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Topic: electrophile/nucleophile and form of amino acid in a a certain pH  (Read 3983 times)

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Offline Zamzen

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In my text book it asks me questions if i can distinguish between electrophiles and nucleophiles. And i get stuck on this one. Br2.
It says its an electrofile which will attack something with negative charge. But i cant see where the Br2 would get its positive charge from. And about Hbr it says that its an elctrophile as well because the Br has higher electronegativity than H. So the H will attack a negativly charged region of a particle. But can this molecule also be a nucleophile? because Br is negatively charged so can it make a nucleophilic attack on something with a positve charge?

I also have problems with this question. aspartic acid which is an amino acid with its Iso electric point at 3.   write its formula in a pH 2.8 solution. its a-aminogroup has a pKa of 9.33. its a-carboxylic acid has pKa = 1.99 and its B-carboxylic acid has a pKa = 3.9.

My progress is that the amino chain will take up a proton and become NH3+. And my reasoning about the carboxylic acids is that for the side chain whose pKa is 3.9 he will serve as a base, and since he can not pick up any protons he stays the same. so, COOH on him. and on the main COOH im thinking that he will serve as an acid so he will donate his proton and become COO-
.
My text book says that the side chain donates its proton and the main COOH doesnt donate its proton. Can the text book be wrong or am i thinking wrong?

Offline Jorriss

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Re: electrophile/nucleophile and form of amino acid in a a certain pH
« Reply #1 on: October 10, 2010, 02:26:12 PM »
In the case of Br2 you have to consider polarization.

For example, the pi bond in an alkene is nucleophilic and will polarize the Bromine molecule so it has an induced dipole. It can now act as an electrophile.

Offline Zamzen

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Re: electrophile/nucleophile and form of amino acid in a a certain pH
« Reply #2 on: October 10, 2010, 02:48:29 PM »
thank you. still need help with the other question if anyone can help me.
« Last Edit: October 10, 2010, 03:07:39 PM by Zamzen »

Offline macman104

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Re: electrophile/nucleophile and form of amino acid in a a certain pH
« Reply #3 on: October 10, 2010, 03:15:26 PM »
I agree with your reasoning for the amino acid.  At a pH of 2.8, the side acid will be protonated because the solution is more acidic than acid group, and the main chain is more acidic than the solution, so exist in it's deprotonated form.

Offline Zamzen

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Re: electrophile/nucleophile and form of amino acid in a a certain pH
« Reply #4 on: October 10, 2010, 05:14:16 PM »
Thank you.

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