Advanced A Level Organic Chemistry: Selective reduction of carboxylic acids

AQA Advanced A level Chemistry Edexcel Advanced A level Chemistry OCR Advanced A level Chemistry A OCR Salters Advanced A level Chemistry B

Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 6.5 Selective reduction of carboxylic acids and use of the products in organic synthesis routes and a unique carboxylic acid oxidation!

INDEX of all carboxylic acids and derivatives notes

All Advanced A Level Organic Chemistry Notes


6.5.1 Reduction of a carboxylic acid to a primary aliphatic alcohol

Sodium tetrahydridoborate(III) NaBH4, (sodium borohydride), is not a powerful enough reducing agent to reduce carboxylic acids.

Lithium tetrahydridoaluminate(III), LiAlH4, (lithium aluminium tetrahydride), is a much more powerful reducing agent than NaBH4, and in ether solvent, readily reduces carboxylic acids to primary alcohols.

The LiAlH4 effectively releases a hydride ion, :H-, a powerful nucleophile - electron pair donor, which can attacks the δ+ carbon of polarised carbonyl bond δ+C=Oδ-. (electronegativity of O > C).

The reaction must be carried with a dry solvent such as ethoxyethane ('ether') because LiAlH4 reacts rapidly with water (and ethanol too).

The reaction is complex and goes through various stages and can be summarised as:

RCOOH  +  4[H]  ===>  RCH2OH  +  H2O  (R = H, alkyl or aryl)

The initial product must then be hydrolysed with water to release the primary alcohol.

e.g. propanoic acid to propan–1–ol: CH3CH2COOH + 4[H] ==> CH3CH2CH2OH + H2O

or benzoic acid to phenylmethanol: C6H5COOH + 4[H] ==> C6H5CH2OH + H2O

(c) doc b  +  4[H]  ===>  (c) doc b  +  H2O


So, LiAlH4 (not NaBH4) readily reduces the carbonyl group (>C=O) in carboxylic acids and derivatives to the primary alcohol functional group.


As far as I know, metal/acid reducing agents like Zn(s)/HCl(aq) is not powerful enough to reduce carboxylic acids.

H2(g)/Ni(s) will NOT reduce carboxylic acids, but there are other specialised catalysts that can effect this reduction using hydrogen gas in the chemical industry.

RCOOH  +  2H2  ===>  RCH2OH  +  H2O   (R = H, alkyl or aryl)


A simple example to illustrate what is, and is not, reduced in terms of the functional groups.

e.g. propenoic acid H2C=CH-COOH ('acrylic acid'),

which has two functional groups - alkene and carboxylic acid

(i) H2C=CH-COOH  +  [H]  == NaBH4 ==>  No reaction, no reduction


(ii) H2C=CH-COOH  +  4[H]  == LiAlH4 ==>  H2C=CH-CH2OH  +  H2O

The alkene is NOT reduced, but the carbonyl group is, giving prop-2-en-1-ol. via the negative hydride ion nucleophile attacking the polarised carbonyl bond.

Nucleophilic attack:  -H: ==> δ+C=Oδ-

This product is an unsaturated primary alcohol, still with two functional groups.

The >C=C< alkene group is NOT reduced by LiAlH4 because the attacking nucleophile is essentially a negative hydride ion (:H-) which would be repelled by the high electron density of the pi electron cloud of the non-polar C=C double bond.


(iii) H2C=CH-COOH  +  H2  == Ni ==>  H3C-CH2-COOH

Only the alkene group is reduced with a nickel catalyst, giving propanoic acid with only one functional group.


This means you can selectively reduce either functional groups or you need two reductions to form propan-1-ol.


The molecule now has the chemistry of primary alcohols

6.5.2 A unique oxidation

Carboxylic acids are usually quite stable against oxidation.

Think of the end product of oxidising alcohols and aldehydes with acidified potassium dichromate(VI) - carboxylic acids.

However, in terms of lower members of the simple aliphatic carboxylic acids, there is one clear exception.

Apart from a carboxylic acid with an aldehyde group as or in a side chain, the first in this homologous series of aliphatic carboxylic acids, methanoic acid (HCOOH) is the only carboxylic acid that is the only one easily oxidised, acting as a reducing agent, it gives the following results with the following reagents:

(i) It gives a silver mirror with ammoniacal silver nitrate (Tollen's reagent).

(ii) It gives a red-brown copper(I) oxide (Cu2O) with blue Fehling's solution.

(iii) However, it does not give a yellow-orange precipitate with 2,4-dinitrophenylhydrazine - carboxylic acids themselves do not usually undergo nucleophilic addition - elimination reactions like aldehydes do.


In tests (i) and (ii) methanoic acid is oxidised to carbon dioxide and water.

HCOOH  +  [O]  ===>  CO2  +  H2O

So methanoic acid acts as a reducing agent and readily oxidised to carbon dioxide and water by these two reagents.

When you write the formula of methanoic acid, HCOOH, the 'usual' abbreviated structural formula, the explanation isn't as obvious until you write it another way ...

(c) doc bi.e.  HOCHO is now a formula of a hydroxy-aldehyde, hence its ease of oxidation, giving positive results for two the simple tests for aldehydes !!!

So methanoic acid behaves like an aldehyde, the left of this particular carboxylic acid molecule has a H-C=O grouping, identical to an aldehyde group.

Methanoic acid esters have the same 'end' grouping e.g. ethyl methanoate, (c) doc b so I assume they can also give a positive 'aldehyde' result?

(c) doc b Ethanoic acid CH3COOH, cannot behave in this way, the left H is replaced by an alkyl group, so there is no equivalent of an aldehyde group present in the molecule.

You can't 'rearrange' any of the other carboxylic acids to be both aldehyde and carboxylic acid.


INDEX of all carboxylic acids and derivatives notes

 All Advanced Organic Chemistry Notes


KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 


best gift deals latest video game release, best gift deals best bargains in shop sales latest pop music releases, best sales deals download free music, latest film releases, best gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best gift deals best bargains in shop salesgirls buy clothes, spend a lot of money on clothes, best sales deals shoes, sweets and chocolates, magazines and make-up best gifts for teenagers Boys buy food and drink, computer games best gift deals best bargains in trainers shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best sales deals best bargains in shop sales buses and trains to transport them going out best gifts for teenagers best bargains in shoes shop sales Girls spend a lot of money on clothes best gift deals color colour preferences in clothes, cool sunglasses best sales deals boys buy expensive thins like best gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best shoes gift deals fashionable quality cool airpods, air pods, fashionable clothes best sales deals the most popular favourite sneakers best gifts for teenagers fancy shoes, sweets, chocolates, magazines, best trainers gift deals best bargains in shop sales lip moisturizer best bargains in  shop sale slots on make-up, well being, teenage decor decorating their room best sales deals teenagers like LED string lantern lights, best gifts for teenagers beauty products for teenagers, denim jackets, scrunchies coolness, fashionable back packs, typical boy’s shopping list includes mobile credit deals, eating out takeaway food and drinks, what teenagers like to buy in clothes best gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best gift deals what teenagers worry about, what teenagers like to do for fun sports best sales deals what cool things do teenagers buy, resale websites like eBay Teenager best high street shop or best online deals currys pc world argos amazon john lewis dell acer samsung raycon bose sony asus huawei HP microsoft in-ear headphones earbuds ipad desktop computer laptop computer for school college university students latest video games consoles apple iphone google high end mobile phones cell phone bargain smartphone xiaomi oppo high tech products latest fashion in trainers latest fashion in shoes latest fashion in mobile phones cell phones