Chemistry - ALCOHOL & ETHER Question with Solution | TestHub

MECHANISM

The reaction probably involves a cyclic transition state (I) in which a hydride ion (H^_ ) from the a - CH

bond of the alkoxide migrates to the carbonyl carbon of the ketone to yield the mixed alkoxide

(II).

b → I,q

REDUCTION WITH DIBAL-H (DI- ISOBUTYL ALUMINIUM HYDRIDE)

is written as AlH(i-Bu)₂ . It reduces R –C ≡ N to R–CHO and also ester

to RCH = O and alcohol. It does not reduce (C = C) or (C ≡ C) bond. Nitriles are

selectively reduced by DIBAL-H to imine hydrochlories followed by hydrolysis to aldehydes.

Similarly, esters are also reduced to aldehydes with DIBAL -H.

c → II,p

TOLLENS, FEHLING’S BENEDICT’S AND SCHIFF’S REAGENTS

Aldehydes are easily oxidised to acids containing the same number of C atoms. Hence, they are oxidised

not only by strong O.A. such as MNO₄ and K₂Cr₂O₇ but also by weak O.A. such as Br water,

Ag⁺ ,Cu²⁺ ions, etc, As a result, aldehydes act as strong reducing agents. They reduce (i) Tollens

reagent to metallic silver (silver mirror), and (ii) Fehling’s or Benedict’s solution (F.S. or B.S.) to a red

precipitate of cuprous oxide (Cu₂O).

a . Reduction of Tollens reagent : T.R. is an ammoniacal solution of ${\mathrm{AgNO}}_3$. When an aldehyde is heated with T.R., the latter is reduced to metallic Ag and aldehydes are oxidised to acids. Thus. T.R. is called an oxidising agent.

The silver, thus deposited, shines like a mirror. Hence, this test is known as silver mirror test T.R. does

not attack olefinic bond. For example

b. Reduction of Fehling's solution : It is an alkaline solution of ${\mathrm{CuSO}}_4$containing some Rochelle salt, i.e. sodium potassium tartarate $\left(\mathrm{NaOOC}-\mathrm{CH}{\left(OH\right)}_{}-\mathrm{C}\left(\mathrm{OH}\right)-\mathrm{COO}K\right)$. When an aldehyde is heated with F.S., the latter is reduced to give a red precipitate of cuprous oxide.

F.S.’ ‘A’ contains aqueous solution of CUSO₄ , while F.S. ‘B’ contains ammoniacal solution of Rochelle

salt. For testing aldehydes. F.S. A and B are mixed in equal amounts.

c. Function of Rochelle salt in Fehling’s solution:

Cu²⁺ ions get precipitated in alkaline medium as Cu(OH)₂ . So, Rochelle salt is added to keep Cu²⁺ in

alkaline solution. The insoluble Cu(OH)₂ first formed goes into the solution due to the formation of a

soluble complex between Cu²⁺ ions and tartarate ion as shown below

Note:

(i) Aromatic aldehyde are not oxidized by F.S. and B.S.

(ii) T.R. test is also given by a - hydroxy ketone .

It is due to rearrangement in aldehydes via enediol intermediate (however this need a terminal a - carbon.

This compound, although a- hydroxy ketone but does not have a- C, does not give positive Tollen’s test.

(iii) All of them do not oxidise olefinic bond.

(iv) T.R. test is also given by alkyl or aryl hydroxyl amine.

d. Reduction of Bendict's solution ; it is an alkaline solution of ${\mathrm{CuSO}}_4$ containing some sodium citrate. $\left(\mathrm{NaOOC}-{\mathrm{CH}}_2-\mathrm{C}(\mathrm{OH})(\mathrm{COONa})-{\mathrm{CH}}_2\mathrm{COONa}\right)$. It reacts in the same way as Fehling's solution.

e. Reduction of Schff's reagent : It is a dilute solution of rosanaline hydrochloride in ${\mathrm{H}}_2\mathrm{O}$, whose red (magenta) colour has been discharged by passing ${\mathrm{SO}}_2$. When this reagent is added to aldehyde, magenta colour is restored.

Note : Formic acid reduces Tollens reagent and Fehling’s and Fehling’s solution. This is because formic

acid can be considered to contain both an aldehyde (–CH = O) and a carboxyl (–COOH) group.

Therefore, formic acid behaves as a reducing agent. Therefore , T.R. and F.S. can distinguish between

HCOOH and CH₃COOH, e.g.

(ii) ${\mathrm{CH}}_3\mathrm{COOH}\stackrel{\text{ T.R. }}{\to }$ No silver mirror

(iii)

$\mathrm{HCOOH}+\underset{\text{F.S }}{\underbrace{2{\mathrm{Cu}}^{2\oplus }+40^{\ominus }\mathrm{H}}}\to {\mathrm{Cu}}_2\underset{\text{ Red.ppt }}{\mathrm{O}}\downarrow +{\mathrm{CO}}_2\uparrow +3{\mathrm{H}}_2\mathrm{O}$

(iv)

${\mathrm{CH}}_3\mathrm{COOH}\stackrel{\text{ F.S. }}{\to }$ No red ppt. of ${\mathrm{Cu}}_2\mathrm{O}$