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A2 Chemistry 1 > 1.9 Derivatives of Carboxylic Acids > Flashcards

1.9 Derivatives of Carboxylic Acids Flashcards

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1
Q

describe the structural change that is undergone when a carboxylic acid is converted to an ester

A

The hydrogen in the -COOH group is replaced with an alkyl group (or possibly some more complex hydrocarbon group)

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2
Q

give the general group formula for an ester

A

RCOOR’

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3
Q

give the most common use for esters

A

artificial and natural flavours due to their fruity odour

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4
Q

Describe the boiling point trends of esters

A
  • Small esters have boiling points similar to those of aldehydes and ketones with a similar RMM
  • Are polar molecules - therefore have dipole-dipole interactions as well as Van Der Waals’ forces. However they don’t form H bonds with each other, so their boiling points aren’t as high as an acid with the same n number.
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5
Q

Describe the solubility of esters in water

A
  • Small esters are readily soluble in water, but solubility falls with chain length increase.
  • Esters can’t H bond with each other, but can form H bonds with water molecules
  • There will also be VdW forces and dipole-dipole attractions
  • As chain length increases, the hydrocarbon parts of the esters get in the way, and force themselves between water molecules, breaking the H bonds between water molecules. This makes the process energetically less profitable, and so solubility decreases.
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6
Q

describe the structural change that is undergone when a carboxylic acid is converted to an acyl chloride

A

-OH is replaced with a Cl

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7
Q

give the general group formula of an acyl chloride

A

RCOCl

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8
Q

describe the reactivity of acyl chlorides

A
  • Very reactive as Cl- is a good leaving group - even poor nucleophiles will react.
  • Will readily react with water, or even moisture in the air, to form a carboxylic acid.
  • Therefore they’ll often ‘fume’ when open in the atmosphere.
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9
Q

how do nucleophiles react with acyl chlorides?

A

nucleophile attacks delta positive carbon-1, electrons move to Cl, which breaks off

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10
Q

give the general equation for a reaction between an acyl chloride and water

A

RCOCl + H2O -> RCOOH + HCl

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11
Q

Name the techniques for the preparation of esters

A
  • from carboxylic acids + alcohol (esterification)

- from acyl chlorides + alcohol (esterification)

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12
Q

give the general equation for esterification of a carboxylic acid

A

RCOOH + R’OH -> RCOOR’ + H2O

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13
Q

what molecules form the water molecule in esterification of a carboxylic acid?

A
  • OH from carboxylic acid

- H from alcohol

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14
Q

what is needed to catalyse the esterification of a carboxylic acid and alcohol and why is this used?

A

strong acid
The addition of the sulfuric acid increases the yield as it is a drying agent and removes the water, shifting equilibrium to the RHS. The yield can also be increased by adding excess acid or alcohol

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15
Q

Describe the conditions necessary in the esterification of an acyl chloride

A

Acyl chlorides undergo esterification readily with alcohols at room temperature. No heating is necessary and there is a good yield (note arrow isn’t reversible) as the reaction cannot reform the very reactive acyl chloride.

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16
Q

give the general equation of esterification of an acyl chloride with an alcohol

A

RCOCl + R’OH -> RCOOR’ + HCl

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17
Q

List some reactions that esters can undergo

A

acid-catalysed hydrolysis

base-catalysed hydrolysis

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18
Q

Describe the use of an acid catalyst in hydrolysis of an ester

A

The reaction needs to be catalysed by a strong acid as water is a very weak nucleophile.

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19
Q

Describe the conditions for acid-catalysed hydrolysis of an ester

A

reflux with HCl

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20
Q

give the general equation for acid catalysed hydrolysis of an ester

A

RCOOR’ + H2O -> RCOOH + R’OH

21
Q

Compare the yield of acid and base catalysed hydrolysis of an ester

A
  • With acid catalysed hydrolysis, equilibrium occurs, with the products reacting together to reform the ester. This gives a very poor yield.
  • Base catalysed hydrolysis is much more useful as the carboxylate salt formed is fairly unreactive therefore the reaction isn’t reversible.
22
Q

Give the general equations for base catalysed hydrolysis of an ester

A
  1. RCOOR’ + NaOH -> RCOONa + R’OH

2. RCOONa + HCl -> RCOOH + NaCl

23
Q

Describe how a base catalysed hydrolysis is carried out in the lab

A
  • The reaction is carried out by refluxing the ester with aqueous alkali (usually NaOH) until the mixture becomes homogenous (two layers become one)
  • After hydrolysis the alcohol is distilled off and the residue acidified with a strong acid.
  • This liberates the weak carboxylic acid from the carboxylate salt. The acid can be distilled off.
24
Q

define triglyceride

A

a triester formed from propane-1,2,3-triol (glycerol), joined to three long chain fatty (carboxylic) acids

25
Q

define trihydric alcohol

A

a molecule with three -OH groups

26
Q

what types of esters can glycerol form and why?

A

Since glycerol have three -OH groups (sometimes called a trihydric alcohol) it can form mono-, di- and triglycerides.

27
Q

what is the structural difference between saturated and unsaturated fats?

A

saturated has no C=C, unsaturated has C=C

28
Q

give examples of saturated fats

A

animal fats such as butter, cheese and yoghurt

29
Q

how can you differentiate between a fat and an oil?

A

A fat is defined as one which is solid at 20ºC, while an oil is liquid at this temp

30
Q

Describe and explain the difference in boiling points between saturated and unsaturated fats

A
  • Saturated fats can line up closely together, resulting in strong VdWs’ forces between them. This makes them solid or very thick viscous liquids at room temp
  • Unsaturated fats can be monounsaturated (one C=C) or polyunsaturated (multiple C=C)
    Presence of C=C bonds produced kinks in the hydrocarbon chain, meaning the molecules can’t get as close together and form strong VdWs’, therefore unsaturated (esp polyunsaturated as there are more kinks) fats are liquids at room temp
31
Q

how can you test for saturation in a fat?

A

Use bromine water or iodine solution. Both will turn colourless in unsaturated fats (as they react with C=C). The volume required can indicate the degree of saturation.

32
Q

What is hydrogenation in an unsaturated fat?

A
  • Reduces the degree of unsaturation to such a level to make an oil solid at room temperature
  • Mainly affects oleic acid residues as vegetable oils contain a high degree of this fatty acid
  • The solid fat still has a high degree of unsaturation, which is regarded as a healthier alternative and is easier to spread
  • During this process only SOME of the C=C bonds are hydrogenated.
33
Q

describe the conditions used in hydrogenation of an unsaturated fat

A
  • Catalyst: Nickel dust (heterogenous catalysis)
  • Temperature: 180ºC
  • Nickel dust is added to the oil at the stated temperature, then hydrogen gas bubbled through whilst stirring.
34
Q

give the general equation for hydrogenation

A

-CH=CH- + H2 -> -CH2-CH2-

35
Q

what is saponification?

A

When a fat or oil is refluxed with dilute alkali, the triglycerides undergo simple alkaline hydrolysis.

36
Q

define transesterification

A

a reaction where the alkyl group of an ester is exchanged with the alkyl group of an alcohol

37
Q

give the general equation for transesterification

A

R’OH + RCOOR” -> R”OH + RCOOR’

38
Q

what is transesterification used for?

A
  • It is widely used to increase the melting point of sunflower oil for use in the manufacture of margarine
  • production of biodiesel
39
Q

describe the two different types of exchange that can occur in transesterification

A

The carboxylate (RCOO-) groups are exchanged between triglycerides (intermolecular exchange) or within the same triglyceride (intramolecular exchange)

40
Q

How is biodiesel made?

A

Biodiesel is made from heating vegetable oils (eg. rapeseed oil) and reacting with an excess of methanol in the presence of a catalyst of aqueous sodium hydroxide
On heating, the glycerol in the triglycerides exchanges with the alcohol to produce a mixture of the fatty acid esters

41
Q

what are the advantages of biodiesel?

A
  • Effective replacement for petrol as it generates a similar amount of energy when burnt
  • Can be burnt in petrol and diesel engines without modifying the engine
  • Renewable and almost carbon neutral
42
Q

what are the disadvantages of biodiesel?

A

Requires a lot of ground to grow crops

Not entirely carbon neutral (machinery and processing needed)

43
Q

what is interesterification?

A

A catalytic reaction that occurs when a catalyst or enzyme is introduced into oil and rearranges the fatty acids on the glycerol backbone of a triglyceride

44
Q

Why is interesterification used in margarine production?

A

Changes boiling point due to a change in intermolecular bonding

45
Q

what is the benefit of using acyl chlorides for esterification instead of carboxylic acids?

A

no catalyst needed
less purification
more reactive - goes to completion/higher yield

46
Q

describe the procedure for preparing a liquid ester from a carboxylic acid and an alcohol

A
  1. add a spatula measure of anti-bumping granules to a 50cm3 pear shaped flask
  2. in a fume cupboard, add 10cm3 ethanol, 12cm3 glacial ethanoic acid and 15 drop of concentrated sulfuric acid to a pear shaped flask
  3. place a 250cm3 beaker containing some water on a tripod and gauze over a bunsen burner
  4. clamp the pear shaped flask in the beaker of water so that the reaction mixture is below the water line
  5. connect the condenser to the pear shaper flask and clamp the condenser
  6. gently boil the reaction mixture in the water bath for 15-20 minutes. turn off the bunsen burner and cool the mixture by removing the hot water bath.
  7. rearrange the apparatus for distillation, heat the mixture gently and collect the distillate around the boiling point of the ester (77-78ºC)
  8. Transfer the distillate to a separating funnel
  9. add 10cm3 of sodium carbonate solution, stopper and shake gently, release pressure built up by formation of gas by opening the tap
  10. remove the stopper, allow the layers to separate and discard the lower aqueous layer
  11. repeat steps 8 and 9 until no more effervescence is observed
  12. transfer the ester to a conical flask
  13. add a spatula of anhydrous sodium sulfate and shake until the ester changes from cloudy to clear
  14. decant the ester into a clean dry pear shaped flask and add a spatula of antibumping granules
  15. clamp in a waterbath and gently heat, collecting the ester at its boiling point (77-78ºC)
47
Q

explain, using two equations, the function of sodium carbonate solution in the preparation of an ester

A

removes the concentrated sulfuric acid and can also react with unreacted ethanoic acid
Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2
Na2CO3 + 2CH3COOH -> 2CH3COONa + H2O + CO2

48
Q

give the equation for the dehydration of ethanol to ether (ethoxyethane)

A

2CH3CH2OH -> (CH3CH2)2O + H2O

A2 Chemistry 1 (10 decks)

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