Question

Write a small summary ( introduction) of what this experiment is going to be about

Experiment 6 Testsfor Aldehydes and Ketones Both aldehydes and ketones have carbonyl group and thus have similar reactions due to this functiona group. The C-o double bond consists of a -bond and a bond. The electrons situated above an below the carbonyl plane are exposure, making the carbonyl bond reactive. Oxygen is more electronegative than carbon and the C-O bond is polar, more negative on oxygen and more positive on carbon. When nucleophile attacks carbonyl carbon, the π electrons shift toward oxygen atom that can accommodate one negative charge while also follows octet rule. This initiates nucleophilic addition and nucleophilic acyl substitution. Attached to the carbonyl carbon, ketone has two alky groups while aldehyde has two or one hydrogen atom. Both hydrogen and alkyl are not good leaving group. Alkyl groups, owing to its electron-donating character, can stabilize the more positive carbon. Hydrogen atom, owing to its smaller size, has less steric hindrance than alkyl group. Aidehyde, therefore, is more reactive than ketone for the same reaction. ched to a carbonyl group can be oxidized while carbonyl group without hydrogen attached cannot. Mild oxidizing agents, Ag' and Cu' can oxidize aldehydes, but not ketones, to acids. Hydrogen atta rong oxidizing agents, chromic acid, H,CrOs, and potassium permanganate in acidic solution, can not oxidize ketones. To oxidize ketones, the C-C bond must be broken and needs stronger oxidizing agents or at high temperature. Silver ion and cupric ion in basic solution form hydroxide precipitates. To make them dissolve in basic solution, silver ion is stabilized by complexing with ammonia Tollen's reagent), and cupric ion is stabilized by complexing with tartrate (Fehling reagent) or with citrate Benedict reagent).

Answer 1

Aldehydes and ketones both contained carbonyl group but reactivity different.

Carbonyl group has polar in nature, (sp2) hybridized, due to difference in electronegativity of Carbon and Oxygen, the pi-bond is shifted towards more electronegative oxygen and creates partial charge separation, hence shows electromeric effect, when any molecule attacks it converts to tetrahedral carbon.

Aldehydes are more reactive towards nucleophilic addition reactions because ketones have two +I effect from 2- alkyl groups and increases electron density towards carbonyl carbon.

Nucleophilic addition reaction takes place like addition of HCN, addition of ROH/ NH3 / R-NH2 / PhNHNH2 like theses are nucleophilic addition reaction.

Examples of these reactions are:

clanィ 2. Sp Lond St s- C+)R mose deueases Clange en arb Casom in c CNan sp Sf 0H C. f2

o R R.

2. Chemical test :

Aldehydes are more reactive therefore easily oxidized by mild oxidizing agent into carboxylic acid by use of reagent like: Tollen's reagent / Fehling solution but ketones do not give these test.

Therefore we can check the sample either aldehyde or ketone by these test.

Reaction with Tollen's reagent:

ammonical silver nitrate solution is known as Tollen's reagent [Ag(NH3)₂]⁺ and active species is Ag2O.

When sample of aldehyde is treated with T.R. in a test tube and heat up in a boiled water bath silver mirror will be formed, a grey-white shine wall is formed on the wall of test tube. (Silver mirror test)

CH3CHO + Ag2O $\rightarrow$ CH3COOH + 2Ag(s) [redox reaction]

ketone does not oxidize with mild ox.agent due to 2(+I effect of alkyl groups) makes less reactive.

Fehling test:

Fehlin solution is made up by A and B reagent:

Fehling A is Copper sulphate solution (blue color)

and Fehling B is sodium potassium tartrate + NaOH solution

Reaction:

When a sample is treated with equal amount of Fehling A and Fehling B and heat up the solution about 10-15 minutes a red brown precipitate will be formed, which indicates that sample is aldehyde.

Ketones do not give this test.

RCHO + 2 CuO $\rightarrow$ RCOOH + Cu2O (s, red) [redox reaction]

Benedict's test also same in place of Sodium pot tartrate here is taken citrate solution.

Reaction with strong ox. agent:

Strong oxidizing agent like conc.HNO3 aldehyde and ketones both will be oxidized.

But ketones are dissociated into smaller fragments due to carbonyl alkyl cleavege and give smaller acids.

CH3COCH3 + [O]  $\rightarrow$ CH3COOH + HCOOH

Acetone acetic acid formic acid

CH3CHO +[O] $\rightarrow$ CH3COOH

Acetaldehyde Acetic acid