production of esters anyone?? (1 Viewer)

[little master]

i was looking at the syallabus and it never ask directly the steps used (method) for esterification. All it asks about on esterification are : reactants used and products formed, need for refluxing, purpose of using acids, safety precautions.

I was using called "DOT POINT" HSC chemistry and it never once states the method/procedure used.
Although this question was in 2001 HSC paper... yet its not in the syallabus now or not that i am aware of.
Can they ask this(procedure for conducting esterification) in the HSC? Correct me if i am mistaken...

thanks

 

[tristambrown]

i dont think it was in the syllabus then either ... aparently they have done this a few times - asked things not in sylabus i mean. apparently most peeps get it wrong so the marking criteria becomes vry leniant and as such peeps who write anything generally get some marks... if you dont know exactly just write what you do know .. correct relevant chemistry scores marks

As far as proceedure - it isnt that difficult ... simply put - throw everything in, boil (reflux) it for 30 mins & isolate your product from the leftover crud.

more detailed overview:
add boiling chips to stop bumping + carboxylic acid + alkanol + drops of conc H2SO4 Catalyst to round bottom flask. Invert and directly attach condensor to boiling flask & attach to water supply (feeding from bottom inlet up to ensure even distribution of water - no bubbles).

Apply heat & wait - done .. u now have yur ester

to seperate pour into seperatory funnel. In sep funnel wash ester with distilled H20 then wash with weak basic solution to neutralise any remaining acid <insert base choice here- we used sodium bicarbonate>, discarding the aqueous layer appropriatley with each wash. Pour ester out of funnel and use for whatever evil purpose you desire lol

 

[loolee]

The thing I know the purpose of using concentrated H2SO4 as a catalyst is instead of speed up its reaction and it also acting as a dehydrating agent. Remember, alkanol and alkanoic acid produce ester and water as a product. If you use H2SO4, it will increase the rate of ellimination of the water. Thus when you taken out the water product, the equilibrium will tend to shift to the right to make more of ester product.

This H2SO4 will not affecting the acidity of the ester, because this concentrated acid will tend to combine with water than with the ester (ester immicible with water). This will seperated by using separation funnel and washed it with distilled water by repeating it twice, to get rid of the unreacted alkanol and alkanoic acid. Drain off the lower aqueous layer and discard.

The excess acid from alkanoic acid could be neutralise by using sodium carbonate. As sodium carbonate is a strong base, and can neutralise the acid, means that alkanoic acid used is strong acid. After neutralising, the ester is purified further by using fractional distillation.

That's my point of view.....please correction for any mistakes...

 

[tristambrown]

loolee - yes the H2SO4 does shift equil to right by eliminating water & with everything apart from residual acididy of ester you are perfectly correct


Depending on the ester's properties the H2SO4 may be prsent in the ester portion. There is no clear cut line with polarity and solubility it's all on a scale of more of less soluble.. just because an acid is much more soluble in an aqueous phase doesnt mean we can assume it is not partially dissolved in the less polar layer (the ester in this case) either directly or indirectly due to water absorption. Many non polar solvents actually absorb water to a degree which will happily carry the acid across (hence the need to chemically dry many solvents before use) - eg from wikipedia "Ethyl acetate can dissolve up to 3% water and has a solubility of 8% in water at room temperature"

Though i dont know that we need so much detail for hsc chemistry

 

[phil2010]

i was looking at the syallabus and it never ask directly the steps used (method) for esterification. All it asks about on esterification are : reactants used and products formed, need for refluxing, purpose of using acids, safety precautions.

There are many ways to make esters - Refluxing is only one of the METHODS - and this method is stipulated by the syllabus.

 

[izzy88]

i thought production of esters was in the syllabus...

doesnt knowing the procedure come under the dot point 'indentify data, plan, select equipment and perform a first-hand investigation to prepare an ester using reflux' it does stipulate reflux, but thats only the first part in the 'preparation of an ester' cause u still have to then separate it through filtering, adding Na(2)CO(3) etc

oh well either way its been in past papers so u have to know it anyway...

 

[tristambrown]

izzy, i think phil's point is that we only use one route (incluuding all steps involved like the cleanup u mentioned) to our esters whuile others are available.

from wikipedia (coz im too lazy to pick up a book lol)

Esters can be prepared in the laboratory in several ways:

• by esterification of carboxylic acid derivatives and alcohols
• by transesterifications between other esters
• by Dieckmann condensation or Claisen condensation of esters carrying acidic α-protons
• by Favorskii rearrangement of α-haloketones in presence of base
• by nucleophilic displacement of alkyl halides with carboxylic acid salts
• by Baeyer-Villiger oxidation of ketones with peroxides
• by Pinner reaction of nitriles with an alcohol

 

[Keskimo]

alkanol + alkanoic acid ---conc. H2SO4---> ester + water
e.g. butanol + propanoic acid ---conc. H2SO4---> butyl propanoate + water

*alkanoic acids contain the carboxyl group, but the former is the correct terminology (for the purposes of the hsc anyway)

often the easiest way to gain marks is to draw a labelled diagram, including retort stand, bosshead and clamps, tripod, gauze, bunsen, water bath (beaker + water), globe flask with reagents, catalyst and boiling chips, and CONDENSOR indicating entry and exit of water. this is a valid way of describing the procedure and saves a lot of space! we might be asked to assess or justify the prodecure, in which case we do need to learn it in some detail.

yeah, the syllabus is like that...