year 13 organic 1

optical isomerism, aldehydes and ketones, carboxylic acid

what are optical isomers?

non- superimposable mirror images

how do optical isomers arise ?

• arises when a molecules mirror image is non-superimposable

• also called enantiomers

• this happens when a molecule contains a carbon atom with 4 different groups attach to it

• that carbon can be called the ‘chiral centre’ therefore the whole molecule can be referred to as ‘chiral’

• we can also refer to these types of molecules as ‘optically active’

is this optically active ?

not optically active

is this optically active ?

optically active

is this optically active ?

not optically active

is this optically active?

optically active

is this optically active ?

not optically active

properties of optically active compounds?

• identical chemical and physical properties

• effect of plane polarised light, one optical isomer will rotate the plane of polarisation clockwise, the other will rotate it anticlockwise

reaction with other chiral molecules ? (mention drugs)

• chiral molecules react differently

• many drugs are optically active with one enantiomer only having the beneficial effect

• in other cases, the other enantiomer can be harmful (thalidomide)

how to distinguish between aldehydes and ketones?

fehling’s/ tollenes reagent

aldehydes/ketones are a better oxidising/reducing agent than the other due to the presence of a ….. atom attached to the …. double bond therefore is easier to get oxidised/ reduced.

aldehydes are a better reducing agent than the other due to the presence of a hydrogen atom attached to the carbon-oxygen double bond therefore is easier to get oxidised

describe test to differentiate aldehydes and ketones. (describe tollens)

Tollen’s reagent can be used to identify aldehydes. It is added to the solution being tested and warmed gently.

If an aldehyde is present, a silver mirror will form in the test tube. If not, the solution will remain colourless.

describe test to differentiate aldehydes and ketones. (describe fehling’s solution)

Fehling's solution is another way of testing for aldehydes. It is added to the solution being tested and heated.

A brick red precipitate will form if an aldehyde is present and if not, the solution remains blue and there is no observed change.

what is formed when an aldehyde and ketone is reduced ?

aldehydes - primary alcohol

ketones - secondary alcohol

what reducing agent is used to reduce aldehydes and ketones and how does it allow this? (what is it dissolved in as well to allow it to work )

whats the name of the mechanism that takes place ?

NaBH4/ LiBH4

provides a source of hydride ions (H-) which is the nucleophile

dissolved in water/ ethanol which is the source of H+ ions

nucleophilic addition

outline the mechanism of reduction of an aldehyde ethanal

what else can be used as a reducing agent other than NaBH4 / LiBH4?

state conditions and what it produces and why its not preferred over the other reducing agent

KCN

• acidified KCN as thats a good source of H+ ions

• produces hydroxy nitriles

• HCN not used as its a toxic gas

outline mechanism of an aldehyde with KCN ethanal

what happens if unsymmetrical ketone/ any aldehyde undergoes nucleophilic addition?

a racemic mixture would be formed

• C=O bond is flat/planar

• nucleophile can attack from above or below with equal probability

• racemic mixture forms (equal amount of two enantiomers of an optically active substance)

• no optical activity/ no effect on plane polarised light as half would rotate it clockwise and the other half anticlockwise

carboxylic acid + metal

carboxylic acid + carbonates

carboxylic acid + bases and alkalis

• salt + hydrogen

• salt + water + CO2

• salt + water

how are esters formed ? what catalyst is used ?

carboxylic acid + alcohol

sulfuric acid catalyst - H2SO4

name these esters

1. ethyl propanoate

2. propyl methanoate

3. methyl butanoate

4. propyl ethanoate

name this ester

butyl butanoate

name this ester

ethyl 2-hydroxypropanoate

outline acid hydrolysis of esters - methyl propanoate

CH3CH2COOCH3 + H2O ~ CH3CH2COOH + CH3OH

~ = reversible reaction sign

! remember H+ acid catalyst on top of reversible reaction sign

acid hydrolysis of an ester would form a carboxylic acid and an alcohol

this is a reversible process so if this is done in a closed system an equilibrium mixture of reactants and products is obtained

outline base hydrolysis of esters - methyl propanoate

what is the product called

CH3CH2COOCH3 + H2O CH3CH2COOH + CH3OH

then with NaOH it forms:

CH3CH3COONa + H2O

overall: CH3CH2COCH3 + NaOH —> CH3CH2COONa + CH3OH

CH3CH2COONa - carboxylate salt

why do scientists make esters? what can they be used for?

• esters give fruits their smell

• they are sweet smelling which are added to foods/perfumes

• they are used as solvents

• they are used plastics to make plastics softer and more flexible

- soaps and biodiesel

how are triesters formed? and what is the IUPAC name for glycerol ?

they’re formed when 3 carboxylic acids react with the alcohol (aka glycerol)

glycerol - propan- 1,2,3 - triol

show how glycerol reacts with carboxylic acid to form a triester

how would you form soap from a triesters ?

hydrolysing triesters is done using conc. NaOH

glycerol (propane-1,2,3 - triol) and sodium carboxylate salts are made

how can triesters be used to make biodiesel ?

triesters react with methanol in the presence of a base catalyst

methyl esters and propan- 1,2,3 triol are produced

what are acyl chlorides?

an acid derivatives where the OH group has been replaced by a chlorine atom

how can you produce an acyl chloride?

mix a carboxylic acid with phosphorus pentachloride (PCl5)

Ethanoyl chloride + water

ethanoic acid + HCl

ethanoyl chloride + methanol

methyl ethanoate + HCl

ethanoyl chloride + ammonia

ethanamide + HCl

ethanoyl chloride + propylamine

n- propylethanamide + HCl

what’s the name of the mechanism which would take place in an ethanoyl chloride ?

nucleophilic addition- elimination

outline the mechanism between ethanoyl chloride and water. and state observation made.

outline the mechanism between ethanoyl chloride and alcohol. and state observation made.

outline the mechanism between ethanoyl chloride and ammonia. and state observation made

outline the mechanism between ethanoyl chloride and primary amine. and state observation made

ethanoic anhydride + water

2x ethanoic acid

ethanoic anhydride + methanol

methyl ethanoate + ethanoic acid

ethanoic anhydride + ammonia

ethanamide + ethanoic anhydride

ethanoic anhydride + methylamine

n- methylethanamide + ethanoic acid

compare acyl chloride and acid anhydride reactions