#MCQ, #Biomolecule, #vitaminC, #ascorbicacid, #Vinylogy,
Hello students i am dr tanva vishash i welcome you all in my channel chemistry the mystery of molecules today’s topic of discussion is one mcq about biomolecule in front of you i request you student please pause the video try by yourself and whatever answer you get please write in the comment box along with few words explaining your answer and remember self
Evaluation is essential for improvement and for the purpose you should try now i believe you have tried by yourself so it’s my turn to give you the right answer by the way before going to details let me tell you the name of this molecule it’s very familiar i mean majority of you have consumed this molecule its name is vitamin c or ascorbic acid okay so these
Molecules ascorbic acids chemical reactivity will see now from this name ascorbic acid you can understand that this molecule is actually acidic in nature and this acidity is mainly because of hydroxyl group now question which hydroxyl group there are two hydroxyl groups i shall give you the answer after some time and by the way this is not exactly vitamin c this
Is actually you can consider acetyl protected vitamin c so these two hydroxyl groups should be free in case of the actual vitamin c molecule since we are doing reaction so this part is protected now in this case dmso and thf is the solvent and potassium carbonate is the base and reagent and rx is the electrophilic reagent you can consider so let’s learn what’s
Happening here so this this is actually potassium carbonate means it is actually a basic medium so we know that acetyls are stable in base i have already discussed a dedicated lecture on this topic please visit for better understanding so they will not this acetyl part will not react or participate in this reaction so it will remain unaffected now question there
Are two hydroxyl group as i told these one and this one so question is which one will react now apparently both looks like similar because both are you can consider in and all while they have already discussed many lectures on in all innovative silinol ethers or such kind of molecules you can see now so this is actually enol now these o h groups are acidic because
They are polar attached to electronegative oxygen not only that these hydroxyl group is repeat this part is slightly more acidic why because if you consider this is actually acid and if i say vinyl gas acid this is phenylicos acid and by the web already discussed the principle of phenology so you may visit for better understanding about this concept now so this
Part is called vinyl logos acid because of this vinyl group ok and because of this vinyl group what happens the property of a molecule is carry forward what do i mean that this is actually acetic acid so we know this is acid as as acidic in nature we can hear about four point seven four and this is actually vinelogas as take as you may consider like that so it is
Also acidic yes obviously yes and if you look at this vitamin c molecule this is also a vinyl logos acid because this part just focus so if it is vinyl gas acid then its acidity is more by the way if you consider the other one i mean this part this is alpha hydroxy ketone derivative it’s not that much acidic like vinelogas acid because after deprotonation this
Vinyl logos acid has a canonical form what do i mean means conjugate base so this is double bond this is o minus and this o minus is stabilized how double bond comes here so it will come here and it will go so because of this resonance it is stabilized so from this thing we can understand that this proton is more acidic so in presence of milder base like k2co3
It will get deprotonated preferentially and not only that the medium if you see that is dmso and thf both are capable because if you look at the structure of dmso the actual structure is like that okay dimethyl sulfoxide so this o minus or if you look at thf this is thf in both case oxygen are there oxygen atoms so one oxygen and one oxygen and these make such
Molecules are capable to stabilize the cations like k plus so consequently k plus is stable so base reactivity is more or carbonates basicity enhances consequently this will abstract these vinyl logos proton or vinyl lucas carboxylic acid will be deprotonated to produce this anion ok and in this case we know these anion can act as a nucleophile and what it does
It attacks this rx from the back side and it opens this is a sn2 type reaction i have already discussed many lectures on this sn2 so this is nothing but you can consider ether for me means a star formation or if you see from carbox venalucus carboxylic acid point of view you can say its an ester formation or if you consider this is from a simple carbon oxygen
Miss alkoxide point of view this is williamson ether synthesis type reaction so after that what will produce it will produce this or so this is the answer now i have explained why that particular hydroxyl group will be deportionated because it is vinyl logos acid so it is more acidic compared to this alpha hydroxy ketone derivative second why the acetyl did not
Hydrolyzed because medium is basic so acetyl didn’t get metallized now so what is the answer of today’s discussion first of all these will remain unaffected so option c and d gone okay and this part will be hydrolyzed so option b is the right answer so if this question is asked in your exam same logic you can apply that basic medium so this will unaffected
So option c and d gone what is left option a and b and now ascorbic acid you have to remember that it is a vinyl gas acid and for that purpose you know you need to know the concept of principle of analogy then only you can come to this ah answer means option b will clearly because this is acidic so it will be deprotonated first and if this is deprotonated then
Alkylation will happened on the oxygen minus because alkoxide is a better nucleophile compared to simple alcohol or which or carboxylate you can say considered is more nucleophilic compared to simple o h so this is the overall discussion so in conclusion what you have learned today that ascorbic acid is a vinyl gas acid and spk is 4.2 it is significantly acidic
And if you look at the acidity of this acetic acid this pka is 4.74 so from this data you can consider that ascorbic acid or vitamin c is more acidic actually here the first deprotonation we are talking so it’s more acidic than acetic acid so it’s significantly strong acid now acetyls are stable in alkali or base but they are unstable in acidic medium potassium
Salts have relatively higher solubility than sodium salt because if you check the size comparison k plus is higher or bigger in size so charge density of k plus is less charge density of na plus is more so that’s why this get more solvated in the organic medium and in this case in the reaction medium there is dmso dimethyl sulfoxide it is actually student very
Polar region very polar solvent high boiling solvent so it is capable to solvent those inorganic salts and if such kind of salts are absent then you need a specific concept that is ptc or phase transfer catalyst i have already discussed this thing also now generally the tendency of leaving character for sn2 reaction is iodide is a better living group than bromide
Then chloride then fluoride ok and dmso can solvent k plus by its oxygen atom i have explained so i believe this video will be useful thanks for watching if you really enjoy please help this channel to grow and if possible please visit my another channel where i upload global warming and climate change related videos so stay happy stay blessed see you in my next
Transcribed from video
MCQ-174: About vitamin-C (ascorbic acid) by Dr. Tanmoy Biswas (Chemistry : The Mystery of Molecules) By Chemistry The Mystery of Molecules
