Solved Examples on Alcohols, Phenols and Ether:

Question:1, Compound (A) gives positive Lucas test in 5 minutes when 6.0 gm of (A) is treated with Na metal, 1120 ml of H₂ is evolved at STP. Assuming (A) to contain one of oxygen per molecule, write structural formula of (A). Compound (A) when treated with PBr₃ gives (B) which when treated with benzene in prsence of anhydrous AlCl₃ gives (C). What are (B) and (C)?

Solution:

i) A gives Lucas test within 5 min. and thus (A) is secondary alcohol

ii) (A) contains one O-atom and thus, only one O-H group in (A).

      Thus, (A) is C_nH_2n+1 OH.

iii) Since, 1120 ml of H₂ is given by the action of Na over 6 gm (A)

 11200 ml H₂ is given by the action of Na = (6×11200)/1120  = 60 gm (A)

M.wt. of (A) is 60

Since one mole of an alcohol having one OH group gives 11200 ml H₂ at STP with Na.

iv) C_nH_2n+1Oh = 60
n = 3
or (A) in C₃H₇OH; being secondary alcohol its formula is CH₃.CHOH.CH₃.

Reactions Involved:

Question:2, 0.396 gm of a bromoderivative of a hydrocarbon (A) when vapourised occupied 67.2 ml at NTP. On reaction with aqueous NaOH (A) gives (B). (B) when passed over alumina at 250°C gives a neutral compound (C), while at 350°C it gives a hydrocarbon (D), (D) when heated with HBr gives an isomer of (A). When (D) is treated with conc. H₂SO₄ and the product is diluted with water and distilled, (E) is obtained. Identify (A) to (E) and explain the reactions involved.

 

Solution: Molecular weight of A = (0.369 ×22.1×1000)/67.2

(A) is a bromiderivative so it may be written and R – Br

    M_RBr = 123, So, R = 43 Q Br = 80 (Atomic weight)

i.e. C_n H_{2n + 1} = 43 or, 12n + 2n + 1 = 43

or n = 3

 A = C₃H₇Br

So, the possible structure of (A) are CH₃CH₂CH₂Br or CH₃CH(Br)CH₃

The reactions are as follows.

Question:3. Explain the lower boiling point and decreased water solubility of o-nitrophenol and o-hydroxybenzaldehyde as compared with their m-and p-isomers.

 

 

Solution: Intramolecular H–bonding (chelation) in the o-isomers inhibits intermolecular attraction, lowering the boiling point, and reduces H-bonding with H₂O, decreasing water solubility. Intramolecular chelation cannot occur in m- and p-isomers.