Isomerism and Carbonyls

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56 Terms

1
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What can aldehydes be reduced to?

Primary alcohols

2
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How can we represent a reducing agent?

[H]

3
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What reducing agent can be used to reduce aldehydes?

Sodium tetrahydridoborate/NaBH4

4
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What is the name of the mechanism for the reduction of aldehydes and ketones?

Nucleophilic addition

5
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What is required for nucleophilic addition?

A slightly positive carbon and slightly negative oxygen joined by a double bond

6
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What happens during nucleophilic addition

  • A hydride (H- ion) with a lone pair of electrons attacks the delta positive carbon

  • The pi bond in the C=O bond breaks and the electrons move from the bond to the oxygen so that carbon will only be bonded to 4 atoms

  • This forms an intermediate with a negative O with a lone pair of electrons and a hydrogen from the hydride ion attached to the carbon

  • A H+ ion from the NaBH4 solution then forms a bond from the lone pair of electrons on the negative oxygen

  • This forms an alcohol

7
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When do optical isomers occur?

  • They occur when a carbon is attached to four different groups

  • We call this a chiral carbon

8
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How do we represent a chiral carbon?

With an asterisk next to the chiral carbon in the compound

9
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What is another name for optical isomers?

Enantiomers

10
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What is the definition of an optical isomer?

Optical isomers have the same physical properties, but they rote the plane of polarised light in equal and opposite directions

11
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How can we describe enantiomers in terms of their image?

  • They are non-superimposable mirror images of each other

  • This means that they are mirror images that cannot be perfectly stacked on top of one another

12
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What is plane polarised light?

  • Light only travelling in one plane

  • This is produced with the use of a polarising filter which has slits to only let one plane of light through

  • When plane polarised light passes through separate solutions of a pair of enantiomers, they rotate the light in equal and opposite directions

13
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Under standard lab conditions, what percentage of each optical isomer will be formed in a reaction? How will this affect a plane of polarised light?

  • 50%

  • If we have a solution like this the effects of each enantiomer will be cancelled out

    • We refer to them as optically inactive

    • A mixture of two enantiomers in a solution is called a racemic mixture

14
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What are hydroxynitriles?

Compounds that contain O-H and C≡N

15
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What is the priority of nitriles in terms of functional groups?

Second only to carboxylic acids

16
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How can hydroxynitriles be formed?

  • By the reaction of an aldehyde with KCN followed by the addition of H2SO4

  • This is a nucleophilic addition reaction where the cyanide ion acts as a nucelophile

17
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What should you remember about the charge on the cyanide ion?

The negative charge should be on the carbon

18
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When do we form hydroxynitriles from KCN and H2SO4 and not HCN?

  • HCN is incredibly lethal

  • Smells like almonds so yk its cooked

19
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Why can we form racemic mixtures with KCN/H2SO4 or NaBH4

  • The nucleophile is equally likely to attack above or below the carbon in the aldehyde group to form a chiral carbon

  • This means we can form two compounds that are non-superimposable mirror images of each other- they are enantiomers

20
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Standard answer for why we form racemic mixtures with KCN/H2SO4 or NaBH4? What are some exceptions to this?

  • The planar carbonyl group is equally likely to be attacked above or below its place

  • This leads to equal quantities of both isomers being formed- a racemic mixture is produced

  • One isomers will rotate plane polarised right clockwise, the other anticlockwise so the effects on plane polarised light cancel out

Exceptions to this are:

  • Methanol

  • Symmetrical ketones

21
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What reactions can carboxylic acids have?

  • Acid + base → salt + water

  • Acid + metal carbonate+ Salt + carbon dioxide + water

  • Acid + metal → salt + hydrogen

22
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How do we draw metal ions attached to a carboxylic acid?

  • You need to draw a negative charge on the oxygen that has lost the hydrogen and a positive charge on the metal ion to show ionic bonding

  • If the charge of the ion is not +1, draw how many carboxylic acid molecules you would need to cancel the charge out in brackets next to the ion

23
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How can we form esters?

  • By condensation reactions between carboxylic acids and alcohols

  • The OH must come from the carboxylic acid, the H must come from the alcohol

24
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What is the general word equation for the reaction of carboxylic acids and alcohol?

Carboxylic acid + alcohol → Ester + water

25
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What does an ester functional group look like?

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26
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How do we name esters?

  • The first component of the name is derived from the alcohol (methanol = methyl)

  • The second component is derived from the carboxylic acid (ethanoic acid = ethanoate)

  • The name in this example would be methyl ethanoate

27
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How do we name esters made from secondary alcohols?

  • We name it as we normally would

  • Then we can give the number 1 to the alkyl group attached- this is the only time this happens

28
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How can we form cyclic esters?

If a molecule has both an alcohol and carboxylic acid group, a condensation reaction can occur between these two groups to form a cyclic ester

29
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How can we from di-esters?

  • Di-esters are formed when a di-carboxylic acid reacts with 2 moles of alcohol

  • Also occurs when a di-alcohol reacts with 2 moles of carboxylic acid

30
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How can tri-esters be formed?

  • Reaction a tri-alcohol with 3 carboxylic acids

  • Essentially making a lipid

31
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What are some uses for esters?

  • Fragrances, essential oils, artificial flavours

  • Used as an organic solvent

  • Natural esters found in pheromones 

  • Naturally occurring fats and oils are fatty acid esters of glycerol

  • Nitrate esters known for explosive properties

  • Polyesters used to make plastics

  • Esters used to make surfactants like soap

32
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What does the hydrolysis of esters produce?

  • Alcohol

  • Carboxylic acid

33
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What are the conditions of the hydrolysis of esters?

  • Acid or alkali in solution

  • Reflux

34
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Why is the % yield of the hydrolysis of esters never 100%

It is a reversible reaction

35
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What is base catalysed hydrolysis of esters?

  • Done with sodium hydroxide under reflux

  • In practice alkaline conditions are normally used as equilibrium lies to the right hand side and the reaction is also quicker

  • In this reaction, the sodium salt of the carboxylic acid and an alcohol is formed

36
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How can base hydrolysis be used in saponification?

  • This is the production of soap

  • Triglycerides can be hydrolysed using hot sodium hydroxide to form glycerol and the sodium salts of long chain carboxylic acids

  • Sodium salts of long chain carboxylic acids are used to make soap

37
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What can base hydrolysis be used for the the manufacture of biodiesel?

  • Referred to as transesterification

  • Triglycerides HAVE to react with methanol

  • This is heated under reflux with concentrated potassium hydroxide which forms glycerol and long chain methyl esters

38
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What is the functional group for acyl chlorides?

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39
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What is the mechanism for the reactions of acyl chlorides with nucleophiles?

Nucleophilic addition elimination

40
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Why can acyl chlorides undergo nucleophilic addition elimination?

Because of the oxygen and the chlorine which both withdraw electron density from the carbon

41
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What are potential nucleophiles which can react with acyl chlorides and what conditions are needed?

  • H2O

  • ROH alcohol

  • NH3

  • RNH2 (primary amine)

The last 3 need anhydrous conditions

42
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What happens during nucleophilic addition elimination?

  • The nucleophile (Nu) with a hydrogen attached attacks the delta positive carbon

  • The pi bond in the C=O bond breaks

  • This forms an intermediate with a positively charged central nucleophile atom and a negative oxygen with a lone pair of electrons

  • The lone pair of electrons move to the C-O bond to reform the C=O bond

  • To compensate, the weakest bond the the carbon, the C-Cl bond, is broken as the electrons move to the chlorine atom

  • The one of the Nu-H bonds breaks with the electrons moving to the positively charged nucleophile in order to discharge it

  • This will form HCl as well as our other product

43
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What are the four word equations for the reaction of acyl chlorides with it’s nucleophiles (water, alcohols, ammonia and primary amines)

Acyl chlorides + water → carboxylic acid + HCl

Acyl chlorides + alcohol → ester + HCl

Acyl chlorides + ammonia → amide+ HCl

Acyl chlorides + primary amine → N-substituted amide + HCl

44
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What is the difference between an amide and an N substituted amide?

  • An N substituted amide has had one or both of the hydrogens swapped for an alkyl group

  • An amide has only hydrogen atoms attached to the nitrogen

45
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How do we name N-substituted amides?

Just put an N in front of the name and name it as normal e.g. N-methyl ehtnamide

46
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Amide group

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47
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Acid anhydride functional group

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48
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How are acid anhydrides formed?

From the condensation reaction between two carboxylic acids

49
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Why are carbons in acid anhydrides susceptible to attack?

The oxygen’s attached to the withdraw electron density, making the carbons slightly positive

50
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By what mechanism do acid anhydrides react with nucleophiles?

Nucleophilic addition elimination

51
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What are the four word equations for the reaction of acid anhydrides with it’s nucleophiles (water, alcohols, ammonia and primary amines)

acid anhydrides + water → 2 x carboxylic acid 

acid anhydrides + alcohol → carboxylic acid + ester

acid anhydrides + ammonia → amide + carboxylic acid

acid anhydrides + primary amine → N-substituted amide + carboxylic acid

52
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Reduced pressure filtration apparatus

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53
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RP10: Steps for preparation of an organic solid

  • Dissolve the impure solid in a minimum volume of hot solvent

    • Hot to increase solubility

    • Minimum to form a saturated solution so that sold will recrystalise upon cooling

  • Filter to remove insoluble impurities

  • Cool the solution down as slowly as possible

    • This allows larger and purer crystals to be formed

  • Scratch the inside of the conical flask and place in ice bath to help formation of crystals

    • Scratching creates grooves which increases the surface area for the formation of crystals

  • Filter sample under reduced pressure with a cold solvent

    • Cold ensures that the crystals don’t dissolve back in solution

  • Leave to dry

    • Drying means that we won’t calculate % yield that could be over 100% which isnt’ possible

54
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RP10: Melting point determination

  • Compare sample to mpt of pure substance

  • Impurities will decrease the mpt as they get in the way of IMF

  • The larger the mpt range, the more impure our sample is

Method

  1. Place sample of organic solvent into a capillary tube sealed at the end

  2. Place capillary tube into melting point apparatus

  3. Heat rapidly at first then slowly when approaching mpt

  4. Record temperature when solid starts and finishes melting

55
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What is the old method of melting point determination, what problems are there and how can we fix it?

  • A test tube filled with water with a thermometer and capillary tube attached above the bulb

  • The sample needs to be in line with the bulb due to the temperature gradient of the water

  • The water will evaporate if our sample has a melting point of of over 100 degrees celsius

  • We can use oil instead

56
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RP10: Purification of an organic solvent- solvent extraction

  • This works as organic solvents are immiscible in water (doesn’t mix)

Method

  1. Place organic liquid in separating funnel and add water, then remove the lower aqueous layer to remove any water soluble impurities

  2. Add sodium carbonate/hydrogen carbonate solution to remove excess acid used as a catalyst and shake with the organic liquid- if fizzing is seen, release the gas from the funnel to prevent an explosion by a build up of gas- remove the lower aqueous layer

  3. Place organic layer into a conical flask and add a drying agent such as anhydrous calcium chloride or magnesium sulfate- filter off the solid

  4. Could do a further distillation

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