yazyj
why is amine a weak bronsted base? How does it differ from a strong bronsted base? What are possible examples of strong bronsted bases?
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BedokFunlandJC
Amines are weak Bronsted-Lowry bases because they only hydrolyze partially.

The levelling effect (beyond H2 syllabus, but if you're serious about getting A grade, you need to be prepared to learn beyond the syllabus) states that any Bronsted-Lowry bases stronger than the OH-(aq) ion will undergo complete hydrolysis to generate the OH-(aq) as the strong remaning basic species. Levelling effect applies to Bronsted-Lowry acids too.

Read : https://en.wikipedia.org/wiki/Leveling_effect

Examples of Bronsted-Lowry bases stronger than OH-(aq) would be soluble metal oxides such as Na2O(s), conjugate bases of ammonia or amines, etc. These will undergo complete hydrolysis to generate the OH-(aq) ion as the strongest remaning basic species. Eg. NH2- + H2O --> NH3 + OH-

Why is ammonia and amines weaker Bronsted-Lowry bases than OH-? Because the OH- has a negative formal charge on the O atom, hence the O atom (in OH- ion) is significantly more electron rich than the N atom (in ammonia and amines), and hence the lone pair is more available to accept a proton. So in this case, electron-richness outweighs the factor of electronegativity (which would predominate ceteris paribus), ie. N- is more basic than O- is more basic than N is more basic than O.

If you still need further clarification, ask during tuition.
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yazyj
DF8E429E-27F5-465A-9526-011EBC2318F5.jpeg

For part b), to get the tripeptide required, do I combine ser and pro with the 2 functional groups labelled a OR the 2 functional groups underlined at the bottom? And also why?  
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BedokFunlandJC
yazyj wrote:
DF8E429E-27F5-465A-9526-011EBC2318F5.jpeg

For part b), to get the tripeptide required, do I combine ser and pro with the 2 functional groups labelled a OR the 2 functional groups underlined at the bottom? And also why?  


Because by agreed convention for polypeptides of proteins, we (ie. scientists, chemists, biologists, biochemists, etc) always start with the amino or N terminus on the left, and the carboxylic acid or C terminus on the right.

This means Ser-Pro-Phe is totally different from Phe-Pro-Ser.

Let the R groups of the amino acids Ser, Pro and Phe be R1, R2 and R3 respectively.

Then the tripeptide Ser-Pro-Phe would be :

H2N-CH(R1)-CON(R2)CH-CONH-CH(R3)-COOH (as unionized molecular form)

+H3N-CH(R1)-CON(R2)CH-CONH-CH(R3)-COO- (as zwitterionic form in neutral pH aqueous solution)

Note that Proline is a special case (the structure of the tripeptide as written above is admittedly misleading, due to the unusual cyclic R group of Proline, see diagram below), as its R group is directly bonded to its alpha amine, making it a secondary amine (while all other proteinogenic alpha amino acids have primary alpha amines).

Proline_in_Polypeptides.png
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