SOLVED PROBLEMS

Subjective: 

Board Types Questions

Prob 1.    In E. coli DNA, the AT/GC ratio is 0.93. If the number of moles of adenine in the DNA sample is 558,000, calculate the number of moles of guanine present

Sol.        Number of moles of adenine must be equal to that of thymine
        (A + T) = 558,000 + 558,000 = 1116000
        Since (A + T)/(C + G ) = 0.93
        Therefore, no of moles of C + G

        C + G = 1116000/0.93 = 1200000 

        Since number of moles of C = number of moles of G
        Number of moles of guanine =1200000/2 = 600000

Prob 2.        How will you synthesize alanine from acetylene?

Sol.     

Prob 3.        Write the structure of alanine at pH =2 and pH = 10.

Sol.    Amino acids exist as zwitter ions (I) in aqueous solution. In presence of acids (pH=2) basic COO– accepts a proton to give cation (II) but in presence of base (pH = 10), the acidic   donates a proton to the base and thus exists as anion (III).

Prob 4.    Glycine exists as (H₃N+CH₂COO–) while anthranilic acid (P–NH₂C₆H₄– COOH) does not exist as dipolar ions. 

Sol.    – COOH is too weakly acidic to transfer H⁺ to the weakly basic – NH2 attached to the electron withdrawing benzene ring. When attached to an aliphatic carbon, the – NH₂ is sufficiently basic to accept H⁺ from – COOH group

Sol.        A = (NH₄)₂S₂O₈

Prob 6.        Complete the following reaction:

Prob 7.        In the structure of aspartame (an artificial sweetner) which is a peptide

(i)  Identify the four functional groups
        (ii) Write the zwitterionic structure
    (iii) Write the structure of amino acids obtained from the hydrolysis of aspartame 

Sol.        (i)    Four functional groups in aspartame are
            – NH₂ (amino group)
            – COOH (carboxyl group)

   (iii)    Hydrolysis of aspartame gives following two amino acids

Prob 8.    Following two amino acids lysine and glutamine form dipeptide linkage. What are two possible dipeptide?

solution:

Prob 9.        Complete the following reactions

Prob 11.    Give the classification of polymers obtained from esters of acrylic acid 
(CH₂ = CH.COOH)

Prob 12.    When primary bromides are treated with a solution of NaI in pure acetone precipitate generates within 3 minutes at 25°C whereas primary chlorides when treated with same reagent precipitate generates within 6 minutes at 50°C. Explain.

Sol.    The reaction is probably of the SN₂ type involving a bimolecular attack of the iodide ion upon the carbon atom carrying the chloride or bromine. Now Br– is better leaving group when Cl– (as it is a weaker base). So the rate of the reaction for bromides is faster.
          

Prob 13.    Glucose and fructose give the same osazone? Explain

Sol.    During osazone formation reaction occurs only at C₁and C₂ while rest of the molecule remains same, and glucose & fructose differ from each other only in arrangement of atoms at C₁ and C₂, so they give same osazone   

Prob 14.    Specific rotations of  -anomer of given mono saccharide is + 112° and for 
 -anomer is + 19°. Specific rotation of equilibrium mixture is 52.7° calculate % composition of  α and β anomers in the equilibrium mixture.

Sol.        Suppose fraction of  and anomers is X and Y respectively
        X + Y = 1 ……………………..(1)
        112X + 19Y = 52.7……………(2)
        By solving above two equations
        X = 36.2%,   Y = 63.8%

Prob 15.    Predict the products of the treatment of glycine with
        (a)    Aqueous NaOH
        (b)    Aqueous HCl
        (c)    Acetic anhydride
        (d)    NaNO₂ + HCl

Sol.    (a)    H₂N.CH₂COO–Na⁺
    (b)    Cl– + H₃N⁺CH₂COOH