Oxone in DMF at room temperature. You can also use aqueous acetone or aqueous alcohols.

B. R. Travis, M. Sivakumar, G. O. Hollist, B. Borhan, Org. Lett., 2003, 5, 1031-1034

The usual process used in chemical synthesis for potentially labile functionalities is to use a Pinnick Oxidation, NaClO2/H2O2 in NaH2PO4/H2O.

Jones' Oxidation is probably overkill and chromium is gross.

NaOCl could chlorinate your electron rich aromatic ring. H2O2 and base could cause a Dakin oxidation resulting in a phenol. The Cannizaro reaction has a 50% yield limitation that converts half of your aldehyde into the alcohol. Hot, dilute nitric acid will destroy your compound.

Hi,

Some can oxidize at r.t. without catalyst in open air with relatively long times. Some can oxidized at high temperature in catalyst-free condition.... Heating at 140-150 can readily oxidize benzaldehyde to benzoic acid...

Dear Dhirendra,

there are many mild methods for aldehydes oxidation. The method selection depends on the reactivity of the rest of molecule connected with -CH=O. In the case of aromatic aldehyde an easy oxidation can be performed via reaction with a mixture of Fehling I and II solutions at elevated temperatures close to 100 oC. The principle of this method dwells in aldehyde oxidation at alkaline pH: 2Cu 2+  + 2e  /taken from aldehydic group/ in the presence of water yields carboxylic aromatic acid and Cu2O precipitate.  Similar mechanisms pays for aldehydes oxidation with AgNO3 dissolved in ammonia water solution / 2Ag+/NH3/2. + 2e = 2Ag + 2NH3.The excessive solution of the reagent is recomended to dillute and precipitate with dilluted inorganic acid. Storing it is hazardous due to formation of Ag3N which is even in wet state very sensitive and explosive moiety!

Another metod of oxidation of aromatic aldehydes might be their oxidation with  water solutions of  HNO3 at elevated temperatures / 60 to 90 oC/. To avoid possible nitration of the aldehyde it is necessary to check the reaction conditions /temperature, HNO3 concentration, other substituents and their position on an aromatic ring/ and perform the preliminary small scale oxidations followed with GLC, TLC or HPLC analyses. 

If aromatic aldehydes do not contain phenolic -OH, maybe  oxidation of -CH=O group with dilluted H2O2 could work well, in mildly acidic or neutral water solutions and moderately elevated temperatures.  

Maybe you may find in the literature even simpler mild aromatic aldehydes oxidations based on direct catalyzed or non-catalyzed reaction with O2.

Best regards,

R.S. 

Another method of aromatic aldehyde oxidation to corresponding acid is so called Cannizari reaction. Aromatic aldehydes and any aldehydes with tercial carbon in position alpha to -CH=O undergo to oxido-reduction reaction in strongly alkaline environment, yielding corresponding alcohol and carboxylic acid, e.g.  2 C6H5-CH=O   =  C6H5-CH2-OH + C6H5-COOH. As alkali medium, 33 % water solution NaOH is usually used. Reaction proceeds at ambient temperature in cooling water bath.  When the the reaction is completed, the reaction mixture can be rid of alcohol via distillation or extraction with diethylether, benzene, THF, chlorophorm and like.

Free acid can be released from the sodium salt dissolved in the residual mixture, or its suspension by addition of excess of mineral acid solution. The crystals of aromatic carboxy acids can be isolated via centrifuge or passing through filter paper.  

Oxone in DMF at room temperature. You can also use aqueous acetone or aqueous alcohols.

B. R. Travis, M. Sivakumar, G. O. Hollist, B. Borhan, Org. Lett., 2003, 5, 1031-1034

The usual process used in chemical synthesis for potentially labile functionalities is to use a Pinnick Oxidation, NaClO2/H2O2 in NaH2PO4/H2O.

Jones' Oxidation is probably overkill and chromium is gross.

NaOCl could chlorinate your electron rich aromatic ring. H2O2 and base could cause a Dakin oxidation resulting in a phenol. The Cannizaro reaction has a 50% yield limitation that converts half of your aldehyde into the alcohol. Hot, dilute nitric acid will destroy your compound.

With silber cation - ammonia complex.

Pyridinium chlorocromate  is good mild oxidising agent for the oxidation of aldehydes to acids.

Oxygen? Hypo (NaOCl)?

Depending on the aldehyde Oxidation of aromatic aldehye to acid can be achieved by using sodium chlorite which can chlorinate your aromatic rich ring however using a scavenger should solve this issue. To aromatic aldehyde (1 mmol) in a mixture of acetonitrile, tert-butanol and water (ratio 5:7:1, 10 ml/mmol) was added  sodium dihydrogen phosphate monohydrate (0.62 mmol).  The reaction mixture was cooled with ice bath added 2-methyl-2-butene (1.5 mmol), followed by a solution of 80% aqueous sodium chlorite (3.2 mol). The reaction mixture was stirred in cold until complete, standard workup and purification gives clean acid.

It depends on what you consider mild. It can go from Jones oxidation (stong acid) to AgO/NaOH (strong basic médium). Both can be run at 0ºC. If you have an electron rich aromatic, the ClO2 oxidation may yield side products. There are also one pot methods to get esters and amides from aldehydes under mild conditions.

A rapid oxidation of primary and secondary alcohols using catalytic amounts of TEMPO and Yb(OTf)3 in combination with a stoichiometric amount of iodosylbenzene afforded carbonyl compounds in excellent yields without over-oxidation. Oxidation of primary alcohols in the presence of secondary alcohols proceeded with good selectivity.

1)J.-M. Vatèle, Synlett, 2006, 2055-2058.

An aerobic oxidation of primary and secondary alcohols to aldehydes and ketones using TEMPO-CuCl as catalyst in the ionic liquid [bmin][PF6] has been developed. The system needs no bubbling of O2 due to its good solubility in the ionic liquid. The resulting aldehydes (with no traces of carboxylic acids) and ketones can be extracted with organic solvents. The ionic liquid can be reused after washing with water and drying under high vacuum (8 runs for the oxidation of benzyl alcohol: yields of 72%, 70, 68, 70, 65, 64, 62, and 60).

2) A. Ansari, R. Gree, Org. Lett., 2001, 1507-1509.

The  metod of oxidation of aromatic aldehydes with water solutions of HNO3 at elevated temperatures / 60 to 90 oC  and further analyzed by HPLC is simpler one, when compared with other methods, las suggeted by Dr. Rastislav Solar in discusison pannel. 

The Pinnick oxidation might work, I think it should be mild enough.

 Dear downvoter,

it is your point of view, OK. Did you read the question attentively? The economy and loss of yield was not the main objective of the process.

The author of this question expected to get an information conerning mild way oxidation of aromatic aldehyde to aromatic acid without any side reactions changing properties of aromatic ring and yielding a number of by-products. 

In this respect, the  Cannizari reaction leads to formation of pure aromatic carboxylic acid free of contaminants, easy to isolate, and ready to use for further purposes. It is known that the oxido-reduction reaction gives only 50 % yield in this case . 

R.S.

Tollen's Reagent- Ammoniacal Silver Nitrate.

Hi

In general it is possible to use UREA-H2O2 (UHP) IN FORMIC ACID. Quite good yield.

Good luck.

You can also take a look at :

Pinnick oxidation is very mild, but produces HOCl, which might chlorinate an electron-rich aromatic ring.  You might need a scavenger (sulfamic acid, or DMSO) that's present in excess, and/or more reactive than your substrate.

Thanks to all of you for giving the valuable suggestions.

Specifically, if it is dialdehyde or trialdehyde, then Cannizzaro method would give trouble; for single aldehyde it may be clean despite of low yield.

Dear Dhirendra.,

pity, that no one could guess the presence of more than 1 aldehydic groups in that aromatic moiety. Best regards,

R.S

Dear D,

NaClO2 in t-BuOH/buffer NaH2PO4 (pH=3.5) and alquene;