Acidity of lactic acid and propanoic acid

[Michael_Light]

Homework Statement

i) Is lactic acid,CH₃CH(OH)COOH a stronger or weaker acid than propanoic acid (CH₃CH₂COOH)

ii) Another question is does ketone reacts with phosphorus pentachloride PCl₅?

Homework Equations

The Attempt at a Solution

(i) I guess lactic acid is a stronger acid because the oxygen atom on the hydroxyl group is very electronegative thus increases the polarity of the -OH bond on the carboxyl group, so H⁺ are more easily released. (Please correct me if i am wrong)

(ii) I can't find any relevant information from my textbook and even on google, but I encountered this type of question from my exercise book.. Can anyone guide me?

 

[Infinitum]

Hi Michael!

For (i) you are correct that lactic acid is stronger. -OH in an alkyl chain acts as electron withdrawing due to -I effect.

For (ii) you probably didn't google it well, as it is quite a standard reaction in organic chemistry.

Ketones react with phosphorus penta-chloride to give acid chlorides.

http://www.ch.ic.ac.uk/local/organic/tutorial/DB_Carbonylnotes1.pdf

 

[chemisttree]

...snip

Ketones react with phosphorus penta-chloride to give acid chlorides.

Close, but not quite. The product isn't called an acid chloride.

 

[Infinitum]

Close, but not quite. The product isn't called an acid chloride.

Whoops, I meant alkyl chlorides, specifically geminal.

 

[DeeNos]

(i) Your reasoning is correct. The presence of the electronegative oxygen atom in lactic acid makes it a stronger acid than propanoic acid, which only has a hydrogen atom attached to the carboxyl group. This allows for easier release of H+ ions in lactic acid, giving it a lower pKa value (3.86) compared to propanoic acid (4.87).

(ii) Ketones do not typically react with phosphorus pentachloride (PCl5) because they do not contain a reactive hydrogen atom. However, in some cases, ketones can react with PCl5 under harsh conditions (high temperature and pressure) to form chlorinated products. This reaction is not commonly studied or used in organic chemistry.