescence of the carbon - oxygen double bond in an aldehyde or a ketone .
  • the test is only positive for aldehydes or ketones . there is no precipitate with a carboxylic acid or an ester
28
Q

what happens when 2,4-DNP reacts with a carbonyl group

A

the NH2 group adds across the c=o bond and oxygen and the 2 hydrogens from NH2 are removed , releasing a molecule of water , also known as an nucleophillic elimination reaction

29
Q

how do you distinguish between an aldehyde and a ketone once the 2,4-DNP test produces a positive result

A

we use tollens reagent to distinguish between aldehydes and ketones . tollens reagent is a solution of silver nitrate in aqeous ammonia, in the prescence of an aldehyde it gives a silver mirror - not with ketone .

30
Q

why does tollens reagent not produce a silver mirror with a ketone

A

tollens reagent is a very weak oxidising agent and only aldehydes are easily oxidised through to carboxylic acids , ketones are not oxidised by tollens reagent

31
Q

how is Tollens reagent made

A
  • aqueous sodium hydroxide and aqeous silver nitrate to form a brown precipitate of silver oxide. dilute aqueous ammonia is added until the precipitate just dissolves
  • in Tollens reagent the oxidising species is the aqueous silver ion Ag+ , when added to an unknown in a water bath , a silver mirror appears
32
Q

why is Tollens reagent made immediately

A

Tollens reagent has a short shelf life so needs to be made immediately before carrying out the test

33
Q

what is produced in the Tollens reaction

A

aldehyde + [O] = carboxylic acid + silver mirror ( the silver is the oxidising agent)
- Ag+ (aq) + e- = Ag(s)

34
Q

how do you identify the compound from the 2,4- DNP test

A

you can purify it by recrystallisation
- the 2,4-dinitrophenyldrazone derivative is slightly impure so the precipitate is filtered and washed with cold solvent
- it is then recrystallsed from a suitable solvent , the crystals are dissolved in the minimum quantity of warm solvent ( any insoluble impuritites could be removed by passing through a warmed filter) then when the solution cools , the crystals are re- precipitated and can be filtered , washed with a small amount of solvent and dried . they should then be pure . then you would find the melting point of the crystals by comparing to a database of melting points of the 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones to find out which you may have got

35
Q

what could you do if the melting point of your derivative is similar to ones in a database

A
  • use the boiling point too so you can confirm the compount
36
Q

what happens to melting and boiling points with impurities

A

-lower the melting point
-increase the boiling point

37
Q

what is the carboxylic acid functional group

A

contains both a carbonyl group , and a hydroxyl group

38
Q

how do you name a carboxylic acid

A

replace the e on the alkane with oic acid
eg ethane , ethanoic acid
-the carboxly carbon atom is always designated carbon -1- and substituents are after this

39
Q

draw the structure for maleic acid

A
40
Q

draw the strucutre of fumaric acid

A
41
Q

Why are carboxylic acids soluble in water

A

They form hydrogen bonds with water , solubility decreases as chain length increases as greater proportion of molecule is non polar
Boiling points higher than expected from their molecular weights as form hydrogen bonds with themselves to form dimmers ( they are soluble up to 4 carbon atoms )

42
Q

What is the trend in solubility in dicarboxylic acids

A
  • to name dicarboxylic acids you keep the alkane name and the e and use dioc eg butanedioic acid
    -solubility increases and melting point increases
43
Q

Why does propanal have a higher boiling point than butane

A

It has a permanent dipole therefore polar and has permanent dipole interactions

44
Q

Why does Ethanoic acid have a greater boiling point than propan-1-ol

A

Ethanoic acid is a carboxylic acid and can therefore form dimers as 2 Ethanoic acid molecules can form 2 hydrogen bonds whereas propan-1-ol can only form 1 hydrogen bond with another molecule

45
Q

What type of acids are carboxylic acids

A

They are weak acids as they only partially dissociate . They participate in typical acid reactions forming carboxylate salts ( eg acid + base —> salt + water )
However make sure you show the charges of the salts because they are ionic

46
Q

what is the salt formed from methanoic acid and a base (eg metal oxide , alkali ect)

A

methanoate

47
Q

what is the salt formed from ethanoic acid

A

ethanoate

48
Q

what is the salt formed from propanoic acid

A

propanoate

49
Q

what is the salt formed from butanoic acid

A

butanoate

50
Q

what is formed from magnesium and propanoic acid

A

magnesium propanoate and hydrogen

51
Q

what is formed from ethanoic acid and barium oxide

A

barium ethanoate (Ba2+ CH3COO-)2 and water

52
Q

what is formed from ethanoic acid and potassium hydroxide

A

potassium ethanoate and water

53
Q

describe the test for a metal carbonate . how would you use this test to distinguish between ethanoic acid and phenol

A

-add dilute acid and fizzing will occur
-carboxylic acids are the only common organic compound sufficiently acidic to react with carbonates , they are weak acids but phenol is a weaker acid and phenol does not react as not acidic enough , phenol only reacts with strong bases , therefore you can disinguish a phenol and carboxylic acid by adding a weak base eg sodium carbonate and only the carboxylic acid will react

chemistry (21 decks)

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