Isomerism and Carbonyls

What can aldehydes be reduced to?

Primary alcohols

How can we represent a reducing agent?

[H]

What reducing agent can be used to reduce aldehydes?

Aqueous Sodium tetrahydridoborate/NaBH₄

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

Nucleophilic addition

What is required for nucleophilic addition?

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

What happens during nucleophilic addition

• A hydride (H^- ion) or another nucleophile 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 or nucleophile 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

When do optical isomers occur?

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

• We call this a chiral carbon

How do we represent a chiral carbon?

With an asterisk next to the chiral carbon in the compound

What is another name for optical isomers?

Enantiomers

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 by the same amount

• They are non-superimposable mirror images of each other

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

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

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

What are hydroxynitriles?

Compounds that contain O-H and C≡N

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

Second only to carboxylic acids

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

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

The negative charge should be on the carbon

Why do we form hydroxynitriles from KCN and H₂SO₄ and not HCN?

• HCN is incredibly lethal

• Smells like almonds so yk its cooked

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

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

What reactions can carboxylic acids have?

• Acid + base → salt + water

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

• Acid + metal → salt + hydrogen

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

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

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

Carboxylic acid + alcohol → Ester + water

What does an ester functional group look like?

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

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

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

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

How can tri-esters be formed?

• Reaction a tri-alcohol with 3 carboxylic acids

• Essentially making a lipid

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

What does the hydrolysis of esters produce?

• Alcohol

• Carboxylic acid

What are the conditions of the hydrolysis of esters?

• Acid or alkali in solution

• Reflux

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

It is a reversible reaction

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

How can base hydrolysis be used in saponification?

• This is the production of soap

• Triglycerides can be hydrolysed using 3 moles of sodium hydroxide under reflux to form glycerol and the sodium salts of long chain carboxylic acids

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

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

What is the functional group for acyl chlorides?

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

Nucleophilic addition elimination

Why can acyl chlorides undergo nucleophilic addition elimination?

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

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

• H₂O

• ROH alcohol

• NH₃

• RNH_{2 (primary amine)}

The last 3 need anhydrous conditions

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 of the carbon is broken as the electrons move to the to them atom that is separated from the compound

• 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

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

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

How do we name N-substituted amides?

• Put an N in front of the name

• The first part of the name comes from the alkyl group attached to the nitrogen atom

• The second part of the name comes from the length of the carbon chain from the C=O bond onwards

• e.g. N-methyl ethnamide

Amide group

Acid anhydride functional group

How are acid anhydrides formed?

From the condensation reaction between two carboxylic acids

Why are carbons in acid anhydrides susceptible to attack?

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

• The lone pair of electrons on a nucleophile are attracted to this carbon, so a bond is formed

By what mechanism do acid anhydrides react with nucleophiles?

Nucleophilic addition elimination

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

Reduced pressure filtration apparatus

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 isn’t possible

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

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

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

Write out a general equation for the reaction of an aldehyde or ketone with a reducing agent

Aldehyde/Ketone + 2[H] → Primary/secondary alcohol

What are some disadvantages to using acyl chloride for the manufacture of aspirin?

• Acyl chlorides are difficult to handle as they react very readily with water- anhydrous conditions needed

• React to produce corrosive HCl

Why are are acid anhydrides favoured over acyl chlorides for the manufacture of aspirin?

• Cheaper to produce

• Not as moisture sensitive

• Less corrosive

• Carboxylic acids produced are safer than HCl